Paste.ee

Selected Operators and Their Precedence
(See Appendix B for the complete list.)
[] Array element access
++ -- ! Increment, decrement, Boolean not
* / % Multiplication, division, remainder
+ - Addition, subtraction
< <= > >= Comparisons
== != Equal, not equal
&& Boolean and
|| Boolean or
= Assignment
Conditional Statement
if (floor >= 13)
{
actualFloor = floor - 1;
}
else if (floor >= 0)
{
actualFloor = floor;
}
else
{
System.out.println("Floor negative");
}
Condition
Executed when condition is true
Second condition (optional)
Executed when
all conditions are
false (optional)
Class Declaration
public class CashRegister
{
private int itemCount;
private double totalPrice;
public void addItem(double price)
{
itemCount++;
totalPrice = totalPrice + price;
}
. . .
}
Method
Instance variables
do
{
System.out.print("Enter a positive integer: ");
input = in.nextInt();
}
while (input <= 0);
for (double value : values)
{
sum = sum + value;
}
An array or collection
Executed for each element
Loop body executed
at least once
Set to a new element in each iteration
Executed while
condition is true
Condition
Initialization Condition Update
Loop Statements
while (balance < TARGET)
{
year++;
balance = balance * (1 + rate / 100);
}
for (int i = 0; i < 10; i++)
{
System.out.println(i);
}
String Operations
String s = "Hello";
int n = s.length(); // 5
char ch = s.charAt(1); // 'e'
String t = s.substring(1, 4); // "ell"
String u = s.toUpperCase(); // "HELLO"
if (u.equals("HELLO")) ... //  Use equals , not ==
for (int i = 0; i < s.length(); i++)
{
char ch = s.charAt(i);
Process  ch
}
Mathematical Operations
Math.pow(x, y) Raising to a power x y
Math.sqrt(x) Square root x
Math.log10(x) Decimal log log 10 (x)
Math.abs(x) Absolute value |x|
Math.sin(x)
Math.cos(x) Sine, cosine, tangent of x (x in radians)
Math.tan(x)
Variable and Constant Declarations
int cansPerPack = 6;
final double CAN_VOLUME = 0.335;
Type Name Initial value
Parameter
type and name
Exits method and
returns result.
Return type Modifiers
Method Declaration
public static double cubeVolume(double sideLength)
{
double volume = sideLength * sideLength * sideLength;
return volume;
}
Java
ConCepts
early objects
seventh edition
© S Sailer/A Sailer/Age Fotostock America, Inc.
© Frans Lemmens/SuperStock
© FLPA/John Holmes/Age Fotostock America, Inc.
Java
ConCepts
early objects
seventh edition
Cay Horstmann
San Jose State University
PUBLISHER  Don Fowley
EXECUTIVE EDITOR  Beth Lang Golub
CONTENT MANAGER  Kevin Holm
EDITORIAL PROGRAM ASSISTANT  Katherine Willis
EXECUTIVE MARKETING MANAGER  Christopher Ruel
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COVER PHOTOS (bird) © FLPA/John Holmes/Age Fotostock America,
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INTERIOR DESIGN  Maureen Eide
This book was set in Stempel Garamond by Publishing Services, and printed and bound by R.R. Donnelley &
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ISBN 978-1-118-43112-2 (Main Book)
ISBN 978-1-118-42301-1 (Binder-Ready Version)
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1
Preface
v
This book is an introduction to Java and computer programming that focuses on the
essentials—and on effective learning. The book is designed to serve a wide range of
student interests and abilities and is suitable for a first course in programming for
computer scientists, engineers, and students in other disciplines. No prior program-
ming experience is required, and only a modest amount of high school algebra is
needed. Here are the key features of this book:
Start objects early, teach object orientation gradually.
In Chapter 2, students learn how to use objects and classes from the standard library.
Chapter 3 shows the mechanics of implementing classes from a given specification.
Students then use simple objects as they master branches, loops, and arrays. Object-
oriented design starts in Chapter 8. This gradual approach allows students to use
objects throughout their study of the core algorithmic topics, without teaching bad
habits that must be un-learned later.
Guidance and worked examples help students succeed.
Beginning programmers often ask “How do I start? Now what do I do?” Of course,
an activity as complex as programming cannot be reduced to cookbook-style instruc-
tions. However, step-by-step guidance is immensely helpful for building confidence
and providing an outline for the task at hand. “Problem Solving” sections stress the
importance of design and planning. “How To” guides help students with common
programming tasks. Additional Worked Examples are available online.
Practice makes perfect.
Of course, programming students need to be able to implement nontrivial programs,
but they first need to have the confidence that they can succeed. This book contains
a substantial number of self-check questions at the end of each section. “Practice It”
pointers suggest exercises to try after each section. And additional practice opportu-
nities, including lab exercises and skill-oriented multiple-choice questions are avail-
able online.
A visual approach motivates the reader and eases navigation.
Photographs present visual analogies that explain the
nature and behavior of computer concepts. Step-by-
step figures illustrate complex program operations.
Syntax boxes and example tables present a variety
of typical and special cases in a compact format. It
is easy to get the “lay of the land” by browsing the
visuals, before focusing on the textual material.
Focus on the essentials while being
technically accurate.
An encyclopedic coverage is not helpful for a begin-
ning programmer, but neither is the opposite—
reducing the material to a list of simplistic bullet points. In this book, the essentials are
presented in digestible chunks, with separate notes that go deeper into good practices
© Terraxplorer/iStockphoto.
Visual features help the reader
with navigation.
vi Preface
or language features when the reader is ready for the additional information. You will
not find artificial over-simplifications that give an illusion of knowledge.
Reinforce sound engineering practices.
A multitude of useful tips on software quality and common errors encourage the
development of good programming habits. The optional testing track focuses on
test-driven development, encouraging students to test their programs systematically.
Provide an optional graphics track.
Graphical shapes are splendid examples of objects. Many students enjoy writing pro-
grams that create drawings or use graphical user interfaces. If desired, these topics can
be integrated into the course by using the materials at the end of Chapters 2, 3, and 10.
New to This edition
Problem Solving Strategies
This edition adds practical, step-by-step illustrations of techniques that can help stu-
dents devise and evaluate solutions to programming problems. Introduced where
they are most relevant, these strategies address barriers to success for many students.
Strategies included are:
• Algorithm Design (with pseudocode)
• Tracing Objects
• First Do It By Hand (doing sample calculations by hand)
• Flowcharts
• Selecting Test Cases
• Hand-Tracing
• Storyboards
• Adapting Algorithms
• Discovering Algorithms by Manipulating Physical Objects
• Patterns for Object Data
• Thinking Recursively
• Estimating the Running Time of an Algorithm
Optional Science and Business exercises
End-of-chapter exercises have been enhanced with problems from scientific and
business domains. Designed to engage students, the exercises illustrate the value of
programming in applied fields.
New and reorganized Topics
All chapters were revised and enhanced to respond to user feedback and improve the
flow of topics. Loop algorithms are now introduced explicitly in Chapter 6. Addi-
tional array algorithms are presented in Chapter 7 and incorporated into the prob-
lem-solving sections. Chapter 8 is more clearly focused on the design of a single class,
whereas Chapter 12 deals with relationships between classes. Chapter 15 shows how
Preface vii
to use the basic data structures from the standard library. New example tables, photo-
graphs, and exercises appear throughout the book.
a Tour of the Book
The book can be naturally grouped into three parts, as illustrated by Figure 1. The
organization of chapters offers the same flexibility as the previous edition; dependen-
cies among the chapters are also shown in the figure.
Figure 1  chapter Dependencies
9. Inheritance
10. Interfaces
13. Recursion
14. Sorting and
Searching
15. The Java
Collections
Framework
6. Iteration
8. Designing
Classes
Fundamentals
Object-Oriented Design
Data Structures & Algorithms
Online Chapters
2. Using Objects
3. Implementing
Classes
4. Fundamental
Data Types
5. Decisions
6. Loops
7. Arrays
and Array Lists
11. Input/Output
and Exception
Handling
Sections 11.1 and 11.2
(text file processing) can be
covered with Chapter 6.
1. Introduction
12. Object-
Oriented Design
viii Preface
Part A: Fundamentals (Chapters 1–7)
Chapter 1 contains a brief introduction to computer science and Java programming.
Chapter 2 shows how to manipulate objects of predefined classes. In Chapter 3,
you will build your own simple classes from given specifications. Fundamental data
types, branches, loops, and arrays are covered in Chapters 4–7.
Part B: Object-Oriented Design (Chapters 8–12)
Chapter 8 takes up the subject of class design in a systematic fashion, and it intro-
duces a very simple subset of the UML notation. The discussion of polymorphism
and inheritance is split into two chapters. Chapter 9 covers inheritance and polymor-
phism, whereas Chapter 10 covers interfaces. Exception handling and basic file input/
output are covered in Chapter 11. The exception hierarchy gives a useful example for
inheritance. Chapter 12 contains an introduction to object-oriented design, including
two significant case studies.
Part C: Data Structures and Algorithms (Chapters 13–15)
Chapters 13 through 15 (available on the book’s companion sites) contain an intro-
duction to algorithms and data structures, covering recursion, sorting and searching,
linked lists, sets, maps, stacks, and queues. These topics may be outside the scope of
a one-semester course, but can be covered as desired after Chapter 7 (see Figure 1).
Recursion, in Chapter 13, starts with simple examples and progresses to meaning-
ful applications that would be difficult to implement iteratively. Chapter 14 covers
quadratic sorting algorithms as well as merge sort, with an informal introduction to
big-Oh notation. Each data structure is presented in the context of the standard Java
collections library. You will learn the essential abstractions of the standard library
(such as iterators, sets, and maps) as well as the performance characteristics of the
various collections.
Appendices
Many instructors find it highly beneficial to require a consistent style for all assign-
ments. If the style guide in Appendix I conflicts with instructor sentiment or local
customs, however, it is available in electronic form so that it can be modified. Appen-
dices E–J are available on the Web.
A.  The Basic Latin and Latin-1 Subsets of Unicode
B.  Java Operator Summary
C.  Java Reserved Word Summary
D.  The Java Library
E.  Java Syntax Summary
F.  Tool Summary
G.  Number Systems
H.  UML Summary
I.  Java Language Coding Guidelines
J.  HTML Summary
Preface ix
Custom Book and eBook Options
Java Concepts may be ordered as a custom print or eBook that includes your choice
of chapters—including those from other Horstmann titles. Visit  customselect.wiley.
com to create your custom book order.
To order the Wiley Select Edition of Java Concepts with all 15 chapters in the
printed book, specify ISBN 978-1-119-93669-5 when you order books.
Java Concepts is available in a variety of eBook formats at prices that are signifi-
cantly lower than the printed book. Please contact your Wiley sales rep for more
information or check  www.wiley.com/college/horstmann for available versions.
Web Resources
This book is complemented by a complete suite of online resources. Go to  www.wiley.
com/college/horstmann to visit the online companion sites, which include
• “CodeCheck,” a new online service currently in development by Cay Horstmann
that students can use to check their homework assignments and to work on addi-
tional practice problems. Visit  http://horstmann.com/codecheck to learn more and to
try it out.
• Source code for all example programs in the book and in online examples.
• Worked Examples that apply the problem-solving steps in the book to other
realistic examples.
• Animations of key concepts.
• Lab exercises that apply chapter concepts (with solutions for instructors only).
• Lecture presentation slides (for instructors only).
• Solutions to all review and programming exercises (for instructors only).
• A test bank that focuses on skills, not just terminology (for instructors only). This
extensive set of multiple-choice questions can be used with a word processor or
imported into a course management system.
FULL CODE EXA
Go to  wiley.com/go/
javacode to download
a program that dem
onstrates variables
and assignments.
WORKED EXAMPLE 6.3  A Sample Debugging Session
Learn how to find bugs in an algorithm for counting the
syllables of a word. Go to  wiley.com/go/javaexamples and
download Worked Example 6.3.
MPLE
-
Pointers in the book
describe what students
will find on the Web.
x Walkthrough
a Walkthrough of the Learning aids
The pedagogical elements in this book work together to focus on and reinforce key
concepts and fundamental principles of programming, with additional tips and detail
organized to support and deepen these fundamentals. In addition to traditional
features, such as chapter objectives and a wealth of exercises, each chapter contains
elements geared to today’s visual learner.
FULL CODE EXAMPLE
Go to  wiley.com/go/
javacode to download
a program that
uses common loop
algorithms.
Additional full code examples
provides complete programs for
students to run and modify.
254 Chapter 6 Loops
6.3 The  for Loop
It often happens that you want to execute a sequence of statements a given number
of times. You can use a  while loop that is controlled by a counter, as in the following
example:
int counter = 1; //  Initialize the counter
while (counter <= 10) //  Check the counter
{
System.out.println(counter);
counter++; //  Update the counter
}
Because this loop type is so common, there is a spe-
cial form for it, called the  for loop (see Syntax 6.2).
for (int counter = 1; counter <= 10; counter++)
{
System.out.println(counter);
}
Some people call this loop count-controlled. In con-
trast, the  while loop of the preceding section can be
called an event-controlled loop because it executes
until an event occurs; namely that the balance reaches
the target. Another commonly used term for a
count-controlled loop is definite. You know from
the outset that the loop body will be executed a
definite number of times; ten times in our example.
In contrast, you do not know how many iterations it
takes to accumulate a target balance. Such a loop is
called indefinite.
The  for loop is
used when a
value runs from a
starting point to an
ending point with a
constant increment
or decrement.
You can visualize the  for loop as
an orderly sequence of steps.
Syntax 6.2  for Statement
for (int i = 5; i <= 10; i++)
{
sum = sum + i;
}
This loop executes 6 times.
See page 260.
This initialization
happens once
before the loop starts.
The condition is
checked before
each iteration.
This update is
executed after
each iteration.
The variable  i is
defined only in this  for loop.
See page 261.
These three
expressions should be related.
See page 259.
for ( initialization ;  condition ;  update )
{
statements
}
Syntax
Throughout each chapter,
margin notes show where
new concepts are introduced
and provide an outline of key ideas.
Annotations explain required
components and point to more
information on common errors
or best practices associated
with the syntax.
Annotated syntax boxes
provide a quick, visual overview
of new language constructs.
Like a variable in a computer
program, a parking space has
an identifier and a contents.
Analogies to everyday objects are
used to explain the nature and behavior
of concepts such as variables, data
types, loops, and more.
Walkthrough xi
7.5 Problem Solving: Discovering Algorithms by Manipulating Physical Objects 339
Now how does that help us with our problem, switching the first and the second
half of the array?
Let’s put the first coin into place, by swapping it with the fifth coin. However, as
Java programmers, we will say that we swap the coins in positions 0 and 4:
Problem Solving sections teach
techniques for generating ideas and
evaluating proposed solutions, often
using pencil and paper or other
artifacts. These sections emphasize
that most of the planning and problem
solving that makes students successful
happens away from the computer.
Next, we swap the coins in positions 1 and 5:
Memorable photos reinforce
analogies and help students
remember the concepts.
In the same way that there can be a street named “Main Street” in different cities,
a Java program can have multiple variables with the same name.
Step 1  Decide what work must be done inside the loop.
Every loop needs to do some kind of repetitive work, such as
• Reading another item.
• Updating a value (such as a bank balance or total).
• Incrementing a counter.
If you can’t figure out what needs to go inside the loop, start by writing down the steps that
HOW TO 6.1  Writing a Loop
This How To walks you through the process of implementing a
loop statement. We will illustrate the steps with the following
example problem.
Problem Statement  Read twelve temperature values (one for
each month) and display the number of the month with the high-
est temperature. For example, according to  worldclimate.com , the
average maximum temperatures for Death Valley are (in order by
month, in degrees Celsius):
18.2 22.6 26.4 31.1 36.6 42.2 45.7 44.5 40.2 33.1 24.2 17.6
In this case, the month with the highest temperature (45.7 degrees
Celsius) is July, and the program should display 7.
How To guides give step-by-step
guidance for common programming
tasks, emphasizing planning and
testing. They answer the beginner’s
question, “Now what do I do?” and
integrate key concepts into a
problem-solving sequence.
WORKED EXAMPLE 6.1  Credit Card Processing
Learn how to use a loop to remove spaces from a credit card
number. Go to  wiley.com/go/javaexamples and download
Worked Example 6.1.
Worked Examples apply
the steps in the How To to a
different example, showing
how they can be used to
plan, implement, and test
a solution to another
programming problem.
Table 1 Variable Declarations in Java
Variable Name Comment
int width = 20; Declares an integer variable and initializes it with 20.
int perimeter = 4 * width; The initial value need not be a fixed value. (Of course,  width
must have been previously declared.)
String greeting = "Hi!"; This variable has the type  String and is initialized with the
string “Hi”.
height = 30; Error: The type is missing. This statement is not a declaration
but an assignment of a new value to an existing variable—see
Section 2.2.5.
int width = "20"; Error: You cannot initialize a number with the string “20”.
(Note the quotation marks.)
int width; Declares an integer variable without initializing it. This can be a
cause for errors—see Common Error 2.1 on page 42.
int width, height; Declares two integer variables in a single statement. In this
book, we will declare each variable in a separate statement.
Example tables support beginners
with multiple, concrete examples.
These tables point out common
errors and present another quick
reference to the section’s topic.
xii Walkthrough
section_1/Investment.java
1 /**
2 A class to monitor the growth of an investment that
3 accumulates interest at a fixed annual rate.
4 */
5 public class Investment
6 {
7 private double balance;
8 private double rate;
9 private int year;
10
11 /**
12 Constructs an  Investment object from a starting balance and
13 interest rate.
14 @param aBalance  the starting balance
15 @param aRate  the interest rate in percent
16 */
17 public Investment(double aBalance, double aRate)
18 {
19 balance = aBalance;
20 rate = aRate;
21 year = 0;
22 }
23
24 /**
25 Keeps accumulating interest until a target balance has
26 been reached.
27 @param targetBalance  the desired balance
28 */
The  for loop neatly groups the initialization, condition, and update expressions
together. However, it is important to realize that these expressions are not executed
together (see Figure 3).
• The initialization is executed once, before the loop is entered.  1
• The condition is checked before each iteration.  2 5
• The update is executed after each iteration.  4
A N IM AT IO N
The  for Loop
Figure 3 
Execution of a
for Loop
for (int counter = 1; counter <= 10; counter++)
{
System.out.println(counter);
}
Initialize counter
1
for (int counter = 1; counter <= 10; counter++)
{
System.out.println(counter);
}
Check condition
2
for (int counter = 1; counter <= 10; counter++)
{
System.out.println(counter);
}
Execute loop body
3
for (int counter = 1; counter <= 10; counter++)
{
System.out.println(counter);
}
Update counter
4
for (int counter = 1; counter <= 10; counter++)
{
System.out.println(counter);
}
Check condition again
5
counter = 1
counter = 1
counter = 1
counter = 2
counter = 2
Students can view animations
of key concepts on the Web.
Self-check exercises at the
end of each section are designed
to make students think through
the new material—and can
spark discussion in lecture.
Optional science and business
exercises engage students with
realistic applications of Java.
Program listings are carefully
designed for easy reading,
going well beyond simple
color coding. Methods are set
off by a subtle outline.
This means “compute the value of  width + 10 1 and store that value in the variable
width  2 ” (see Figure 4).
In Java, it is not a problem that the variable  width is used on both sides of the  = sym-
bol. Of course, in mathematics, the equation width = width + 10 has no solution.
Figure 4 
Executing the Statement
width = width + 10
1
width =
width + 10
40
30
2
width = 40
Compute the value of the right-hand side
Store the value in the variable
Progressive figures trace code
segments to help students visualize
the program flow. Color is used
consistently to make variables and
other elements easily recognizable.
11.  Write the  for loop of the  Investment class as a  while loop.
12.  How many numbers does this loop print?
for (int n = 10; n >= 0; n--)
{
System.out.println(n);
}
13.  Write a  for loop that prints all even numbers between 10 and 20 (inclusive).
14.  Write a  for loop that computes the sum of the integers from 1 to  n .
Practice It  Now you can try these exercises at the end of the chapter: R6.4, R6.10, E6.8, E6.12.
S E L F C H E C K
•• Business E6.17  Currency conversion. Write a program
that first asks the user to type today’s
price for one dollar in Japanese yen,
then reads U.S. dollar values and
converts each to yen. Use 0 as a sentinel.
• Science P6.15  Radioactive decay of radioactive materials can be
modeled by the equation A = A 0 e -t(log 2/h) , where A is
the amount of the material at time t, A 0 is the amount
at time 0, and h is the half-life.
Technetium-99 is a radioisotope that is used in imaging
of the brain. It has a half-life of 6 hours. Your program
should display the relative amount A / A 0 in a patient
body every hour for 24 hours after receiving a dose.
Walkthrough xiii
Length and Size
Unfortunately, the Java syntax for
determining the number of elements
in an array, an array list, and a string
is not at all consistent. It is a com-
mon error to confuse these. You just
have to remember the correct syntax
for every data type.
Common Error 7.4
Data Type Number of Elements
Array a.length
Array list  a.size()
String a.length()
Common Errors describe the kinds
of errors that students often make,
with an explanation of why the errors
occur, and what to do about them.
Hand-Tracing
A very useful technique for understanding whether a pro-
gram works correctly is called hand-tracing. You simulate
the program’s activity on a sheet of paper. You can use this
method with pseudocode or Java code.
Get an index card, a cocktail napkin, or whatever sheet
of paper is within reach. Make a column for each variable.
Have the program code ready. Use a marker, such as a
paper clip, to mark the current statement. In your mind,
execute statements one at a time. Every time the value of a
variable changes, cross out the old value and write the new
value below the old one.
For example, let’s trace the  getTax method with the data
from the program run above.
When the  TaxReturn object is constructed, the  income
instance variable is set to 80,000 and  status is set to  MARRIED . Then the  getTax method is called.
In lines 31 and 32 of  TaxReturn.java ,  tax1 and  tax2 are initialized to 0.
29 public double getTax()
30 {
31 double tax1 = 0;
32 double tax2 = 0;
33
Programming Tip 5.5
Hand-tracing helps you
understand whether a
program works correctly.
income  status  tax1  tax2
80000 MARRIED  0  0
Because  status is not  SINGLE , we move to the  else
branch of the outer if statement (line 46).
34 if (status == SINGLE)
35 {
36 if (income <= RATE1_SINGLE_LIMIT)
37 {
38 tax1 = RATE1 * income;
39 }
40 else
41 {
42 tax1 = RATE1 * RATE1_SINGLE_LIMIT;
43 tax2 = RATE2 * (income - RATE1_SINGLE_LIMIT);
44 }
45 }
46 else
47 {
File Dialog Boxes
In a program with a graphical user interface, you will want to use a file dialog box (such as the
one shown in the figure below) whenever the users of your program need to pick a file. The
JFileChooser class implements a file dialog box for the Swing user-interface toolkit.
The  JFileChooser class has many options to fine-tune the display of the dialog box, but in its
most basic form it is quite simple: Construct a file chooser object; then call the  showOpenDialog
or  showSaveDialog method. Both methods show the same dialog box, but the button for select-
ing a file is labeled “Open” or “Save”, depending on which method you call.
For better placement of the dialog box on the screen, you can specify the user-interface
component over which to pop up the dialog box. If you don’t care where the dialog box pops
up, you can simply pass  null . The  showOpenDialog and  showSaveDialog methods return either
JFileChooser.APPROVE_OPTION , if the user has chosen a file, or  JFi leChooser.CANCEL_OPTION , if the
user canceled the selection. If a file was chosen, then you call the  getSelectedFile method to
obtain a  File object that describes the file. Here is a complete example:
JFileChooser chooser = new JFileChooser();
Scanner in = null;
if (chooser.showOpenDialog(null) == JFileChooser.APPROVE_OPTION)
{
File selectedFile = chooser.getSelectedFile();
in = new Scanner(selectedFile);
. . .
}
Special Topic 11.2
A  JFileChooser Dialog Box
Call with
showOpenDialog
method
Button is “Save” when
showSaveDialog method
is called
FULL CODE EXAMPLE
Go to  wiley.com/go/
javacode to download
a program that
demonstrates how to
use a file chooser.
When  computers
were first invented
in the 1940s, a computer filled an
entire room. The photo below shows
the ENIAC (electronic numerical inte-
grator and computer), completed in
1946 at the University of Pennsylvania.
The ENIAC was used by the military
to compute the trajectories of projec-
tiles. Nowadays, computing facilities
of search engines, Internet shops, and
social networks fill huge buildings
called data centers. At the other end of
the spectrum, computers are all around
us. Your cell phone has a computer
inside, as do many credit cards and fare
cards for public transit. A modern car
has several computers––to control the
engine, brakes, lights, and the radio.
This transit card contains a computer.
The advent of ubiqui-
tous computing changed
many aspects of our
lives.  Factories  used
to employ people to
do repetitive assembly
tasks that are today car-
ried out by computer-
controlled robots, oper-
ated by a few people
who know how to work
with those computers.
Books, music, and mov-
ies are nowadays often
consumed  on  com-
puters,  and  comput-
ers are almost always
involved in their production. The
book that you are reading right now
could not have been written without
computers.
Computing & Society 1.1 Computers Are Everywhere
Computing & Society presents social
and historical topics on computing—for
interest and to fulfill the “historical and
social context” requirements of the
ACM/IEEE curriculum guidelines.
Special Topics present optional
topics and provide additional
explanation of others. New
features of Java 7 are also
covered in these notes.
Programming Tips explain
good programming practices,
and encourage students to be
more productive with tips and
techniques such as hand-tracing.
xiv Walkthrough
Test Bank Instructors can use quiz and
test questions designed to exercise
students’ code reading and writing skills.
Web Resources
CodeCheck “CodeCheck” is a new
online service currently in development
by Cay Horstmann that students can
use to check their homework and to
work on additional practice problems.
Visit http://horstmann.com/codecheck
to learn more and to try it out.
10) What is displayed after executing the given code snippet?
int[] mymarks = new int[10];
int total = 0;
Scanner in = new Scanner(System.in);
for (int cnt = 1; cnt <= 10; cnt++)
{
System.out.print("Enter the marks: ");
mymarks[cnt] = in.nextInt();
total = total + mymarks[cnt];
}
System.out.println(total);
a) The code snippet displays the total marks of all ten subjects.
b) The for loop causes a run-time time error on the first iteration.
c) The code snippet causes a bounds error.
d) The code snippet displays zero.
1.1) Consider the following  Card  class.
public class Card
{
private String name;
public Card()
{
name = "";
}
public Card(String n)
{
name = n;
}
public String getName()
{
return name;
}
public boolean isExpired()
{
return false;
}
public String format()
{
return "Card holder: " + name;
}
}
Use this class as a superclass to implement a hierarchy of related classes:
Class  Data
IDCard
ID number
CallingCard
Card number, PIN
DriverLicense
Expiration year
Write declarations for each of the subclasses. For each subclass, supply private instance variables. Leave the
bodies of the constructors and the  format methods blank for now.
Lab Exercises These multi-part
exercises ask students to apply
chapter concepts. They can serve
as “warm-ups” in the lab or to
provide additional practice.
Animations Students can
play and replay dynamic
explanations of concepts
and program flow.
http://horstmann.com/codecheck/
acknowledgments xv
acknowledgments
Many thanks to Beth Lang Golub, Don Fowley, Elizabeth Mills, Katherine Willis,
Jenny Welter, Wendy Ashenberg, Lisa Gee, Kevin Holm, and Tim Lindner at John
Wiley & Sons, and Vickie Piercey at Publishing Services for their help with this proj-
ect. An especially deep acknowledgment and thanks goes to Cindy Johnson for her
hard work, sound judgment, and amazing attention to detail.
I am grateful to Jose Cordova, University of Louisiana, James Johnson, Norm
Krumpe, Miami University Ohio, Kathy Liszka, University of Akron, Donald Smith,
Columbia College, Brent Wilson, George Fox University, and David Woolbright,
Columbus State University, for their excellent contributions to the supplementary
materials.
Many thanks to the individuals who reviewed the manuscript for this edition,
made valuable suggestions, and brought an embarrassingly large number of errors
and omissions to my attention. They include:
Eric Aaron, Wesleyan University
James Agnew, Anne Arundel
Community College
Greg Ballinger, Miami Dade College
Jon Beck, Truman State University
Matt Boutell, Rose-Hulman Institute
of Technology
John Bundy, DeVry University Chicago
Michael Carney, Finger Lakes
Community College
Christopher Cassa, Massachusetts
Institute of Technology
Dr. Suchindran S. Chatterjee, Arizona
State University
Tina Comston, Franklin University
Lennie Cooper, Miami Dade College
Sherif Elfayoumy, University of
North Florida
Henry A Etlinger, Rochester Institute
of Technology
Guy Helmer, Iowa State University
Ed Holden, Rochester Institute
of Technology
Steven Janke, Colorado College
Mark Jones, Lock Haven University of
Pennsylvania
Dr. Mustafa Kamal, University of
Central Missouri
Gary J. Koehler, University of Florida
Ronald Krawitz, DeVry University
Norm Krumpe, Miami University Ohio
Jim Leone, Rochester Institute
of Technology
Kevin Lillis, St. Ambrose University
Darren Lim, Siena College
Hong Lin, DeVry University
Kuber Maharjan, Purdue University
College of Technology at Columbus
Patricia McDermott-Wells, Florida
International University
Bill Mongan, Drexel University
George Novacky, University
of Pittsburgh
Mimi Opkins, California State
University Long Beach
Derek Pao, City University of
Hong Kong
Katherine Salch, Illinois Central
College
Javad Shakib, DeVry University
Charlie Shu, Franklin University
Joslyn A. Smith, Florida International
University
Robert Strader, Stephen F. Austin
State University
Jonathan S. Weissman, Finger Lakes
Community College
Katherine H. Winters, University of
Tennessee Chattanooga
Tom Wulf, University of Cincinnati
Qi Yu, Rochester Institute of
Technology
xvi acknowledgments
Tim Andersen, Boise State University
Ivan Bajic, San Diego State University
Ted Bangay, Sheridan Institute
of Technology
Ian Barland, Radford University
George Basham, Franklin University
Sambit Bhattacharya, Fayetteville
State University
Rick Birney, Arizona State University
Paul Bladek, Edmonds Community
College
Joseph Bowbeer, Vizrea Corporation
Timothy A. Budd, Oregon State
University
Robert P. Burton, Brigham Young
University
Frank Butt, IBM
Jerry Cain, Stanford University
Adam Cannon, Columbia University
Nancy Chase, Gonzaga University
Archana Chidanandan, Rose-Hulman
Institute of Technology
Vincent Cicirello, The Richard Stockton
College of New Jersey
Teresa Cole, Boise State University
Deborah Coleman, Rochester Institute
of Technology
Jose Cordova, University of Louisiana,
Monroe
Valentino Crespi, California State
University, Los Angeles
Jim Cross, Auburn University
Russell Deaton, University of Arkansas
Geoffrey Decker, Northern Illinois
University
H. E. Dunsmore, Purdue University
Robert Duvall, Duke University
Eman El-Sheikh, University of
West Florida
John Fendrich, Bradley University
David Freer, Miami Dade College
John Fulton, Franklin University
David Geary, Sabreware, Inc.
Margaret Geroch, Wheeling Jesuit
University
Ahmad Ghafarian, North Georgia
College & State University
Rick Giles, Acadia University
Stacey Grasso, College of San Mateo
Jianchao Han, California State
University, Dominguez Hills
Lisa Hansen, Western New
England College
Elliotte Harold
Eileen Head, Binghamton University
Cecily Heiner, University of Utah
Brian Howard, Depauw University
Lubomir Ivanov, Iona College
Norman Jacobson, University of
California, Irvine
Curt Jones, Bloomsburg University
Aaron Keen, California Polytechnic
State University, San Luis Obispo
Mugdha Khaladkar, New Jersey
Institute of Technology
Elliot Koffman, Temple University
Kathy Liszka, University of Akron
Hunter Lloyd, Montana State
University
Youmin Lu, Bloomsburg University
John S. Mallozzi, Iona College
John Martin, North Dakota
State University
Jeanna Matthews, Clarkson University
Scott McElfresh, Carnegie Mellon
University
Joan McGrory, Christian Brothers
University
Carolyn Miller, North Carolina State
University
Sandeep R. Mitra, State University of
New York, Brockport
Teng Moh, San Jose State University
John Moore, The Citadel
Jose-Arturo Mora-Soto, Jesica Rivero-
Espinosa, and Julio-Angel Cano-
Romero, University of Madrid
Faye Navabi, Arizona State University
Parviz Partow-Navid, California State
University, Los Angeles
Every new edition builds on the suggestions and experiences of prior reviewers and
users. I am grateful for the invaluable contributions these individuals have made:
acknowledgments xvii
Kevin O’Gorman, California
Polytechnic State University,
San Luis Obispo
Michael Olan, Richard Stockton College
Kevin Parker, Idaho State University
Jim Perry, Ulster County Community
College
Cornel Pokorny, California Polytechnic
State University, San Luis Obispo
Roger Priebe, University of Texas,
Austin
C. Robert Putnam, California State
University, Northridge
Kai Qian, Southern Polytechnic
State University
Cyndi Rader, Colorado School of Mines
Neil Rankin, Worcester Polytechnic
Institute
Brad Rippe, Fullerton College
Pedro I. Rivera Vega, University of
Puerto Rico, Mayaguez
Daniel Rogers, SUNY Brockport
Chaman Lal Sabharwal, Missouri
University of Science and Technology
John Santore, Bridgewater
State College
Carolyn Schauble, Colorado
State University
Brent Seales, University of Kentucky
Christian Shin, SUNY Geneseo
Jeffrey Six, University of Delaware
Don Slater, Carnegie Mellon University
Ken Slonneger, University of Iowa
Donald Smith, Columbia College
Stephanie Smullen, University of
Tennessee, Chattanooga
Monica Sweat, Georgia Institute
of Technology
Peter Stanchev, Kettering University
Shannon Tauro, University of
California, Irvine
Ron Taylor, Wright State University
Russell Tessier, University of
Massachusetts, Amherst
Jonathan L. Tolstedt, North Dakota
State University
David Vineyard, Kettering University
Joseph Vybihal, McGill University
Xiaoming Wei, Iona College
Todd Whittaker, Franklin University
Robert Willhoft, Roberts Wesleyan
College
Lea Wittie, Bucknell University
David Womack, University of Texas
at San Antonio
David Woolbright, Columbus State
University
Catherine Wyman, DeVry University
Arthur Yanushka, Christian Brothers
University
Salih Yurttas, Texas A&M University
cONTeNTS
xix
Preface  v
SPecIaL feaTureS  xxiv
INTrODucTION 1
1.1  computer Programs  2
1.2  The anatomy of a computer  3
1.3  The Java Programming Language  6
1.4  Becoming familiar with Your Programming environment  8
1.5  analyzing Your first Program  12
1.6  errors  15
1.7  Problem Solving: algorithm Design  16
uSING OBJecTS 33
2.1  Objects and classes  34
2.2  Variables  36
2.3  calling Methods  43
2.4  constructing Objects  48
2.5  accessor and Mutator Methods  50
2.6  The aPI Documentation  52
2.7  Implementing a Test Program 55
2.8  Object references 57
2.9  Graphical applications 61
2.10  ellipses, Lines, Text, and color 66
IMPLeMeNTING cLaSSeS 81
3.1  Instance Variables and encapsulation  82
3.2  Specifying the Public Interface of a class  86
3.3  Providing the class Implementation  93
3.4  unit Testing 102
3.5  Problem Solving: Tracing Objects  105
3.6  Local Variables  107
3.7  The  this  reference  109
3.8  Shape classes 112
ChAPteR 1
ChAPteR 2
ChAPteR 3
xx contents
fuNDaMeNTaL DaTa TYPeS 131
4.1  Numbers  132
4.2  arithmetic  139
4.3  Input and Output  147
4.4  Problem Solving: first Do it By Hand  154
4.5  Strings  156
DecISIONS 179
5.1  The  if  Statement  180
5.2  comparing Values  186
5.3  Multiple alternatives  196
5.4  Nested Branches  200
5.5  Problem Solving: flowcharts  207
5.6  Problem Solving: Selecting Test cases 210
5.7  Boolean Variables and Operators  213
5.8  application: Input Validation  218
LOOPS 241
6.1  The  while  Loop  242
6.2  Problem Solving: Hand-Tracing  249
6.3  The  for  Loop  254
6.4  The  do  Loop  262
6.5  application: Processing Sentinel Values  263
6.6  Problem Solving: Storyboards  269
6.7  common Loop algorithms  272
6.8  Nested Loops  279
6.9  application: random Numbers and Simulations  283
6.10 using a Debugger 286
 arraYS aND arraY LISTS 311
7.1  arrays  312
7.2  The enhanced  for  Loop  321
7.3  common array algorithms  322
7.4  Problem Solving: adapting algorithms  331
7.5  Problem Solving: Discovering algorithms by Manipulating
Physical Objects  336
7.6  Two-Dimensional arrays  340
ChAPteR 4
ChAPteR 5
ChAPteR 6
ChAPteR 7
contents xxi
7.7  array Lists  347
7.8  regression Testing 356
DeSIGNING cLaSSeS 379
8.1  Discovering classes  380
8.2  Designing Good Methods  381
8.3  Problem Solving: Patterns for Object Data  390
8.4  Static Variables and Methods  395
8.5  Packages  400
8.6  unit Test frameworks  407
INHerITaNce 421
9.1  Inheritance Hierarchies  422
9.2  Implementing Subclasses  426
9.3  Overriding Methods  431
9.4  Polymorphism  437
9.5  Object : The cosmic Superclass  448
INTerfaceS 463
10.1  using Interfaces for algorithm reuse  464
10.2  Working with Interface Variables  471
10.3  The  Comparable  Interface  473
10.4  using Interfaces for callbacks  477
10.5  Inner classes  481
10.6  Mock Objects  483
10.7  event Handling  484
10.8  Building applications with Buttons  490
10.9  Processing Timer events  494
10.10 Mouse events  497
INPuT/OuTPuT aND excePTION HaNDLING 513
11.1  reading and Writing Text files  514
11.2  Text Input and Output  519
11.3  command Line arguments  527
11.4  exception Handling  534
11.5  application: Handling Input errors  545
ChAPteR 8
ChAPteR 9
ChAPteR 10
ChAPteR 11
xxii contents
OBJecT-OrIeNTeD DeSIGN 559
12.1  classes and Their responsibilities  560
12.2  relationships Between classes  563
12.3  application: Printing an Invoice  569
recurSION (WeB ONLY)
13.1 Triangle Numbers 
13.2 recursive Helper Methods 
13.3 The efficiency of recursion 
13.4 Permutations 
13.5 Mutual recursion 
13.6 Backtracking 
SOrTING aND SearcHING (WeB ONLY)
14.1 Selection Sort 
14.2 Profiling the Selection Sort algorithm 
14.3 analyzing the Performance of the Selection Sort algorithm 
14.4 Merge Sort 
14.5 analyzing the Merge Sort algorithm 
14.6 Searching 
14.7 Problem Solving: estimating the running Time of an algorithm 
14.8 Sorting and Searching in the Java Library 
THe JaVa cOLLecTIONS fraMeWOrk
(WeB ONLY)
15.1 an Overview of the collections framework 
15.2 Linked Lists 
15.3 Sets 
15.4 Maps 
15.5 Stacks, Queues, and Priority Queues 
15.6 Stack and Queue applications 
aPPeNDIx a  THe BaSIc LaTIN aND LaTIN-1 SuBSeTS Of uNIcODe   A-1
aPPeNDIx B  JaVa OPeraTOr SuMMarY  A-5
aPPeNDIx c  JaVa reSerVeD WOrD SuMMarY  A-7
aPPeNDIx D  THe JaVa LIBrarY  A-9
aPPeNDIx e  JaVa SYNTax SuMMarY
ChAPteR 12
ChAPteR 13
ChAPteR 14
ChAPteR 15
APPendiCeS
contents xxiii
aPPeNDIx f  TOOL SuMMarY
aPPeNDIx G  NuMBer SYSTeMS
aPPeNDIx H  uML SuMMarY
aPPeNDIx I  JaVa LaNGuaGe cODING GuIDeLINeS
aPPeNDIx J  HTML SuMMarY
GLOSSarY  G-1
INDex  i-1
creDITS  C-1
SYNTax BOxeS
arrays   313
array Lists   347
assignment   41
calling a Superclass Method  431
cast  143
catching exceptions   536
class Declaration  89
comparisons   187
constant Declaration  136
constructor with Superclass Initializer  436
Declaring an Interface  465
for  Statement   254
if  Statement   182
Implementing an Interface  467
Importing a class from a Package  54
Input Statement   147
Instance Variable Declaration  83
Java Program  13
Object construction  49
Package Specification  402
Subclass Declaration  428
The enhanced  for  Loop  322
The  finally  clause  540
The  instanceof  Operator  451
The  throws  clause  539
Throwing an exception  534
Two-Dimensional array Declaration  341
while  Statement  243
Variable Declaration  37
AlPhAbetiCAl liSt oF
xxiv Special features
© Steve Simzer/iStockphoto.
cHaPTer
Available online at  www.wiley.com/college/horstmann .
© John Bell/iStockphoto.
common
errors
How Tos
  and
Worked examples
1  Introduction
Omitting Semicolons  14
Misspelling Words  16
Describing an algorithm 
with Pseudocode  20
Writing an algorithm for 
Tiling a floor  22
2  using Objects
using undeclared or
uninitialized Variables  42
confusing Variable Declarations
and assignment Statements  42
Trying to Invoke a constructor
Like a Method  50
How Many Days Have You 
Been alive?
Working with Pictures
3  Implementing classes
Declaring a constructor 
as  void    92
Ignoring Parameter Variables 98
Duplicating Instance Variables
in Local Variables  108
Providing unnecessary
Instance Variables  108
forgetting to Initialize Object 
references in a constructor 109
Implementing a class  98
Making a Simple Menu
Drawing Graphical Shapes  116
4  fundamental 
Data Types
unintended Integer Division 144
unbalanced Parentheses  144
carrying out computations 151
computing the Volume and
Surface area of a Pyramid
computing Travel Time
Special features xxv
© Eric Isselé/iStockphoto.
© Eric Isselé/iStockphoto.
Media Bakery.
Programming
Tips
Special Topics
computing &
Society
Backup copies   11
computers are everywhere  5
choose Descriptive 
Variable Names  43
Learn By Trying   47
Don’t Memorize—use 
Online Help  55
Testing classes in an Interactive
environment  56
computer Monopoly   60
The  javadoc  utility  92 calling One constructor 
from another  112
electronic Voting Machines  104
Do Not use Magic Numbers 139
Spaces in expressions  145
reading exception reports  162
Big Numbers  138
combining assignment 
and arithmetic  145
Instance Methods and 
Static Methods  145
using Dialog Boxes for Input 
and Output  162
The Pentium floating-Point
Bug   146
International alphabets 
and unicode   163
xxvi Special features
© Steve Simzer/iStockphoto.
cHaPTer
Available online at  www.wiley.com/college/horstmann .
© John Bell/iStockphoto.
common
errors
How Tos
  and
Worked examples
5  Decisions
a Semicolon after the 
if  condition  184
using  ==  to compare Strings 192
The Dangling  else  Problem 204
combining Multiple 
relational Operators  216
confusing  &&  and
||  conditions  216
Implementing an 
if  Statement  193
extracting the Middle
6  Loops
Don’t Think “are We
There Yet?”  247
Infinite Loops  248
Off-by-One errors  248
Writing a Loop  276
credit card Processing
Manipulating the Pixels
in an Image
Debugging  289
a Sample Debugging Session
7  arrays and array Lists
Bounds errors   318
uninitialized and 
unfilled arrays  318
underestimating the 
Size of a Data Set  331
Length and Size  356
Working with arrays  334
rolling the Dice
a World Population Table
8  Designing classes
Trying to access Instance 
Variables in Static Methods 398
confusing Dots  403
Programming with Packages 404
Special features xxvii
© Eric Isselé/iStockphoto.
© Eric Isselé/iStockphoto.
Media Bakery.
Programming
Tips
Special Topics
computing &
Society
Brace Layout  184
always use Braces  184
Tabs  185
avoid Duplication 
in Branches  186
Hand-Tracing  203
Make a Schedule and 
Make Time for 
unexpected Problems  212
The conditional Operator  185
The  switch  Statement  199
Block Scope  205
enumeration Types  206
Logging  212
Short-circuit evaluation of 
Boolean Operators  217
De Morgan’s Law  217
Denver’s Luggage
Handling System  195
artificial Intelligence  221
use  for  Loops for Their
Intended Purpose Only  259
choose Loop Bounds That
Match Your Task  260
count Iterations  260
flowcharts for Loops  263
Variables Declared in a
for  Loop Header  261
redirection of Input 
and Output  266
The Loop-and-a-Half Problem 266
The  break  and  continue 
Statements  267
Software Piracy  253
The first Bug  291
use arrays for Sequences of 
related Items  318
Make Parallel arrays into 
arrays of Objects  318
Batch files and Shell Scripts 358
Methods with a Variable 
Number of arguments  319
Sorting with the Java Library 331
Two-Dimensional arrays 
with Variable row Lengths 345
Multidimensional arrays  347
The Diamond Syntax in 
Java 7  356
computer Viruses  320
The Therac-25 Incidents  359
consistency  385
Minimize the use of 
Static Methods  397
call by Value and call 
by reference  386
Static Imports  398
alternative forms of 
Instance and Static 
Variable Initialization   399
Package access  404
Personal computing   406
xxviii Special features
© Steve Simzer/iStockphoto.
cHaPTer
Available online at  www.wiley.com/college/horstmann .
© John Bell/iStockphoto.
common
errors
How Tos
  and
Worked examples
9  Inheritance
replicating Instance Variables
from the Superclass  430
confusing Super- and 
Subclasses  430
accidental Overloading  435
forgetting to use  super 
When Invoking a 
Superclass Method  435
Don’t use Type Tests   452
Developing an 
Inheritance Hierarchy  443
Implementing an 
employee Hierarchy for 
Payroll Processing
10  Interfaces
forgetting to Declare Implement-
ing Methods as Public  470
Trying to Instantiate
an Interface  470
Modifying Parameter Types in 
the Implementing Method 489
Trying to call
Listener Methods   490
forgetting to attach 
a Listener  493
forgetting to repaint  496
Investigating Number 
Sequences
11  Input/Output and 
exception Handling
Backslashes in file Names  517
constructing a Scanner 
with a String  517
Processing Text files   530
analyzing Baby Names 
12  Object-Oriented Design
using crc cards and 
uML Diagrams in 
Program Design  566
Simulating an automatic 
Teller Machine
Special features xxix
© Eric Isselé/iStockphoto.
© Eric Isselé/iStockphoto.
Media Bakery.
Programming
Tips
Special Topics
computing &
Society
use a Single class for Variation
in Values, Inheritance for 
Variation in Behavior  426
calling the Superclass 
constructor  436
Dynamic Method Lookup and 
the Implicit Parameter   440
abstract classes   441
final Methods and classes   442
Protected access   442
Inheritance and the 
toString  Method   453
Inheritance and the 
equals  Method   454
Who controls the Internet?  454
Don’t use a container 
as a Listener  493
constants in Interfaces  470
The  clone  Method and the
Cloneable  Interface  475
anonymous classes  482
keyboard events  500
event adapters  501
Open Source and 
free Software  502
Throw early, catch Late  542
Do Not Squelch exceptions  542
Do Not use  catch  and  finally 
in the Same  try  Statement 542
Do Throw Specific 
exceptions  543
reading Web Pages  517
file Dialog Boxes  517
character encodings  518
regular expressions  526
assertions  543
automatic resource Management
in Java 7  544
encryption algorithms  533
The ariane rocket Incident  544
attributes and Methods in 
uML Diagrams  567
Multiplicities  568
aggregation, association, 
and composition  568
Databases and Privacy  580
xxx Special features
© Steve Simzer/iStockphoto.
cHaPTer
Available online at  www.wiley.com/college/horstmann .
© John Bell/iStockphoto.
common
errors
How Tos
  and
Worked examples
13  recursion 
(WeB ONLY)
Infinite recursion
Tracing Through recursive 
Methods
Thinking recursively 
finding files
Towers of Hanoi
14  Sorting and Searching
(WeB ONLY)
The  compareTo  Method can
return any Integer, 
Not Just –1, 0, and 1
enhancing the Insertion 
Sort algorithm
15  The Java collections
framework 
(WeB ONLY)
choosing a collection
Word frequency
Simulating a Queue of 
Waiting customers 
Special features xxxi
Available online at  www.wiley.com/college/horstmann .
© Eric Isselé/iStockphoto.
© Eric Isselé/iStockphoto.
Media Bakery.
Programming
Tips
Special Topics
computing &
Society
The Limits of computation
Oh, Omega, and Theta
Insertion Sort
The Quicksort algorithm
The Parameterized 
Comparable  Interface
The  Comparator  Interface
The first Programmer
use Interface references to 
Manipulate Data Structures
Hash functions
reverse Polish Notation
Standardization
1
C h a p t e r
1
IntroduCtIon
to learn about computers
and programming
to compile and run your first Java program
to recognize compile-time and run-time errors
to describe an algorithm with pseudocode
C h a p t e r G o a l s
C h a p t e r C o n t e n t s
1.1 Computer programs 2
1.2 the anatomy of a Computer 3
Computing & Society 1.1: Computers are
everywhere 5
1.3 the Java programming
Language 6
1.4 BeComing famiLiar with your
programming environment 8
Programming Tip 1.1: Backup Copies 11
1.5 anaLyzing your
first program 12
Syntax 1.1: Java program 13
Common Error 1.1: omitting semicolons 14
1.6 errors 15
Common Error 1.2: Misspelling Words 16
1.7 proBLem soLving:
aLgorithm Design 16
How To 1.1: describing an algorithm with
pseudocode 20
Worked Example 1.1: Writing an algorithm for
tiling a Floor 22
© JanPietruszka/iStockphoto.
2
Just as you gather tools, study a project, and make a plan for
tackling it, in this chapter you will gather up the basics you
need to start learning to program. after a brief introduction
to computer hardware, software, and programming in
general, you will learn how to write and run your first
Java program. You will also learn how to diagnose and
fix programming errors, and how to use pseudocode to
describe an algorithm—a step-by-step description of how
to solve a problem—as you plan your computer programs.
1.1 Computer programs
You have probably used a computer for work or fun. Many people use computers
for everyday tasks such as electronic banking or writing a term paper. Computers are
good for such tasks. They can handle repetitive chores, such as totaling up numbers
or placing words on a page, without getting bored or exhausted.
The flexibility of a computer is quite an amazing phenomenon. The same machine
can balance your checkbook, lay out your term paper, and play a game. In contrast,
other machines carry out a much nar rower range of tasks; a car drives and a toaster
toasts. Computers can carry out a wide range of tasks because they execute different
programs, each of which directs the computer to work on a specific task.
The computer itself is a machine that stores data (numbers, words, pictures), inter-
acts with devices (the monitor, the sound system, the printer), and executes programs.
A computer program tells a computer, in minute detail, the sequence of steps that are
needed to fulfill a task. The physical computer and periph eral devices are collectively
called the hardware. The programs the computer executes are called the soft ware.
Today’s computer programs are so sophisticated that it is hard to believe that they
are composed of extremely primitive instructions. A typical instruction may be one
of the following:
• Put a red dot at a given screen position.
• Add up two numbers.
• If this value is negative, continue the program at a certain instruction.
The computer user has the illusion of smooth interaction because a program contains
a huge number of such instructions, and because the computer can execute them at
great speed.
The act of designing and implementing computer programs is called program-
ming. In this book, you will learn how to program a computer—that is, how to direct
the computer to execute tasks.
To write a computer game with motion and sound effects or a word processor
that supports fancy fonts and pictures is a complex task that requires a team of many
highly-skilled programmers. Your first programming efforts will be more mundane.
The concepts and skills you learn in this book form an important foundation, and
you should not be disappointed if your first programs do not rival the sophis ticated
software that is familiar to you. Actually, you will find that there is an immense thrill
even in sim ple programming tasks. It is an amazing experience to see the computer
precisely and quickly carry out a task that would take you hours of drudgery, to
Computers
execute very basic
instructions in
rapid succession.
a computer program
is a sequence
of instructions
and decisions.
programming is the
act of designing
and implementing
computer programs.
JanPietruszka/iStockphoto.
1.2 the anatomy of a Computer 3
make small changes in a program that lead to immediate improvements, and to see the
computer become an extension of your mental powers.
1.  What is required to play music on a computer?
2.  Why is a CD player less flexible than a computer?
3.  What does a computer user need to know about programming in order to play a
video game?
1.2 the anatomy of a Computer
To understand the programming process, you need to have a rudimentary under-
standing of the building blocks that make up a computer. We will look at a personal
computer. Larger computers have faster, larger, or more powerful components, but
they have fundamentally the same design.
At the heart of the computer lies the central
processing unit (CPU) (see Figure 3). The inside
wiring of the CPU is enormously complicated.
For example, the Intel Core processor (a popular
CPU for per sonal computers at the time of this
writing) is composed of several hundred million
structural elements, called transistors.
The CPU performs program control and data
processing. That is, the CPU locates and executes
the program instructions; it carries out arith-
metic operations such as addition, subtraction,
multiplication, and division; it fetches data from
external memory or devices and places processed
data into storage.
There are two kinds of storage. Primary storage
or memory is made from electronic circuits that can store data, provided they are
supplied with electric power. Secondary storage, usually a hard disk (see Figure 2)
© Nicholas Homrich/iStockphoto.
s e L f  C h e C k
figure 1  Central processing unit
© Amorphis/iStockphoto.
the central
processing unit (Cpu)
performs program
control and
data processing.
storage devices
include memory and
secondary storage.
figure 2  a hard disk
© PhotoDisc, Inc./Getty Images.
4 Chapter 1 Introduction
or a solid-state drive, provides
figure 3  schematic design of a personal Computer
Printer
Mouse/Trackpad
Keyboard
Microphone
Ports
CPU
Memory
Disk
controller
Secondary storage
Monitor
Speakers
Internet
Network
controller
slower and less expensive storage that persists without
electricity. A hard disk consists of rotating platters, which are coated with a mag netic
material. A solid-state drive uses electronic components that can retain information
without power, and without moving parts.
To interact with a human user, a computer requires peripheral devices. The com-
puter transmits infor mation (called output) to the user through a display screen,
speakers, and printers. The user can enter information (called input) for the computer
by using a keyboard or a pointing device such as a mouse.
Some computers are self-contained units, whereas others are interconnected
through networks. Through the network cabling, the computer can read data and
programs from central storage locations or send data to other computers. To the user
of a networked computer, it may not even be obvious which data reside on the com-
puter itself and which are transmitted through the network.
Figure 3 gives a schematic overview of the architecture of a personal computer.
Program instructions and data (such as text, numbers, audio, or video) reside in sec-
ondary storage or elsewhere on the network. When a program is started, its instruc-
tions are brought into memory, where the CPU can read them. The CPU reads and
executes one instruction at a time. As directed by these instructions, the CPU reads
data, modifies it, and writes it back to memory or secondary storage. Some program
instruc tions will cause the CPU to place dots on the display screen or printer or to
vibrate the speaker. As these actions happen many times over and at great speed, the
human user will perceive images and sound. Some program instructions read user
input from the keyboard, mouse, touch sensor, or microphone. The program ana-
lyzes the nature of these inputs and then executes the next appropriate instruction.
1.2 The Anatomy of a Computer  5
4.  Where is a program stored when it is not currently running?
5.  Which part of the computer carries out arithmetic operations, such as addition
and multiplication?
6.  A modern smartphone is a computer, comparable to a desktop computer. Which
components of a smartphone correspond to those shown in Figure 3?
Practice It  Now you can try these exercises at the end of the chapter: R1.2, R1.3.
© Nicholas Homrich/iStockphoto.
S e l f C h e C k
When  computers
were first invented
in the 1940s, a computer filled an
entire room. The photo below shows
the ENIAC (electronic numerical inte-
grator and computer), completed in
1946 at the University of Pennsylvania.
The ENIAC was used by the military
to compute the trajectories of projec-
tiles. Nowadays, computing facilities
of search engines, Internet shops, and
social networks fill huge buildings
called data centers. At the other end of
the spectrum, computers are all around
us. Your cell phone has a computer
inside, as do many credit cards and fare
cards for public transit. A modern car
has several computers––to control the
engine, brakes, lights, and the radio.
© UPPA/Photoshot.
The ENIAC
© Maurice Savage/Alamy Limited.
This transit card contains a computer.
The advent of ubiqui-
tous computing changed
many aspects of our
lives.  Factories  used
to employ people to
do repetitive assembly
tasks that are today car-
ried out by computer-
controlled robots, oper-
ated by a few people
who know how to work
with those computers.
Books, music, and mov-
ies nowadays are often
consumed  on  com-
puters,  and  comput-
ers are almost always
involved in their production. The
book that you are reading right now
could not have been written without
computers.
Knowing about computers and
how to program them has become
an essential skill in many careers.
Engineers design computer-controlled
cars and medical equipment that
preserve lives. Computer scientists
develop programs that help people
come together to support social
causes. For example, activists used
social  networks  to  share  videos
showing abuse by repressive regimes,
and this information was instrumental
in changing public opinion.
As computers, large and small,
become ever more embedded in our
everyday lives, it is increasingly impor-
tant for everyone to understand how
they work, and how to work with them.
As you use this book to learn how to
program a computer, you will develop
a good understanding of computing
fundamentals that will make you a
more informed citizen and, perhaps, 
a computing professional.
Computing & Society 1.1  Computers Are Everywhere
© Media Bakery.
6 Chapter 1 Introduction
1.3 the Java programming language
In order to write a computer program, you need
to provide a sequence of instructions that the CPU
can execute. A computer program consists of a large
number of simple CPU instructions, and it is tedious
and error-prone to specify them one by one. For that
reason, high-level programming languages have
been created. In a high-level language, you specify
the actions that your program should carry out. A
compiler translates the high-level instructions into
the more detailed instructions (called machine code)
required by the CPU. Many different programming
languages have been designed for different purposes.
In 1991, a group led by James Gosling and Patrick
Naughton at Sun Microsystems designed a program-
ming language, code-named “Green”, for use in con-
sumer devices, such as intelligent television “set-top”
boxes. The language was designed to be simple, secure, and usable for many dif ferent
processor types. No customer was ever found for this technology.
Gosling recounts that in 1994 the team realized, “We could write a really cool
browser. It was one of the few things in the client/server main stream that needed
some of the weird things we’d done: architecture neu tral, real-time, reliable, secure.”
Java was introduced to an enthusiastic crowd at the SunWorld exhibition in 1995,
together with a browser that ran applets—Java code that can be located anywhere on
the Internet. Figure 4 shows a typical example of an applet.
© James Sullivan/Getty Images.
James Gosling
Java was originally
designed for
programming
consumer devices,
but it was first
successfully used
to write Internet
applets.
figure 4  an applet for Visualizing Molecules running in a
Browser Window (http://jmol.sourceforge.net/)
1.3 the Java programming language 7
table 1 Java Versions
Version Year Important new Features
1.0 1996
1.1 1997 Inner classes
1.2 1998 Swing, Collections framework
1.3 2000 Performance enhancements
1.4 2002 Assertions, XML support
5 2004 Generic classes, enhanced  for loop, auto-boxing, enumerations, annotations
6 2006 Library improvements
7 2011 Small language changes and library improvements
Since then, Java has grown at a phenomenal rate. Programmers have embraced the
language because it is easier to use than its closest rival, C++. In addition, Java has a
rich library that makes it possible to write portable programs that can bypass pro-
prietary operating systems—a feature that was eagerly sought by those who wanted
to be independent of those proprietary systems and was bitterly fought by their ven-
dors. A “micro edition” and an “enterprise edition” of the Java library allow Java
programmers to target hardware ranging from smart cards and cell phones to the
largest Internet servers.
Because Java was designed for the Internet, it has two attributes that make it very
suitable for begin ners: safety and portability.
You can run a Java program in your browser without fear. The safety features
of the Java language ensure that a program is terminated if it tries to do something
unsafe. Having a safe environment is also helpful for anyone learning Java. When you
make an error that results in unsafe behavior, your program is terminated and you
receive an accurate error report.
The other benefit of Java is portability. The same Java program will run, without
change, on Windows, UNIX, Linux, or Macintosh. In order to achieve portability,
the Java compiler does not translate Java programs directly into CPU instructions.
Instead, compiled Java programs contain instructions for the Java virtual machine,
a program that simulates a real CPU. Portability is another benefit for the begin ning
student. You do not have to learn how to write programs for different platforms.
At this time, Java is firmly established as one of the most important languages for
general-purpose pro gramming as well as for computer science instruction. However,
although Java is a good language for beginners, it is not perfect, for three reasons.
Because Java was not specifically designed for students, no thought was given to
making it really sim ple to write basic programs. A certain amount of technical machin-
ery is necessary to write even the simplest programs. This is not a problem for pro-
fessional programmers, but it can be a nuisance for beginning students. As you learn
how to program in Java, there will be times when you will be asked to be satisfied with
a preliminary explanation and wait for more complete detail in a later chapter.
Java has been extended many times during its life—see Table 1. In this book, we
assume that you have Java version 5 or later.
Java was designed to
be safe and portable,
benefiting both
Internet users
and students.
Java programs
are distributed as
instructions for a
virtual machine,
making them
platform-independent.
8 Chapter 1 Introduction
Finally, you cannot hope to learn all of Java in one course. The Java language itself
is relatively simple, but Java contains a vast set of library packages that are required
to write useful programs. There are pack ages for graphics, user-interface design,
cryptography, networking, sound, database storage, and many other purposes. Even
expert Java programmers cannot hope to know the contents of all of the packages—
they just use those that they need for particular projects.
Using this book, you should expect to learn a good deal about the Java language
and about the most important packages. Keep in mind that the central goal of this
book is not to make you memorize Java minutiae, but to teach you how to think
about programming.
7.  What are the two most important benefits of the Java language?
8.  How long does it take to learn the entire Java library?
practice it  Now you can try this exercise at the end of the chapter: R1.5.
1.4 Becoming Familiar with Your
programming environment
Many students find that the tools they need as programmers are very different from
the software with which they are familiar. You should spend some time making your-
self familiar with your programming environment. Because computer systems vary
widely, this book can only give an outline of the steps you need to follow. It is a good
idea to participate in a hands-on lab, or to ask a knowledgeable friend to give you a
tour.
step 1  Start the Java development environment.
Computer systems differ greatly in this regard. On many computers there is an inte-
grated development environment in which you can write and test your programs.
On other computers you first launch an editor, a program that functions like a word
processor, in which you can enter your Java instructions; you then open a console
window and type commands to execute your program. You need to find out how to
get started with your environment.
step 2  Write a simple program.
The traditional choice for the very first program in a new programming language is
a program that dis plays a simple greeting: “Hello, World!”. Let us follow that tradi-
tion. Here is the “Hello, World!” pro gram in Java:
public class HelloPrinter
{
public static void main(String[] args)
{
System.out.println("Hello, World!");
}
}
We will examine this program in the next section.
Java has a very
large library. Focus
on learning those
parts of the library
that you need for
your programming
projects.
© Nicholas Homrich/iStockphoto.
s e L f  C h e C k
set aside some
time to become
familiar with the
programming
environment that
you will use for your
class work.
an editor is a
program for entering
and modifying
text, such as a Java
program.
1.4 Becoming Familiar with Your programming environment 9
figure 5 
running the
HelloPrinter
program in an
Integrated
development
environment
Java program
Program output
Click to compile and run
No matter which programming environment you use, you begin your activity by
typing the program statements into an editor window.
Create a new file and call it  HelloPrinter.java , using the steps that are appropriate
for your environ ment. (If your environment requires that you supply a project name
in addition to the file name, use the name  hello for the project.) Enter the program
instructions exactly as they are given above. Alternatively, locate the electronic copy
in this book’s companion code and paste it into your editor.
As you write this program, pay careful attention to the various symbols, and
keep in mind that Java is case sensitive. You must enter upper- and lowercase letters
exactly as they appear in the program listing. You cannot type  MAIN or  PrintLn . If you
are not careful, you will run into problems—see Common Error 1.2 on page 16.
step 3  Run the program.
The process for running a program depends greatly on your programming environ-
ment. You may have to click a button or enter some commands. When you run the
test program, the message
Hello, World!
will appear somewhere on the screen (see Figures 5 and 6).
Java is case sensitive.
You must be careful
about distinguishing
between upper- and
lowercase letters.
figure 6 
running the  HelloPrinter
program in a Console Window
10 Chapter 1 Introduction
figure 7  From source Code to running program
Compiler Editor
Virtual
Machine
Running
Program
Source File
Class files
Library files
In order to run your program, the Java compiler translates your source files (that
is, the statements that you wrote) into class files. (A class file contains instructions for
the Java virtual machine.) After the compiler has translated your source code into
virtual machine instructions, the virtual machine executes them. During execution,
the virtual machine accesses a library of pre-written code, including the implementa-
tions of the  System and  PrintStream classes that are necessary for displaying the pro-
gram’s output. Figure 7 summarizes the process of creating and running a Java pro-
gram. In some programming environments, the compiler and virtual machine are
essentially invisible to the programmer—they are automatically executed whenever
you ask to run a Java program. In other environments, you need to launch the com-
piler and virtual machine explicitly.
step 4  Organize your work.
As a programmer, you write programs, try them out, and improve them. You store
your programs in files. Files are stored in folders or directories. A folder can contain
files as well as other folders, which themselves can contain more files and folders (see
Figure 8). This hierarchy can be quite large, and you need not be concerned with all of
its branches. However, you should create folders for organizing your work. It is a
good idea to make a separate folder for your programming class. Inside that folder,
make a separate folder for each program.
the Java compiler
translates source
code into class
files that contain
instructions for the
Java virtual machine.
Camera: © james steidl/iStockphoto.
Globe: © Alex Slobodkin/iStockphoto.
A N IM AT IO N
Compilation Process
figure 8 
a Folder hierarchy
1.4 Becoming Familiar with Your programming environment 11
Some programming environments place your programs into a default location if
you don’t specify a folder yourself. In that case, you need to find out where those files
are located.
Be sure that you understand where your files are located in the folder hierarchy.
This information is essential when you submit files for grading, and for making
backup copies (see Programming Tip 1.1).
9.  Where is the  HelloPrinter.java file stored on your computer?
10.  What do you do to protect yourself from data loss when you work on program-
ming projects?
practice it  Now you can try this exercise at the end of the chapter: R1.6.
Backup Copies
You will spend many hours creating and improving Java pro-
grams. It is easy to delete a file by accident, and occasionally files
are lost because of a computer malfunction. Retyping the contents
of lost files is frustrating and time-consuming. It is therefore cru-
cially impor tant that you learn how to safeguard files and get in
the habit of doing so before disaster strikes. Backing up files on a
memory stick is an easy and convenient storage method for many
people. Another increasingly popular form of backup is Internet
file storage. Here are a few pointers to keep in mind:
• Back up often. Backing up a file takes only a few seconds, and you will hate yourself if you
have to spend many hours recreating work that you could have saved easily. I rec ommend
that you back up your work once every thirty minutes.
• Rotate backups. Use more than one directory for backups, and
rotate them. That is, first back up onto the first directory. Then
back up onto the second directory. Then use the third, and then
go back to the first. That way you always have three recent
backups. If your recent changes made matters worse, you can
then go back to the older version.
• Pay attention to the backup direction. Backing up involves copying files from one place to
another. It is important that you do this right—that is, copy from your work location to
the backup location. If you do it the wrong way, you will overwrite a newer file with an
older version.
• Check your backups once in a while. Double-check that your backups are where you think
they are. There is nothing more frustrating than to find out that the backups are not there
when you need them.
• Relax, then restore. When you lose a file and need to restore it from a backup, you are
likely to be in an unhappy, nervous state. Take a deep breath and think through the
recovery process before you start. It is not uncommon for an agitated computer user to
wipe out the last backup when trying to restore a damaged file.
© Nicholas Homrich/iStockphoto.
s e L f  C h e C k
programming tip 1.1
© Eric Isselé/iStockphoto.
© Tatiana Popova/iStockphoto.
develop a strategy
for keeping backup
copies of your
work before
disaster strikes.
12 Chapter 1 Introduction
1.5 analyzing Your First program
In this section, we will analyze the first Java program in detail. Here again is the
source code:
section_5/helloprinter.java
1 public class HelloPrinter
2 {
3 public static void main(String[] args)
4 {
5 //  Display a greeting in the console window
6
7 System.out.println("Hello, World!");
8 }
9 }
The line
public class HelloPrinter
indicates the declaration of a class called  HelloPrinter .
Every Java program consists of one or more classes. We will discuss classes in more
detail in Chapters 2 and 3.
The word  public denotes that the class is usable by the “public”. You will later
encounter  private fea tures.
In Java, every source file can contain at most one public class, and the name of the
public class must match the name of the file containing the class. For example, the
class  HelloPrinter must be contained in a file named  HelloPrinter.java .
The construction
public static void main(String[] args)
{
. . .
}
declares a method called  main . A method contains a collection of programming
instructions that describe how to carry out a particular task. Every Java application
must have a  main method. Most Java programs contain other methods besides  main ,
and you will see in Chapter 3 how to write other methods.
The term  static is explained in more detail in Chapter 8, and the meaning of
String[] args is covered in Chapter 11. At this time, simply consider
public class  ClassName
{
public static void main(String[] args)
{
. . .
}
}
as a part of the “plumbing” that is required to create a Java program. Our first pro-
gram has all instructions inside the  main method of the class.
The  main method contains one or more instructions called statements. Each state-
ment ends in a semi colon ( ; ). When a program runs, the statements in the  main method
are executed one by one.
© Amanda Rohde/iStockphoto.
Classes are the
fundamental
building blocks of
Java programs.
every Java
application contains
a class with a  main
method. When the
application starts,
the instructions in
the  main method
are executed.
each class contains
declarations of
methods. each
method contains
a sequence
of instructions.
1.5 analyzing Your First program 13
syntax 1.1  Java program
public class HelloPrinter
{
public static void main(String[] args)
{
System.out.println("Hello, World!");
}
}
Every program contains at least one class.
Choose a class name that describes
the program action.
The statements inside the
main method are executed
when the program runs.
Every Java program
contains a  main method
with this header.
Replace this
statement when you
write your own
programs.
Be sure to match the
opening and closing braces.
Each statement
ends in a semicolon.
See page 14.
In our example program, the  main method has a single statement:
System.out.println("Hello, World!");
This statement prints a line of text, namely “Hello, World!”. In this statement, we call
a method which, for reasons that we will not explain here, is specified by the rather
long name  System.out.println .
We do not have to implement this method—the programmers who wrote the Java
library already did that for us. We simply want the method to perform its intended
task, namely to print a value.
Whenever you call a method in Java, you need to specify
1.  The method you want to use (in this case,  System.out.println ).
2.  Any values the method needs to carry out its task (in this case,  "Hello, World!" ).
The technical term for such a value is an argument. Arguments are enclosed in
parentheses. Multiple arguments are sep arated by commas.
A sequence of characters enclosed in quotation marks
"Hello, World!"
is called a  string. You must enclose the contents of the string inside quotation marks
so that the compiler knows you literally mean  "Hello, World!" . There is a reason for
this requirement. Suppose you need to print the word main. By enclosing it in quota-
tion marks,  "main" , the compiler knows you mean the sequence of characters  m a i n ,
not the method named  main . The rule is simply that you must enclose all text strings
in quotation marks, so that the compiler considers them plain text and does not try to
inter pret them as program instructions.
You can also print numerical values. For example, the statement
System.out.println(3 + 4);
evaluates the expression 3 + 4 and displays the number 7.
a method is called
by specifying the
method and
its arguments.
a string is a sequence
of characters
enclosed in
quotation marks.
14 Chapter 1 Introduction
The  System.out.println method prints a string or a number and then starts a new
line. For example, the sequence of statements
System.out.println("Hello");
System.out.println("World!");
prints two lines of text:
Hello
World!
There is a second method,  System.out.print , that you can use to print an item without
starting a new line. For example, the output of the two statements
System.out.print("00");
System.out.println(3 + 4);
is the single line
007
11.  How do you modify the  HelloPrinter program to greet you instead?
12.  How would you modify the  HelloPrinter program to print the word “Hello”
vertically?
13.  Would the program continue to work if you replaced line 7 with this statement?
System.out.println(Hello);
14.  What does the following set of statements print?
System.out.print("My lucky number is");
System.out.println(3 + 4 + 5);
15.  What do the following statements print?
System.out.println("Hello");
System.out.println("");
System.out.println("World");
practice it  Now you can try these exercises at the end of the chapter: R1.7, R1.8, E1.5, E1.7.
omitting semicolons
In Java every statement must end in a semicolon. Forgetting to type a semicolon is a common
error. It confuses the compiler, because the compiler uses the semicolon to find where one
statement ends and the next one starts. The compiler does not use line breaks or closing braces
to recognize the end of statements. For example, the compiler considers
System.out.println("Hello")
System.out.println("World!");
a single statement, as if you had written
System.out.println("Hello") System.out.println("World!");
Then it doesn’t understand that statement, because it does not expect the word  System follow-
ing the closing parenthe sis after  "Hello" .
The remedy is simple. Scan every statement for a terminating semicolon, just as you would
check that every English sentence ends in a period. However, do not add a semicolon at the
end of public class Hello  or public static void main . These lines are not statements.
fuLL CoDe exampLe
Go to  wiley.com/go/
javacode to download
a program to
demonstrate print
commands.
© Nicholas Homrich/iStockphoto.
s e L f  C h e C k
Common error 1.1
© John Bell/iStockphoto.
1.6 errors 15
1.6 errors
Experiment a little with the  HelloPrinter program.
What happens if you make a typing error such as
System.ou.println("Hello, World!");
System.out.println("Hello, Word!");
In the first case, the compiler will complain. It will
say that it has no clue what you mean by  ou . The
exact wording of the error message is dependent
on your development environment, but it might be
something like “Cannot find symbol ou”. This is a
compile-time error. Something is wrong accord-
ing to the rules of the language and the compiler
finds it. For this reason, compile-time errors are
often called syntax errors. When the compiler
finds one or more errors, it refuses to translate the program into Java virtual machine
instructions, and as a consequence you have no program that you can run. You must
fix the error and compile again. In fact, the compiler is quite picky, and it is common
to go through several rounds of fixing compile-time errors before compila tion suc-
ceeds for the first time.
If the compiler finds an error, it will not simply stop and give up. It will try to
report as many errors as it can find, so you can fix them all at once.
Sometimes, an error throws the compiler off track. Suppose, for example, you
forget the quotation marks around a string:  System.out.println(Hello, World!) . The
compiler will not complain about the missing quotation marks. Instead, it will report
“Cannot find symbol Hello”. Unfortunately, the com piler is not very smart and it
does not realize that you meant to use a string. It is up to you to realize that you need
to enclose strings in quotation marks.
The error in the second line above is of a different kind. The program will compile
and run, but its output will be wrong. It will print
Hello, Word!
This is a run-time error. The program is syntactically correct and does something,
but it doesn’t do what it is supposed to do. Because run-time errors are caused by
logical flaws in the program, they are often called logic errors.
This particular run-time error did not include an error message. It simply pro-
duced the wrong output. Some kinds of run-time errors are so severe that they gen-
erate an exception: an error message from the Java virtual machine. For example, if
your program includes the statement
System.out.println(1 / 0);
you will get a run-time error message “Division by zero”.
During program development, errors are unavoidable. Once a program is longer
than a few lines, it would require superhuman concentration to enter it correctly
without slipping up once. You will find yourself omitting semicolons or quotation
marks more often than you would like, but the compiler will track down these prob-
lems for you.
Run-time errors are more troublesome. The compiler will not find them—in fact,
the compiler will cheerfully translate any program as long as its syntax is correct—
© CarlssonInc/iStockphoto.
Programmers spend a fair amount
of time fixing compile-time and run-
time errors.
a compile-time error
is a violation of
the programming
language rules that
is detected by
the compiler.
a run-time error
causes a program to
take an action that
the programmer did
not intend.
fuLL CoDe exampLe
Go to  wiley.com/go/
javacode to download
three programs to
illustrate errors.
16 Chapter 1 Introduction
but the resulting program will do some thing wrong. It is the responsibility of the
program author to test the program and find any run-time errors.
16.  Suppose you omit the  "" characters around  Hello, World! from the  HelloPrinter.
java program. Is this a compile-time error or a run-time error?
17.  Suppose you change  println to  printline in the  HelloPrinter.java program. Is this
a compile-time error or a run-time error?
18.  Suppose you change  main to  hello in the  HelloPrinter.java program. Is this a
compile-time error or a run-time error?
19.  When you used your computer, you may have experienced a program that
“crashed” (quit spontane ously) or “hung” (failed to respond to your input). Is
that behavior a compile-time error or a run-time error?
20.  Why can’t you test a program for run-time errors when it has compiler errors?
practice it  Now you can try these exercises at the end of the chapter: R1.9, R1.10, R1.11.
misspelling words
If you accidentally misspell a word, then strange things may happen, and it may not always be
completely obvious from the error messages what went wrong. Here is a good example of how
simple spelling errors can cause trouble:
public class HelloPrinter
{
public static void  Main (String[] args)
{
System.out.println("Hello, World!");
}
}
This class declares a method called  Main . The compiler will not consider this to be the same as
the  main method, because  Main starts with an uppercase letter and the Java language is case sen-
sitive. Upper- and lowercase letters are considered to be completely different from each other,
and to the compiler  Main is no better match for  main than  rain . The compiler will cheerfully
compile your  Main method, but when the Java virtual machine reads the compiled file, it will
complain about the missing  main method and refuse to run the program. Of course, the mes-
sage “missing main method” should give you a clue where to look for the error.
If you get an error message that seems to indicate that the compiler or virtual machine is on
the wrong track, check for spelling and capitalization. If you misspell the name of a symbol
(for example,  ou instead of  out ), the compiler will produce a message such as “cannot find sym-
bol ou”. That error message is usually a good clue that you made a spelling error.
1.7 problem solving: algorithm design
You will soon learn how to program calculations and decision making in Java. But
before we look at the mechanics of implementing computations in the next chapter,
let’s consider how you can describe the steps that are necessary for finding the solu-
tion to a problem.
© Nicholas Homrich/iStockphoto.
s e L f  C h e C k
Common error 1.2
© John Bell/iStockphoto.
1.7 problem solving: algorithm design 17
1.7.1 the algorithm Concept
You may have run across advertisements that
encourage you to pay for a computerized service
that matches you up with a love partner. Think
how this might work. You fill out a form and send
it in. Others do the same. The data are processed
by a computer program. Is it reasonable to assume
that the computer can perform the task of finding
the best match for you? Suppose your younger
brother, not the computer, had all the forms on his
desk. What instructions could you give him? You
can’t say, “Find the best-looking person who likes
inline skating and browsing the Internet”. There
is no objective standard for good looks, and your
brother’s opinion (or that of a computer program
analyzing the photos of prospective partners) will likely be different from yours. If
you can’t give written instructions for someone to solve the problem, there is no way
the com puter can magically find the right solution. The computer can only do what
you tell it to do. It just does it faster, without getting bored or exhausted.
For that reason, a computerized match-making service cannot guarantee to find
the optimal match for you. Instead, you may be presented with a set of potential part-
ners who share common interests with you. That is a task that a computer program
can solve.
In order for a computer program to provide an answer to a problem that computes
an answer, it must follow a sequence of steps that is
• Unambiguous
• Executable
• Terminating
The step sequence is unambiguous when there are precise instructions for what to do
at each step and where to go next. There is no room for guesswork or personal opin-
ion. A step is executable when it can be carried out in practice. For example, a com-
puter can list all people that share your hobbies, but it can’t predict who will be your
life-long partner. Finally, a sequence of steps is terminating if it will eventually come
to an end. A program that keeps working without delivering an answer is clearly not
useful.
A sequence of steps that is unambiguous,
executable, and terminating is called an algorithm.
Although there is no algorithm for finding a part-
ner, many problems do have algorithms for solving
them. The next section gives an example.
An algorithm is a recipe for
finding a solution.
© mammamaart/iStockphoto.
Finding the perfect partner is not a
problem that a computer can solve.
an algorithm for
solving a problem is
a sequence of steps
that is unambiguous,
executable, and
terminating.
© Claudiad/iStockphoto.
18 Chapter 1 Introduction
1.7.2 an algorithm for solving an Investment problem
Consider the following investment problem:
You put $10,000 into a bank account that earns 5 percent interest per year. How many
years does it take for the account balance to be double the original?
Could you solve this problem by hand? Sure, you could. You figure out the balance
as follows:
year  interest  balance
0  10000
1  10000.00 x 0.05 = 500.00  10000.00 + 500.00 = 10500.00
2  10500.00 x 0.05 = 525.00  10500.00 + 525.00 = 11025.00
3  11025.00 x 0.05 = 551.25  11025.00 + 551.25 = 11576.25
4  11576.25 x 0.05 = 578.81  11576.25 + 578.81 = 12155.06
You keep going until the balance is at least $20,000. Then the last number in the year
column is the answer.
Of course, carrying out this computation is intensely boring to you or your
younger brother. But computers are very good at carrying out repetitive calcula-
tions quickly and flawlessly. What is important to the computer is a description of the
steps for finding the solution. Each step must be clear and unam biguous, requiring no
guesswork. Here is such a description:
Start with a year value of 0, a column for the interest, and a balance of $10,000.
year interest  balance
0  10000
Repeat the following steps while the balance is less than $20,000
Add 1 to the year value.
Compute the interest as balance x 0.05 (i.e., 5 percent interest).
Add the interest to the balance.
year  interest  balance
0  10000
1  500.00  10500.00
14  942.82  19799.32
15  989.96  20789.28
Report the final year value as the answer.
These steps are not yet in a language that a computer can understand, but you will
soon learn how to formulate them in Java. This informal description is called pseudo-
code. We examine the rules for writing pseudocode in the next section.
1.7 problem solving: algorithm design 19
1.7.3 pseudocode
There are no strict requirements for pseudocode because it is read by human readers,
not a computer program. Here are the kinds of pseudocode statements and how we
will use them in this book:
• Use statements such as the following to describe how a value is set or changed:
total cost = purchase price + operating cost
Multiply the balance value by 1.05.
Remove the first and last character from the word.
• Describe decisions and repetitions as follows:
If total cost 1 < total cost 2
While the balance is less than $20,000
For each picture in the sequence
Use indentation to indicate which statements should be selected or repeated:
For each car
operating cost = 10 x annual fuel cost
total cost = purchase price + operating cost
Here, the indentation indicates that both statements should be executed for
each car.
• Indicate results with statements such as:
Choose car1.
Report the final year value as the answer.
1.7.4 From algorithms to programs
In Section 1.7.2, we developed pseudocode for finding how long it takes to double an
investment. Let’s double-check that the pseudocode represents an algorithm; that is,
that it is unambiguous, executable, and terminating.
Our pseudocode is unambiguous. It simply tells how to update values in each step.
The pseudocode is executable because we use a fixed interest rate. Had we said to use
the actual interest rate that will be charged in years to come, and not a fixed rate of 5
percent per year, the instructions would not have been executable. There is no way
for anyone to know what the interest rate will be in the future. It requires a bit of
thought to see that the steps are terminating: With every step, the balance goes up by
at least $500, so eventually it must reach $20,000.
Therefore, we have found an algorithm to solve our investment problem, and
we know we can find the solution by programming a computer. The existence of
an algorithm is an essential prerequisite for programming a task. You need to first
discover and describe an algorithm for the task before you start programming (see
Figure 9). In the chapters that follow, you will learn how to express algorithms in the
Java language.
figure 9  the software development process
pseudocode is an
informal description
of a sequence of
steps for solving
a problem.
Understand
the problem
Develop and
describe an
algorithm
Translate
the algorithm
into Java
Test the
algorithm with
simple inputs
Compile and test
your program
20 Chapter 1  Introduction
21.  Suppose the interest rate was 20 percent. How long would it take for the invest-
ment to double?
22.  Suppose your cell phone carrier charges you $29.95 for up to 300 minutes of
calls, and $0.45 for each additional minute, plus 12.5 percent taxes and fees. Give
an algorithm to compute the monthly charge from a given number of minutes.
23.  Consider the following pseudocode for finding the most attractive photo from a
sequence of photos:
Pick the first photo and call it "the best so far".
For each photo in the sequence
If it is more attractive than the "best so far"
Discard "the best so far".
Call this photo "the best so far".
The photo called "the best so far" is the most attractive photo in the sequence.
Is this an algorithm that will find the most attractive photo?
24.  Suppose each photo in Self Check 23 had a price tag. Give an algorithm for find-
ing the most expen sive photo.
25.  Suppose you have a random sequence of black and white marbles and want to
rearrange it so that the black and white marbles are grouped together. Consider
this algorithm:
Repeat until sorted
Locate the first black marble that is preceded by a white marble, and switch them.
What does the algorithm do with the sequence mlmll? Spell out the steps
until the algorithm stops.
26.  Suppose you have a random sequence of colored marbles. Consider this pseudo-
code:
Repeat until sorted
Locate the first marble that is preceded by a marble of a different color, and switch them.
Why is this not an algorithm?
Practice It  Now you can try these exercises at the end of the chapter: R1.15, E1.4, P1.1.
© Nicholas Homrich/iStockphoto.
S e l f C h e C k
© Steve Simzer/iStockphoto.
How To 1.1  Describing an Algorithm with Pseudocode
This is the first of many “How To” sections in this book that give you step-by-step proce-
dures for carrying out important tasks in developing computer programs.
Before you are ready to write a program in Java, you need to develop an algorithm—a
method for arriving at a solution for a particular problem. Describe the algorithm in pseudo-
code: a sequence of precise steps formulated in English. To illustrate, we’ll devise an algorithm
for this problem:
Problem Statement  You have the choice of buying two
cars. One is more fuel efficient than the other, but also more
expensive. You know the price and fuel efficiency (in miles per
gallon, mpg) of both cars. You plan to keep the car for ten years.
Assume a price of $4 per gallon of gas and usage of 15,000 miles
per year. You will pay cash for the car and not worry about
financing costs. Which car is the better deal?
© dlewis33/iStockphoto.
1.7 problem solving: algorithm design 21
step 1  Determine the inputs and outputs.
In our sample problem, we have these inputs:
• purchase price1 and fuel efficiency1, the price and fuel efficiency (in mpg) of the first car
• purchase price2 and fuel efficiency2, the price and fuel efficiency of the second car
We simply want to know which car is the better buy. That is the desired output.
step 2  Break down the problem into smaller tasks.
For each car, we need to know the total cost of driving it. Let’s do this computation separately
for each car. Once we have the total cost for each car, we can decide which car is the better deal.
The total cost for each car is purchase price + operating cost.
We assume a constant usage and gas price for ten years, so the operating cost depends on the
cost of driving the car for one year.
The operating cost is 10 x annual fuel cost.
The annual fuel cost is  price per gallon x annual fuel consumed.
The annual fuel consumed is annual miles driven / fuel efficiency. For example, if you drive the car
for 15,000 miles and the fuel efficiency is 15 miles/gallon, the car consumes 1,000 gallons.
step 3  Describe each subtask in pseudocode.
In your description, arrange the steps so that any intermediate values are computed before
they are needed in other computations. For example, list the step
total cost = purchase price + operating cost
after you have computed operating cost.
Here is the algorithm for deciding which car to buy:
For each car, compute the total cost as follows:
annual fuel consumed = annual miles driven / fuel efficiency
annual fuel cost = price per gallon x annual fuel consumed
operating cost = 10 x annual fuel cost
total cost = purchase price + operating cost
If total cost1 < total cost2
Choose car1.
Else
Choose car2.
step 4  Test your pseudocode by working a problem.
We will use these sample values:
Car 1: $25,000, 50 miles/gallon
Car 2: $20,000, 30 miles/gallon
Here is the calculation for the cost of the first car:
annual fuel consumed = annual miles driven / fuel efficiency = 15000 / 50 = 300
annual fuel cost = price per gallon x annual fuel consumed = 4 x 300 = 1200
operating cost = 10 x annual fuel cost = 10 x 1200 = 12000
total cost = purchase price + operating cost = 25000 + 12000 = 37000
Similarly, the total cost for the second car is $40,000. Therefore, the output of the algorithm is
to choose car 1.
22 Chapter 1 Introduction
The following Worked Example demonstrates how to use the concepts in this chap-
ter and the steps in the How To feature to solve another problem. In this case, you
will see how to develop an algorithm for laying tile in an alternating pattern of colors.
You should read the Worked Example to review what you have learned, or for help in
tackling another problem.
In future chapters, Worked Examples are provided for you on the book’s compan-
ion Web site. A brief description of the problem tackled in the example will appear
with the reminder to download it from  www.wiley.com/go/javaexamples . You will find
any code related to the Worked Example included with the companion code for the
chapter. When you see the Worked Example description, download the example and
the code to learn how the problem was solved.
step 1  Determine the inputs and outputs.
The inputs are the floor dimensions (length × width),
measured in inches. The output is a tiled floor.
step 2  Break down the problem into smaller tasks.
A natural subtask is to lay one row of tiles. If you can
solve that task, then you can solve the problem by lay-
ing one row next to the other, starting from a wall, until
you reach the opposite wall.
How do you lay a row? Start with a tile at one wall.
If it is white, put a black one next to it. If it is black, put
a white one next to it. Keep going until you reach the
opposite wall. The row will contain width / 4 tiles.
step 3  Describe each subtask in pseudocode.
In the pseudocode, you want to be more precise about exactly where the tiles are placed.
Place a black tile in the northwest corner.
While the floor is not yet filled, repeat the following steps:
Repeat this step width / 4 – 1 times:
Place a tile east of the previously placed tile. If the previously placed tile was white, pick a black one;
otherwise, a white one.
Locate the tile at the beginning of the row that you just placed. If there is space to the south, place a tile of
the opposite color below it.
step 4  Test your pseudocode by working a problem.
Suppose you want to tile an area measuring 20 × 12 inches.
WorKed exaMple 1.1  writing an algorithm for tiling a floor
problem statement  Write an algorithm for tiling a rectangular bathroom floor with
alternating black and white tiles measuring 4 × 4 inches. The floor dimensions, measured in
inches, are multiples of 4.
© rban/iStockphoto.
1.7 problem solving: algorithm design 23
The first step is to place a black tile in the northwest corner.
1
20 inches
12
Next, alternate four tiles until reaching the east wall. (width / 4 – 1 = 20 / 4 – 1 = 4)
1 2 3 4 5
There is room to the south. Locate the tile at the beginning of the completed row. It is black.
Place a white tile south of it.
1
6
2 3 4 5
Complete the row.
1
6
2 3 4 5
7 8 9 10
There is still room to the south. Locate the tile at the beginning of the completed row. It is
white. Place a black tile south of it.
1
6
2 3 4 5
11
7 8 9 10
Complete the row.
1
6
2 3 4 5
11 12 13 14 15
7 8 9 10
Now the entire floor is filled, and you are done.
24 Chapter 1 Introduction
Define “computer program” and programming.
• Computers execute very basic instructions in rapid succession.
• A computer program is a sequence of instructions and decisions.
• Programming is the act of designing and implementing computer programs.
Describe the components of a computer.
• The central processing unit (CPU) performs program control and data
processing.
• Storage devices include memory and secondary storage.
Describe the process of translating high-level languages to machine code.
• Java was originally designed for programming consumer devices, but it was first
successfully used to write Internet applets.
• Java was designed to be safe and portable, benefiting both Internet users and
stu dents.
• Java programs are distributed as instructions for a virtual machine, making them
platform-independent.
• Java has a very large library. Focus on learning those parts of the library that you
need for your programming projects.
Become familiar with your Java programming environment.
• Set aside some time to become familiar with the programming environment that
you will use for your class work.
• An editor is a program for entering and modifying text, such as a Java program.
• Java is case sensitive. You must be careful about distinguishing between upper-
and lowercase letters.
• The Java compiler translates source code into class files that contain instructions
for the Java virtual machine.
• Develop a strategy for keeping backup copies of your
work before disaster strikes.
Describe the building blocks of a simple program.
• Classes are the fundamental building blocks of Java programs.
• Every Java application contains a class with a  main method. When the application
starts, the instructions in the  main method are executed.
• Each class contains declarations of methods. Each method contains a sequence of
instructions.
• A method is called by specifying the method and its arguments.
• A string is a sequence of characters enclosed in quotation marks.
C h a p t e r s u M M a rY
© Amorphis/iStockphoto.
© James Sullivan/Getty Images.
© Tatiana Popova/iStockphoto.
© Amanda Rohde/iStockphoto.
Review Questions 25
Classify program errors as compile-time and run-time errors.
• A compile-time error is a violation of the programming language rules that is
detected by the compiler.
• A run-time error causes a program to take an action that the programmer did
not intend.
Write pseudocode for simple algorithms.
• An algorithm for solving a problem is a sequence of steps that
is unambiguous, executable, and terminating.
• Pseudocode is an informal description of a sequence of steps
for solving a problem.
• R1.1  Explain the difference between using a computer program and programming a
computer.
• R1.2  Which parts of a computer can store program code? Which can store user data?
• R1.3  Which parts of a computer serve to give information to the user? Which parts take
user input?
•• R1.4  A toaster is a single-function device, but a computer can be programmed to carry out
different tasks. Is your cell phone a single-function device, or is it a programma ble
computer? (Your answer will depend on your cell phone model.)
•• R1.5  Explain two benefits of using Java over machine code.
•• R1.6  On your own computer or on a lab computer, find the exact location (folder or
directory name) of
a.  The sample file  HelloPrinter.java , which you wrote with the editor
b.  The Java program launcher  java.exe or  java
c.  The library file  rt.jar that contains the run-time library
•• R1.7  What does this program print?
public class Test
{
public static void main(String[] args)
{
System.out.println("39 + 3");
System.out.println(39 + 3);
}
}
© CarlssonInc/iStockphoto.
© Claudiad/iStockphoto.
java.io.PrintStream
print
println
java.lang.System
out
S ta n d a R d L i b R a Ry i t e m S i n t R o d u c e d i n t h i S c h ap t e R
R e v i e w Q u e S t i o n S
26 Chapter 1 Introduction
•• r1.8  What does this program print? Pay close attention to spaces.
public class Test
{
public static void main(String[] args)
{
System.out.print("Hello");
System.out.println("World");
}
}
•• r1.9  What is the compile-time error in this program?
public class Test
{
public static void main(String[] args)
{
System.out.println("Hello", "World!");
}
}
•• r1.10  Write three versions of the  HelloPrinter.java program that have different compile-
time errors. Write a version that has a run-time error.
• r1.11  How do you discover syntax errors? How do you discover logic errors?
•• r1.12  Write an algorithm to settle the following question: A bank account starts out with
$10,000. Interest is compounded monthly at 6 percent per year (0.5 percent per
month). Every month, $500 is withdrawn to meet college expenses. After how many
years is the account depleted?
••• r1.13  Consider the question in Exercise R1.12. Suppose the numbers ($10,000, 6 percent,
$500) were user selectable. Are there values for which the algorithm you developed
would not terminate? If so, change the algorithm to make sure it always terminates.
••• r1.14  In order to estimate the cost of painting a house, a painter needs to know the surface
area of the exterior. Develop an algorithm for computing that value. Your inputs are
the width, length, and height of the house, the number of windows and doors, and
their dimensions. (Assume the windows and doors have a uniform size.)
•• r1.15  In How To 1.1, you made assumptions about the price of gas and annual usage to
compare cars. Ideally, you would like to know which car is the better deal without
making these assumptions. Why can’t a computer program solve that problem?
•• r1.16  Suppose you put your younger brother in charge of backing up your work. Write a
set of detailed instructions for carrying out his task. Explain how often he should do
it, and what files he needs to copy from which folder to which location. Explain how
he should verify that the backup was carried out correctly.
• r1.17  Write pseudocode for an algorithm that describes how to prepare Sunday breakfast
in your household.
•• r1.18  The ancient Babylonians had an algorithm for determining the square root of a num-
ber a. Start with an initial guess of a / 2. Then find the average of your guess g and
a / g. That’s your next guess. Repeat until two consecutive guesses are close enough.
Write pseudocode for this algorithm.
practice exercises 27
• e1.1  Write a program that prints a greeting of your choice, perhaps in a language other
than English.
•• e1.2  Write a program that prints the sum of the first ten positive integers, 1 + 2 + … + 10.
•• e1.3  Write a program that prints the product of the first ten positive integers, 1 × 2 × … ×
10. (Use  * to indicate multiplication in Java.)
•• e1.4  Write a program that prints the balance of an account after the first, second, and
third year. The account has an initial balance of $1,000 and earns 5 percent interest
per year.
• e1.5  Write a program that displays your name inside a box on the screen, like this:
Dave
Do your best to approximate lines with characters such as  | - + .
••• e1.6  Write a program that prints your name in large letters, such as
* * ** **** **** * *
* * * * * * * * * *
***** * * **** **** * *
* * ****** * * * * *
* * * * * * * * *
•• e1.7  Write a program that prints a face similar to (but different from) the following:
/////
+"""""+
(| o o |)
| ^ |
| ‘-’ |
+-----+
•• e1.8  Write a program that prints an imitation of a Piet Mondrian painting. (Search the
Internet if you are not familiar with his paintings.) Use character sequences such as
@@@ or  ::: to indicate different colors, and use  - and  | to form lines.
•• e1.9  Write a program that prints a house that looks exactly like the following:
+
+ +
+ +
+-----+
| .-. |
| | | |
+-+-+-+
••• e1.10  Write a program that prints an animal speaking a greeting, similar to (but different
from) the following:
/\_/\ -----
( ‘ ’ ) / Hello \'
( - ) < Junior |
| | | \ Coder!/
(__|__) -----
p r a C t I C e e x e r C I s e s
28 Chapter 1 Introduction
• e1.11  Write a program that prints three items, such as the names of your three best friends
or favorite movies, on three separate lines.
• e1.12  Write a program that prints a poem of your choice. If you don’t have a favorite
poem, search the Internet for “Emily Dickinson” or “e e cummings”.
•• e1.13  Write a program that prints the United States flag, using  * and  = characters.
•• e1.14  Type in and run the following program:
import javax.swing.JOptionPane;
public class DialogViewer
{
public static void main(String[] args)
{
JOptionPane.showMessageDialog(null, "Hello, World!");
}
}
Then modify the program to show the message “Hello, your name!”.
•• e1.15  Type in and run the following program:
import javax.swing.JOptionPane;
public class DialogViewer
{
public static void main(String[] args)
{
String name = JOptionPane.showInputDialog("What is your name?");
System.out.println(name);
}
}
Then modify the program to print “Hello, name!”, displaying the name that the user
typed in.
••• e1.16  Modify the program from Exercise E1.15 so that the dialog continues with the mes-
sage “My name is Hal! What would you like me to do?” Discard the user’s input and
display a message such as
I'm sorry, Dave. I'm afraid I can't do that.
Replace  Dave with the name that was provided by the user.
•• e1.17  Type in and run the following program:
import java.net.URL;
import javax.swing.ImageIcon;
import javax.swing.JOptionPane;
public class Test
{
public static void main(String[] args) throws Exception
{
URL imageLocation = new URL(
"http://horstmann.com/java4everyone/duke.gif");
JOptionPane.showMessageDialog(null, "Hello", "Title",
JOptionPane.PLAIN_MESSAGE, new ImageIcon(imageLocation));
}
}
programming projects 29
Then modify it to show a different greeting and image.
• Business e1.18  Write a program that prints a two-column list of your friends’ birthdays. In the
first column, print the names of your best friends; in the second column, print their
birthdays.
• Business e1.19  In the United States there is no federal sales tax, so every state may impose its own
sales taxes. Look on the Internet for the sales tax charged in five U.S. states, then
write a program that prints the tax rate for five states of your choice.
Sales Tax Rates
---------------
Alaska: 0%
Hawaii: 4%
. . .
• Business e1.20  To speak more than one language is a valuable skill in the labor market today. One of
the basic skills is learning to greet people. Write a program that prints a two-column
list with the greeting phrases shown in the following table; in the first column, print
the phrase in English, in the second column, print the phrase in a language of your
choice. If you don’t speak any language other than English, use an online translator
or ask a friend.
list of phrases to translate
Good morning.
It is a pleasure to meet you.
Please call me tomorrow.
Have a nice day!
•• p1.1  You want to decide whether you should drive your car to work or take the train.
You know the one-way distance from your home to your place of work, and the
fuel efficiency of your car (in miles per gallon). You also know the one-way price
of a train ticket. You assume the cost of gas at $4 per gallon, and car maintenance at
5 cents per mile. Write an algorithm to decide which commute is cheaper.
•• p1.2  You want to find out which fraction of your car’s use is for commuting to work,
and which is for personal use. You know the one-way distance from your home to
work. For a particular period, you recorded the beginning and ending mileage on the
odometer and the number of work days. Write an algorithm to settle this question.
••• p1.3  The value of  p can be computed according to the following formula:
π
4
1
1
3
1
5
1
7
1
9
= − + − + − ?
Write an algorithm to compute  p . Because the formula is an infinite series and an
algorithm must stop after a finite number of steps, you should stop when you have
the result determined to six significant dig its.
p r o G r a M M I n G p r o J e C t s
30 Chapter 1 Introduction
• Business p1.4  Imagine that you and a number of friends go to a luxury restaurant, and when you
ask for the bill you want to split the amount and the tip (15 percent) between all.
Write pseudocode for calculating the amount of money that everyone has to pay.
Your program should print the amount of the bill, the tip, the total cost, and the
amount each person has to pay. It should also print how much of what each person
pays is for the bill and for the tip.
•• p1.5  Write an algorithm to create a tile pattern composed of black
and white tiles, with a fringe of black tiles all around and
two or three black tiles in the center, equally spaced from the
boundary. The inputs to your algorithm are the total number
of rows and columns in the pattern.
••• p1.6  Suppose you received a loyalty promotion that lets you purchase one item, valued
up to $100, from an online catalog. You want to make the best of the offer. You have
a list of all items for sale, some of which are less than $100, some more. Write an
algorithm to produce the item that is closest to $100. If there is more than one such
item, list them all. Remember that a computer will inspect one item at a time––it
can’t just glance at a list and find the best one.
••• p1.7  Consider a robot that is placed in a room. The robot can do the following:
• Move forward by one unit
• Turn left or right
• Sense what is in front of it: a wall, a window, or
neither
Write an algorithm that enables the robot, placed any-
where in the room, to count the number of windows. For
example, in the room at right, the robot (marked as  R )
should find that it has two windows.
••• p1.8  Consider a robot that has been placed in a maze. The right-hand rule tells you how
to escape from a maze: Always have the right hand next to a wall, and eventually you
will find an exit.
© Skip ODonnell/iStockphoto.
The robot can do the following:
• Move forward by one unit
• Turn left or right
• Sense what is in front of it: a wall, an exit, or neither
Write an algorithm that lets the robot escape the maze. You may assume that there is
an exit that is reachable by the right-hand rule. Your challenge is to deal with situa-
tions in which the path turns. The robot can’t see turns. It can only see what is
directly in front of it.
R
answers to self-Check Questions 31
•• science p1.9  A television manufacturer advertises that a televi-
sion set has a certain size, measured diagonally.
You wonder how the set will fit into your living
room. Write an algorithm that yields the horizontal
and vertical size of the television. Your inputs are
the diagonal size and the aspect ratio (the ratio of
width to height, usually 16 : 9 for television sets).
••• science p1.10  Cameras today can correct “red eye” problems caused when the photo flash makes
eyes look red. Write pseudocode for an algorithm that can detect red eyes. Your
input is a pattern of colors, such as
You are given the number of rows and columns. For any row or column number,
you can query the color, which will be red, black, or something else. If you find that
the center of the black pixels coincides with the center of the red pixels, you have
found a red eye, and your output should be “yes”. Otherwise, your output is “no”.
© Don Bayley/iStockPhoto.
a n s W e rs t o s e l F - C h e C K Q u e s t I o n s
1.  A program that reads the data on the CD and
sends output to the speakers and the screen.
2.  A CD player can do one thing—play music
CDs. It cannot execute programs.
3.  Nothing.
4.  In secondary storage, typically a hard disk.
5.  The central processing unit.
6.  A smartphone has a CPU and memory, like
any computer. A few smartphones have key-
boards. Generally, the touchpad is used instead
of a mouse. Secondary storage is in the form
of a solid state drive. Of course, smartphones
have a display, speaker, and microphone. The
network connection uses the wireless radio to
connect to a cell tower.
7.  Safety and portability.
8.  No one person can learn the entire library—it
is too large.
9.  The answer varies among systems. A typical
answer might be  /home/dave/cs1/hello/Hello-
Printer.java or  c:\Users\Dave\Workspace\hello\
HelloPrinter.java
10.  You back up your files and folders.
11.  Change  World to your name (here,  Dave ):
System.out.println("Hello, Dave!");
12.  System.out.println("H");
System.out.println("e");
System.out.println("l");
System.out.println("l");
System.out.println("o");
13.  No. The compiler would look for an
item whose name is  Hello . You need to
enclose  Hello in quotation marks:
System.out.println("Hello");
14.  The printout is  My lucky number is12 . It would
be a good idea to add a space after the  is .
15.  Hello
a blank line
World
32 Chapter 1 Introduction
16.  This is a compile-time error. The compiler will
complain that it does not know the meanings
of the words  Hello and  World .
17.  This is a compile-time error. The compiler
will complain that  System.out does not have a
method called  printline .
18.  This is a run-time error. It is perfectly legal to
give the name  hello to a method, so the com-
piler won’t complain. But when the program
is run, the virtual machine will look for a  main
method and won’t find one.
19.  It is a run-time error. After all, the program
had been compiled in order for you to run it.
20.  When a program has compiler errors, no class
file is produced, and there is nothing to run.
21.  4 years:
0 10,000
1 12,000
2 14,400
3 17,280
4 20,736
22.  Is the number of minutes at most 300?
a.  If so, the answer is $29.95  × 1.125 = $33.70.
b.  If not,
1.  Compute the difference: (number of
minutes) – 300.
2.  Multiply that difference by 0.45.
3.  Add $29.95.
4.  Multiply the total by 1.125. That is the
answer.
23.  No. The step  If it is more attractive than the "best
so far" is not executable because there is no
objective way of deciding which of two photos
is more attractive.
24.  Pick the first photo and call it "the most expensive so far" .
For each photo in the sequence
If it is more expensive than "the most expensive so far"
Discard "the most expensive so far".
Call this photo "the most expensive so far".
The photo called "the most expensive so far" is the most
expensive photo in the sequence.
25.  The first black marble that is preceded by a
white one is marked in blue:
m l mll
Switching the two yields
lmmll
The next black marble to be switched is
lmmll
yielding
lmlml
The next steps are
llmml
llmlm
lllmm
Now the sequence is sorted.
26.  The sequence doesn’t terminate. Consider the
input mlmlm. The first two marbles keep
getting switched.
2
C h a p t e r
33
© Lisa F. Young/iStockphoto.
Using ObjeCts
to learn about variables
to understand the concepts of classes
and objects
to be able to call methods
to learn about arguments and return values
to be able to browse the api documentation
to implement test programs
to understand the difference between objects and
object references
to write programs that display simple shapes
C h a p t e r g O a l s
C h a p t e r C O n t e n t s
2.1 Objects and classes 34
2.2 Variables 36
Syntax 2.1: Variable Declaration 37
Syntax 2.2: assignment 41
Common Error 2.1: Using Undeclared or
Uninitialized Variables 42
Common Error 2.2: Confusing Variable
Declarations and assignment statements 42
Programming Tip 2.1: Choose Descriptive
Variable names 43
2.3 calling MethOds 43
Programming Tip 2.2: learn by trying 47
2.4 cOnstructing Objects 48
Syntax 2.3: Object Construction 49
Common Error 2.3: trying to invoke a
Constructor like a Method 50
2.5 accessOr and MutatOr
MethOds 50
2.6 the aPi dOcuMentatiOn 52
Syntax 2.4: importing a Class from a package 54
Programming Tip 2.3: Don’t Memorize—Use
Online help 55
2.7 iMPleMenting a test PrOgraM 55
Special Topic 2.1: testing Classes in an interactive
environment 56
Worked Example 2.1: how Many Days have You
been alive?
Worked Example 2.2: Working with pictures
2.8 Object references 57
Computing & Society 2.1: Computer Monopoly 60
2.9 graPhical aPPlicatiOns 61
2.10 elliPses, lines, text,
and cOlOr 66
34
Most useful programs don’t just manipulate numbers and
strings. instead, they deal with data items that are more
complex and that more closely represent entities in the real
world. examples of these data items include bank accounts,
employee records, and graphical shapes.
the java language is ideally suited for designing and
manipulating such data items, or objects. in java, you
implement classes that describe the behavior of these
objects. in this chapter, you will learn how to manipulate
objects that belong to classes that have already been
implemented. this will prepare you for the next chapter, in
which you will learn how to implement your own classes.
2.1 Objects and Classes
When you write a computer program, you put
it together from certain “building blocks”. In
Java, you build programs from objects. Each
object has a particular behavior, and you can
manipulate it to achieve certain effects.
As an analogy, think of a home builder who
constructs a house from certain parts: doors,
windows, walls, pipes, a furnace, a water heater,
and so on. Each of these elements has a particu-
lar function, and they work together to fulfill a
common purpose. Note that the home builder
is not concerned with how to build a window or
a water heater. These elements are readily avail-
able, and the builder’s job is to integrate them
into the house.
Of course, computer programs are more
abstract than houses, and the objects that make
up a computer program aren’t as tangible as a
window or a water heater. But the analogy
holds well: A programmer produces a working
program from elements with the desired functionality—the objects. In this chapter,
you will learn the basics about using objects written by other programmers.
2.1.1 Using Objects
An object is an entity that you can manipulate by calling one or more of its methods.
A method consists of a sequence of instructions that can access the internal data of
an object. When you call the method, you do not know exactly what those instruc-
tions are, or even how the object is organized internally. However, the behavior of the
method is well defined, and that is what matters to us when we use it.
© Luc Meaille/iStockphoto. Each part that a home builder uses,
such as a furnace or a water heater,
fulfills a particular function. Similarly,
you build programs from objects, each
of which has a particular behavior.
Objects are entities
in your program that
you manipulate by
calling methods.
 F. Young/iStockphoto.
2.1 Objects and Classes 35
figure 1  representation of the  System.out Object
data =
PrintStream
println
print
10101110
11110110
01101011
00110101
The class that the
System.out object belongs to
Methods you can
call on  System.out
The object’s internal data
For example, you saw in Chapter 1 that  System.out refers to an object. You manipu-
late it by calling the  println method. When the  println method is called, some activi-
ties occur inside the object, and the ulti mate effect is that text appears in the console
window. You don’t know how that happens, and that’s OK. What matters is that the
method carries out the work that you requested.
Figure 1 shows a representation of the  System.out object. The internal data is sym-
bolized by a sequence of zeroes and ones. Think of each method (symbolized by the
gears) as a piece of machinery that carries out its assigned task.
In general, think of an object as an entity that can do work for you when you call
its methods. How the work is done is not important to the programmer using the
object.
In the remainder of this chapter, you will see other objects
and the methods that they can carry out.
You can think of a water heater as an object that can carry out the “get
hot water” method. When you call that method to enjoy a hot shower,
you don’t care whether the water heater uses gas or solar power.
2.1.2 Classes
In Chapter 1, you encountered two objects:
•  System.out
•  “Hello, World!”
Each of these objects belongs to a different class. The  System.out object belongs to
the  PrintStream class. The  “Hello, World!” object belongs to the  String class. Of course,
there are many more  String objects, such as  “Goodbye” or  “Mississippi” . They all have
something in common––you can invoke the same methods on all strings. You will see
some of these methods in Section 2.3.
As you will see in Chapter 11, you can construct objects of the  PrintStream  class
other than  System.out . Those objects write data to files or other destinations instead of
the console. Still, all  PrintStream objects share common behavior. You can invoke the
println and  print methods on any  PrintStream object, and the printed values are sent to
their destination.
a method is a
sequence of
instructions that
accesses the data
of an object.
© Steven Frame/iStockphoto.
a class describes
a set of objects with
the same behavior.
36 Chapter 2 Using Objects
Of course, the objects of the  PrintStream
class have a completely different behavior
than the objects of the  String class. You could
not call  println on a  String object. A string
wouldn’t know how to send itself to a console
window or file.
As you can see, different classes have dif-
ferent responsibilities. A string knows about
the letters that it contains, but it does not
know how to display them to a human or to
save them to a file.
1.  In Java, objects are grouped into classes according to their behavior. Would a
window object and a water heater object belong to the same class or to different
classes? Why?
2.  Some light bulbs use a glowing filament, others use a fluorescent gas. If you
consider a light bulb a Java object with an “illuminate” method, would you need
to know which kind of bulb it is?
Practice it  Now you can try these exercises at the end of the chapter: R2.1, R2.2.
2.2 Variables
Before we continue with the main topic of this chapter—the behavior of objects—we
need to go over some basic programming termi nology. In the following sections, you
will learn about the concepts of variables, types, and assignment.
2.2.1 Variable Declarations
When your program manipulates objects, you will want to store the objects and the
values that their methods return, so that you can use them later. In a Java program,
you use variables to store values. The following statement declares a variable named
width :
int width = 20;
© Peter Mukherjee/iStockphoto. All objects of a  Window  class share the
same behavior.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
Like a variable in a computer
program, a parking space has
an identifier and a contents.
Javier Larrea/Age Fotostock.
2.2 Variables 37
syntax 2.1  Variable Declaration
typeName variableName =  value ;
or
typeName variableName ;
Syntax
String greeting = "Hello, Dave!";
A variable declaration ends
with a semicolon.
The type specifies
what can be done
with values stored
in this variable.
Supplying an initial value is optional,
but it is usually a good idea.
See page 39 for rules and
examples of valid names.
Use a descriptive
variable name.
See page 43.
A variable is a storage location in a computer program. Each variable has a name and
holds a value.
A variable is similar to a parking space in a parking garage. The parking space has
an identifier (such as “J 053”), and it can hold a vehicle. A variable has a name (such
as  width ), and it can hold a value (such as 20). When declaring a variable, you usually
want to initialize it. That is, you specify the value that should be stored in the vari-
able. Consider again this variable declaration:
int width = 20;
The variable  width is initialized with the value 20.
Like a parking space that is restricted to a certain type of vehicle (such as a compact
car, motorcycle, or electric vehicle), a variable in Java stores data of a specific type.
Java supports quite a few data types: num bers, text strings, files, dates, and many oth-
ers. You must specify the type whenever you declare a variable (see Syntax 2.1).
The  width variable is an integer, a whole number without a fractional part. In Java,
this type is called  int .
Note that the type comes before the variable name:
int width = 20;
After you have declared and initialized a variable,
you can use it. For example,
int width = 20;
System.out.println(width);
int area = width * width;
Table 1 shows several examples of variable
declarations.
Each parking space is suitable for a particular type of vehicle,
just as each variable holds a value of a particular type.
a variable is a
storage location
with a name.
When declaring a
variable, you
usually specify an
initial value.
When declaring a
variable, you also
specify the type of
its values.
© Ingenui/iStockphoto.
38 Chapter 2 Using Objects
table 1 Variable Declarations in java
Variable name Comment
int width = 20; Declares an integer variable and initializes it with 20.
int perimeter = 4 * width; The initial value need not be a fixed value. (Of course,  width
must have been previously declared.)
String greeting = "Hi!"; This variable has the type  String and is initialized with the
string “Hi”.
height = 30; error: The type is missing. This statement is not a declaration
but an assignment of a new value to an existing variable—see
Section 2.2.5.
int width = "20"; error: You cannot initialize a number with the string “20”.
(Note the quotation marks.)
int width; Declares an integer variable without initializing it. This can be a
cause for errors—see Common Error 2.1 on page 42.
int width, height; Declares two integer variables in a single statement. In this
book, we will declare each variable in a separate statement.
2.2.2 types
In Java, there are several different types of numbers. You use the  int type to denote a
whole number with out a fractional part. For example, suppose you count the num-
ber of cars in a parking lot. The counter must be an integer number—you can not have
a fraction of a car.
When a fractional part is required (such as in the number 22.5), we use floating-
point numbers. The most commonly used type for floating-point numbers in Java is
called  double . Here is the declaration of a floating-point variable:
double milesPerGallon = 22.5;
You can combine numbers with the  + and  - operators, as in  width + 10 or  width - 1 .
To multiply two numbers, use the  * operator. For example, 2 × width is written as
2 * width . Use the / operator for division, such as width / 2 .
As in mathematics, the  * and  / operator bind more strongly than the  + and  -
operators. That is,  width + height * 2 means the sum of  width and the product  height * 2 .
If you want to multiply the sum by 2, use parentheses:  (width + height) * 2 .
Not all types are number types. For example, the value  "Hello" has the type  String .
You need to specify that type when you define a variable that holds a string:
String greeting = "Hello";
A type specifies the operations that can be carried out with its values.
Types are important because they indicate what you can do with a variable. For
example, consider the variable  width . It’s type is  int . Therefore, you can multiply the
value that it holds with another number. But the type of  greeting is  String . You can’t
multiply a string with another number. (You will see in Section 2.3.1 what you can do
with strings.)
Use the  int type
for numbers that
cannot have a
fractional part.
Use the  double
type for floating-
point numbers.
numbers can
be combined by
arithmetic operators
such as  + ,  - , and  * .
2.2 Variables 39
2.2.3 names
When you declare a variable, you should pick a name that explains its purpose. For
example, it is better to use a descriptive name, such as  milesPerGallon , than a terse
name, such as  mpg .
In Java, there are a few simple rules for the names of variables, methods, and classes:
1.  Names must start with a letter or the underscore (_) character, and the remain-
ing characters must be letters, numbers, or underscores. (Technically, the  $
symbol is allowed as well, but you should not use it—it is intended for names
that are automatically generated by tools.)
2.  You cannot use other symbols such as  ? or  % . Spaces
are not permitted inside names either. You can use
uppercase letters to denote word bound aries, as in
milesPerGallon . This naming convention is called
camel case because the uppercase letters in the
middle of the name look like the humps of a camel.)
3.  Names are case sensitive, that is,  milesPerGallon and
milespergallon are differ ent names.
4.  You cannot use reserved words such as  double or  class as names; these words
are reserved exclusively for their special Java meanings. (See Appendix C for a
listing of all reserved words in Java.)
It is a convention among Java programmers that names of variables and methods
start with a lowercase letter (such as  milesPerGallon ). Class names should start with an
uppercase letter (such as  HelloPrinter ). That way, it is easy to tell them apart.
Table 2 shows examples of legal and illegal variable names in Java.
table 2 Variable names in java
Variable name Comment
distance_1 Names consist of letters, numbers, and the underscore
character.
x In mathematics, you use short variable names such as x or
y. This is legal in Java, but not very common, because it can
make programs harder to understand (see Programming Tip
2.1 on page 43).
!
CanVolume caution: Names are case sensitive. This variable name is
different from  canVolume , and it violates the convention that
variable names should start with a lowercase letter.
6pack  error: Names cannot start with a number.
can volume error: Names cannot contain spaces.
double error: You cannot use a reserved word as a name.
miles/gal error: You cannot use symbols such as / in names.
© GlobalP/iStockphoto.
by convention,
variable names
should start with a
lowercase letter.
40 Chapter 2  Using Objects
2.2.4 Comments
As your programs get more complex, you should add comments, explanations for
human readers of your code. For example, here is a comment that explains the value
used to initialize a variable:
double milesPerGallon = 33.8; //  The average fuel efficiency of new U.S. cars in 2011
This comment explains the significance of the value 33.8 to a human reader. The com-
piler does not pro cess comments at all. It ignores everything from a  // delimiter to the
end of the line.
It is a good practice to provide comments. This helps programmers who read your
code understand your intent. In addition, you will find comments helpful when you
review your own programs.
You use the  // delimiter for short comments. If you have a longer comment,
enclose it between  /* and  */ delimiters. The compiler ignores these delimiters and
everything in between. For example,
/*
In most countries, fuel efficiency is measured in liters per hundred
kilometer. Perhaps that is more useful—it tells you how much gas you need
to purchase to drive a given distance. Here is the conversion formula.
*/
double fuelEfficiency = 235.214583 / milesPerGallon;
2.2.5 Assignment
You can change the value of a variable with the assignment operator ( = ). For example,
consider the vari able declaration
int width = 10;  1
If you want to change the value of the variable, simply assign the new value:
width = 20;  2
The assignment replaces the original value of the variable (see Figure 2).
It is an error to use a variable that has never had a value assigned to it. For example,
the following assign ment statement has an error:
int height;
int width = height; //  ERROR—uninitialized variable  height
The compiler will complain about an “uninitialized variable” when you use a vari-
able that has never been assigned a value. (See Figure 3.)
Use comments to
add explanations
for humans who
read your code. The
compiler ignores
comments.
Use the assignment
operator ( = ) to
change the value 
of a variable.
Figure 2  
Assigning a New Value to a Variable
width = 10
width = 20
1
2
Camera: © james steidl/iStockphoto.
Globe: © Alex Slobodkin/iStockphoto.
A N IM AT IO N
Variable Initialization
and Assignment
Figure 3  
An Uninitialized Variable
height =
No value has been assigned.
2.2 Variables  41
Syntax 2.2  Assignment
variableName =  value ; Syntax
double width = 20;
.
.
width = 30;
.
.
.
width = width + 10;
The value of this variable is changed.
The same name
can occur on both sides.
See Figure 4.
The new value of the variable
This is a variable declaration.
This is an assignment statement.
The remedy is to assign a value to the variable before you use it:
int height = 20;
int width = height; //  OK
The right-hand side of the  = symbol can be a mathematical expression. For example,
width = height + 10;
This means “compute the value of  height + 10 and store that value in the variable  width ”.
In the Java programming language, the  = operator denotes an action, namely to
replace the value of a variable. This usage differs from the mathematical usage of the
= symbol as a statement about equality. For exam ple, in Java, the following statement
is entirely legal:
width = width + 10;
This means “compute the value of  width + 10 1 and store that value in the variable
width  2 ” (see Figure 4).
In Java, it is not a problem that the variable  width is used on both sides of the  = sym-
bol. Of course, in mathematics, the equation width = width + 10 has no solution.
All variables must be
initialized before you
access them.
The assignment
operator  =  does not
denote mathematical
equality.
Full Code example
Go to  wiley.com/go/
javacode  to download
a program that dem-
onstrates variables
and assignments.
Figure 4  
Executing the Statement 
width = width + 10
1
width =
width + 10
40
30
2
width = 40
Compute the value of the right-hand side
Store the value in the variable
42 Chapter 2 Using Objects
3.  What is wrong with the following variable declaration?
int miles per gallon = 39.4
4.  Declare and initialize two variables,  unitPrice and  quantity , to contain the unit
price of a single item and the number of items purchased. Use reasonable initial
values.
5.  Use the variables declared in Self Check 4 to display the total purchase price.
6.  What are the types of the values  0 and  "0" ?
7.  Which number type would you use for storing the area of a circle?
8.  Which of the following are legal identifiers?
Greeting1
g
void
101dalmatians
Hello, World
<greeting>
9.  Declare a variable to hold your name. Use camel case in the variable name.
10.  Is  12 = 12 a valid expression in the Java language?
11.  How do you change the value of the  greeting variable to  "Hello, Nina!" ?
12.  How would you explain assignment using the parking space analogy?
Practice it  Now you can try these exercises at the end of the chapter: R2.3, R2.4, R2.6.
using undeclared or uninitialized Variables
You must declare a variable before you use it for the first time. For example, the following
sequence of statements would not be legal:
int perimeter = 4 * width; //  ERROR: width  not yet declared
int width = 20;
In your program, the statements are compiled in order. When the compiler reaches the first
statement, it does not know that  width will be declared in the next line, and it reports an error.
The remedy is to reorder the decla rations so that each variable is declared before it is used.
A related error is to leave a variable uninitialized:
int width;
int perimeter = 4 * width; //  ERROR: width  not yet initialized
The Java compiler will complain that you are using a variable that has not yet been given a
value. The remedy is to assign a value to the variable before it is used.
confusing Variable declarations and assignment statements
Suppose your program declares a variable as follows:
int width = 20;
If you want to change the value of the variable, you use an assignment statement:
width = 30;
It is a common error to accidentally use another variable declaration:
int width = 30; // ERROR—starts with  int and is therefore a declaration
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
Common error 2.1
© John Bell/iStockphoto.
Common error 2.2
© John Bell/iStockphoto.
2.3 Calling Methods  43
But there is already a variable named  width . The compiler will complain that you are trying to
declare another vari able with the same name.
Choose Descriptive Variable Names
In algebra, variable names are usually just one letter long, such as p or A, maybe with a sub-
script such as p 1 . You might be tempted to save yourself a lot of typing by using short variable
names in your Java programs:
int a = w * h;
Compare that statement with the following one:
int area = width * height;
The advantage is obvious. Reading  width is much easier than reading  w and then figuring out
that it must mean “width”.
In practical programming, descriptive variable names are particularly important when pro-
grams are written by more than one person. It may be obvious to you that  w stands for width,
but is it obvious to the person who needs to update your code years later? For that matter, will
you yourself remember what  w means when you look at the code a month from now?
2.3 Calling Methods
A program performs useful work by calling methods on its objects. In this section, we
examine how to supply values in a method, and how to obtain the result of the method.
2.3.1 The Public Interface of a Class
You use an object by calling its methods. All objects of a given class share a common
set of methods. For example, the  PrintStream class provides methods for its objects
(such as  println and  print ). Similarly, the  String class provides methods that you can
apply to  String objects. One of them is the  length method. The  length method counts
the number of characters in a string. You can apply that method to any object of type
String . For example, the sequence of statements:
String greeting = “Hello, World!”;
int numberOfCharacters = greeting.length();
sets  numberOfCharacters to the length of the  String object  “Hello, World!” . After the
instructions in the  length method are executed,  numberOfCharacters is set to 13. (The quo-
tation marks are not part of the string, and the  length method does not count them.)
When calling the  length method, you do not supply any values inside the parenthe-
ses. Also note that the  length method does not produce any visible output. It returns a
value that is subsequently used in the program.
Let’s look at another method of the  String class. When you apply the  toUpperCase
method to a  String object, the method creates another  String object that contains the
characters of the original string, with lowercase letters converted to uppercase. For
example, the sequence of statements
String river = “Mississippi”;
String bigRiver = river.toUpperCase();
sets  bigRiver to the  String object  “MISSISSIPPI” .
Programming Tip 2.1
© Eric Isselé/iStockphoto.
44 Chapter 2  Using Objects
The  String class declares many other
methods besides the  length and  toUpper-
Case methods—you will learn about
many of them in Chapter 4. Collectively,
the methods form the public interface
of the class, telling you what you can do
with the objects of the class. A class also
declares a private implementation,
describ ing the data inside its objects and
the instructions for its methods. Those
details are hidden from the pro grammers
who use objects and call methods.
Figure 5 shows two objects of the
String class. Each object stores its own
data (drawn as boxes that contain characters). Both objects support the same set of
methods—the public interface that is specified by the  String class.
2.3.2 Method Arguments
Most methods require values that give details about the work that the method needs
to do. For exam ple, when you call the  println method, you must supply the string that
should be printed. Computer scientists use the techni cal term argument for method
inputs. We say that the string  greeting is an argument of the method call
System.out.println(greeting);
Figure 6 illustrates passing the argument to the method.
The public interface
of a class specifies
what you can do 
with its objects. 
The hidden imple
men ta   tion describes
how these actions
are carried out.
© Damir Cudic/iStockphoto.
The controls of a car form its public interface.
The private implementation is under the hood.
Figure 5  A Representation of Two  String  Objects
length
toUpperCase
H e l l o ...
String
data =
length
toUpperCase
String
M i s s i ... data =
An argument is a
value that is supplied
in a method call.
Figure 6  Passing an Argument to the  println  Method
PrintStream
println
print
10101110
11110110
01101011
00110101
"Hello, World"
2.3 Calling Methods 45
At this tailor shop, the customer’s measurements
and the fabric are the arguments of the  sew method.
The return value is the finished garment.
© Loentura/iStockphoto.
Some methods require multiple arguments; others don’t require any arguments at
all. An example of the latter is the  length method of the  String class (see Figure 7). All
the information that the  length method requires to do its job—namely, the character
sequence of the string—is stored in the object that carries out the method.
2.3.3 return Values
Some methods, such as the  println method, carry out an action for you. Other methods
compute and return a value. For example, the  length method returns a value, namely
the number of characters in the string. You can store the return value in a variable:
int numberOfCharacters = greeting.length();
You can also use the return value of one method as an argument of another method:
System.out.println(greeting.length());
The method call  greeting.length() returns a value—the integer 13. The return value
becomes an argument of the  println method. Figure 8 shows the process.
figure 7 
invoking the  length
Method on a  String Object
13 length
toUpperCase
String
(no argument)
H e l l o ...
the return value of
a method is a result
that the method has
computed.
figure 8  passing the result of a Method Call to another Method
13 length
toUpperCase
String
(no argument)
H e l l o ...
PrintStream
println
print
10101110
11110110
01101011
00110101
46 Chapter 2  Using Objects
Not all methods return values. One example is the  println method. The  println
method interacts with the operating system, causing characters to appear in a win-
dow. But it does not return a value to the code that calls it.
Let us analyze a more complex method call. Here, we will call the  replace method
of the  String class. The  replace method carries out a search-and-replace operation,
similar to that of a word processor. For example, the call
river.replace("issipp", "our")
constructs a new string that is obtained by replacing all occurrences of  "issipp" in
"Mississippi" with  "our" . (In this situation, there was only one replacement.) The
method returns the  String object  "Missouri" . You can save that string in a variable:
river = river.replace("issipp", "our");
Or you can pass it to another method:
System.out.println(river.replace("issipp", "our"));
As Figure 9 shows, this method call
• Is invoked on a  String object:  "Mississippi"
• Has two arguments: the strings  "issipp" and  "our"
• Returns a value: the string  “Missouri”
Table 3  Method Arguments and Return Values
Example Comments
System.out.println(greeting) greeting is an argument of the  println method.
greeting.replace("e","3") The  replace method has two arguments, in this
case  "e" and  "3" .
greeting.length() The  length method has no arguments.
int n = greeting.length(); The  length method returns an integer value.
System.out.println(n); The  println method returns no value. In the
API documentation, its return type is  void .
System.out.println(greeting.length()); The return value of one method can become the
argument of another.
Camera: © james steidl/iStockphoto.
Globe: © Alex Slobodkin/iStockphoto.
A N IM AT IO N
Parameter Passing
Figure 9  Calling the  replace  Method
length
toUpperCase
replace
String
M i s s i ...
"issipp"
"our"
"Missouri"
2.3 Calling Methods 47
2.3.4 Method Declarations
When a method is declared in a class, the declaration specifies the types of the argu-
ments and the return value. For example, the  String class declares the  length method as
public int length()
That is, there are no arguments, and the return value has the type  int . (For now, all
the methods that we consider will be “public” methods—see Chapter 9 for more
restricted methods.)
The  replace method is declared as
public String replace(String target, String replacement)
To call the  replace method, you supply two arguments,  target and  replacement , which
both have type  String . The returned value is another string.
When a method returns no value, the return type is declared with the reserved
word  void . For example, the  PrintStream class declares the  println method as
public void println(String output)
Occasionally, a class declares two methods with the same name and different argu-
ment types. For example, the  PrintStream class declares a second method, also called
println , as
public void println(int output)
That method is used to print an integer value. We say that the  println name is over-
loaded because it refers to more than one method.
13.  How can you compute the length of the string  “Mississippi” ?
14.  How can you print out the uppercase version of  “Hello, World!” ?
15.  Is it legal to call  river.println() ? Why or why not?
16.  What are the arguments in the method call  river.replace("p", "s") ?
17.  What is the result of the call  river.replace("p", "s") ?
18.  What is the result of the call  greeting.replace("World", "Dave").length() ?
19.  How is the  toUpperCase method declared in the  String class?
Practice it  Now you can try these exercises at the end of the chapter: R2.7, R2.8, R2.9.
learn by trying
When you learn about a new method, write a small program to try it out. For example, you can
go right now to your Java development environment and run this program:
public class ReplaceDemo
{
public static void main(String[] args)
{
String river = "Mississippi";
System.out.println(river.replace("issipp", "our"));
}
}
full cOde exaMPle
go to  wiley.com/go/
javacode to download
a program that
demonstrates
method calls.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
programming tip 2.2
© Eric Isselé/iStockphoto.
48 Chapter 2 Using Objects
Then you can see with your own eyes what the  replace method does. Also, you can run experi-
ments. Does  replace change every match, or only the first one? Try it out:
System.out.println(river.replace("i", "x"));
Set up your work environment to make this kind of experimentation easy and natural. Keep a
file with the blank outline of a Java program around, so you can copy and paste it when needed.
Alternatively, some development environments will automatically type the class and  main
method. Find out if yours does. Some environments even let you type commands into a win-
dow and show you the result right away, without having to make a  main method to call  System.
out.println (see Figure 10).
2.4 Constructing Objects
Generally, when you want to use objects in your pro-
gram, you need to specify their initial properties by
constructing them.
To learn about object construction, we need to go
beyond  String objects and the  System.out object. Let us
turn to another class in the Java library: the  Rectangle
class. Objects of type  Rectangle describe rectangular
shapes. These objects are useful for a variety of pur-
poses. You can assemble rectangles into bar charts, and
you can program simple games by moving rectangles
inside a window.
Note that a  Rectangle object isn’t a rectangular
shape—it’s an object that contains a set of numbers. The
numbers describe the rectangle (see Figure 11). Each
rectangle is described by the x- and y-coordinates of its
top-left corner, its width, and its height.
figure 10  the Code pad in bluej
© sinankocasian/iStockphoto.
Objects of the  Rectangle class
describe rectangular shapes.
2.4 Constructing Objects 49
figure 11  Rectangle Objects
x =
Rectangle
y =
width =
height =
5
10
20
30
x =
Rectangle
y =
width =
height =
45
0
30
3 20
x =
Rectangle
y =
width =
height =
35
30
20
3 20
It is very important that you understand this distinction. In the computer, a  Rect-
angle object is a block of memory that holds four numbers, for example x = 5, y = 10,
width = 20, height = 30. In the imagination of the programmer who uses a  Rectangle
object, the object describes a geometric figure.
To make a new rectangle, you need to specify the x, y, width, and height values.
Then invoke the  new operator, specifying the name of the class and the argument(s)
required for constructing a new object. For example, you can make a new rectangle
with its top-left corner at (5, 10), width 20, and height 30 as follows:
new Rectangle(5, 10, 20, 30)
Here is what happens in detail:
1.  The  new operator makes a  Rectangle object.
2.  It uses the arguments (in this case, 5, 10, 20, and 30) to initialize the
object’s data.
3.  It returns the object.
The process of creating a new object is called construction. The four values 5, 10, 20,
and 30 are called the construction arguments.
The  new expression yields an object, and you need to store the object if you want
to use it later. Usually you assign the output of the  new operator to a variable. For
example,
Rectangle box = new Rectangle(5, 10, 20, 30);
Use the  new
operator, followed
by a class name
and arguments,
to construct
new objects.
syntax 2.3  Object Construction
new  ClassName ( arguments ) Syntax
Rectangle box = new Rectangle(5, 10, 20, 30);
System.out.println(new Rectangle());
Construction arguments
Usually, you save
the constructed object
in a variable.
The  new expression yields an object.
Supply the parentheses even when
there are no arguments.
You can also
pass a constructed object
to a method.
50 Chapter 2 Using Objects
Some classes let you construct objects in multiple ways. For example, you can also
obtain a  Rectangle object by supplying no construction arguments at all (but you must
still supply the parentheses):
new Rectangle()
This expression constructs a (rather useless) rectangle with its top-left corner at the
origin (0, 0), width 0, and height 0.
20.  How do you construct a square with center (100, 100) and side length 20?
21.  Initialize the variables  box and  box2 with two rectangles that touch each other.
22.  The  getWidth method returns the width of a  Rectangle object. What does the fol-
lowing statement print?
System.out.println(new Rectangle().getWidth());
23.  The  PrintStream class has a constructor whose argument is the name of a file.
How do you construct a  PrintStream object with the construction argument
"output.txt" ?
24.  Write a statement to save the object that you constructed in Self Check 23 in a
variable.
Practice it  Now you can try these exercises at the end of the chapter: R2.11, R2.14, R2.16.
trying to invoke a constructor like a Method
Constructors are not methods. You can only use a constructor with the  new operator, not to
reinitialize an existing object:
box.Rectangle(20, 35, 20, 30); //  Error—can’t reinitialize object
The remedy is simple: Make a new object and overwrite the current one stored by  box .
box = new Rectangle(20, 35, 20, 30); //  OK
2.5 accessor and Mutator Methods
In this section we introduce a useful terminology for the methods of a class. A method
that accesses an object and returns some information about it, without changing the
object, is called an accessor method. In contrast, a method whose purpose is to mod-
ify the internal data of an object is called a mutator method.
For example, the  length method of the  String class is an accessor method. It returns
information about a string, namely its length. But it doesn’t modify the string at all
when counting the characters.
The  Rectangle class has a number of accessor methods. The  getX ,  getY ,  getWidth , and
getHeight methods return the x- and y-coordinates of the top-left corner, the width,
and the height values. For example,
double width = box.getWidth();
full cOde exaMPle
go to  wiley.com/go/
javacode to download
a program that
demonstrates
constructors.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
Common error 2.3
© John Bell/iStockphoto.
an accessor method
does not change the
internal data of the
object on which it is
invoked. a mutator
method changes
the data.
2.5 accessor and Mutator Methods 51
Now let us consider a mutator method. Programs that manipulate rectangles fre-
quently need to move them around, for example, to display animations. The  Rectangle
class has a method for that purpose, called  translate . (Mathematicians use the term
“translation” for a rigid motion of the plane.) This method moves a rectangle by a
certain distance in the x- and y-directions. The method call,
box.translate(15, 25);
moves the rectangle by 15 units in the x-direction and 25 units in the y-direction (see
Figure 12). Moving a rectangle doesn’t change its width or height, but it changes the
top-left corner. Afterward, the rectangle that had its top-left corner at (5, 10) now has
it at (20, 35).
This method is a mutator because it modifies the object on which the method is
invoked.
25.  What does this sequence of statements print?
Rectangle box = new Rectangle(5, 10, 20, 30);
System.out.println("Before: " + box.getX());
box.translate(25, 40);
System.out.println("After: " + box.getX());
26.  What does this sequence of statements print?
Rectangle box = new Rectangle(5, 10, 20, 30);
System.out.println("Before: " + box.getWidth());
box.translate(25, 40);
System.out.println("After: " + box.getWidth());
27.  What does this sequence of statements print?
String greeting = "Hello";
System.out.println(greeting.toUpperCase());
System.out.println(greeting);
28.  Is the  toUpperCase method of the  String class an accessor or a mutator?
29.  Which call to  translate is needed to move the rectangle declared by  Rectangle
box = new Rectangle(5, 10, 20, 30) so that its top-left corner is the origin (0, 0)?
Practice it  Now you can try these exercises at the end of the chapter: R2.17, E2.6, E2.8.
full cOde exaMPle
go to  wiley.com/go/
javacode to down-
load a program that
demonstrates acces-
sors and mutators.
figure 12  Using the  translate Method to Move a rectangle
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
52 Chapter 2 Using Objects
2.6 the api Documentation
The classes and methods of the Java library are listed in the API documentation.
The API is the “applica tion programming interface”. A programmer who uses the
Java classes to put together a computer pro gram (or application) is an application pro-
grammer. That’s you. In contrast, the programmers who designed and implemented
the library classes such as  PrintStream and  Rectangle are system programmers.
You can find the API documentation on the Web. Point your web browser to
http://docs.oracle.com/javase/7/docs/api/index.html . An abbreviated version of the
API documentation is provided in Appendix D that may be easier to use at first, but
you should eventually move on to the real thing.
2.6.1 browsing the api Documentation
The API documentation documents all classes in the Java library—there are thou-
sands of them (see Figure 13, top). Most of the classes are rather specialized, and only
a few are of interest to the beginning programmer.
Locate the  Rectangle link in the left pane, preferably by using the search function of
your browser. Click on the link, and the right pane shows all the features of the  Rect-
angle class (see Figure 13, bottom).
the api (application
programming inter-
face) documentation
lists the classes
and methods of
the java library.
figure 13  the api Documentation of the standard java library
API documentation of
the  Rectangle class
Scroll down
2.6 the api Documentation 53
figure 14  the Method summary for the  Rectangle Class
API documentation of
the  translate method
1
2
The API documentation for each class starts out with a section that describes the
purpose of the class. Then come summary tables for the constructors and methods
(see Figure 14, top). Click on a method’s link to get a detailed description (see Figure
14, bottom).
The detailed description of a method shows
• The action that the method carries out.  1
• The types and names of the parameter variables that receive the arguments when
the method is called.  2
• The value that it returns (or the reserved word  void if the method doesn’t return
any value).
As you can see, the  Rectangle class has quite a few methods. While occasionally intim-
idating for the begin ning programmer, this is a strength of the standard library. If you
ever need to do a computation involv ing rectangles, chances are that there is a method
that does all the work for you.
For example, suppose you want to change the width or height of a rectangle. If
you browse through the API documentation, you will find a  setSize method with
the description “Sets the size of this  Rectan gle to the specified width and height.” The
method has two arguments, described as
•  width - the new width for this  Rectangle
•  height - the new height for this  Rectangle
54 Chapter 2 Using Objects
We can use this information to change the  box object so that it is a square of side length
40. The name of the method is  setSize , and we supply two arguments: the new width
and height:
box.setSize(40, 40);
2.6.2 packages
The API documentation contains another important piece of information about each
class. The classes in the standard library are organized into packages. A package is a
collection of classes with a related pur pose. The  Rectangle class belongs to the pack-
age  java.awt (where  awt is an abbreviation for “Abstract Win dowing Toolkit”), which
contains many classes for drawing windows and graphical shapes. You can see the
package name  java.awt in Figure 13, just above the class name.
To use the  Rectangle class from the  java.awt package, you must import the package.
Simply place the fol lowing line at the top of your program:
import java.awt.Rectangle;
Why don’t you have to import the  System and  String classes? Because the  System and
String classes are in the  java.lang package, and all classes from this package are auto-
matically imported, so you never need to import them yourself.
30.  Look at the API documentation of the  String class. Which method would you
use to obtain the string  "hello, world!" from the string  "Hello, World!" ?
31.  In the API documentation of the  String class, look at the description of the  trim
method. What is the result of applying  trim to the string  " Hello, Space ! " ? (Note
the spaces in the string.)
32.  Look into the API documentation of the  Rectangle class. What is the difference
between the methods  void translate(int x, int y) and  void setLocation(int x,
int y) ?
33.  The  Random class is declared in the  java.util package. What do you need to do in
order to use that class in your program?
java classes are
grouped into
packages. Use the
import statement
to use classes that
are declared in
other packages.
syntax 2.4  importing a Class from a package
import  packageName . ClassName ; Syntax
import java.awt.Rectangle;
Class name
You can look up the package name
in the API documentation.
Package name
Import statements
must be at the top of
the source file.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
testing track 2.7 implementing a test program 55
34.  In which package is the  BigInteger class located? Look it up in the API
documentation.
Practice it  Now you can try these exercises at the end of the chapter: R2.18, E2.4, E2.11.
don’t Memorize—use Online help
The Java library has thousands of classes and methods. It is neither necessary nor useful trying
to memorize them. Instead, you should become familiar with using the API documentation.
Because you will need to use the API docu mentation all the time, it is best to download and
install it onto your computer, particularly if your computer is not always connected to the
Internet. You can download the documentation from  http://www.oracle.com/technetwork/java/
javase/downloads/index.html .
2.7 implementing a test program
In this section, we discuss the steps that are necessary to implement a test program.
The purpose of a test program is to verify that one or more methods have been imple-
mented correctly. A test program calls methods and checks that they return the
expected results. Writing test programs is a very important skill.
In this section, we will develop a simple program that tests a method in the  Rect-
angle class using these steps:
1.  Provide a tester class.
2.  Supply a  main method.
3.  Inside the  main method, construct one or more objects.
4.  Apply methods to the objects.
5.  Display the results of the method calls.
6.  Display the values that you expect to get.
Our sample test program tests the behavior of the  translate method. Here are the key
steps (which have been placed inside the  main method of the  Move Tester class).
Rectangle box = new Rectangle(5, 10, 20, 30);
//  Move the rectangle
box.translate(15, 25);
//  Print information about the moved rectangle
System.out.print("x: ");
System.out.println(box.getX());
System.out.println("Expected: 20");
We print the value that is returned by the  getX method, and then we print a message
that describes the value we expect to see.
This is a very important step. You want to spend some time thinking about the
expected result before you run a test program. This thought process will help you
understand how your program should behave, and it can help you track down errors
at an early stage. Finding and fixing errors early is a very effective strategy that can
save you a great deal of time.
programming tip 2.3
© Eric Isselé/iStockphoto.
a test program
verifies that methods
behave as expected.
Determining the
expected result
in advance is an
important part
of testing.
56 Chapter 2 Using Objects  testing track
In our case, the rectangle has been constructed with the top-left corner at (5, 10).
The x-direction is moved by 15, so we expect an x-value of 5 + 15 = 20 after the move.
Here is the program that tests the moving of a rectangle:
section_7/Movetester.java
1 import java.awt.Rectangle;
2
3 public class MoveTester
4 {
5 public static void main(String[] args)
6 {
7 Rectangle box = new Rectangle(5, 10, 20, 30);
8
9 //  Move the rectangle
10 box.translate(15, 25);
11
12 //  Print information about the moved rectangle
13 System.out.print("x: ");
14 System.out.println(box.getX());
15 System.out.println("Expected: 20");
16
17 System.out.print("y: ");
18 System.out.println(box.getY());
19 System.out.println("Expected: 35");
20 }
21 }
Program run
x: 20
Expected: 20
y: 35
Expected: 35
35.  Suppose we had called  box.translate(25, 15) instead of  box.translate(15, 25) .
What are the expected outputs?
36.  Why doesn’t the  MoveTester program need to print the width and height of the
rectangle?
Practice it  Now you can try these exercises at the end of the chapter: E2.1, E2.7, E2.13.
testing classes in an interactive environment
Some development environments are specifically designed to help students explore objects
without having to pro vide tester classes. These environments can be very helpful for gaining
insight into the behavior of objects, and for promoting object-oriented thinking. The BlueJ
environment (shown in the figure) displays objects as blobs on a workbench.
You can construct new objects, put them on the workbench, invoke methods, and see the
return values, all without writing a line of code. You can download BlueJ at no charge from
www.bluej.org . Another excellent envi ronment for interactively exploring objects is Dr. Java at
drjava.sourceforge.net .
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
special topic 2.1
© Eric Isselé/iStockphoto.
2.8 Object references 57
Testing a Method Call in BlueJ
2.8 Object references
In Java, an object variable (that is, a variable whose type is a class) does not actually
hold an object. It merely holds the memory loca tion of an object. The object itself is
stored elsewhere—see Figure 15.
WOrkeD exaMple 2.1  how Many days have You been alive?
Explore the API of a class  Day that represents a calendar day. Using
that class, learn to write a program that computes how many days have
elapsed since the day you were born. Go to  wiley.com/go/javaexamples
and download Worked Example 2.1.
© Constance Bannister Corp/Hulton Arch
WOrkeD exaMple 2.2  Working with Pictures
Learn how to use the API of a  Picture class to edit photos. Go to
wiley.com/go/javaexamples and download Worked Example 2.2.
Cay Horstmann.
figure 15  an Object Variable Containing an Object reference
box =
x =
Rectangle
y =
width =
height =
5
10
20
30
58 Chapter 2 Using Objects
There is a reason for this behavior. Objects can be very large. It is more efficient to
store only the memory location instead of the entire object.
We use the technical term object reference to denote the memory location of an
object. When a vari able contains the memory location of an object, we say that it
refers to an object. For example, after the statement
Rectangle box = new Rectangle(5, 10, 20, 30);
the variable  box refers to the  Rectangle object that the  new operator constructed. Tech-
nically speaking, the  new operator returned a reference to the new object, and that
reference is stored in the  box variable.
It is very important that you remember that the  box variable does not contain the
object. It refers to the object. Two object variables can refer to the same object:
Rectangle box2 = box;
Now you can access the same  Rectangle object as  box and as  box2 , as shown in Figure 16.
In Java, numbers are not objects. Number variables actually store numbers. When
you declare
int luckyNumber = 13;
then the  luckyNumber variable holds the number 13, not a reference to the number (see
Figure 17). The rea son is again efficiency. Because numbers require little storage, it is
more efficient to store them directly in a variable.
You can see the difference between number variables and object variables when you
make a copy of a variable. When you copy a number, the original and the copy of the
number are independent values. But when you copy an object reference, both the
original and the copy are references to the same object.
Consider the following code, which copies a number and then changes the copy
(see Figure 18):
int luckyNumber = 13;  1
int luckyNumber2 = luckyNumber;  2
luckyNumber2 = 12;  3
Now the variable  luckyNumber contains the value 13, and  luckyNumber2 contains 12.
an object reference
describes the
location of an object.
© Jacob Wackerhausen/iStockphoto.
Multiple object
variables can contain
references to the
same object.
figure 16  two Object Variables referring to the same Object
box =
box2 =
x =
Rectangle
y =
width =
height =
5
10
20
30
figure 17  a number Variable stores a number
luckyNumber = 13
number variables
store numbers.
Object variables
store references.
2.8 Object References  59
Figure 18  Copying Numbers
luckyNumber = 13
luckyNumber2 = 13
luckyNumber = 13
1
2
luckyNumber = 13
luckyNumber2 = 12
3
Now consider the seemingly analogous code with  Rectangle objects (see Figure 19).
Rectangle box = new Rectangle(5, 10, 20, 30);  1
Rectangle box2 = box;  2
box2.translate(15, 25); 3
Because  box and  box2 refer to the same rectangle after step  2 , both variables refer to
the moved rectangle after the call to the  translate method.
Full Code example
Go to  wiley.com/go/
javacode  to down-
load a program that
demonstrates the
difference between
copying numbers and
object references.
Camera: © james steidl/iStockphoto.
Globe: © Alex Slobodkin/iStockphoto.
A N IM AT IO N
Object References
Figure 19  Copying Object References
box =
box2 =
x =
Rectangle
y =
width =
height =
5
10
20
30
box =
box2 =
x =
Rectangle
y =
width =
height =
20
35
20
30
box =
x =
Rectangle
y =
width =
height =
5
10
20
30
1
2
3
60 Chapter 2 Using Objects
You need not worry too much about the difference between objects and object
references. Much of the time, you will have the correct intuition when you think of
the “object  box ” rather than the technically more accurate “object reference stored
in variable  box ”. The difference between objects and object references only becomes
apparent when you have multiple variables that refer to the same object.
37.  What is the effect of the assignment String greeting2 = greeting ?
38.  After calling  greeting2.toUpperCase() , what are the contents of  greeting and
greeting2 ?
Practice it  Now you can try these exercises at the end of the chapter: R2.15, R2.19.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
When  international
business  Machines
Cor poration (ibM), a successful manu-
facturer of punched-card equipment
for tabulating data, first turned its
attention to designing computers in
the early 1950s, its planners assumed
that there was a market for perhaps
50 such devices, for installation by
the government, the military, and a
few of the country’s largest corpora-
tions. instead, they sold about 1,500
machines of their system 650 model
and went on to build and sell more
powerful computers.
these computers, called main-
frames, were huge. they filled rooms,
which had to be climate-controlled to
protect the delicate equipment. ibM
was not the first company to build
mainframe computers; that honor
belongs to the Univac Corporation.
however, ibM soon became the major
player, partially because of its techni-
cal excellence and attention to cus-
tomer needs and partially because it
exploited its strengths and structured
its products and services in a way that
made it difficult for customers to mix
them with those of other vendors.
as all of ibM’s competitors fell
on hard times, the U.s. government
brought an antitrust suit against ibM in
1969. in the United states, it is legal
to be a monopoly supplier, but it is
not legal to use one’s monopoly in one
market to gain supremacy in another.
ibM was accused of forcing custom-
ers to buy bundles of computers,
software, and peripherals, making it
impossible for other vendors of soft-
ware and peripherals to compete.
the suit went to trial in 1975 and
dragged on until 1982, when it was
abandoned,  largely  because  new
waves of smaller computers had made
it irrelevant.
in fact, when ibM offered its first
personal computers, its operating
system was supplied by an outside
vendor, Microsoft, which became so
dominant that it too was sued by the
U.s. goverment for abusing its monop-
oly position in 1998. Microsoft was
accused of bundling
its  web  browser
with  its  operating
system. at the time,
Microsoft  allegedly
threatened hardware
makers  that  they
would not receive a
Windows license if
they distributed the
competing netscape
browser. in 2000, the
company was found
guilty  of  antitrust
violations, and the
judge ordered it bro-
ken up into an oper-
ating systems unit
and an applications
unit.  the  breakup
was  reversed  on
appeal, and a settle-
ment in 2001 was
largely unsuccessful
in establishing alternatives for desk-
top software.
now the computing landscape is
shifting once again, toward mobile
devices and cloud computing. as you
observe that change, you may well see
new monopolies in the making. When
a software vendor needs the permis-
sion of a hardware vendor in order to
place a product into an “app store”, or
when a maker of a digital book reader
tries to coerce publishers into a par-
ticular pricing structure, the question
arises whether such conduct is illegal
exploitation of a monopoly position.
Corbis Digital Stock.
A Mainframe Computer
Computing & Society 2.1  Computer Monopoly
© MediaBakery.
graphics track 2.9 graphical applications 61
2.9 graphical applications
The following optional sections teach you how to write graphical applications:
applica tions that display drawings inside a window. The drawings are made up of
shape objects: rectangles, ellipses, and lines. The shape objects provide another source
of examples, and many students enjoy the visual feedback.
2.9.1 Frame Windows
A graphical application shows information inside a frame:
a window with a title bar, as shown in Figure 20. In this sec-
tion, you will learn how to display a frame. In Section 2.9.2,
you will learn how to cre ate a drawing inside the frame.
A graphical application shows
information inside a frame.
To show a frame, carry out the following steps:
1.  Construct an object of the  JFrame class:
JFrame frame = new JFrame();
2.  Set the size of the frame:
frame.setSize(300, 400);
This frame will be 300 pixels wide and 400 pixels tall. If you omit this step the
frame will be 0 by 0 pixels, and you won’t be able to see it. (Pixels are the tiny
dots from which digital images are com posed.)
3.  If you’d like, set the title of the frame:
frame.setTitle("An empty frame");
If you omit this step, the title bar is simply left blank.
4.  Set the “default close operation”:
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
When the user closes the frame, the program automatically exits. Don’t omit
this step. If you do, the program keeps running even after the frame is closed.
5.  Make the frame visible:
frame.setVisible(true);
The simple program below shows all of these steps. It produces the empty frame
shown in Figure 20.
The  JFrame class is a part of the  javax.swing package. Swing is the nickname for the
graphical user inter face library in Java. The “ x ” in  javax denotes the fact that Swing
started out as a Java extension before it was added to the standard library.
© Eduardo Jose Bernardino/iStockphoto.
to show a frame,
construct a  JFrame
object, set its size,
and make it visible.
62 Chapter 2 Using Objects  graphics track
figure 20  a Frame Window
Title bar
Close button
We will go into much greater detail about Swing programming in Chapters 3, 10,
and 19. For now, consider this program to be the essential plumbing that is required
to show a frame.
section_9_1/emptyframeViewer.java
1 import javax.swing.JFrame;
2
3 public class EmptyFrameViewer
4 {
5 public static void main(String[] args)
6 {
7 JFrame frame = new JFrame();
8 frame.setSize(300, 400);
9 frame.setTitle("An empty frame");
10 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
11 frame.setVisible(true);
12 }
13 }
2.9.2 Drawing on a Component
In this section, you will learn how to make shapes appear inside a frame window. The
first drawing will be exceedingly modest: just two rectangles (see Figure 21). You’ll
soon see how to produce more interesting drawings. The purpose of this example is
to show you the basic outline of a program that creates a drawing.
You cannot draw directly onto a frame. Instead, drawing happens in a component
object. In the Swing toolkit, the  JComponent class represents a blank component.
Because we don’t want to add a blank component, we have to modify the  JComponent
class and specify how the component should be painted. The solution is to declare a
new class that extends the  JComponent class. You will learn about the process of extend-
ing classes in Chapter 9.
in order to display a
drawing in a frame,
declare a class
that extends the
JComponent class.
graphics track 2.9 graphical applications 63
figure 21 
Drawing rectangles
For now, simply use the following code as a template:
public class RectangleComponent extends JComponent
{
public void paintComponent(Graphics g)
{
Drawing instructions
}
}
The  extends reserved word indicates that our component class,  RectangleComponent ,
can be used like a  JCom ponent . However, the  RectangleComponent class will be different
from the plain  JComponent class in one respect: Its  paintComponent method will contain
instructions to draw the rectangles.
When the component is shown for the first time, the  paintComponent method is
called automatically. The method is also called when the window is resized, or when
it is shown again after it was hidden.
The  paintComponent method receives an object of type  Graphics as its argument. The
Graphics object stores the graphics state—the current color, font, and so on—that are
used for drawing operations. However, the  Graphics class is not very useful. When
programmers clamored for a more object-oriented approach to drawing graphics, the
designers of Java created the  Graphics2D class, which extends the  Graphics class. When-
ever the Swing toolkit calls the  paintComponent method, it actually passes an object of
type  Graphics2D as the argument. Because we want to use the more sophisticated meth-
ods to draw two-dimensional graphics objects, we need to use the  Graphics2D class.
This is accomplished by using a cast:
public class RectangleComponent extends JComponent
{
public void paintComponent(Graphics g)
{
//  Recover  Graphics2D
Graphics2D g2 = (Graphics2D) g;
. . .
}
}
place drawing
instructions inside
the  paintComponent
method. that
method is called
whenever the
component needs
to be repainted.
Use a cast to recover
the  Graphics2D object
from the  Graphics
argument of the
paintComponent
method.
64 Chapter 2 Using Objects  graphics track
Chapter 9 has more information about casting. For now, you should simply include
the cast at the top of your  paintComponent methods.
Now you are ready to draw shapes. The  draw method of the  Graphics2D class can
draw shapes, such as rectangles, ellipses, line segments, polygons, and arcs. Here we
draw a rectangle:
public class RectangleComponent extends JComponent
{
public void paintComponent(Graphics g)
{
. . .
Rectangle box = new Rectangle(5, 10, 20, 30);
g2.draw(box);
. . .
}
}
When positioning the shapes, you need to pay attention to the coordinate system.
It is different from the one used in mathematics. The origin (0, 0) is at the upper-left
corner of the component, and the y-coordinate grows downward.
(0, 0)
(5, 10)
(25, 40)
x
y
Following is the source code for the  RectangleComponent class. Note that the  paint-
Component method of the  RectangleComponent class draws two rectangles. As you can see
from the  import statements, the  Graphics and  Graphics2D classes are part of the  java.awt
package.
section_9_2/rectanglecomponent.java
1 import java.awt.Graphics;
2 import java.awt.Graphics2D;
3 import java.awt.Rectangle;
4 import javax.swing.JComponent;
5
6 /**
7 A component that draws two rectangles.
8 */
9 public class RectangleComponent extends JComponent
10 {
11 public void paintComponent(Graphics g)
12 {
13 //  Recover  Graphics2D
14 Graphics2D g2 = (Graphics2D) g;
15
graphics track 2.9 graphical applications 65
16 //  Construct a rectangle and draw it
17 Rectangle box = new Rectangle(5, 10, 20, 30);
18 g2.draw(box);
19
20 //  Move rectangle 15 units to the right and 25 units down
21 box.translate(15, 25);
22
23 //  Draw moved rectangle
24 g2.draw(box);
25 }
26 }
2.9.3 Displaying a Component in a Frame
In a graphical application, you need a frame to show the application, and you need
a component for the drawing. In this section, you will see how to combine the two.
Follow these steps:
1.  Construct a frame object and configure it.
2.  Construct an object of your component class:
RectangleComponent component = new RectangleComponent();
3.  Add the component to the frame:
frame.add(component);
4.  Make the frame visible.
The following listing shows the complete process.
section_9_3/rectangleViewer.java
1 import javax.swing.JFrame;
2
3 public class RectangleViewer
4 {
5 public static void main(String[] args)
6 {
7 JFrame frame = new JFrame();
8
9 frame.setSize(300, 400);
10 frame.setTitle("Two rectangles");
11 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
12
13 RectangleComponent component = new RectangleComponent();
14 frame.add(component);
15
16 frame.setVisible(true);
17 }
18 }
Note that the rectangle drawing program consists of two classes:
• The  RectangleComponent class, whose  paintComponent method produces the drawing.
• The  RectangleViewer class, whose  main method constructs a frame and a  Rectangle-
Component , adds the com ponent to the frame, and makes the frame visible.
66 Chapter 2 Using Objects  graphics track
39.  How do you display a square frame with a title bar that reads “Hello, World!”?
40.  How can a program display two frames at once?
41.  How do you modify the program to draw two squares?
42.  How do you modify the program to draw one rectangle and one square?
43.  What happens if you call  g.draw(box) instead of  g2.draw(box) ?
Practice it  Now you can try these exercises at the end of the chapter: R2.20, R2.24, E2.17.
2.10 ellipses, lines, text, and Color
In Section 2.9 you learned how to write a program that draws
rectangles. In the following sections, you will learn how to
draw other shapes: ellipses and lines. With these graphical ele-
ments, you can draw quite a few interesting pictures.
2.10.1 ellipses and Circles
To draw an ellipse, you specify its bounding box (see Fig-
ure 22) in the same way that you would specify a rectangle,
namely by the x- and y-coordinates of the top-left corner and
the width and height of the box.
However, there is no simple  Ellipse class that you can use.
Instead, you must use one of the two classes  Ellipse2D.Float
and  Ellipse2D.Double , depending on whether you want to store the ellipse coordinates
as single- or double-precision floating-point values. Because the latter are more con-
venient to use in Java, we will always use the  Ellipse2D.Double class.
Here is how you construct an ellipse:
Ellipse2D.Double ellipse = new Ellipse2D.Double(x, y, width, height);
The class name  Ellipse2D.Double looks different from the class names that you have
encountered up to now. It consists of two class names  Ellipse2D and  Double separated
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
© Alexey Avdeev/iStockphoto. You can make simple
drawings out of lines,
rectangles, and circles.
The Ellipse2D.Double
and  Line2D.Double
classes describe
graphical shapes.
figure 22  an ellipse and its bounding box
(x, y)
Height
Width
graphics track 2.10 ellipses, lines, text, and Color 67
by a period ( . ). This indicates that  Ellipse2D.Double is a so-called inner class inside
Ellipse2D . When constructing and using ellipses, you don’t actually need to worry
about the fact that  Ellipse2D.Double is an inner class—just think of it as a class with a
long name. However, in the  import statement at the top of your program, you must be
careful that you import only the outer class:
import java.awt.geom.Ellipse2D;
Drawing an ellipse is easy: Use exactly the same  draw method of the  Graphics2D class
that you used for drawing rectangles.
g2.draw(ellipse);
To draw a circle, simply set the width and height to the same values:
Ellipse2D.Double circle = new Ellipse2D.Double(x, y, diameter, diameter);
g2.draw(circle);
Notice that (x, y) is the top-left corner of the bounding box, not the center of the
circle.
2.10.2 lines
To draw a line, use an object of the  Line2D.Double class. A line is constructed by speci-
fying its two end points. You can do this in two ways. Give the x- and y-coordinates
of both end points:
Line2D.Double segment = new Line2D.Double(x1, y1, x2, y2);
Or specify each end point as an object of the  Point2D.Double class:
Point2D.Double from = new Point2D.Double(x1, y1);
Point2D.Double to = new Point2D.Double(x2, y2);
Line2D.Double segment = new Line2D.Double(from, to);
The second option is more object-oriented and is often more useful, particularly if
the point objects can be reused elsewhere in the same drawing.
2.10.3 Drawing text
You often want to put text inside a drawing, for example, to label some of the parts.
Use the  drawString method of the  Graphics2D class to draw a string anywhere in a win-
dow. You must specify the string and the x- and y-coordinates of the basepoint of the
first character in the string (see Figure 23). For example,
g2.drawString("Message", 50, 100);
the  drawString
method draws a
string, starting at
its basepoint.
figure 23  basepoint and baseline
Baseline
Basepoint
68 Chapter 2 Using Objects  graphics track
2.10.4 Colors
When you first start drawing, all shapes and strings are drawn with a black pen. To
change the color, you need to supply an object of type  Color . Java uses the RGB color
model. That is, you specify a color by the amounts of the primary colors—red, green,
and blue—that make up the color. The amounts are given as integers between 0 (pri-
mary color not present) and 255 (maximum amount present). For example,
Color magenta = new Color(255, 0, 255);
constructs a  Color object with maximum red, no green, and maximum blue, yielding a
bright purple color called magenta.
For your convenience, a variety of colors have been declared in the  Color class.
Table 4 shows those col ors and their RGB values. For example,  Color.PINK has been
declared to be the same color as  new Color(255, 175, 175) .
To draw a shape in a different color, first set the color of the  Graphics2D object, then
call the  draw method:
g2.setColor(Color.RED);
g2.draw(circle); //  Draws the shape in red
If you want to color the inside of the shape, use the  fill method instead of the  draw
method. For example,
g2.fill(circle);
fills the inside of the circle with the current color.
table 4 predefined Colors
Color rgb Values
Color.BLACK 0, 0, 0
Color.BLUE 0, 0, 255
Color.CYAN 0, 255, 255
Color.GRAY 128, 128, 128
Color.DARK_GRAY 64, 64, 64
Color.LIGHT_GRAY 192, 192, 192
Color.GREEN 0, 255, 0
Color.MAGENTA 255, 0, 255
Color.ORANGE 255, 200, 0
Color.PINK 255, 175, 175
Color.RED 255, 0, 0
Color.WHITE 255, 255, 255
Color.YELLOW 255, 255, 0
When you set a new
color in the graphics
context, it is used for
subsequent drawing
operations.
graphics track 2.10 ellipses, lines, text, and Color 69
The following program puts all these shapes to work, creating a simple drawing
(see Figure 24).
section_10/facecomponent.java
1 import java.awt.Color;
2 import java.awt.Graphics;
3 import java.awt.Graphics2D;
4 import java.awt.Rectangle;
5 import java.awt.geom.Ellipse2D;
6 import java.awt.geom.Line2D;
7 import javax.swing.JComponent;
8
9 /**
10 A component that draws an alien face.
11 */
12 public class FaceComponent extends JComponent
13 {
14 public void paintComponent(Graphics g)
15 {
16 //  Recover  Graphics2D
17 Graphics2D g2 = (Graphics2D) g;
18
19 //  Draw the head
20 Ellipse2D.Double head = new Ellipse2D.Double(5, 10, 100, 150);
21 g2.draw(head);
22
23 //  Draw the eyes
24 g2.setColor(Color.GREEN);
25 Rectangle eye = new Rectangle(25, 70, 15, 15);
26 g2.fill(eye);
27 eye.translate(50, 0);
28 g2.fill(eye);
29
30 //  Draw the mouth
31 Line2D.Double mouth = new Line2D.Double(30, 110, 80, 110);
32 g2.setColor(Color.RED);
33 g2.draw(mouth);
34
35 //  Draw the greeting
36 g2.setColor(Color.BLUE);
37 g2.drawString("Hello, World!", 5, 175);
38 }
39 }
figure 24  an alien Face
70 Chapter 2 Using Objects  graphics track
section_10/faceViewer.java
1 import javax.swing.JFrame;
2
3 public class FaceViewer
4 {
5 public static void main(String[] args)
6 {
7 JFrame frame = new JFrame();
8 frame.setSize(150, 250);
9 frame.setTitle("An Alien Face");
10 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
11
12 FaceComponent component = new FaceComponent();
13 frame.add(component);
14
15 frame.setVisible(true);
16 }
17 }
44.  Give instructions to draw a circle with center (100, 100) and radius 25.
45.  Give instructions to draw a letter “V” by drawing two line segments.
46.  Give instructions to draw a string consisting of the letter “V”.
47.  What are the RGB color values of  Color.BLUE ?
48.  How do you draw a yellow square on a red background?
Practice it  Now you can try these exercises at the end of the chapter: R2.25, E2.18, E2.19.
identify objects, methods, and classes.
• Objects are entities in your program that you manipulate by
calling methods.
• A method is a sequence of instructions that accesses the data
of an object.
• A class describes a set of objects with the same behavior.
Write variable declarations and assignments.
• A variable is a storage location with a name.
• When declaring a variable, you usually specify an initial value.
• When declaring a variable, you also specify the type of its values.
• Use the  int type for numbers that cannot have a fractional part.
• Use the  double type for floating-point numbers.
• Numbers can be combined by arithmetic operators such as  + ,  - , and  * .
• By convention, variable names should start with a lowercase letter.
• Use comments to add explanations for humans who read your
code. The compiler ignores comments.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
C h a p t e r s U M M a rY
© Luc Meaille/iStockphoto.
© Ingenui/iStockphoto.
© GlobalP/iStockphoto.
Chapter summary 71
• Use the assignment operator ( = ) to change the value of a variable.
• All variables must be initialized before you access them.
• The assignment operator  = does not denote mathematical equality.
recognize arguments and return values of methods.
• The public interface of a class specifies what you can do
with its objects. The hidden imple men ta tion describes
how these actions are carried out.
• An argument is a value that is supplied in a method call.
• The return value of a method is a result that the method
has computed.
use constructors to construct new objects.
• Use the  new operator, followed by a class name and arguments, to construct
new objects.
classify methods as accessor and mutator methods.
• An accessor method does not change the internal data of the object on which it is
invoked. A mutator method changes the data.
use the aPi documentation for finding method descriptions and packages.
• The API (Application Programming Interface) documentation lists the classes
and methods of the Java library.
• Java classes are grouped into packages. Use the  import statement to use classes that
are declared in other packages.
Write programs that test the behavior of methods.
• A test program verifies that methods behave as expected.
• Determining the expected result in advance is an important part of testing.
describe how multiple object references can refer to the same object.
• An object reference describes the location of an object.
• Multiple object variables can contain references to the same object.
• Number variables store numbers. Object variables store references.
Write programs that display frame windows.
• To show a frame, construct a  JFrame object, set its size, and make it visible.
• In order to display a drawing in a frame, declare a class that extends the
JComponent class.
© Loentura/iStockphoto.
© sinankocasian/iStockphoto.
© Jacob Wackerhausen/iStockphoto.
72 Chapter 2 Using Objects
• Place drawing instructions inside the  paintComponent method. That method is called
whenever the component needs to be repainted.
• Use a cast to recover the  Graphics2D object from the  Graphics argument of the  paint-
Component method.
use the java aPi for drawing simple figures.
• The  Ellipse2D.Double and  Line2D.Double classes describe graphical shapes.
• The  drawString method draws a string, starting at its basepoint.
• When you set a new color in the graphics context, it is used for subsequent draw-
ing operations.
• r2.1  Explain the difference between an object and a class.
• r2.2  What is the public interface of a class? How does it differ from the implementation of
a class?
• r2.3  Declare and initialize variables for holding the price and the description of an article
that is available for sale.
• r2.4  What is the value of  mystery after this sequence of statements?
int mystery = 1;
mystery = 1 - 2 * mystery;
mystery = mystery + 1;
• r2.5  What is wrong with the following sequence of statements?
int mystery = 1;
mystery = mystery + 1;
int mystery = 1 - 2 * mystery;
© Eduardo Jose Bernardino/iStockphoto.
© Alexey Avdeev/iStockphoto.
java.awt.Color
java.awt.Component
getHeight
getWidth
setSize
setVisible
java.awt.Frame
setTitle
java.awt.geom.Ellipse2D.Double
java.awt.geom.Line2D.Double
java.awt.geom.Point2D.Double
java.awt.Graphics
setColor
java.awt.Graphics2D
draw
drawString
fill
java.awt.Rectangle
getX
getY
getHeight
getWidth
setSize
translate
java.lang.String
length
replace
toLowerCase
toUpperCase
javax.swing.JComponent
paintComponent
javax.swing.JFrame
setDefaultCloseOperation
s ta n D a r D l i b r a rY i t e M s i n t r O D U C e D i n t h i s C h a p t e r
r e V i e W Q U e s t i O n s
review Questions 73
•• r2.6  Explain the difference between the  = symbol in Java and in mathematics.
•• r2.7  Give an example of a method that has an argument of type  int . Give an example of a
method that has a return value of type  int . Repeat for the type  String .
•• r2.8  Write Java statements that initialize a string  message with  “Hello” and then change it to
“HELLO” . Use the  toUpperCase method.
•• r2.9  Write Java statements that initialize a string  message with  “Hello” and then change it to
“hello” . Use the  replace method.
• r2.10  Explain the difference between an object and an object variable.
•• r2.11  Give the Java code for constructing an object of class  Rectangle , and for declaring an
object variable of class  Rectangle .
•• r2.12  Give Java code for objects with the following descriptions:
a.  A rectangle with center (100, 100) and all side lengths equal to 50
b.  A string with the contents “Hello, Dave”
Create objects, not object variables.
•• r2.13  Repeat Exercise R2.12, but now declare object variables that are initialized with the
required objects.
•• r2.14  Write a Java statement to initialize a variable  square with a rectangle object whose top
left corner is (10, 20) and whose sides all have length 40. Then write a statement that
replaces  square with a rectangle of the same size and top left corner (20, 20).
•• r2.15  Write Java statements that initialize two variables  square1 and  square2 to refer to the
same square with center (20, 20) and side length 40.
•• r2.16  Find the errors in the following statements:
a.  Rectangle r = (5, 10, 15, 20);
b.  double width = Rectangle(5, 10, 15, 20).getWidth();
c.  Rectangle r;
r.translate(15, 25);
d.  r = new Rectangle();
r.translate("far, far away!");
• r2.17  Name two accessor methods and two mutator methods of the  Rectangle class.
•• r2.18  Consult the API documentation to find methods for
• Concatenating two strings, that is, making a string consisting of the first string,
followed by the second string.
• Removing leading and trailing white space of a string.
• Converting a rectangle to a string.
• Computing the smallest rectangle that contains two given rectangles.
• Returning a random floating-point number.
For each method, list the class in which it is defined, the return type, the method
name, and the types of the arguments.
• r2.19  Explain the difference between an object and an object reference.
• graphics r2.20  What is the difference between a console application and a graphical application?
74 Chapter 2 Using Objects
•• graphics r2.21  Who calls the  paintComponent method of a component? When does the call to the
paintComponent method occur?
•• graphics r2.22  Why does the argument of the  paintComponent method have type  Graphics and not
Graphics2D ?
•• graphics r2.23  What is the purpose of a graphics context?
•• graphics r2.24  Why are separate viewer and component classes used for graphical programs?
• graphics r2.25  How do you specify a text color?
• testing e2.1  Write an  AreaTester program that constructs a  Rectangle object and then computes
and prints its area. Use the  getWidth and  getHeight methods. Also print the expected
answer.
• testing e2.2  Write a  PerimeterTester program that constructs a  Rectangle object and then com-
putes and prints its perimeter. Use the  getWidth and  getHeight methods. Also print the
expected answer.
e2.3  Write a program that constructs a rectangle with area 42 and a rectangle with perim-
eter 42. Print the widths and heights of both rectangles.
•• testing e2.4  Look into the API documentation of the  Rectangle class and locate the method
void add(int newx, int newy)
Read through the method documentation. Then determine the result of the follow-
ing statements:
Rectangle box = new Rectangle(5, 10, 20, 30);
box.add(0, 0);
Write a program  AddTester that prints the expected and actual location, width, and
height of  box after the call to  add .
•• testing e2.5  Write a program  ReplaceTester that encodes a string by replacing all letters  "i" with
"!" and all letters  "s" with  "$" . Use the  replace method. Demonstrate that you can
correctly encode the string  "Mississippi" . Print both the actual and expected result.
••• e2.6  Write a program  HollePrinter that switches the letters  "e" and  "o" in a string. Use the
replace method repeatedly. Demonstrate that the string  "Hello, World!" turns into
"Holle, Werld!"
• testing e2.7  The  StringBuilder class has a method for reversing a string.
In a  ReverseTester class, construct a  StringBuilder from a
given string (such as  "desserts" ), call the  reverse method
followed by the  toString method, and print the result. Also
print the expected value.
•• e2.8  In the Java library, a color is specified by its red, green, and blue components
between 0 and 255 (see Table 4 on page 68). Write a program  BrighterDemo that con-
structs a  Color object with red, green, and blue values of 50, 100, and 150. Then apply
the  brighter method of the  Color class and print the red, green, and blue values of
the resulting color. (You won’t actually see the color—see Exercise E2.9 on how to
display the color.)
p r a C t i C e e x e r C i s e s
© PeskyMonkey/iStockphoto.
practice exercises 75
•• graphics e2.9  Repeat Exercise E2.8, but place your code into the following class. Then the color
will be displayed.
import java.awt.Color;
import javax.swing.JFrame;
public class BrighterDemo
{
public static void main(String[] args)
{
JFrame frame = new JFrame();
frame.setSize(200, 200);
Color myColor = ...;
frame.getContentPane().setBackground(myColor);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
}
•• e2.10  Repeat Exercise E2.8, but apply the  darker method of the  Color class twice to the
object  Color.RED . Call your class  DarkerDemo .
•• e2.11  The  Random class implements a random number generator, which produces sequences
of numbers that appear to be random. To generate random integers, you construct
an object of the  Random class, and then apply the  nextInt method. For example, the call
generator.nextInt(6) gives you a random number between 0 and 5.
Write a program  DieSimulator that uses the  Random class to simulate the cast of a die,
printing a random number between 1 and 6 every time that the program is run.
•• e2.12  Write a program  RandomPrice that prints a random price between $10.00 and $19.95
every time the program is run.
•• testing e2.13  Look at the API of the  Point class and find out how to construct a  Point object. In a
PointTester program, construct two points with coordinates (3, 4) and (–3, –4). Find
the distance between them, using the  distance method. Print the distance, as well as
the expected value. (Draw a sketch on graph paper to find the value you will expect.)
• e2.14  Using the  Day class of Worked Example 2.1, write a  DayTester program that constructs
a  Day object representing today, adds ten days to it, and then computes the difference
between that day and today. Print the difference and the expected value.
•• e2.15  Using the  Picture class of Worked Example 2.2, write a  HalfSizePicture program that
loads a picture and shows it at half the original size, centered in the window.
•• e2.16  Using the  Picture class of Worked Example 2.2, write a  DoubleSizePicture program
that loads a picture, doubles its size, and shows the center of the picture in the
window.
•• graphics e2.17  Write a graphics program that draws two squares, both with the same center. Pro-
vide a class  TwoSquareViewer and a class  TwoSquareComponent .
•• graphics e2.18  Write a program that draws two solid squares: one in pink and one in purple. Use a
standard color for one of them and a custom color for the other. Provide a class  Two-
SquareViewer and a class  TwoSquareComponent .
•• graphics e2.19  Write a graphics program that draws your name in red, contained inside a blue rect-
angle. Provide a class  NameViewer and a class  NameComponent .
76 Chapter 2  Using Objects
•• P2.1  Write a program called  FourRectanglePrinter that constructs a  Rectangle
object, prints its location by calling  System.out.println(box) , and then
translates and prints it three more times, so that, if the rectangles were
drawn, they would form one large rectangle, as shown at right.
Your program will not produce a drawing. It will simply print the
locations of the four rectangles.
•• P2.2  Write a  GrowSquarePrinter program that constructs a  Rectangle object  square
repre senting a square with top-left corner (100, 100) and side
length 50, prints its location by calling  System.out.println(square) ,
applies the  translate and  grow methods, and calls  System.out.
println(square) again. The calls to  translate and  grow should
modify the square so that it has twice the size and the same
top-left corner as the original. If the squares were drawn, they
would look like the figure at right.
Your program will not produce a drawing. It will simply print the locations of  square
before and after calling the mutator methods.
Look up the description of the  grow method in the API documentation.
••• P2.3  The  intersection method computes the intersection of
two rectangles—that is, the rectangle that would be
formed by two overlapping rectangles if they were
drawn, as shown at right.
You call this method as follows:
Rectangle r3 = r1.intersection(r2);
Write a program  IntersectionPrinter that constructs
two rectangle objects, prints them as described in
Exercise P2.1, and then prints the rectangle object that
describes the intersection. Then the program should
print the result of the  intersection method when the
rectangles do not overlap. Add a comment to your program that explains how you
can tell whether the resulting rectangle is empty.
••• Graphics P2.4  In this exercise, you will explore a simple way of visualizing a  Rectangle object. The
setBounds method of the  JFrame class moves a frame window to a given rectangle.
Complete the following program to visually show the  translate method of the
Rect angle class:
import java.awt.Rectangle;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
public class TranslateDemo
{
public static void main(String[] args)
{
//  Construct a frame and show it
JFrame frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
P r O g r a m m i n g  P r O j e C t s
Intersection
Programming Projects 77
frame.setVisible(true);
//  Your work goes here:
//  Construct a rectangle and set the frame bounds
JOptionPane.showMessageDialog(frame, "Click OK to continue");
//  Your work goes here:
//  Move the rectangle and set the frame bounds again
}
}
••• P2.5  Write a program  LotteryPrinter that picks a
combination in a lottery. In this lottery,
players can choose 6 numbers (possibly
repeated) between 1 and 49. Construct an
object of the  Random class and invoke an
appropriate method to generate each num-
ber. (In a real lot tery, repetitions aren’t
allowed, but we haven’t yet discussed the
programming con structs that would be
required to deal with that problem.) Your program should print out a sentence such
as “Play this combination—it’ll make you rich!”, followed by a lottery combination.
•• P2.6  Using the  Day class of Worked Example 1, write a program that generates a  Day object
representing February 28 of this year, and three more such objects that represent
February 28 of the next three years. Advance each object by one day, and print each
object. Also print the expected values:
2012-02-29
Expected: 2012-02-29
2013-03-01
Expected: 2013-03-01
. . .
••• P2.7  The  GregorianCalendar class describes a point in time, as measured by the Gregorian
calendar, the standard calendar that is commonly used throughout the world today.
You construct a  GregorianCalendar object from a year, month, and day of the month,
like this:
GregorianCalendar cal = new GregorianCalendar(); //  Today’s date
GregorianCalendar eckertsBirthday = new GregorianCalendar(1919,
Calendar.APRIL, 9);
Use the values  Calendar.JANUARY . . . Calendar.DECEMBER to specify the month.
The  add method can be used to add a number of days to a  GregorianCalendar object:
cal.add(Calendar.DAY_OF_MONTH, 10); //  Now  cal is ten days from today
This is a mutator method—it changes the  cal object.
The  get method can be used to query a given  GregorianCalendar object:
int dayOfMonth = cal.get(Calendar.DAY_OF_MONTH);
int month = cal.get(Calendar.MONTH);
int year = cal.get(Calendar.YEAR);
int weekday = cal.get(Calendar.DAY_OF_WEEK);
//  1 is Sunday, 2 is Monday, . . . , 7 is Saturday
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78 Chapter 2  Using Objects
Your task is to write a program that prints:
• The date and weekday that is 100 days from today.
• The weekday of your birthday.
• The date that is 10,000 days from your birthday.
Use the birthday of a computer scientist if you don’t want to reveal your own
birthday.
Hint: The  GregorianCalendar class is complex, and it is a really good idea to write a
few test programs to explore the API before tackling the whole problem. Start with
a program that constructs today’s date, adds ten days, and prints out the day of the
month and the weekday.
••• Testing P2.8  Write a program  LineDistanceTester that constructs a line joining the points (100, 100)
and (200, 200), then constructs points (100, 200), (150, 150), and (250, 50). Print the
distance from the line to each of the three points, using the  ptSegDist method of the
Line2D class. Also print the expected values. (Draw a sketch on graph paper to find
what values you expect.)
•• Graphics P2.9  Repeat Exercise P2.8, but now write a graphical application that shows the line and
the points. Draw each point as a tiny circle. Use the  drawString method to draw each
distance next to the point, using calls
g2.drawString("Distance: " + distance, p.getX(), p.getY());
•• Graphics P2.10  Write a graphics program that draws 12 strings, one each for the 12 standard colors
(except  Color.WHITE) , each in its own color. Provide a class  ColorNameViewer and a class
ColorNameComponent .
•• Graphics P2.11  Write a program to plot the face at right. Provide a class  FaceViewer and a
class  FaceComponent .
•• Graphics P2.12  Write a graphical program that draws a traffic light.
•• Graphics P2.13  Run the following program:
import java.awt.Color;
import javax.swing.JFrame;
import javax.swing.JLabel;
public class FrameViewer
{
public static void main(String[] args)
{
JFrame frame = new JFrame();
frame.setSize(200, 200);
JLabel label = new JLabel("Hello, World!");
label.setOpaque(true);
label.setBackground(Color.PINK);
frame.add(label);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
}
answers to self-Check Questions 79
Modify the program as follows:
• Double the frame size.
• Change the greeting to “Hello, your name!”.
• Change the background color to pale green (see Exercise E2.9).
• For extra credit, add an image of yourself. (Hint: Construct an  ImageIcon .)
a n s w e r s  t O  s e l f - C h e C k  Q U e s t i O n s
1.  Objects with the same behavior belong to
the same class. A window lets in light while
protecting a room from the outside wind and
heat or cold. A water heater has completely
different behavior. It heats water. They belong
to different classes.
2.  When one calls a method, one is not concerned
with how it does its job. As long as a light bulb
illuminates a room, it doesn’t matter to the
occupant how the photons are produced.
3.  There are three errors:
•  You cannot have spaces in variable names.
•  The variable type should be  double because it
holds a fractional value.
•  There is a semicolon missing at the end of the
statement.
4.  double unitPrice = 1.95;
int quantity = 2;
5.  System.out.print("Total price: ");
System.out.println(unitPrice * quantity);
6.  int and  String
7.  double
8.  Only the first two are legal identifiers.
9.  String myName = "John Q. Public";
10.  No, the left-hand side of the  = operator must
be a variable.
11.  greeting = "Hello, Nina!";
Note that
String greeting = "Hello, Nina!";
is not the right answer—that statement
declares a new variable.
12.  Assignment would occur when one car is
replaced by another in the parking space.
13.  river.length() or  "Mississippi".length()
14.  System.out.println(greeting.toUpperCase());
or
System.out.println(
"Hello, World!".toUpperCase());
15.  It is not legal. The variable  river has type
String . The  println method is not a method of
the  String class.
16.  The arguments are the strings “p” and “s”.
17.  "Missississi"
18.  12
19.  As  public String toUpperCase() , with no argu-
ment and return type  String .
20.  new Rectangle(90, 90, 20, 20)
21.  Rectangle box = new Rectangle(5, 10, 20, 30);
Rectangle box2 = new Rectangle(25, 10, 20, 30);
22.  0
23.  new PrintStream("output.txt");
24.  PrintStream out = new PrintStream("output.txt");
25.  Before: 5
After: 30
26.  Before: 20
After: 20
Moving the rectangle does not affect its width
or height. You can change the width and height
with the  setSize method.
27.  HELLO
hello
Note that calling  toUpperCase doesn’t modify
the string.
28.  An accessor—it doesn’t modify the original
string but returns a new string with uppercase
letters.
29.  box.translate(-5, -10) , provided the method
is called immediately after storing the new
rectangle into  box .
80 Chapter 2  Using Objects
30.  toLowerCase
31.  "Hello, Space !" —only the leading and trailing
spaces are trimmed.
32.  The arguments of the  translate method tell
how far to move the rectangle in the x- and
y - directions. The arguments of the  setLocation
method indicate the new x- and y - values for
the top-left corner.
For example,  box.move(1, 1) moves the  box
one pixel down and to the right.  box.setLoca-
tion(1, 1) moves  box to the top-left corner of
the screen.
33.  Add the statement  import java.util.Random; at
the top of your program.
34.  In the  java.math package.
35.  x: 30 ,  y: 25
36.  Because the  translate method doesn’t modify
the shape of the rectangle.
37.  Now  greeting and  greeting2 both refer to the
same  String object.
38.  Both variables still refer to the same string,
and the string has not been modified. Recall
that the  toUpperCase method constructs a new
string that contains uppercase characters,
leaving the original string unchanged.
39.  Modify the  EmptyFrameViewer program as
follows:
frame.setSize(300, 300);
frame.setTitle("Hello, World!");
40.  Construct two  JFrame objects, set each of
their sizes, and call  setVisible(true) on each
of them.
41.  Change line 17 of  RectangleComponent to
Rectangle box = new Rectangle(5, 10, 20, 20);
42.  Replace the call to  box.translate(15, 25) with
box = new Rectangle(20, 35, 20, 20);
43.  The compiler complains that  g doesn’t have a
draw method.
44.  g2.draw(new Ellipse2D.Double(75, 75, 50, 50));
45.  Line2D.Double segment1
= new Line2D.Double(0, 0, 10, 30);
g2.draw(segment1);
Line2D.Double segment2
= new Line2D.Double(10, 30, 20, 0);
g2.draw(segment2);
46.  g2.drawString("V", 0, 30);
47.  0, 0, 255
48.  First fill a big red square, then fill a small
yellow square inside:
g2.setColor(Color.RED);
g2.fill(new Rectangle(0, 0, 200, 200));
g2.setColor(Color.YELLOW);
g2.fill(new Rectangle(50, 50, 100, 100));
3
C h a p t e r
81
© Kris Hanke/iStockphoto.
ImplementIng
Classes
to become familiar with the process of
implementing classes
to be able to implement and test
simple methods
to understand the purpose and use of constructors
to understand how to access instance variables and
local variables
to be able to write  javadoc comments
to implement classes for drawing graphical shapes
C h a p t e r g o a l s
C h a p t e r C o n t e n t s
3.1 Instance VarIables and
encapsulatIon 82
Syntax 3.1: Instance Variable Declaration 83
3.2 specIfyIng the publIc Interface
of a class 86
Syntax 3.2: Class Declaration 89
Common Error 3.1: Declaring a Constructor
as void 92
Programming Tip 3.1: the javadoc Utility 92
3.3 proVIdIng the class
ImplementatIon 93
Common Error 3.2: Ignoring parameter
Variables 98
How To 3.1: Implementing a Class 98
Worked Example 3.1: making a simple menu
3.4 unIt testIng 102
Computing & Society 3.1: electronic Voting
machines 104
3.5 problem solVIng: tracIng
objects 105
3.6 local VarIables 107
Common Error 3.3: Duplicating Instance
Variables in local Variables 108
Common Error 3.4: providing Unnecessary
Instance Variables 108
Common Error 3.5: Forgetting to Initialize object
references in a Constructor 109
3.7 the thIs reference 109
Special Topic 3.1: Calling one Constructor from
another 112
3.8 shape classes 112
How To 3.2: Drawing graphical shapes 116
82
In this chapter, you will learn how to implement your own
classes. You will start with a given design that specifies
the public interface of the class—that is, the methods
through which programmers can manipulate the objects
of the class. Then you will learn the steps to completing
the class—creating the internal “workings” like the inside of
an air conditioner shown here. You need to implement the
methods, which entails finding a data representation for the
objects and supplying the instructions for each method. You
need to document your efforts so that other programmers
can understand and use your creation. And you need to
provide a tester to validate that your class works correctly.
3.1 Instance Variables and Encapsulation
In Chapter 2, you learned how to use objects from
existing classes. In this chapter, you will start imple
menting your own classes. We begin with a very
simple example that shows you how objects store
their data, and how methods access the data of an
object. Our first example is a class that models a tally
counter, a mechanical device that is used to count
peo ple—for example, to find out how many people
attend a concert or board a bus (see Figure 1).
3.1.1 Instance Variables
Whenever the operator clicks the button of a tally counter, the counter value advances
by one. We model this operation with a  click method of a  Counter class. A physical
counter has a display to show the current value. In our simulation, we use a  getValue
method to get the current value. For example,
Counter tally = new Counter();
tally.click();
tally.click();
int result = tally.getValue(); //  Sets result to 2
When implementing the  Counter class, you need to determine the data that each coun
ter object contains. In this simple example, that is very straightforward. Each counter
needs a variable that keeps track of the number of simulated button clicks.
An object stores its data in instance variables. An instance of a class is an object of
the class. Thus, an instance variable is a storage location that is present in each object
of the class.
You specify instance variables in the class declaration:
public class Counter
{
private int value;
. . .
}
Figure 1  A Tally Counter
© Jasmin Awad/iStockphoto.
An object’s instance
variables store
the data required
for executing
its methods.
3.1 Instance Variables and encapsulation 83
syntax 3.1  Instance Variable Declaration
public class  ClassName
{
private  typeName variableName ;
. . .
}
Syntax
public class Counter
{
private int value;
. . .
}
Each object of this class
has a separate copy of
this instance variable.
Instance variables should
always be private.
Type of the variable
An instance variable declaration consists of the following parts:
• An access specifier ( private )
• The type of the instance variable (such as  int )
• The name of the instance variable (such as  value )
Each object of a class has its own set of instance variables. For example, if  concert-
Counter and  boarding Counter are two objects of the  Counter class, then each object has its
own  value variable (see Figure 2). As you will see in Section 3.3, the instance variable
value is set to 0 when a  Counter object is constructed.
each object of a class
has its own set of
instance variables.
figure 2 
Instance Variables
concertCounter =
value =
Counter
value =
Counter
boardingCounter =
Instance
variables
These clocks have common behavior, but each of them has a different state. Similarly, objects of
a class can have their instance variables set to different values.
84 Chapter 3 Implementing Classes
3.1.2 the methods of the Counter Class
In this section, we will look at the implementation of the methods of the  Counter class.
The  click method advances the counter value by 1. You have seen the method
header syntax in Chapter 2. Now, focus on the body of the method inside the braces.
public void click()
{
value = value + 1;
}
Note how the  click method accesses the instance variable  value . Which instance vari
able? The one belong ing to the object on which the method is invoked. For example,
consider the call
concertCounter.click();
This call advances the  value variable of the  concertCounter object.
The  getValue method returns the current value:
public int getValue()
{
return value;
}
The  return statement is a special statement that terminates the method call and returns
a result (the return value) to the method’s caller.
Instance variables are generally declared with the access specifier  private . That
specifier means that they can be accessed only by the methods of the same class, not
by any other method. For example, the  value variable can be accessed by the  click
and  getValue methods of the  Counter class but not by a method of another class. Those
other methods need to use the  Counter class methods if they want to manipulate a
counter’s internal data.
3.1.3 encapsulation
In the preceding section, you learned that you should hide instance variables by mak
ing them private. Why would a programmer want to hide something?
The strategy of information hiding is not unique to computer programming—it is
used in many engi neering disciplines. Consider the thermostat that you find in your
home. It is a device that allows a user to set temperature preferences and that controls
the furnace and the air conditioner. If you ask your contractor what is inside the ther
mostat, you will likely get a shrug.
The thermostat is a black box, something that magically does its thing. A contrac
tor would never open the control module—it contains electronic parts that can only
be serviced at the factory. In general, engi neers use the term “black box” to describe
any device whose inner workings are hidden. Note that a black box is not totally
mysterious. Its interface with the outside world is welldefined. For example, the
contractor understands how the thermostat must be connected with the furnace and
air conditioner.
The process of hiding implementation details while publishing an interface is
called encapsulation. In Java, the  class construct provides encapsulation. The pub
lic methods of a class are the interface through which the private implementation is
manipulated.
private instance
variables can only be
accessed by methods
of the same class.
encapsulation is the
process of hiding
implementation
details and providing
methods for
data access.
3.1 Instance Variables and encapsulation 85
Why do contractors use prefabricated com
ponents such as thermostats and furnaces?
These “black boxes” greatly simplify the work
of the contractor. In ancient times, builders had
to know how to construct furnaces from brick
and mortar, and how to produce some rudimen
tary temperature controls. Nowadays, a con
tractor just makes a trip to the hardware store,
without needing to know what goes on inside
the components.
Similarly, a programmer using a class is not
burdened by unnecessary detail, as you know
from your own experience. In Chapter 2, you
used classes for strings, streams, and windows without worrying how these classes
are implemented.
Encapsulation also helps with diagnosing errors. A large program may consist of
hundreds of classes and thousands of methods, but if there is an error with the inter
nal data of an object, you only need to look at the methods of one class. Finally,
encapsulation makes it possible to change the implementation of a class without hav
ing to tell the programmers who use the class.
In Chapter 2, you learned to be an object user. You saw how to obtain objects, how
to manipulate them, and how to assemble them into a program. In that chapter, you
treated objects as black boxes. Your role was roughly analogous to the contractor
who installs a new thermostat.
In this chapter, you will move on to implementing classes. In these sections, your
role is analogous to the hardware manufacturer who puts together a thermostat from
buttons, sensors, and other electronic parts. You will learn the necessary Java pro
gramming techniques that enable your objects to carry out the desired behavior.
section_1/counter.java
1 /**
2 This class models a tally counter.
3 */
4 public class Counter
5 {
6 private int value;
7
8 /**
9 Gets the current value of this counter.
10 @return  the current value
11 */
12 public int getValue()
13 {
14 return value;
15 }
16
17 /**
18 Advances the value of this counter by 1.
19 */
20 public void click()
21 {
22 value = value + 1;
23 }
24
© yenwen/iStockphoto.
A thermostat functions as a “black
box” whose inner workings are hidden.
encapsulation allows
a programmer to
use a class without
having to know its
implementation.
Information hiding
makes it simpler for
the implementor
of a class to locate
errors and change
implementations.
full code example
go to  wiley.com/go/
javacode to download
a demonstration of
the  Counter class.
86 Chapter 3 Implementing Classes
25 /**
26 Resets the value of this counter to 0.
27 */
28 public void reset()
29 {
30 value = 0;
31 }
32 }
1.  Supply the body of a method  public void unclick() that undoes an unwanted
button click.
2.  Suppose you use a class  Clock with private instance variables  hours and  minutes .
How can you access these variables in your program?
3.  Consider the  Counter class. A counter’s value starts at 0 and is advanced by the
click method, so it should never be negative. Suppose you found a negative  value
variable during testing. Where would you look for the error?
4.  In Chapters 1 and 2, you used  System.out as a black box to cause output to appear
on the screen. Who designed and implemented  System.out ?
5.  Suppose you are working in a company that produces personal finance software.
You are asked to design and implement a class for representing bank accounts.
Who will be the users of your class?
practice It  Now you can try these exercises at the end of the chapter: R3.1, R3.3, E3.1.
3.2 specifying the public Interface of a Class
In the following sections, we will discuss the process of specifying the public inter
face of a class. Imagine that you are a member of a team that works on banking soft
ware. A fundamental concept in banking is a bank account. Your task is to design a
BankAccount class that can be used by other programmers to manipulate bank accounts.
What methods should you provide? What information should you give the program
mers who use this class? You will want to settle these questions before you imple
ment the class.
3.2.1 specifying methods
You need to know exactly what operations of a bank account need to be imple
mented. Some operations are essential (such as taking deposits), whereas others are
not important (such as giving a gift to a customer who opens a bank account). Decid
ing which operations are essential is not always an easy task. We will revisit that issue
in Chapters 8 and 12. For now, we will assume that a competent designer has decided
that the following are considered the essential operations of a bank account:
• Deposit money
• Withdraw money
• Get the current balance
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
In order to
implement a class,
you first need to
know which methods
are required.
3.2 specifying the public Interface of a Class 87
In Java, you call a method when you want to apply an operation to an object. To fig
ure out the exact specification of the method calls, imagine how a programmer would
carry out the bank account operations. We’ll assume that the variable  harrysChecking
contains a reference to an object of type  BankAccount . We want to support method calls
such as the following:
harrysChecking.deposit(2240.59);
harrysChecking.withdraw(500);
double currentBalance = harrysChecking.getBalance();
The first two methods are mutators. They modify the balance of the bank account
and don’t return a value. The third method is an accessor. It returns a value that you
store in a variable or pass to a method.
From the sample calls, we decide the  BankAccount class should declare three methods:
•  public void deposit(double amount)
•  public void withdraw(double amount)
•  public double getBalance()
Recall from Chapter 2 that  double denotes the doubleprecision floatingpoint type,
and  void indicates that a method does not return a value.
Here we only give the method headers. When you declare a method, you also need
to provide the method body, which consists of statements that are executed when the
method is called.
public void deposit(double amount)
{
method body—implementation filled in later
}
We will supply the method bodies in Section 3.3.
Note that the methods have been declared as  public , indicating that all other meth
ods in a program can call them. Occasionally, it can be useful to have  private methods.
They can only be called from other methods of the same class.
Some people like to fill in the bodies so that they compile, like this:
public double getBalance()
{
//  TODO: fill in implementation
return 0;
}
That is a good idea if you compose your specification in your development environ
ment––you won’t get warnings about incorrect code.
3.2.2 specifying Constructors
As you know from Chapter 2, constructors are used to initialize objects. In Java, a
constructor is very similar to a method, with two important differences:
• The name of the constructor is always the same as the name of the class (e.g.,
BankAccount ).
• Constructors have no return type (not even  void ).
We want to be able to construct bank accounts that initially have a zero balance, as
well as accounts that have a given initial balance.
Constructors set
the initial data
for objects.
88 Chapter 3 Implementing Classes
For this purpose, we specify two constructors:
•  public BankAccount()
•  public BankAccount(double initialBalance)
They are used as follows:
BankAccount harrysChecking = new BankAccount();
BankAccount momsSavings = new BankAccount(5000);
Don’t worry about the fact that there are two constructors with the same name—all
constructors of a class have the same name, that is, the name of the class. The compiler
can tell them apart because they take different arguments. The first constructor takes
no arguments at all. Such a constructor is called a no-argument constructor. The
second constructor takes an argument of type  double .
Just like a method, a constructor also has a body—a sequence of statements that is
executed when a new object is constructed.
public BankAccount()
{
constructor body—implementation filled in later
}
The statements in the constructor body will set the instance variables of the object
that is being con structed—see Section 3.3.
When declaring a class, you place all constructor and method declarations inside,
like this:
public class BankAccount
{
private instance variables—filled in later
//  Constructors
public BankAccount()
{
implementation—filled in later
}
public BankAccount(double initialBalance)
{
implementation—filled in later
}
//  Methods
public void deposit(double amount)
{
implementation—filled in later
}
public void withdraw(double amount)
{
implementation—filled in later
}
public double getBalance()
{
implementation—filled in later
}
}
the constructor
name is always
the same as the
class name.
3.2 specifying the public Interface of a Class 89
syntax 3.2  Class Declaration
accessSpecifier class  ClassName
{
instance variables
constructors
methods
}
Syntax
public class Counter
{
private int value;
public Counter(int initialValue) { value = initialValue; }
public void click() { value = value + 1; }
public int getValue() { return value; }
}
Public interface
Private
implementation
The public constructors and methods of a class form the public interface of the class.
These are the oper ations that any programmer can use to create and manipulate
BankAccount objects.
3.2.3 Using the public Interface
Our  BankAccount class is simple, but it allows programmers to carry out all of the
important operations that commonly occur with bank accounts. For example, con
sider this program segment, authored by a programmer who uses the  BankAccount class.
These statements transfer an amount of money from one bank account to another:
//  Transfer from one account to another
double transferAmount = 500;
momsSavings.withdraw(transferAmount);
harrysChecking.deposit(transferAmount);
And here is a program segment that adds interest to a savings account:
double interestRate = 5; //  5 percent interest
double interestAmount = momsSavings.getBalance() * interestRate / 100;
momsSavings.deposit(interestAmount);
As you can see, programmers can use objects of the  BankAccount class to carry out
meaningful tasks, with out knowing how the  BankAccount objects store their data or
how the  BankAccount methods do their work.
Of course, as implementors of the  BankAccount class, we will need to supply the pri
vate implementation. We will do so in Section 3.3. First, however, an important step
remains: documenting the public interface. That is the topic of the next section.
3.2.4 Commenting the public Interface
When you implement classes and methods, you should get into the habit of thor
oughly commenting their behaviors. In Java there is a very useful standard form for
90 Chapter 3 Implementing Classes
documentation comments. If you use this form in your classes, a program called
javadoc can automatically generate a neat set of HTML pages that describe them. (See
Programming Tip 3.1 on page 92 for a description of this utility.)
A documentation comment is placed before the class or method declaration that is
being documented. It starts with a  /** , a special comment delimiter used by the  java-
doc utility. Then you describe the method’s purpose. Then, for each argument, you
supply a line that starts with  @param , followed by the name of the variable that holds
the argument (which is called a parameter variable). Supply a short explanation
for each argument after the variable name. Finally, you supply a line that starts with
@return , describing the return value. You omit the  @param tag for methods that have no
arguments, and you omit the  @return tag for methods whose return type is  void .
The  javadoc utility copies the first sentence of each comment to a summary table in
the HTML docu mentation. Therefore, it is best to write that first sentence with some
care. It should start with an upper case letter and end with a period. It does not have to
be a grammatically complete sentence, but it should be meaningful when it is pulled
out of the comment and displayed in a summary.
Here are two typical examples:
/**
Withdraws money from the bank account.
@param amount  the amount to withdraw
*/
public void withdraw(double amount)
{
implementation—filled in later
}
/**
Gets the current balance of the bank account.
@return  the current balance
*/
public double getBalance()
{
implementation—filled in later
}
The comments you have just seen explain individual methods. Supply a brief com
ment for each class, too, explaining its purpose. Place the documenta tion comment
above the class declaration:
/**
A bank account has a balance that can be changed by
deposits and withdrawals.
*/
public class BankAccount
{
. . .
}
Your first reaction may well be “Whoa! Am I supposed to write all this stuff?” Some
times, documentation comments seem pretty repetitive, but in most cases, they are
informative. Even with seemingly repetitive comments, you should take the time to
write them.
It is always a good idea to write the method comment first, before writing the code
in the method body. This is an excellent test to see that you firmly understand what
Use documentation
comments to
describe the classes
and public methods
of your programs.
3.2 specifying the public Interface of a Class 91
you need to program. If you can’t explain what a class or method does, you aren’t
ready to implement it.
What about very simple methods? You can easily spend more time pondering
whether a comment is too trivial to write than it takes to write it. In practical pro
gramming, very simple methods are rare. It is harmless to have a trivial method over
commented, whereas a complicated method without any comment can cause real
grief to future maintenance programmers. According to the standard Java documen
tation style, every class, every method, every parameter variable, and every return
value should have a comment.
The  javadoc utility formats your comments into a neat set of documents that you
can view in a web browser. It makes good use of the seemingly repetitive phrases. The
first sentence of the comment is used for a summary table of all methods of your class
(see Figure 3). The  @param and  @return comments are neatly formatted in the detail
description of each method (see Figure 4). If you omit any of the comments, then
javadoc generates documents that look strangely empty.
provide documen
tation comments
for every class,
every method,
every parameter
variable, and every
return value.
figure 3  a method summary generated by  javadoc
full code example
go to  wiley.com/go/
javacode to download
the  BankAccount class
with documentation
but without
implementation.
figure 4  method Detail generated by  javadoc
92 Chapter 3 Implementing Classes
This documentation format should look familiar. The programmers who imple
ment the Java library use  javadoc themselves. They too document every class, every
method, every parameter variable, and every return value, and then use  javadoc to
extract the documentation in HTML format.
6.  How can you use the methods of the public interface to empty the  harrysChecking
bank account?
7.  What is wrong with this sequence of statements?
BankAccount harrysChecking = new BankAccount(10000);
System.out.println(harrysChecking.withdraw(500));
8.  Suppose you want a more powerful bank account abstraction that keeps track of
an account number in addition to the balance. How would you change the
public interface to accommodate this enhance ment?
9.  Suppose we enhance the  BankAccount class so that each account has an account
number. Supply a docu mentation comment for the constructor
public BankAccount(int accountNumber, double initialBalance)
10.  Why is the following documentation comment questionable?
/**
Each account has an account number.
@return  the account number of this account
*/
public int getAccountNumber()
practice It  Now you can try these exercises at the end of the chapter: R3.7, R3.8, R3.9.
declaring a constructor as void
Do not use the  void reserved word when you declare a constructor:
public void BankAccount() //  Error—don’t use  void !
This would declare a method with return type  void and not a constructor. Unfortunately, the
Java compiler does not consider this a syntax error.
the  javadoc  utility
Always insert documentation comments in your code, whether or not you use  javadoc to pro
duce HTML documen tation. Most people find the HTML documentation convenient, so it
is worth learning how to run  javadoc . Some programming environments (such as BlueJ) can
execute  javadoc for you. Alternatively, you can invoke the  javadoc utility from a shell window,
by issuing the command
javadoc MyClass.java
or, if you want to document multiple Java files,
javadoc *.java
The  javadoc utility produces files such as  MyClass.html in HTML format, which you can inspect
in a browser. If you know HTML (see Appendix H), you can embed HTML tags into the
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
Common error 3.1
© John Bell/iStockphoto.
programming tip 3.1
© Eric Isselé/iStockphoto.
3.3 providing the Class Implementation 93
comments to specify fonts or add images. Per haps most importantly,  javadoc automatically
provides hyperlinks to other classes and methods.
You can run  javadoc before implementing any methods. Just leave all the method bodies
empty. Don’t run the compiler—it would complain about missing return values. Simply run
javadoc on your file to generate the documen tation for the public interface that you are about
to implement.
The  javadoc tool is wonderful because it does one thing right: It allows you to put the docu
mentation together with your code. That way, when you update your programs, you can see
right away which documentation needs to be updated. Hopefully, you will update it right then
and there. Afterward, run  javadoc again and get updated infor mation that is timely and nicely
formatted.
3.3 providing the Class Implementation
Now that you understand the specification of the public interface of the  Bank Account
class, let’s provide the implementation.
3.3.1 providing Instance Variables
First, we need to determine the data that each bank account object contains. In the
case of our simple bank account class, each object needs to store a single value, the
current balance. (A more complex bank account class might store additional data—
perhaps an account number, the interest rate paid, the date for mailing out the next
statement, and so on.)
public class BankAccount
{
private double balance;
//  Methods and constructors below
. . .
}
In general, it can be challenging to find a good
set of instance variables. Ask yourself what an
object needs to remember so that it can carry
out any of its methods.
Like a wilderness explorer who needs to carry all
items that may be needed, an object needs to store
the data required for its method calls.
the private
implementation of
a class consists of
instance variables,
and the bodies
of constructors
and methods.
© migin/iStockphoto.
94 Chapter 3 Implementing Classes
3.3.2 providing Constructors
A constructor has a simple job: to initialize the instance variables of an object.
Recall that we designed the  BankAccount class to have two constructors. The first
constructor simply sets the balance to zero:
public BankAccount()
{
balance = 0;
}
The second constructor sets the balance to the value supplied as the construction
argument:
public BankAccount(double initialBalance)
{
balance = initialBalance;
}
To see how these constructors work, let us trace the statement
BankAccount harrysChecking = new BankAccount(1000);
one step at a time.
Here are the steps that are carried out when the statement executes (see Figure 5):
• Create a new object of type  BankAccount .  1
• Call the second constructor (because an argument is supplied in the
constructor call).
• Set the parameter variable  initialBalance to 1000.  2
• Set the  balance instance variable of the newly created object
to  initialBalance .  3
• Return an object reference, that is, the memory location of the object,
as the value of the  new expres sion.
• Store that object reference in the  harrysChecking variable.  4
In general, when you implement constructors, be sure that each constructor initial
izes all instance variables, and that you make use of all parameter variables (see Com
mon Error 3.2 on page 98).
A constructor is like a set of
assembly instructions for an object.
© Ann Marie Kurtz/iStockphoto.
3.3 providing the Class Implementation 95
3.3.3
figure 5 
how a Constructor Works
2
1
initialBalance =
balance =
BankAccount
balance =
BankAccount
1000
4
harrysChecking =
balance =
BankAccount
3
initialBalance =
balance =
BankAccount
1000
1000
1000
providing methods
In this section, we finish implementing the methods of the  BankAccount class.
When you implement a method, ask yourself whether it is an accessor or mutator
method. A mutator method needs to update the instance variables in some way. An
accessor method retrieves or computes a result.
Here is the  deposit method. It is a mutator method, updating the balance:
public void deposit(double amount)
{
balance = balance + amount;
}
The  withdraw method is very similar to the  deposit method:
public void withdraw(double amount)
{
balance = balance - amount;
}
96 Chapter 3 Implementing Classes
table 1 Implementing Classes
example Comments
public class BankAccount { . . . }  This is the start of a class declaration. Instance variables,
methods, and constructors are placed inside the braces.
private double balance; This is an instance variable of type  double . Instance variables
should be declared as  private .
public double getBalance() { . . . } This is a method declaration. The body of the method must
be placed inside the braces.
. . . { return balance; }  This is the body of the  getBalance method. The  return
statement returns a value to the caller of the method.
public void deposit(double amount) { . . . } This is a method with a parameter variable ( amount ). Because
the method is declared as  void , it has no return value.
. . . { balance = balance + amount; } This is the body of the  deposit method. It does not have a
return statement.
public BankAccount() { . . . } This is a constructor declaration. A constructor has the same
name as the class and no return type.
. . . { balance = 0; } This is the body of the constructor. A constructor should
initialize the instance variables.
There is one method left,  getBalance . Unlike the  deposit and  withdraw methods, which
modify the instance variable of the object on which they are invoked, the  getBalance
method returns a value:
public double getBalance()
{
return balance;
}
We have now completed the implementation of the  BankAccount class—see the code
listing below. There is only one step remaining: testing that the class works correctly.
That is the topic of the next section.
section_3/bankaccount.java
1 /**
2 A bank account has a balance that can be changed by
3 deposits and withdrawals.
4 */
5 public class BankAccount
6 {
7 private double balance;
8
9 /**
10 Constructs a bank account with a zero balance.
11 */
12 public BankAccount()
13 {
14 balance = 0;
15 }
3.3 providing the Class Implementation 97
16
17 /**
18 Constructs a bank account with a given balance.
19 @param initialBalance  the initial balance
20 */
21 public BankAccount(double initialBalance)
22 {
23 balance = initialBalance;
24 }
25
26 /**
27 Deposits money into the bank account.
28 @param amount  the amount to deposit
29 */
30 public void deposit(double amount)
31 {
32 balance = balance + amount;
33 }
34
35 /**
36 Withdraws money from the bank account.
37 @param amount  the amount to withdraw
38 */
39 public void withdraw(double amount)
40 {
41 balance = balance - amount;
42 }
43
44 /**
45 Gets the current balance of the bank account.
46 @return  the current balance
47 */
48 public double getBalance()
49 {
50 return balance;
51 }
52 }
11.  Suppose we modify the  BankAccount class so that each bank account has an
account number. How does this change affect the instance variables?
12.  Why does the following code not succeed in robbing mom’s bank account?
public class BankRobber
{
public static void main(String[] args)
{
BankAccount momsSavings = new BankAccount(1000);
momsSavings.balance = 0;
}
}
13.  The  Rectangle class has four instance variables:  x ,  y ,  width , and  height . Give a pos
sible implementation of the  getWidth method.
14.  Give a possible implementation of the  translate method of the  Rectangle class.
practice It  Now you can try these exercises at the end of the chapter: R3.4, R3.10, E3.4.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
98 Chapter 3 Implementing Classes
Ignoring parameter Variables
A surprisingly common beginner’s error is to ignore parameter variables of methods or con
structors. This usually happens when an assignment gives an example with specific values. For
example, suppose you are asked to provide a class  Letter with a recipient and a sender, and you
are given a sample letter like this:
Dear John:
I am sorry we must part.
I wish you all the best.
Sincerely,
Mary
Now look at this incorrect attempt:
public class Letter
{
private String recipient;
private String sender;
public Letter(String aRecipient, String aSender)
{
recipient = "John"; //  Error—should use parameter variable
sender = "Mary"; //  Same error
}
. . .
}
The constructor ignores the names of the recipient and sender arguments that were provided
to the constructor. If a user constructs a
new Letter("John", "Yoko")
the sender is still set to  "Mary" , which is bound to be embarrassing.
The constructor should use the parameter variables, like this:
public Letter(String aRecipient, String aSender)
{
recipient = aRecipient;
sender = aSender;
}
Common error 3.2
© John Bell/iStockphoto.
© Steve Simzer/iStockphoto.
hoW to 3.1  Implementing a class
This “How To” section tells you how you
implement a class from a given specification.
problem statement  Implement a class
that models a selfservice cash register. The
customer scans the price tags and deposits
money in the machine. The machine dis
penses the change.
Z5006 Karlheinz Schindler Deutsche Presse Agentur/NewsCom
3.3 providing the Class Implementation 99
step 1  Find out which methods you are asked to supply.
In a simulation, you won’t have to provide every feature that occurs in the real world—there
are too many. In the cash register example, we don’t deal with sales tax or credit card payments.
The assignment tells you which aspects of the selfservice cash register your class should simu
late. Make a list of them:
• Process the price of each purchased item.
• Receive payment.
• Calculate the amount of change due to the customer.
step 2  Specify the public interface.
Turn the list in Step 1 into a set of methods, with specific types for the parameter variables and
the return values. Many pro grammers find this step simpler if they write out method calls that
are applied to a sample object, like this:
CashRegister register = new CashRegister();
register.recordPurchase(29.95);
register.recordPurchase(9.95);
register.receivePayment(50);
double change = register.giveChange();
Now we have a specific list of methods:
•  public void recordPurchase(double amount)
•  public void receivePayment(double amount)
•  public double giveChange()
To complete the public interface, you need to specify the constructors. Ask yourself what
information you need in order to construct an object of your class. Sometimes you will want
two constructors: one that sets all instance vari ables to a default and one that sets them to user
supplied values.
In the case of the cash register example, we can get by with a single constructor that creates
an empty register. A more realistic cash register might start out with some coins and bills so
that we can give exact change, but that is well beyond the scope of our assignment.
Thus, we add a single constructor:
•  public CashRegister()
step 3  Document the public interface.
Here is the documentation, with comments, that describes the class and its methods:
/**
A cash register totals up sales and computes change due.
*/
public class CashRegister
{
/**
Constructs a cash register with no money in it.
*/
public CashRegister()
{
}
/**
Records the sale of an item.
@param amount  the price of the item
*/
public void recordPurchase(double amount)
{
100 Chapter 3 Implementing Classes
}
/**
Processes a payment received from the customer.
@param amount  the amount of the payment
*/
public void receivePayment(double amount)
{
}
/**
Computes the change due and resets the machine for the next customer.
@return  the change due to the customer
*/
public double giveChange()
{
}
}
step 4  Determine instance variables.
Ask yourself what information an object needs to store to do its job. Remember, the methods
can be called in any order. The object needs to have enough internal memory to be able to
process every method using just its instance variables and the parameter variables. Go through
each method, perhaps starting with a simple one or an interesting one, and ask yourself what
you need to carry out the method’s task. Make instance variables to store the information that
the method needs.
Just as importantly, don’t introduce unnecessary instance variables (see Common Error 3.3).
If a value can be computed from other instance variables, it is generally better to compute it on
demand than to store it.
In the cash register example, you need to keep track of the total purchase amount and the
payment. You can compute the change due from these two amounts.
public class CashRegister
{
private double purchase;
private double payment;
. . .
}
step 5  Implement constructors and methods.
Implement the constructors and methods in your class, one at a time, starting with the easiest
ones. Here is the implementation of the  recordPurchase method:
public void recordPurchase(double amount)
{
purchase = purchase + amount;
}
The  receivePayment method looks almost the same,
public void receivePayment(double amount)
{
payment = payment + amount;
}
but why does the method add the amount, instead of simply setting  payment = amount ? A
customer might provide two separate payments, such as two $10 bills, and the machine must
process them both. Remember, methods can be called more than once, and they can be called
in any order.
3.3 providing the Class Implementation 101
Finally, here is the  giveChange method. This method is a bit more sophisticated—it com
putes the change due, and it also resets the cash register for the next sale.
public double giveChange()
{
double change = payment - purchase;
purchase = 0;
payment = 0;
return change;
}
If you find that you have trouble with the implementation, you may need to rethink your
choice of instance vari ables. It is common for a beginner to start out with a set of instance vari
ables that cannot accurately reflect the state of an object. Don’t hesitate to go back and add or
modify instance variables.
You can find the complete implementation in the  how_to_1  directory of the book’s compan
ion code.
step 6  Test your class.
Write a short tester program and execute it. The tester program should carry out the method
calls that you found in Step 2.
public class CashRegisterTester
{
public static void main(String[] args)
{
CashRegister register = new CashRegister();
register.recordPurchase(29.50);
register.recordPurchase(9.25);
register.receivePayment(50);
double change = register.giveChange();
System.out.println(change);
System.out.println("Expected: 11.25");
}
}
The output of this test program is:
11.25
Expected: 11.25
WorkeD example 3.1  making a simple menu
Learn how to implement a class that constructs simple
textbased menus. Go to  wiley.com/go/javaexamples and
download Worked Example 3.1.
© Mark Evans/iStockphoto.
102 Chapter 3 Implementing Classes testing track
3.4 Unit testing
In the preceding section, we completed the imple
mentation of the  BankAccount class. What can you
do with it? Of course, you can compile the file
BankAccount.java . However, you can’t execute the
resulting  BankAc count.class file. It doesn’t contain
a  main method. That is normal—most classes don’t
contain a  main method.
In the long run, your class may become a part
of a larger program that interacts with users, stores
data in files, and so on. However, before integrat
ing a class into a program, it is always a good idea
to test it in isolation. Testing in isolation, outside a
complete program, is called unit testing.
To test your class, you have two choices. Some
interactive development environments have com
mands for constructing objects and invoking methods (see Special Topic 2.1). Then
you can test a class simply by constructing an object, calling methods, and verifying
that you get the expected return values. Figure 6 shows the result of calling the  get-
Balance method on a  BankAccount object in BlueJ.
Alternatively, you can write a tester class. A tester class is a class with a  main method
that contains state ments to run methods of another class. As discussed in Section 2.7,
a tester class typically carries out the following steps:
1.  Construct one or more objects of the class that is being tested.
2.  Invoke one or more methods.
3.  Print out one or more results.
4.  Print the expected results.
© Chris Fertnig/iStockphoto.
An engineer tests a part in isolation.
This is an example of unit testing.
a unit test verifies
that a class works
correctly in isolation,
outside a complete
program.
to test a class, use
an environment for
interactive testing,
or write a tester
class to execute
test instructions.
figure 6  the return Value of the  getBalance method in BlueJ
testing track 3.4 Unit testing 103
The  MoveTester class in Section 2.7 is a good example of a tester class. That class runs
methods of the  Rect angle class—a class in the Java library.
Following is a class to run methods of the  BankAccount class. The  main method con
structs an object of type  BankAccount , invokes the  deposit and  withdraw methods, and
then displays the remaining balance on the con sole.
We also print the value that we expect to see. In our sample program, we deposit
$2,000 and withdraw $500. We therefore expect a balance of $1,500.
section_4/bankaccounttester.java
1 /**
2 A class to test the  BankAccount class.
3 */
4 public class BankAccountTester
5 {
6 /**
7 Tests the methods of the  BankAccount class.
8 @param args  not used
9 */
10 public static void main(String[] args)
11 {
12 BankAccount harrysChecking = new BankAccount();
13 harrysChecking.deposit(2000);
14 harrysChecking.withdraw(500);
15 System.out.println(harrysChecking.getBalance());
16 System.out.println("Expected: 1500");
17 }
18 }
program run
1500
Expected: 1500
To produce a program, you need to combine the  BankAccount and the  BankAccountTester
classes. The details for building the program depend on your compiler and develop
ment environment. In most environ ments, you need to carry out these steps:
1.  Make a new subfolder for your program.
2.  Make two files, one for each class.
3.  Compile both files.
4.  Run the test program.
Many students are surprised that such a simple program contains two classes. How
ever, this is normal. The two classes have entirely different purposes. The  BankAccount
class describes objects that compute bank balances. The  BankAccountTester class runs a
test that puts a  BankAccount object through its paces.
15.  When you run the BankAccountTester program, how many objects of class  Bank-
Account are constructed? How many objects of type  BankAccountTester?
16.  Why is the  BankAccountTester class unnecessary in development environments
that allow interactive testing, such as BlueJ?
practice It  Now you can try these exercises at the end of the chapter: E3.3, E3.10.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
104 Chapter 3 Implementing Classes
In the 2000 presiden
tial elections in the
United states, votes were tallied by a
variety of machines. some machines
processed  cardboard  ballots  into
which voters punched holes to indi cate
their choices (see below). When voters
were not careful, remains of paper—
the now infamous “chads”—were par
tially stuck in the punch cards, caus
ing votes to be mis counted. a manual
recount was neces sary, but it was not
carried out everywhere due to time
constraints and procedural wrangling.
the elec tion was very close, and there
remain doubts in the minds of many
people whether the election outcome
would have been different if the voting
machines had accurately counted the
intent of the voters.
© Peter Nguyen/iStockphoto.
Punch Card Ballot
subsequently, voting machine man
ufacturers have argued that electronic
voting machines would avoid the prob
lems caused by punch cards or opti
cally scanned forms. In an elec tronic
voting machine, voters indicate their
preferences by pressing buttons or
touching icons on a computer screen.
typically, each voter is pre sented with
a summary screen for review before
casting the ballot. the process is very
similar to using a bank’s automated
teller machine.
It  seems  plausible  that  these
machines make it more likely that a
vote is counted in the same way that
the voter intends. however, there
has  been  significant  controversy
surrounding some types of electronic
voting machines. If a machine simply
records the votes and prints out the
totals after the election has been com
pleted, then how do you know that the
machine worked correctly? Inside the
machine is a computer that executes a
program, and, as you may know from
your own experience, programs can
have bugs.
In fact, some electronic voting
machines do have bugs. there have
been isolated cases where machines
reported tallies that were impossible.
When a machine reports far more or far
fewer votes than voters, then it is clear
that it malfunctioned. Unfortu nately, it
is then impossible to find out the
actual votes. over time, one would
expect these bugs to be fixed in the
software. more insidiously, if the
results are plausible, nobody may ever
investigate.
many computer scientists have spo
ken out on this issue and con firmed
that it is impossible, with today’s tech
nology, to tell that soft ware is error
free and has not been tampered with.
many of them recom mend that elec
tronic voting machines should employ
a voter verifiable audit trail. (a good
source of information is  http://veri-
fiedvoting.org .) typically, a voter
verifiable machine prints out a ballot.
each voter has a chance to review the
printout, and then deposits it in an
oldfashioned ballot box. If there is
a problem with the electronic equip
ment, the printouts can be scanned or
counted by hand.
as this book is written, this con
cept is strongly resisted both by
man ufacturers of electronic voting
machines and by their customers,
the cities and counties that run elec
tions. manufacturers are reluctant
to increase the cost of the machines
because they may not be able to pass
the cost increase on to their custom
ers, who tend to have tight budgets.
election officials fear problems with
malfunctioning printers, and some of
them have publicly stated that they
actually prefer equipment that elimi
nates bothersome recounts.
What do you think? You probably
use an automated bank teller machine
to get cash from your bank account.
Do you review the paper record that
the machine issues? Do you check your
bank statement? even if you don’t, do
you put your faith in other people who
doublecheck their bal ances, so that
the bank won’t get away with wide
spread cheating?
Is the integrity of banking equip
ment more important or less impor
tant than that of voting machines?
Won’t every voting process have some
room for error and fraud anyway? Is
the added cost for equip ment, paper,
and staff time rea
sonable to combat
a potentially slight
risk of malfunction
and fraud? Computer
sci entists  cannot
answer these ques
tions—an  informed
society must make
these tradeoffs. But,
like all profes sionals,
they have an obliga
tion to speak out
and give accurate
testimony about the
capabilities and limi
tations of computing
equipment.
Touch Screen Voting Machine
© Lisa F. Young/iStockphoto.
Computing & Society 3.1  electronic Voting machines
© MediaBakery.
3.5 problem solving: tracing objects 105
3.5 problem solving: tracing objects
Researchers have studied why some students have an easier time learning how to pro
gram than others. One important skill of successful programmers is the ability to
simulate the actions of a program with pencil and paper. In this section, you will see
how to develop this skill by tracing method calls on objects.
Use an index card or a sticky note for each object. On the front, write the methods
that the object can execute. On the back, make a table for the values of the instance
variables.
Here is a card for a  CashRegister object:
CashRegister reg1
recordPurchase
receivePayment
giveChange
reg1.purchase reg1.payment
front back
In a small way, this gives you a feel for encapsulation. An object is manipulated
through its public inter face (on the front of the card), and the instance variables are
hidden in the back.
When an object is constructed, fill in the initial values of the instance variables:
reg1.purchase reg1.payment
0 0
Whenever a mutator method is executed, cross out the old values and write the new
ones below. Here is what happens after a call to the  recordPurchase method:
reg1.purchase reg1.payment
0 0
19.95
Write the methods
on the front of a card
and the instance
variables on the back.
Update the values
of the instance
variables when a
mutator method
is called.
106 Chapter 3 Implementing Classes
If you have more than one object in your program, you will have multiple cards, one
for each object:
0 0
29.50 50.00
9.25
reg1.purchase reg1.payment
0 0
19.95 19.95
reg2.purchase reg2.payment
These diagrams are also useful when you design a class. Suppose you are asked to
enhance the  CashRegister class to compute the sales tax. Add methods  recordTaxable-
Purchase and  getSalesTax to the front of the card. Now turn the card over, look over the
instance variables, and ask yourself whether the object has sufficient information to
com pute the answer. Remember that each object is an autonomous unit. Any value
that can be used in a computation must be
• An instance variable.
• A method argument.
• A static variable (uncommon; see Section 8.4).
To compute the sales tax, we need to know the tax rate and the total of the taxable
items. (Food items are usually not subject to sales tax.) We don’t have that informa
tion available. Let us introduce additional instance variables for the tax rate and the
taxable total. The tax rate can be set in the constructor (assuming it stays fixed for the
lifetime of the object). When adding an item, we need to be told whether the item is
taxable. If so, we add its price to the taxable total.
For example, consider the following statements.
CashRegister reg3(7.5); //  7.5 percent sales tax
reg3.recordPurchase(3.95); //  Not taxable
reg3.recordTaxablePurchase(19.95); //  Taxable
When you record the effect on a card, it looks like this:
reg3.taxRate
0 7.5
reg3.taxablePurchase
0 0
19.95 3.95
reg3.purchase reg3.payment
With this information, we can compute the tax. It is  taxablePurchase x taxRate / 100 . Trac
ing the object helped us understand the need for additional instance variables.
17.  Consider a  Car class that simulates fuel consumption in a car. We will assume a
fixed efficiency (in miles per gallon) that is supplied in the constructor. There are
methods for adding gas, driv ing a given distance, and checking the amount of gas
left in the tank. Make a card for a  Car object, choosing suitable instance variables
and showing their values after the object was constructed.
full code example
go to  wiley.com/go/
javacode to down
load an enhanced
CashRegister class
that computes the
sales tax.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
3.6 Local Variables  107
18.  Trace the following method calls:
Car myCar(25);
myCar.addGas(20);
myCar.drive(100);
myCar.drive(200);
myCar.addGas(5);
19.  Suppose you are asked to simulate the odometer of
the car, by adding a method  getMilesDriven . Add an
instance variable to the object’s card that is suitable
for computing this method’s result.
20.  Trace the methods of Self Check 18, updating the
instance variable that you added in Self Check 19.
Practice It  Now you can try these exercises at the end of the chapter: R3.18, R3.19, R3.20.
3.6 Local Variables
In this section, we discuss the behavior of local variables. A local variable is a variable
that is declared in the body of a method. For example, the  giveChange method in How
To 3.1 declares a local vari able  change :
public double giveChange()
{
double change = payment - purchase;
purchase = 0;
payment = 0;
return change;
}
Parameter variables are similar to local variables, but they are declared in method
headers. For example, the following method declares a parameter variable  amount :
public void receivePayment(double amount)
Local and parameter variables belong to methods. When a method runs, its local and
parameter variables come to life. When the method exits, they are removed immedi-
ately. For example, if you call  regis ter.giveChange() , then a variable  change is created.
When the method exits, that variable is removed.
In contrast, instance variables belong to objects, not methods. When an object is
constructed, its instance variables are created. The instance variables stay alive until
no method uses the object any longer. (The Java virtual machine contains an agent
called a garbage collector that periodically reclaims objects when they are no longer
used.)
An important difference between instance variables and local variables is initial-
ization. You must ini tialize all local variables. If you don’t initialize a local variable,
the compiler complains when you try to use it. (Note that parameter variables are
initialized when the method is called.)
Instance variables are initialized with a default value before a constructor is
invoked. Instance variables that are numbers are initialized to 0. Object references are
set to a special value called  null . If an object reference is  null , then it refers to no object
at all. We will discuss the  null value in greater detail in Section 5.2.5.
© plusphoto/iStockphoto.
Local variables are
declared in the body
of a method.
Camera: © james steidl/iStockphoto.
Globe: © Alex Slobodkin/iStockphoto.
A N IM AT IO N
Lifetime of Variables
When a method exits,
its local variables are
removed.
Instance variables
are initialized to a
default value, but
you must initialize
local variables.
Full Code examPle
Go to  wiley.com/go/
javacode  to download
a demonstration of
local variables.
108 Chapter 3 Implementing Classes
21.  What do local variables and parameter variables have in common? In which
essential aspect do they differ?
22.  Why was it necessary to introduce the local variable  change in the  giveChange
method? That is, why didn’t the method simply end with the statement
return payment - purchase;
23.  Consider a  CashRegister object  reg1 whose  payment instance variable has the
value 20 and whose  purchase instance variable has the value 19.5. Trace the call
reg1.giveChange() . Include the local variable  change . Draw an X in its column
when the variable ceases to exist.
practice It  Now you can try these exercises at the end of the chapter: R3.14, R3.15.
duplicating Instance Variables in local Variables
Beginning programmers commonly add types to assignment statements, thereby changing
them into local variable declarations. For example,
public double giveChange()
{
double change = payment - purchase;
double purchase = 0; //  ERROR! This declares a local variable.
double payment = 0; //  ERROR! The instance variable is not updated.
return change;
}
Another common error is to declare a parameter variable with the same name as an instance
variable. For example, consider this  BankAccount constructor:
public BankAccount(double balance)
{
balance = balance; //  ERROR! Does not set the instance variable
}
This constructor simply sets the parameter variable to itself, leaving it unchanged. A simple
remedy is to come up with a different name for the parameter variable:
public BankAccount(double initialBalance)
{
balance = initialBalance; //  OK
}
providing unnecessary Instance Variables
A common beginner’s mistake is to use instance variables when local variables would be more
appropriate. For example, consider the  change variable of the  giveChange method. It is not
needed anywhere else––that’s why it is local to the method. But what if it had been declared as
an instance variable?
public class CashRegister
{
private double purchase;
private double payment;
private double change; //  Not appropriate
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
Common error 3.3
© John Bell/iStockphoto.
Common error 3.4
© John Bell/iStockphoto.
3.7 the this reference 109
public double giveChange()
{
change = payment - purchase;
purchase = 0;
payment = 0;
return change;
}
...
}
This class will work, but there is a hidden danger. Other methods can read and write to the
change instance variable, which can be a source of confusion.
Use instance variables for values that an object needs to remember between method calls.
Use local variables for values that don’t need to be retained when a method has completed.
forgetting to Initialize object references in a constructor
Just as it is a common error to forget to initialize a local variable, it is easy to forget about
instance variables. Every constructor needs to ensure that all instance variables are set to
appropriate values.
If you do not initialize an instance variable, the Java compiler will initialize it for you.
Numbers are initialized with 0, but object references—such as string variables—are set to the
null reference.
Of course, 0 is often a convenient default for numbers. However,  null is hardly ever a con
venient default for objects. Consider this “lazy” constructor for a modified version of the
BankAccount class:
public class BankAccount
{
private double balance;
private String owner;
. . .
public BankAccount(double initialBalance)
{
balance = initialBalance;
}
}
Then  balance is initialized, but the  owner variable is set to a  null reference. This can be a prob
lem—it is illegal to call methods on the  null reference.
To avoid this problem, it is a good idea to initialize every instance variable:
public BankAccount(double initialBalance)
{
balance = initialBalance;
owner = "None";
}
3.7 the  this reference
When you call a method, you pass two kinds of inputs to the method:
• The object on which you invoke the method
• The method arguments
Common error 3.5
© John Bell/iStockphoto.
110 Chapter 3 Implementing Classes
For example, when you call
momsSavings.deposit(500);
the  deposit method needs to know the account object ( momsSavings ) as well as the
amount that is being deposited (500).
When you implement the method, you provide a parameter variable for each argu
ment. But you don’t need to provide a parameter variable for the object on which
the method is being invoked. That object is called the implicit parameter. All other
parameter variables (such as the amount to be deposited in our example) are called
explicit parameters.
Look again at the code of the  deposit method:
public void deposit(double amount)
{
balance = balance + amount;
}
Here,  amount is an explicit parameter. You don’t see the implicit parameter–that is
why it is called “implicit”. But consider what  balance means exactly. After all, our
program may have multiple  BankAccount objects, and each of them has its own balance.
Because we are depositing the money into  momsSavings ,  balance must mean  moms-
Savings.balance . In general, when you refer to an instance variable inside a method, it
means the instance variable of the implicit parameter.
In any method, you can access the implicit parameter—the object on which the
method is called—with the reserved word  this . For example, in the preceding method
invocation,  this refers to the same object as  momsSavings (see Figure 7).
The statement
balance = balance + amount;
actually means
this.balance = this.balance + amount;
When you refer to an instance variable in a method, the compiler automatically
applies it to the  this refer ence. Some programmers actually prefer to manually insert
the  this reference before every instance vari able because they find it makes the code
clearer. Here is an example:
public BankAccount(double initialBalance)
{
this.balance = initialBalance;
}
You may want to try it out and see if you like that style.
Use of an instance
variable name in a
method denotes
the instance
variable of the
implicit parameter.
the  this reference
denotes the implicit
parameter.
figure 7  the Implicit parameter of a method Call
momsSavings =
balance =
BankAccount
1000
this =
amount = 500
3.7 the this reference 111
The  this reference can also be used to distinguish between instance variables and
local or parameter variables. Consider the constructor
public BankAccount(double balance)
{
this.balance = balance;
}
The expression  this.balance clearly refers to the  balance instance variable. However,
the expression  balance by itself seems ambiguous. It could denote either the param
eter variable or the instance variable. The Java language specifies that in this situation
the local variable wins out. It “shadows” the instance variable. Therefore,
this.balance = balance;
means: “Set the instance variable  balance to the parameter variable  balance ”.
There is another situation in which it is important to understand implicit param
eters. Consider the following modification to the  BankAccount class. We add a method
to apply the monthly account fee:
public class BankAccount
{
. . .
public void monthlyFee()
{
withdraw(10); //  Withdraw $10 from this account
}
}
That means to withdraw from the  same bank account object that is carrying out the
monthlyFee operation. In other words, the implicit parameter of the  withdraw method is
the (invisible) implicit parameter of the  monthlyFee method.
If you find it confusing to have an invisible parameter, you can use the  this refer
ence to make the method easier to read:
public class BankAccount
{
. . .
public void monthlyFee()
{
this.withdraw(10); //  Withdraw $10 from this account
}
}
You have now seen how to use objects and implement classes, and you have learned
some important tech nical details about variables and method parameters. The
remainder of this chapter continues the optional graphics track. In the next chapter,
you will learn more about the most fundamental data types of the Java language.
24.  How many implicit and explicit parameters does the  withdraw method of the
BankAccount class have, and what are their names and types?
25.  In the  deposit method, what is the meaning of  this.amount ? Or, if the expression
has no meaning, why not?
26.  How many implicit and explicit parameters does the  main method of the  Bank-
AccountTester class have, and what are they called?
practice It  Now you can try these exercises at the end of the chapter: R3.11, R3.12.
a local variable
shadows an instance
variable with the
same name. You
can access the
instance variable
name through the
this reference.
a method call
without an implicit
parameter is applied
to the same object.
full code example
go to  wiley.com/go/
javacode to download
a program that
demonstrates the
this reference.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
112 Chapter 3 Implementing Classes graphics track
calling one constructor from another
Consider the  BankAccount class. It has two constructors: a noargument constructor to initialize
the balance with zero, and another constructor to supply an initial balance. Rather than explic
itly setting the balance to zero, one constructor can call another constructor of the same class
instead. There is a shorthand notation to achieve this result:
public class BankAccount
{
public BankAccount(double initialBalance)
{
balance = initialBalance;
}
public BankAccount()
{
this(0);
}
. . .
}
The command t his(0); means “Call another constructor of this class and supply the value  0 ”.
Such a call to another constructor can occur only as the first line in a constructor.
This syntax is a minor convenience. We will not use it in this book. Actually, the use of
the reserved word  this is a little confusing. Normally,  this denotes a reference to the implicit
parameter, but if  this is followed by parentheses, it denotes a call to another constructor of the
same class.
3.8 shape Classes
In this section, we continue the optional graphics track by discussing how to orga
nize complex drawings in a more objectoriented fashion.
When you produce a drawing that has multiple shapes, or parts made of multiple
shapes, such as the car in Figure 8, it is a good idea to make a separate class for each
part. The class should have a  draw method that draws the shape, and a constructor to
set the position of the shape. For example, here is the outline of the  Car class:
public class Car
{
public Car(int x, int y)
{
//  Remember position
. . .
}
public void draw(Graphics2D g2)
{
//  Drawing instructions
. . .
}
}
You will find the complete class declaration at the end of this section. The  draw method
contains a rather long sequence of instructions for drawing the body, roof, and tires.
special topic 3.1
© Eric Isselé/iStockphoto.
It is a good idea to
make a class for any
part of a drawing
that can occur
more than once.
graphics track 3.8 shape Classes 113
figure 8  the Car Component Draws two Car shapes
The coordinates of the car parts seem a bit arbitrary. To come up with suitable values,
draw the image on graph paper and read off the coordi nates (Figure 9).
The program that produces Figure 8 is composed of three classes.
• The  Car class is responsible for drawing a single car. Two objects of this class are
constructed, one for each car.
• The  CarComponent class displays the drawing.
• The  CarViewer class shows a frame that contains a  CarComponent .
Let us look more closely at the  CarComponent class. The  paintComponent method draws
two cars. We place one car in the topleft corner of the window, and the other car in
the bottomright corner. To compute the bottomright position, we call the  getWidth
and  getHeight methods of the  JComponent class. These methods return the dimensions
of the component. We subtract the dimensions of the car to determine the position of
car2 :
Car car1 = new Car(0, 0);
int x = getWidth() - 60;
int y = getHeight() - 30;
Car car2 = new Car(x, y);
to figure out how
to draw a complex
shape, make a sketch
on graph paper.
figure 9 
Using graph paper to
Find shape Coordinates
0
10
20
30
40
0 10 20 30 40 50 60
114 Chapter 3 Implementing Classes graphics track
Pay close attention to the call to  getWidth inside the  paintComponent method of  Car-
Component . The method call has no implicit parameter, which means that the method is
applied to the same object that executes the  paintComponent method. The component
simply obtains its own width.
Run the program and resize the window. Note that the second car always ends
up at the bottomright corner of the window. Whenever the window is resized, the
paintComponent method is called and the car position is recomputed, taking the current
component dimensions into account.
section_8/carcomponent.java
1 import java.awt.Graphics;
2 import java.awt.Graphics2D;
3 import javax.swing.JComponent;
4
5 /**
6 This component draws two car shapes.
7 */
8 public class CarComponent extends JComponent
9 {
10 public void paintComponent(Graphics g)
11 {
12 Graphics2D g2 = (Graphics2D) g;
13
14 Car car1 = new Car(0, 0);
15
16 int x = getWidth() - 60;
17 int y = getHeight() - 30;
18
19 Car car2 = new Car(x, y);
20
21 car1.draw(g2);
22 car2.draw(g2);
23 }
24 }
section_8/car.java
1 import java.awt.Graphics2D;
2 import java.awt.Rectangle;
3 import java.awt.geom.Ellipse2D;
4 import java.awt.geom.Line2D;
5 import java.awt.geom.Point2D;
6
7 /**
8 A car shape that can be positioned anywhere on the screen.
9 */
10 public class Car
11 {
12 private int xLeft;
13 private int yTop;
14
15 /**
16 Constructs a car with a given top left corner.
17 @param x  the xcoordinate of the topleft corner
18 @param y  the ycoordinate of the topleft corner
19 */
20 public Car(int x, int y)
21 {
graphics track 3.8 shape Classes 115
22 xLeft = x;
23 yTop = y;
24 }
25
26 /**
27 Draws the car.
28 @param g2  the graphics context
29 */
30 public void draw(Graphics2D g2)
31 {
32 Rectangle body = new Rectangle(xLeft, yTop + 10, 60, 10);
33 Ellipse2D.Double frontTire
34 = new Ellipse2D.Double(xLeft + 10, yTop + 20, 10, 10);
35 Ellipse2D.Double rearTire
36 = new Ellipse2D.Double(xLeft + 40, yTop + 20, 10, 10);
37
38 //  The bottom of the front windshield
39 Point2D.Double r1 = new Point2D.Double(xLeft + 10, yTop + 10);
40 //  The front of the roof
41 Point2D.Double r2 = new Point2D.Double(xLeft + 20, yTop);
42 //  The rear of the roof
43 Point2D.Double r3 = new Point2D.Double(xLeft + 40, yTop);
44 //  The bottom of the rear windshield
45 Point2D.Double r4 = new Point2D.Double(xLeft + 50, yTop + 10);
46
47 Line2D.Double frontWindshield = new Line2D.Double(r1, r2);
48 Line2D.Double roofTop = new Line2D.Double(r2, r3);
49 Line2D.Double rearWindshield = new Line2D.Double(r3, r4);
50
51 g2.draw(body);
52 g2.draw(frontTire);
53 g2.draw(rearTire);
54 g2.draw(frontWindshield);
55 g2.draw(roofTop);
56 g2.draw(rearWindshield);
57 }
58 }
section_8/carViewer.java
1 import javax.swing.JFrame;
2
3 public class CarViewer
4 {
5 public static void main(String[] args)
6 {
7 JFrame frame = new JFrame();
8
9 frame.setSize(300, 400);
10 frame.setTitle("Two cars");
11 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
12
13 CarComponent component = new CarComponent();
14 frame.add(component);
15
16 frame.setVisible(true);
17 }
18 }
116 Chapter 3  Implementing Classes   Graphics Track
27.  Which class needs to be modified to have the two cars positioned next to each
other?
28.  Which class needs to be modified to have the car tires painted in black, and what
modification do you need to make?
29.  How do you make the cars twice as big?
Practice It  Now you can try these exercises at the end of the chapter: E3.16, E3.21.
Step 1  Determine the shapes that you need for the drawing.
You can use the following shapes:
• Squares and rectangles
• Circles and ellipses
• Lines
The outlines of these shapes can be drawn in any color, and you can fill the insides of these
shapes with any color. You can also use text to label parts of your drawing.
Some national flags consist of three equally wide sections of different colors, side by side.
Punchstock.
You could draw such a flag using three rectangles. But if the middle rectangle is white, as it
is, for example, in the flag of Italy (green, white, red), it is easier and looks better to draw a line
on the top and bottom of the middle portion:
Two rectangles
Two lines
© Nicholas Homrich/iStockphoto.
S e l f C h e C k
© Steve Simzer/iStockphoto.
How To 3.2  Drawing Graphical Shapes
Suppose you want to write a program that displays graphical shapes such as cars, aliens, charts,
or any other images that can be obtained from rectangles, lines, and ellipses. These instructions
give you a step-by-step procedure for decomposing a drawing into parts and implementing a
program that produces the drawing.
Problem Statement  Create a program that draws a national flag.
graphics track 3.8 shape Classes 117
step 2  Find the coordinates for the shapes.
You now need to find the exact positions for the geometric shapes.
• For rectangles, you need the x- and y-position of the topleft corner, the width, and the
height.
• For ellipses, you need the topleft corner, width, and height of the bounding rectangle.
• For lines, you need the x- and y-positions of the start and end points.
• For text, you need the x- and y-position of the basepoint.
A commonlyused size for a window is 300 by 300 pixels. You may not want the flag crammed
all the way to the top, so perhaps the upperleft corner of the flag should be at point (100, 100).
Many flags, such as the flag of Italy, have a width : height ratio of 3 : 2. (You can often find
exact proportions for a particular flag by doing a bit of Internet research on one of several
Flags of the World sites.) For example, if you make the flag 90 pixels wide, then it should be 60
pixels tall. (Why not make it 100 pixels wide? Then the height would be 100 · 2 / 3 ≈ 67, which
seems more awkward.)
Now you can compute the coordinates of all the important points of the shape:
(100, 100) (130, 100) (160, 100) (190, 100)
(100, 160) (130, 160) (160, 160) (190, 160)
step 3  Write Java statements to draw the shapes.
In our example, there are two rectangles and two lines:
Rectangle leftRectangle = new Rectangle(100, 100, 30, 60);
Rectangle rightRectangle = new Rectangle(160, 100, 30, 60);
Line2D.Double topLine = new Line2D.Double(130, 100, 160, 100);
Line2D.Double bottomLine = new Line2D.Double(130, 160, 160, 160);
If you are more ambitious, then you can express the coordinates in terms of a few variables.
In the case of the flag, we have arbitrarily chosen the topleft corner and the width. All other
coordinates follow from those choices. If you decide to follow the ambitious approach, then
the rectangles and lines are determined as follows:
Rectangle leftRectangle = new Rectangle(
xLeft, yTop,
width / 3, width * 2 / 3);
Rectangle rightRectangle = new Rectangle(
xLeft + 2 * width / 3, yTop,
width / 3, width * 2 / 3);
Line2D.Double topLine = new Line2D.Double(
xLeft + width / 3, yTop,
xLeft + width * 2 / 3, yTop);
Line2D.Double bottomLine = new Line2D.Double(
xLeft + width / 3, yTop + width * 2 / 3,
xLeft + width * 2 / 3, yTop + width * 2 / 3);
118 Chapter 3 Implementing Classes graphics track
Now you need to fill the rectangles and draw the lines. For the flag of Italy, the left rectangle is
green and the right rectangle is red. Remember to switch colors before the filling and drawing
operations:
g2.setColor(Color.GREEN);
g2.fill(leftRectangle);
g2.setColor(Color.RED);
g2.fill(rightRectangle);
g2.setColor(Color.BLACK);
g2.draw(topLine);
g2.draw(bottomLine);
step 4  Combine the drawing statements with the component “plumbing”.
public class MyComponent extends JComponent
{
public void paintComponent(Graphics g)
{
Graphics2D g2 = (Graphics2D) g;
//  Drawing instructions
. . .
}
}
In our simple example, you could add all shapes and drawing instructions inside the  paint-
Component method:
public class ItalianFlagComponent extends JComponent
{
public void paintComponent(Graphics g)
{
Graphics2D g2 = (Graphics2D) g;
Rectangle leftRectangle = new Rectangle(100, 100, 30, 60);
. . .
g2.setColor(Color.GREEN);
g2.fill(leftRectangle);
. . .
}
}
That approach is acceptable for simple drawings, but it is not very objectoriented. After all,
a flag is an object. It is better to make a separate class for the flag. Then you can draw different
flags at different positions. Specify the sizes in a constructor and supply a  draw method:
public class ItalianFlag
{
private int xLeft;
private int yTop;
private int width;
public ItalianFlag(int x, int y, int aWidth)
{
xLeft = x;
yTop = y;
width = aWidth;
}
public void draw(Graphics2D g2)
{
Rectangle leftRectangle = new Rectangle(
xLeft, yTop,
width / 3, width * 2 / 3);
graphics track Chapter summary 119
. . .
g2.setColor(Color.GREEN);
g2.fill(leftRectangle);
. . .
}
}
You still need a separate class for the component, but it is very simple:
public class ItalianFlagComponent extends JComponent
{
public void paintComponent(Graphics g)
{
Graphics2D g2 = (Graphics2D) g;
ItalianFlag flag = new ItalianFlag(100, 100, 90);
flag.draw(g2);
}
}
step 5  Write the viewer class.
Provide a viewer class, with a  main method in which you construct a frame, add your compo
nent, and make your frame visible. The viewer class is completely routine; you only need to
change a single line to show a different com ponent.
public class ItalianFlagViewer
{
public static void main(String[] args)
{
JFrame frame = new JFrame();
frame.setSize(300, 400);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
ItalianFlagComponent component = new ItalianFlagComponent();
frame.add(component);
frame.setVisible(true);
}
}
understand instance variables and the methods that access them.
• An object’s instance variables store the data required for executing its methods.
• Each object of a class has its own set of instance variables.
• Private instance variables can only be accessed by meth
ods of the same class.
• Encapsulation is the process of hiding implementation
details and providing methods for data access.
• Encapsulation allows a programmer to use a class without having to know its
implementation.
• Information hiding makes it simpler for the implementor of a class to locate
errors and change implementations.
full code example
go to  wiley.com/go/
javacode to download
the complete flag
drawing program.
C h a p t e r s U m m a rY
© Mark Evans/iStockphoto.
© yenwen/iStockphoto.
120 Chapter 3 Implementing Classes
Write method and constructor headers that describe the pub lic interface of a class.
• In order to implement a class, you first need to know which methods are required.
• Constructors set the initial data for objects.
• The constructor name is always the same as the class name.
• Use documentation comments to describe the classes and public methods of your
programs.
• Provide documen tation comments for every class, every method, every parame ter
variable, and every return value.
Implement a class.
• The private implementation of a class consists of instance variables, and the
bod ies of constructors and methods.
Write tests that verify that a class works correctly.
• A unit test verifies that a class works correctly in isolation, outside a complete
program.
• To test a class, use an environment for interactive testing, or write a tester class to
execute test instructions.
use the technique of object tracing for visualizing object behavior.
• Write the methods on the front of a card and the instance variables on the back.
• Update the values of the instance variables when a mutator method is called.
compare initialization and lifetime of instance, local, and parameter variables.
• Local variables are declared in the body of a method.
• When a method exits, its local variables are removed.
• Instance variables are initialized to a default value, but you must initialize local
variables.
recognize the use of the implicit parameter in method decla rations.
• Use of an instance variable name in a method denotes the instance variable of the
implicit parameter.
• The  this reference denotes the implicit parameter.
• A local variable shadows an instance variable with the same name. You can access
the instance variable name through the  this reference.
• A method call without an implicit parameter is applied to the same object.
Implement classes that draw graphical shapes.
• It is a good idea to make a class for any part of a drawing that can occur more than
once.
• To figure out how to draw a complex shape, make a sketch on graph paper.
© migin/iStockphoto.
© Chris Fertnig/iStockphoto.
Punchstock.
review Questions 121
• r3.1  What is the public interface of the  Counter class in Section 3.1? How does it differ
from the implementation of the class?
• r3.2  What is encapsulation? Why is it useful?
• r3.3  Instance variables are a part of the hidden implementation of a class, but they aren’t
actually hidden from programmers who have the source code of the class. Explain to
what extent the  private reserved word provides information hiding.
• r3.4  Consider a class  Grade that represents a letter grade, such as A+ or B. Give two
choices of instance variables that can be used for implementing the  Grade class.
•• r3.5  Consider a class  Time that represents a point in time, such as 9  a . m . or 3:30  p . m . Give
two different sets of instance variables that can be used for implementing the  Time
class.
• r3.6  Suppose the implementor of the  Time class of Exercise R3.5 changes from one imple
mentation strategy to another, keeping the public interface unchanged. What do the
programmers who use the  Time class need to do?
•• r3.7  You can read the  value instance variable of the  Counter class with the  getValue acces sor
method. Should there be a  setValue mutator method to change it? Explain why or
why not.
•• r3.8  a. Show that the  BankAccount(double initialBalance) constructor is not strictly nec
essary. That is, if we removed that constructor from the public interface, how
could a programmer still obtain  BankAccount objects with an arbitrary balance?
b.  Conversely, could we keep only the  BankAccount(double initialBalance) con
structor and remove the  BankAccount() constructor?
•• r3.9  Why does the  BankAccount class not have a  reset method?
• r3.10  What happens in our implementation of the  BankAccount class when more money is
withdrawn from the account than the current balance?
•• r3.11  What is the  this reference? Why would you use it?
•• r3.12  What does the following method do? Give an example of how you can call the
method.
public class BankAccount
{
public void mystery(BankAccount that, double amount)
{
this.balance = this.balance - amount;
that.balance = that.balance + amount;
}
. . . //  Other bank account methods
}
•• r3.13  Suppose you want to implement a class  TimeDepositAccount . A time deposit account
has a fixed interest rate that should be set in the constructor, together with the initial
balance. Provide a method to get the current balance. Provide a method to add the
earned interest to the account. This method should have no arguments because the
interest rate is already known. It should have no return value because you already
r e V I e W Q U e s t I o n s
122 Chapter 3 Implementing Classes
provided a method for obtaining the current balance. It is not possible to deposit
additional funds into this account. Provide a  withdraw method that removes the entire
balance. Partial withdrawals are not allowed.
• r3.14  Consider the following implementation of a class  Square :
public class Square
{
private int sideLength;
private int area; //  Not a good idea
public Square(int length)
{
sideLength = length;
}
public int getArea()
{
area = sideLength * sideLength;
return area;
}
}
Why is it not a good idea to introduce an instance variable for the area? Rewrite the class so
that  area is a local variable.
•• r3.15  Consider the following implementation of a class  Square :
public class Square
{
private int sideLength;
private int area;
public Square(int initialLength)
{
sideLength = initialLength;
area = sideLength * sideLength;
}
public int getArea() { return area; }
public void grow() { sideLength = 2 * sideLength; }
}
What error does this class have? How would you fix it?
•• testing r3.16  Provide a unit test class for the  Counter class in Section 3.1.
•• testing r3.17  Read Exercise E3.9, but do not implement the  Car class yet. Write a tester class that
tests a scenario in which gas is added to the car, the car is driven, more gas is added,
and the car is driven again. Print the actual and expected amount of gas in the tank.
• r3.18  Using the object tracing technique described in Section 3.5, trace the program at the
end of Section 3.4.
•• r3.19  Using the object tracing technique described in Section 3.5, trace the program in
How To 3.1.
•• r3.20  Using the object tracing technique described in Section 3.5, trace the program in
Worked Example 3.1.
practice exercises 123
••• r3.21  Design a modification of the  BankAccount class in which the first five transactions per
month are free and a $1 fee is charged for every additional transaction. Provide a
method that deducts the fee at the end of a month. What additional instance variables
do you need? Using the object tracing technique described in Section 3.5, trace a
scenario that shows how the fees are computed over two months.
•• graphics r3.22  Suppose you want to extend the car viewer program in Section 3.8 to show a subur
ban scene, with several cars and houses. Which classes do you need?
••• graphics r3.23  Explain why the calls to the  getWidth and  getHeight methods in the  CarComponent class
have no explicit parameter.
•• graphics r3.24  How would you modify the  Car class in order to show cars of varying sizes?
• e3.1  We want to add a button to the tally counter in Section 3.1 that allows an operator to
undo an accidental button click. Provide a method
public void undo()
that simulates such a button. As an added precaution, make sure that clicking the
undo button more often than the click button has no effect. (Hint: The call
Math.max(n, 0) returns  n if  n is greater than zero, zero otherwise.)
• e3.2  Simulate a tally counter that can be used to admit a limited number of people. First,
the limit is set with a call
public void setLimit(int maximum)
If the click button is clicked more often than the limit, it has no effect. (Hint: The call
Math.min(n, limit) returns  n if  n is less than  limit , and  limit otherwise.).
• testing e3.3  Write a  BankAccountTester class whose  main method constructs a bank account, depos
its $1,000, withdraws $500, withdraws another $400, and then prints the remaining
balance. Also print the expected result.
• e3.4  Add a method
public void addInterest(double rate)
to the  BankAccount class that adds interest at the given rate. For example, after the
statements
BankAccount momsSavings = new BankAccount(1000);
momsSavings.addInterest(10); //  10 percent interest
the balance in  momsSavings is $1,100. Also supply a  BankAccountTester class that prints
the actual and expected balance.
• e3.5  Write a class  SavingsAccount that is similar to the  BankAccount class, except that it has an
added instance variable  interest . Supply a constructor that sets both the initial bal
ance and the interest rate. Supply a method  addInterest (with no explicit parameter)
that adds interest to the account. Write a  SavingsAccountTester class that con structs a
savings account with an initial balance of $1,000 and an interest rate of 10 percent.
Then apply the  addInterest method and print the resulting balance. Also com pute the
expected result by hand and print it.
p r a C t I C e e x e r C I s e s
124 Chapter 3 Implementing Classes
••• e3.6  Add a method  printReceipt to the  CashRegister class. The method should print the
prices of all purchased items and the total amount due. Hint: You will need to form
a string of all prices. Use the  concat method of the  String class to add additional items
to that string. To turn a price into a string, use the call  String.valueOf(price) .
• e3.7  After closing time, the store manager would like to know how much business was
transacted during the day. Modify the  CashRegister class to enable this functionality.
Supply methods  getSalesTotal and  getSalesCount to get the total amount of all sales
and the number of sales. Supply a method  reset that resets any counters and totals so
that the next day’s sales start from zero.
•• e3.8  Implement a class  Employee . An employee has a name (a string) and a salary (a  dou ble ).
Provide a constructor with two arguments
public Employee(String employeeName, double currentSalary)
and methods
public String getName()
public double getSalary()
public void raiseSalary(double byPercent)
These methods return the name and salary, and raise the employee’s salary by a
certain percentage. Sam ple usage:
Employee harry = new Employee("Hacker, Harry", 50000);
harry.raiseSalary(10); //  Harry gets a 10 percent raise
Supply an  EmployeeTester class that tests all methods.
•• e3.9  Implement a class  Car with the following properties. A car has a certain fuel effi
ciency (measured in miles̸gallon or liters̸km—pick one) and a certain amount of
fuel in the gas tank. The efficiency is specified in the constructor, and the initial fuel
level is 0. Supply a method  drive that simulates driving the car for a certain distance,
reducing the amount of gasoline in the fuel tank. Also supply methods  getGasInTank ,
returning the current amount of gasoline in the fuel tank, and  addGas , to add gasoline
to the fuel tank. Sample usage:
Car myHybrid = new Car(50); //  50 miles per gallon
myHybrid.addGas(20); //  Tank 20 gallons
myHybrid.drive(100); //  Drive 100 miles
double gasLeft = myHybrid.getGasInTank(); //  Get gas remaining in tank
You may assume that the  drive method is never called with a distance that consumes
more than the avail able gas. Supply a  CarTester class that tests all methods.
• e3.10  Implement a class  Product . A product has a name and a price, for example  new Prod-
uct("Toaster", 29.95) . Supply methods  getName ,  getPrice , and  reducePrice . Supply a
program  ProductPrinter that makes two products, prints the name and price, reduces
their prices by $5.00, and then prints the prices again.
•• e3.11  Provide a class for authoring a simple letter. In the constructor, supply the names of
the sender and the recipient:
public Letter(String from, String to)
Supply a method
public void addLine(String line)
to add a line of text to the body of the letter.
practice exercises 125
Supply a method
public String getText()
that returns the entire text of the letter. The text has the form:
Dear recipient name :
blank line
first line of the body
second line of the body
. . .
last line of the body
blank line
Sincerely,
blank line
sender name
Also supply a class  LetterPrinter that prints this letter.
Dear John:
I am sorry we must part.
I wish you all the best.
Sincerely,
Mary
Construct an object of the  Letter class and call  addLine twice.
Hints: (1) Use the  concat method to form a longer string from two shorter strings.
(2) The special string  "\n" represents a new line. For example, the statement
body = body.concat("Sincerely,").concat("\n");
adds a line containing the string  "Sincerely," to the body.
•• e3.12  Write a class  Bug that models a bug moving along a horizontal line. The bug moves
either to the right or left. Initially, the bug moves to the right, but it can turn to
change its direction. In each move, its position changes by one unit in the current
direction. Provide a constructor
public Bug(int initialPosition)
and methods
public void turn()
public void move()
public int getPosition()
Sample usage:
Bug bugsy = new Bug(10);
bugsy.move(); //  Now the position is 11
bugsy.turn();
bugsy.move(); //  Now the position is 10
Your  BugTester should construct a bug, make it move and turn a few times, and print
the actual and expected position.
•• e3.13  Implement a class  Moth that models a moth flying along a straight line. The moth has
a position, which is the distance from a fixed origin. When the moth moves toward a
point of light, its new position is halfway between its old position and the position of
the light source. Supply a constructor
public Moth(double initialPosition)
126 Chapter 3 Implementing Classes
and methods
public void moveToLight(double lightPosition)
public double getPosition()
Your  MothTester should construct a moth, move it toward a couple of light sources,
and check that the moth’s position is as expected.
••• graphics e3.14  Write a program that fills the window with a large ellipse, with a black outline and
filled with your favorite color. The ellipse should touch the window boundaries,
even if the window is resized. Call the  getWidth and  getHeight methods of the
JComponent class in the  paintComponent method.
•• graphics e3.15  Draw a shooting target—a set of concentric rings in alternating black
and white colors. Hint: Fill a black circle, then fill a smaller white circle
on top, and so on. Your program should be composed of classes  Target ,
TargetComponent , and  TargetViewer .
•• graphics e3.16  Write a program that draws a picture of a house. It could be as
simple as the accom panying figure, or if you like, make it more
elaborate (3D, skyscraper, marble col umns in the entryway,
whatever). Implement a class  House  and supply a method
draw(Graphics2D g2) that draws the house.
•• graphics e3.17  Extend Exercise E3.16 by supplying a  House constructor for specifying the position
and size. Then populate your screen with a few houses of different sizes.
•• graphics e3.18  Change the car viewer program in Section 3.8 to make the cars appear in different
colors. Each  Car object should store its own color. Supply modified  Car and  Car -
Component classes.
•• graphics e3.19  Change the  Car class so that the size of a car can be specified in the constructor.
Change the  CarComponent class to make one of the cars appear twice the size of the
original example.
•• graphics e3.20  Write a program to plot the string “HELLO”, using only lines and circles. Do not
call  drawString , and do not use  System.out . Make classes  LetterH ,  LetterE ,  LetterL, and
LetterO .
•• graphics e3.21  Write a program that displays the Olympic rings. Color
the rings in the Olympic colors. Provide classes  OlympicRing ,
OlympicRingViewer and  OlympicRingComponent .
•• graphics e3.22  Make a bar chart to plot the following data set. Label each bar. Make the bars hori
zontal for easier labeling. Provide a class  BarChartViewer and a class  BarChartComponent .
Bridge name longest span (ft)
Golden Gate 4,200
Brooklyn 1,595
Delaware Memorial 2,150
Mackinac 3,800
programming projects 127
• p3.1  Enhance the  CashRegister class so that it counts the purchased items. Provide a  get-
ItemCount method that returns the count.
••• p3.2  Support computing sales tax in the  CashRegister class. The tax rate should be supplied
when constructing a  CashRegister object. Add  recordTaxablePurchase  and  getTotal-
Tax  methods. (Amounts added with  recordPurchase are not taxable.) The  giveChange
method should correctly reflect the sales tax that is charged on taxable items.
•• p3.3  Implement a class  Balloon . A balloon starts out with radius 0. Supply a method
public void inflate(double amount)
that increases the radius by the given amount. Supply a method
public double getVolume()
that returns the current volume of the balloon. Use  Math.PI for the value of  π . To
compute the cube of a value  r , just use  r * r * r .
••• p3.4  Implement a class  Student . For the purpose of this exercise, a student has a name
and a total quiz score. Supply an appropriate constructor and methods  getName() ,
addQuiz(int score) ,  getTotalScore() , and  getAverageScore() . To compute the average,
you also need to store the number of quizzes that the student took.
Supply a  StudentTester class that tests all methods.
• p3.5  Write a class  Battery that models a rechargeable battery. A battery has a constructor
public Battery(double capacity)
where capacity is a value measured in milliampere hours. A typical AA battery has a
capacity of 2000 to 3000 mAh. The method
public void drain(double amount)
drains the capacity of the battery by the given amount. The method
public void charge()
charges the battery to its original capacity.
The method
public double getRemainingCapacity()
gets the remaining capacity of the battery.
•• graphics p3.6  Write a program that draws three stars. Use classes  Star ,  StarComponent , and  StarViewer .
Each star should look like this:
•• p3.7  Implement a class  RoachPopulation that simulates the growth of a roach population.
The constructor takes the size of the initial roach population. The  breed method
simulates a period in which the roaches breed, which doubles their population. The
p r o g r a m m I n g p r o J e C t s
128 Chapter 3 Implementing Classes
spray(double percent) method simulates spraying with insecticide, which reduces the
population by the given percentage. The  getRoaches method returns the current num
ber of roaches. A pro gram called  RoachSimulation simulates a population that starts
out with 10 roaches. Breed, spray to reduce the population by 10 percent, and print
the roach count. Repeat three more times.
•• p3.8  Implement a  VotingMachine class that can be used for a simple election. Have meth ods
to clear the machine state, to vote for a Democrat, to vote for a Republican, and to
get the tallies for both parties.
••• p3.9  In this project, you will enhance the  BankAccount class and see how abstraction and
encapsulation enable evolutionary changes to software.
Begin with a simple enhancement: charging a fee for every deposit and withdrawal.
Supply a mechanism for setting the fee and modify the  deposit and  withdraw methods
so that the fee is levied. Test your resulting class and check that the fee is computed
correctly.
Now make a more complex change. The bank will allow a fixed number of free
transactions (deposits or withdrawals) every month, and charge for transactions
exceeding the free allotment. The charge is not levied immediately but at the end of
the month.
Supply a new method  deductMonthlyCharge to the  BankAccount class that deducts the
monthly charge and resets the transaction count. (Hint: Use  Math.max(actual transac-
tion count, free transaction count) in your computation.)
Produce a test program that verifies that the fees are calculated correctly over several
months.
••• p3.10  In this project, you will explore an objectoriented alternative to the “Hello, World”
program in Chapter 1.
Begin with a simple  Greeter class that has a single method,  sayHello . That method
should return a string, not print it. Create two objects of this class and invoke their
sayHello methods. Of course, both objects return the same answer.
Enhance the  Greeter class so that each object produces a customized greeting. For
example, the object constructed as  new Greeter("Dave") should say  "Hello, Dave" . (Use
the  concat method to combine strings to form a longer string, or peek ahead at
Section 4.5 to see how you can use the  + operator for the same pur pose.)
Add a method  sayGoodbye to the  Greeter class.
Finally, add a method  refuseHelp to the  Greeter class. It should return a string such as
"I am sorry, Dave. I am afraid I can’t do that."
If you use BlueJ, place two  Greeter objects on the workbench (one that greets the
world and one that greets Dave) and invoke methods on them. Otherwise, write a
tester program that constructs these objects, invokes methods, and prints the results.
answers to selfCheck Questions 129
a n s W e rs t o s e l F  C h e C k Q U e s t I o n s
1.  public void unclick()
{
value = value - 1;
}
2.  You can only access them by invoking the
methods of the  Clock class.
3.  In one of the methods of the  Counter class.
4.  The programmers who designed and imple
mented the Java library.
5.  Other programmers who work on the per
sonal finance application.
6.  harrysChecking.withdraw(
harrysChecking.getBalance())
7.  The  withdraw method has return type  void .
It doesn’t return a value. Use the  getBalance
method to obtain the balance after the with
drawal.
8.  Add an  accountNumber parameter variable to
the constructors, and add a  getAccount Number
method. There is no need for a  setAccountNumber
method—the account number never changes
after construction.
9.  /**
Constructs a new bank account with a given
initial balance.
@param accountNumber  the account number for
this account
@param initialBalance  the initial balance for
this account
*/
10.  The first sentence of the method description
should describe the method—it is dis played in
isolation in the summary table.
11.  An instance variable needs to be added to the
class:
private int accountNumber;
12.  Because the balance instance variable is
accessed from the  main method of  BankRobber .
The compiler will report an error because  main
is not a method of the  BankAccount class and has
no access to  BankAccount instance variables.
13.  public int getWidth()
{
return width;
}
14.  There is more than one correct answer. One
possible implementation is as follows:
public void translate(int dx, int dy)
{
int newx = x + dx;
x = newx;
int newy = y + dy;
y = newy;
}
15.  One  BankAccount object, no  BankAccountTester
object. The purpose of the  BankAccount Tester
class is merely to hold the  main method.
16.  In those environments, you can issue inter
active commands to construct  BankAccount
objects, invoke methods, and display their
return values.
17.
Car myCar
Car(mpg)
addGas(amount)
drive(distance)
getGasLeft
front
gasLeft milesPerGallon
0 25
back
18.
gasLeft milesPerGallon
0
20
16
8
13
25
130 Chapter 3 Implementing Classes
19.
gasLeft milesPerGallon
0 25
totalMiles
0
20.
0
20
16
8
13
25
0
100
300
gasLeft milesPerGallon totalMiles
21.  Variables of both categories belong to meth
ods—they come alive when the method is
called, and they die when the method exits.
They differ in their initialization. Parameter
variables are initialized with the values sup
plied as arguments in the call; local variables
must be explicitly initialized.
22.  After computing the change due,  payment
and  purchase were set to zero. If the method
returned  payment - purchase , it would always
return zero.
23.
change
20
0
0.5
X
reg1.purchase
19.5
0
reg1.payment
24.  One implicit parameter, called  this , of type
BankAccount , and one explicit parameter, called
amount , of type  double .
25.  It is not a legal expression.  this is of type
BankAccount and the  BankAccount class has no
instance variable named amount .
26.  No implicit parameter—the  main method is
not invoked on any object—and one explicit
parameter, called  args .
27.  CarComponent
28.  In the  draw method of the  Car class, call
g2.fill(frontTire);
g2.fill(rearTire);
29.  Double all measurements in the  draw method of
the  Car class.
4
C h a p t e r
131
Fundamental
data types
to understand integer and floating-point
numbers
to recognize the limitations of the numeric types
to become aware of causes for overflow and roundoff errors
to understand the proper use of constants
to write arithmetic expressions in Java
to use the  String type to manipulate character strings
to write programs that read input and produce
formatted output
C h a p t e r G o a l s
C h a p t e r C o n t e n t s
4.1 Numbers 132
Syntax 4.1: Constant declaration 136
Special Topic 4.1: Big numbers 138
Programming Tip 4.1: do not use
magic numbers 139
4.2 Arithmetic 139
Syntax 4.2: Cast 143
Common Error 4.1: unintended Integer
division 144
Common Error 4.2: unbalanced parentheses 144
Programming Tip 4.2: spaces in expressions 145
Special Topic 4.2: Combining assignment
and arithmetic 145
Special Topic 4.3: Instance methods and
static methods 145
Computing & Society 4.1: the pentium
Floating-point Bug 146
4.3 iNput ANd Output 147
Syntax 4.3: Input statement 147
How To 4.1: Carrying out Computations 151
Worked Example 4.1: Computing the Volume and
surface area of a pyramid
4.4 prOblem sOlviNg: First dO it
by hANd 154
Worked Example 4.2: Computing travel time
4.5 striNgs 156
Programming Tip 4.3: reading exception
reports 162
Special Topic 4.4: using dialog Boxes for Input
and output 162
Computing & Society 4.2: International alphabets
and unicode 163
© Eyeidea/iStockphoto.
132
numbers and character strings (such as the ones on this
display board) are important data types in any Java program.
In this chapter, you will learn how to work with numbers and
text, and how to write simple programs that perform useful
tasks with them. We also cover the important topic of input
and output, which enables you to implement interactive
programs.
4.1 numbers
We start this chapter with information about numbers. The following sections tell
you how to choose the most appropriate number types for your numeric values, and
how to work with constants––numeric values that do not change.
4.1.1 number types
In Java, every value is either a reference to an object, or it belongs to one of the eight
primitive types shown in Table 1.
Six of the primitive types are number types; four of them for integers and two for
floating-point num bers.
Each of the number types has a different range. Appendix G explains why the
range limits are related to powers of two. The largest number that can be represented
in an  int is denoted by  Integer.MAX_VALUE . Its value is about 2.14 billion. Similarly, the
smallest integer is  Integer.MIN_VALUE , about –2.14 billion.
table 1 primitive types
type description size
int The integer type, with range
–2,147,483,648 ( Integer.MIN_VALUE ) . . . 2,147,483,647
( Integer.MAX_VALUE , about 2.14 billion)
4 bytes
byte The type describing a single byte, with range –128 . . . 127 1 byte
short The short integer type, with range –32,768 . . . 32,767 2 bytes
long The long integer type, with range
–9,223,372,036,854,775,808 . . . 9,223,372,036,854,775,807
8 bytes
double The double-precision floating-point type, with a range of
about ±10 308 and about 15 significant decimal digits
8 bytes
float The single-precision floating-point type, with a range of
about ±10 38 and about 7 significant decimal digits
4 bytes
char The character type, representing code units in the Unicode
encoding scheme (see Computing & Society 4.2 on page 163)
2 bytes
boolean The type with the two truth values  false and  true (see Chapter 5) 1 bit
Java has eight
primitive types,
including four
integer types
and two floating-
point types.
kphoto.
4.1 Numbers  133
Table 2  Number Literals in Java
Number Type Comment
6 int An integer has no fractional part.
–6 int Integers can be negative.
0 int Zero is an integer.
0.5 double A number with a fractional part has type  double .
1.0 double An integer with a fractional part .0 has type  double .
1E6 double A number in exponential notation: 1 × 10 6 or 1000000.
Numbers in exponential notation always have type  double .
2.96E-2 double Negative exponent: 2.96 × 10 –2 = 2.96 / 100 = 0.0296
100,000 Error: Do not use a comma as a decimal separator.
3 1/2 Error: Do not use fractions; use decimal notation: 3.5
When a value such as  6 or  0.335 occurs in a Java program, it is called a number
literal. If a number literal has a decimal point, it is a floating-point number; other-
wise, it is an integer. Table 2 shows how to write integer and floating-point literals in
Java.
Generally, you will use the  int type for integer quantities. Occasionally, however,
calculations involv ing integers can overflow. This happens if the result of a computa-
tion exceeds the range for the number type. For example,
int n = 1000000;
System.out.println(n * n); //  Prints –727379968, which is clearly wrong
The product  n * n is 10 12 , which is larger than the largest integer (about 2 · 10 9 ). The
result is truncated to fit into an  int , yielding a value that is completely wrong. Unfor-
tunately, there is no warning when an inte ger overflow occurs.
If you run into this problem, the simplest remedy is to use the  long type. Special
Topic 4.1 on page 138 shows you how to use the  BigInteger type in the unlikely event
that even the  long type overflows.
Overflow is not usually a problem for double-precision floating-point numbers.
The  double type has a range of about ±10 308 . Floating-point numbers have a different
problem––limited precision. The  double type has about 15 significant digits, and there
are many numbers that cannot be accurately represented as  double values.
When a value cannot be represented exactly, it is rounded to the nearest match.
Consider this example:
double f = 4.35;
System.out.println(100 * f); //  Prints 434.99999999999994
If a computation yields an integer that is larger than the
largest  int value (about 2.14 billion), it overflows.
A numeric
computation
overflows if the
result falls outside
the range for the
number type.
© Douglas Allen/iStockphoto.
Rounding errors
occur when an 
exact representation
of a floating-point
number is not
possible.
134 Chapter 4 Fundamental data types
Floating-point numbers have limited precision.
Not every value can be represented precisely,
and roundoff errors can occur.
© caracterdesign/iStockphoto.
The problem arises because computers represent numbers in the binary number sys-
tem. In the binary number system, there is no exact representation of the fraction
1/10, just as there is no exact representa tion of the fraction 1/3 = 0.33333 in the deci-
mal number system. (See Appendix G for more information.)
For this reason, the  double type is not appropriate for financial calculations. In
this book, we will con tinue to use  double values for bank balances and other financial
quantities so that we keep our programs as simple as possible. However, professional
programs need to use the  BigDecimal type for this purpose—see Special Topic 4.1.
In Java, it is legal to assign an integer value to a floating-point variable:
int dollars = 100;
double balance = dollars; // OK
But the opposite assignment is an error: You cannot assign a floating-point expres-
sion to an integer vari able.
double balance = 13.75;
int dollars = balance; //  Error
You will see in Section 4.2.5 how to convert a value of type  double into an integer.
In this book, we do not use the  float type. It has less than 7 significant digits, which
greatly increases the risk of roundoff errors. Some programmers use  float to save
on memory if they need to store a huge set of numbers that do not require much
precision.
4.1.2 Constants
In many programs, you need to use numerical constants—values that do not change
and that have a spe cial significance for a computation.
A typical example for the use of constants is a computation that involves coin val-
ues, such as the fol lowing:
payment = dollars + quarters * 0.25 + dimes * 0.1
+ nickels * 0.05 + pennies * 0.01;
Most of the code is self-documenting. However, the four numeric quantities, 0.25,
0.1, 0.05, and 0.01 are included in the arithmetic expression without any explana-
tion. Of course, in this case, you know that the value of a nickel is five cents, which
explains the 0.05, and so on. However, the next person who needs to maintain this
code may live in another country and may not know that a nickel is worth five cents.
Thus, it is a good idea to use symbolic names for all values, even those that appear
obvious. Here is a clearer version of the computation of the total:
double quarterValue = 0.25;
double dimeValue = 0.1;
double nickelValue = 0.05;
double pennyValue = 0.01;
4.1 numbers 135
payment = dollars + quarters * quarterValue + dimes * dimeValue
+ nickels * nickelValue + pennies * pennyValue;
There is another improvement we can make. There is a difference between the  nickels
and  nickelValue variables. The  nickels variable can truly vary over the life of the pro-
gram, as we calculate different pay ments. But  nickelValue is always 0.05.
In Java, constants are identified with the reserved word  final . A variable tagged as
final can never change after it has been set. If you try to change the value of a  final
variable, the compiler will report an error and your program will not compile.
Many programmers use all-uppercase names for constants ( final variables), such
as  NICKEL_VALUE . That way, it is easy to distinguish between variables (with mostly low-
ercase letters) and constants. We will fol low this convention in this book. However,
this rule is a matter of good style, not a requirement of the Java language. The com-
piler will not complain if you give a  final variable a name with lowercase letters.
Here is an improved version of the code that computes the value of a payment.
final double QUARTER_VALUE = 0.25;
final double DIME_VALUE = 0.1;
final double NICKEL_VALUE = 0.05;
final double PENNY_VALUE = 0.01;
payment = dollars + quarters * QUARTER_VALUE + dimes * DIME_VALUE
+ nickels * NICKEL_VALUE + pennies * PENNY_VALUE;
Frequently, constant values are needed in several methods. Then you should declare
them together with the instance variables of a class and tag them as  static and final .
As before,  final indicates that the value is a constant. The  static reserved word means
that the constant belongs to the class—this is explained in greater detail in Chapter 8.)
public class CashRegister
{
//  Constants
public static final double QUARTER_VALUE = 0.25;
public static final double DIME_VALUE = 0.1;
public static final double NICKEL_VALUE = 0.05;
public static final double PENNY_VALUE = 0.01;
//  Instance variables
private double purchase;
private double payment;
//  Methods
. . .
}
We declared the constants as  public . There is no danger in doing this because con-
stants cannot be modi fied. Methods of other classes can access a public constant by
first specifying the name of the class in which it is declared, then a period, then the
name of the constant, such as  CashRegister.NICKEL_VALUE .
The  Math class from the standard library declares a couple of useful constants:
public class Math
{
. . .
public static final double E = 2.7182818284590452354;
public static final double PI = 3.14159265358979323846;
}
You can refer to these constants as  Math.PI and  Math.E in any method. For example,
double circumference = Math.PI * diameter;
a  final variable is
a constant. once its
value has been set, it
cannot be changed.
use named constants
to make your
programs easier to
read and maintain.
136 Chapter 4 Fundamental data types
syntax 4.1  Constant declaration
Syntax
final double NICKEL_VALUE = 0.05;
public static final double LITERS_PER_GALLON = 3.785;
The  final
reserved word
indicates that this
value cannot
be modified.
Declared in a class
Declared in a method: final  typeName variableName =  expression ;
Declared in a class:  accessSpecifier static final  typeName variableName =  expression ;
Use uppercase letters for constants.
Declared in a method
The sample program below puts constants to work. The program shows a refinement
of the  CashRegister class of How To 3.1. The public interface of that class has been
modified in order to solve a common business problem.
Busy cashiers sometimes make mistakes totaling up coin values. Our  CashRegister
class features a method whose inputs are the coin counts. For example, the call
register.receivePayment(1, 2, 1, 1, 4);
processes a payment consisting of one dollar, two quarters, one dime, one nickel, and
four pennies. The  receivePayment method figures out the total value of the payment,
$1.69. As you can see from the code list ing, the method uses named constants for the
coin values.
section_1/cashregister.java
1 /**
2 A cash register totals up sales and computes change due.
3 */
4 public class CashRegister
5 {
6 public static final double QUARTER_VALUE = 0.25;
7 public static final double DIME_VALUE = 0.1;
8 public static final double NICKEL_VALUE = 0.05;
9 public static final double PENNY_VALUE = 0.01;
10
11 private double purchase;
12 private double payment;
13
14 /**
15 Constructs a cash register with no money in it.
16 */
17 public CashRegister()
18 {
19 purchase = 0;
20 payment = 0;
4.1 numbers 137
21 }
22
23 /**
24 Records the purchase price of an item.
25 @param amount  the price of the purchased item
26 */
27 public void recordPurchase(double amount)
28 {
29 purchase = purchase + amount;
30 }
31
32 /**
33 Processes the payment received from the customer.
34 @param dollars  the number of dollars in the payment
35 @param quarters  the number of quarters in the payment
36 @param dimes  the number of dimes in the payment
37 @param nickels  the number of nickels in the payment
38 @param pennies  the number of pennies in the payment
39 */
40 public void receivePayment(int dollars, int quarters,
41 int dimes, int nickels, int pennies)
42 {
43 payment = dollars + quarters * QUARTER_VALUE + dimes * DIME_VALUE
44 + nickels * NICKEL_VALUE + pennies * PENNY_VALUE;
45 }
46
47 /**
48 Computes the change due and resets the machine for the next customer.
49 @return  the change due to the customer
50 */
51 public double giveChange()
52 {
53 double change = payment - purchase;
54 purchase = 0;
55 payment = 0;
56 return change;
57 }
58 }
section_1/cashregistertester.java
1 /**
2 This class tests the  CashRegister class.
3 */
4 public class CashRegisterTester
5 {
6 public static void main(String[] args)
7 {
8 CashRegister register = new CashRegister();
9
10 register.recordPurchase(0.75);
11 register.recordPurchase(1.50);
12 register.receivePayment(2, 0, 5, 0, 0);
13 System.out.print("Change: ");
14 System.out.println(register.giveChange());
15 System.out.println("Expected: 0.25");
16
17 register.recordPurchase(2.25);
18 register.recordPurchase(19.25);
19 register.receivePayment(23, 2, 0, 0, 0);
138 Chapter 4 Fundamental data types
20 System.out.print("Change: ");
21 System.out.println(register.giveChange());
22 System.out.println("Expected: 2.0");
23 }
24 }
program run
Change: 0.25
Expected: 0.25
Change: 2.0
Expected: 2.0
1.  Which are the most commonly used number types in Java?
2.  Suppose you want to write a program that works with population data from
various countries. Which Java data type should you use?
3.  Which of the following initializations are incorrect, and why?
a.  int dollars = 100.0;
b.  double balance = 100;
4.  What is the difference between the following two statements?
final double CM_PER_INCH = 2.54;
and
public static final double CM_PER_INCH = 2.54;
5.  What is wrong with the following statement sequence?
double diameter = . . .;
double circumference = 3.14 * diameter;
practice it  Now you can try these exercises at the end of the chapter: R4.1, R4.21, E4.20.
big Numbers
If you want to compute with really large numbers, you can use big number objects. Big num-
ber objects are objects of the  BigInteger and  BigDecimal classes in the  java.math package. Unlike
the number types such as  int or  double , big number objects have essentially no limits on their
size and precision. However, computations with big number objects are much slower than
those that involve number types. Perhaps more importantly, you can’t use the familiar arith-
metic operators such as ( + - * ) with them. Instead, you have to use methods called  add ,  sub-
tract , and  multiply . Here is an example of how to create a  BigInteger object and how to call the
multiply method:
BigInteger n = new BigInteger("1000000");
BigInteger r = n.multiply(n);
System.out.println(r); //  Prints 1000000000000
The  BigDecimal type carries out floating-point computations without roundoff errors. For
example,
BigDecimal d = new BigDecimal("4.35");
BigDecimal e = new BigDecimal("100");
BigDecimal f = d.multiply(e);
System.out.println(f); //  Prints 435.00
© Nicholas Homrich/iStockphoto.
s e l F  c h e c k
special topic 4.1
© Eric Isselé/iStockphoto.
4.2 arithmetic 139
do Not use magic Numbers
A magic number is a numeric constant that appears in your code without explanation. For
example, consider the fol lowing scary example that actually occurs in the Java library source:
h = 31 * h + ch;
Why 31? The number of days in January? One less than the number of bits in an integer?
Actually, this code com putes a “hash code” from a string—a number that is derived from
the characters in such a way that different strings are likely to
yield different hash codes. The value 31 turns out to scramble
the character values nicely.
A better solution is to use a named constant:
final int HASH_MULTIPLIER = 31;
h = HASH_MULTIPLIER * h + ch;
You should never use magic numbers in your code. Any number
that is not completely self-explanatory should be declared as a
named constant. Even the most reasonable cosmic constant is
going to change one day. You think there are 365 days in a year?
Your customers on Mars are going to be pretty unhappy about
your silly prejudice. Make a constant
final int DAYS_PER_YEAR = 365;
4.2 arithmetic
In this section, you will learn how to carry out arithmetic calculations in Java.
4.2.1 arithmetic operators
Java supports the same four basic arithmetic operations as a calculator—addition,
subtraction, multiplica tion, and division—but it uses different symbols for the multi-
plication and division operators.
You must write  a * b to denote multiplication. Unlike in mathematics, you can-
not write  a b ,  a ·  b , or  a ×  b . Similarly, division is always indicated with the  /  operator,
never a ÷ or a fraction bar. For example,
a b +
2
becomes  (a + b) / 2 .
The combination of variables, literals, operators, and/or method calls is called an
expression. For exam ple,  (a + b) / 2 is an expression.
Parentheses are used just as in algebra: to indicate in which order the parts of the
expression should be computed. For example, in the expression  (a + b) / 2 , the sum
a + b is computed first, and then the sum is divided by 2. In contrast, in the expression
a + b / 2
only  b is divided by 2, and then the sum of  a and  b / 2 is formed. As in regular alge-
braic notation, multi plication and division have a higher precedence than addition
and subtraction. For example, in the expres sion  a + b / 2 , the  / is carried out first, even
though the  + operation occurs further to the left (see Appendix B).
If you mix integer and floating-point values in an arithmetic expression, the result
is a floating-point value. For example,  7 + 4.0 is the floating-point value  11.0 .
programming tip 4.1
© Eric Isselé/iStockphoto.
We prefer programs that
are easy to understand
over those that appear
to work by magic.
© FinnBrandt/iStockphoto.
© arakonyunus/iStockphoto.
mixing integers and
floating-point values
in an arithmetic
expression yields a
floating-point value.
140 Chapter 4 Fundamental data types
4.2.2 Increment and decrement
Changing a variable by adding or subtracting 1 is so common that there is a special
shorthand for it. The  ++ operator increments a variable (see Figure 1):
counter++; //  Adds 1 to the variable  counter
Similarly, the  -- operator decrements a variable:
counter--; //  Subtracts 1 from  counter
4.2.3 Integer division and remainder
Division works as you would expect, as long as at least
one of the numbers involved is a floating-point number.
That is,
7.0 / 4.0
7 / 4.0
7.0 / 4
all yield 1.75. However, if both numbers are integers,
then the result of the integer division is always an
integer, with the remainder discarded. That is,
7 / 4
evaluates to 1 because 7 divided by 4 is 1 with a remain-
der of 3 (which is discarded). This can be a source of
subtle programming errors—see Common Error 4.1.
If you are interested in the remainder only, use the  % operator:
7 % 4
is 3, the remainder of the integer division of 7 by 4. The  % symbol has no analog in alge-
bra. It was chosen because it looks similar to  / , and the remainder operation is related
to division. The operator is called modulus. (Some people call it modulo or mod.) It
has no relationship with the percent operation that you find on some calculators.
Here is a typical use for the integer  / and  % operations. Suppose you have an amount
of pennies in a piggybank:
int pennies = 1729;
You want to determine the value in dollars and cents. You obtain the dollars through
an integer division by 100:
int dollars = pennies / 100; //  Sets  dollars to 17
the  ++ operator adds
1 to a variable; the  --
operator subtracts 1.
Figure 1  Incrementing a Variable
1
counter =
counter + 1
3
2
counter =
4
4
counter + 1
© Michael Flippo/iStockphoto.
Integer division and the %
operator yield the dollar and
cent values of a piggybank
full of pennies.
If both arguments
of  / are integers,
the remainder
is discarded.
the  % operator
computes the
remainder of an
integer division.
4.2 arithmetic 141
table 3 Integer division and remainder
expression
(where  n = 1729)
Value Comment
n % 10 9 n % 10  is always the last digit of  n.
n / 10 172 This is always  n without the last digit.
n % 100 29 The last two digits of  n .
n / 10.0 172.9 Because 10.0 is a floating-point number, the fractional part is not discarded.
–n % 10 -9 Because the first argument is negative, the remainder is also negative.
n % 2 1 n % 2 is 0 if  n is even, 1 or –1 if  n is odd.
The integer division discards the remainder. To obtain the remainder, use the  %
operator:
int cents = pennies % 100; //  Sets  cents to 29
See Table 3 for additional examples.
4.2.4 powers and roots
In Java, there are no symbols for powers and roots. To compute them, you must call
methods. To take the square root of a number, you use the  Math.sqrt method. For
example,  x is written as  Math.sqrt(x) . To compute  x n , you write  Math.pow(x, n) .
In algebra, you use fractions, exponents, and roots to arrange expressions in a
compact two-dimen sional form. In Java, you have to write all expressions in a linear
arrangement. For example, the mathematical expression
b
r
n
× +






1
100
becomes
b * Math.pow(1 + r / 100, n)
Figure 2 shows how to analyze such an expression. Table 4 shows additional mathe-
matical methods.
the Java library
declares many
mathematical
functions, such as
Math.sqrt (square
root) and  Math.pow
(raising to a power).
Figure 2 
analyzing an expression
b * Math.pow(1 + r / 100, n)
r
100
r
1 +
100
r
n






1 +
100
b
r
n
× +






1
100
142 Chapter 4 Fundamental data types
table 4 mathematical methods
method returns method returns
Math.sqrt(x) Square root of x (≥ 0) Math.abs(x) Absolute value | x |
Math.pow(x, y) x y (x > 0, or x = 0 and y > 0, or
x < 0 and y is an integer)
Math.max(x, y) The larger of x and y
Math.sin(x) Sine of x (x in radians) Math.min(x, y) The smaller of x and y
Math.cos(x) Cosine of x Math.exp(x) e x
Math.tan(x) Tangent of x Math.log(x) Natural log (ln(x), x > 0)
Math.round(x) Closest integer to x (as a  long ) Math.log10(x) Decimal log (log 10  (x), x > 0)
Math.ceil(x) Smallest integer ≥ x
(as a  double )
Math.floor(x) Largest integer ≤ x
(as a  double )
Math.toRadians(x) Convert x degrees to radians
(i.e., returns x ·  π /180)
Math.toDegrees(x) Convert x radians to degrees
(i.e., returns x · 180/ π )
4.2.5 Converting Floating-point numbers to Integers
Occasionally, you have a value of type  double that you need to convert to the type  int .
It is an error to assign a floating-point value to an integer:
double balance = total + tax;
int dollars = balance; //  Error: Cannot assign  double to  int
The compiler disallows this assignment because it is potentially dangerous:
• The fractional part is lost.
• The magnitude may be too large. (The largest integer is about 2 billion, but a
floating-point number can be much larger.)
You must use the cast operator  (int) to convert a convert floating-point value to an
integer. Write the cast operator before the expression that you want to convert:
double balance = total + tax;
int dollars = (int) balance;
The cast  (int) converts the floating-point value  balance to an integer by discarding the
fractional part. For example, if  balance is 13.75, then  dollars is set to 13.
When applying the cast operator to an arithmetic expression, you need to place the
expression inside parentheses:
int dollars = (int) (total + tax);
Discarding the fractional part is not always appropriate. If you want to round a
floating-point number to the nearest whole number, use the  Math.round method. This
method returns a  long integer, because large floating-point numbers cannot be stored
in an  int .
long rounded = Math.round(balance);
If  balance is 13.75, then  rounded is set to 14.
you use a cast
(typeName) to
convert a value to
a different type.
Full cOde exAmple
Go to  wiley.com/go/
javacode to download
a program that
demonstrates casts,
rounding, and the
% operator.
4.2 arithmetic 143
syntax 4.2  Cast
Syntax
(int) (balance * 100)
This is the type of the expression after casting.
These parentheses are a
part of the cast operator.
Use parentheses here if
the cast is applied to an expression
with arithmetic operators.
( typeName )  expression
If you know that the result can be stored in an  int and does not require a  long , you
can use a cast:
int rounded = (int) Math.round(balance);
table 5 arithmetic expressions
mathematical
expression
Java
expression
Comments
x y +
2
(x + y) / 2 The parentheses are required;  x + y / 2
computes x
y
+
2 .
xy
2
x * y / 2 Parentheses are not required; operators with
the same precedence are evaluated left to right.
1
100
+






r
n
Math.pow(1 + r / 100, n) Use  Math.pow(x, n) to compute x n .
a b
2 2
+
Math.sqrt(a * a + b * b) a * a is simpler than  Math.pow(a, 2) .
i j k + +
3
(i + j + k) / 3.0 If i, j, and k are integers, using a denominator
of 3.0 forces floating-point division.
π Math.PI Math.PI is a constant declared in the  Math class.
6.  A bank account earns interest once per year. In Java, how do you compute the
interest earned in the first year? Assume variables  percent and  balance of type
double have already been declared.
7.  In Java, how do you compute the side length of a square whose area is stored in
the variable  area ?
8.  The volume of a sphere is given by
V r =
4
3
3
π
If the radius is given by a variable  radius of type  double , write a Java expression
for the volume.
© Nicholas Homrich/iStockphoto.
s e l F  c h e c k
144 Chapter 4 Fundamental data types
9.  What is the value of  1729 / 100 and  1729 % 100 ?
10.  If  n is a positive number, what is (n / 10) % 10 ?
practice it  Now you can try these exercises at the end of the chapter: R4.4, R4.6, E4.4, E4.23.
unintended integer division
It is unfortunate that Java uses the same symbol, namely  / , for both integer and floating-
point division. These are really quite different operations. It is a common error to use integer
division by accident. Consider this segment that computes the average of three integers:
int score1 = 10;
int score2 = 4;
int score3 = 9;
double average = (score1 + score2 + score3) / 3; //  Error
System.out.println("Average score: " + average); //  Prints 7.0 , not 7.666666666666667
What could be wrong with that? Of course, the average of  score1 ,  score2 , and  score3 is
score1 score2 score3 + +
3
Here, however, the  / does not mean division in the mathematical sense. It denotes integer divi-
sion because both 3 and the sum of  score1 + score2 + score3 are integers. Because the scores
add up to 23, the average is computed to be 7, the result of the integer division of 23 by 3. That
integer 7 is then moved into the floating-point variable  average . The remedy is to make the
numerator or denominator into a floating-point number:
double total = score1 + score2 + score3;
double average = total / 3;
or
double average = (score1 + score2 + score3) / 3.0;
unbalanced parentheses
Consider the expression
((a + b) * t / 2 * (1 - t)
What is wrong with it? Count the parentheses. There are three  ( and two  ) . The parenthe-
ses are unbalanced. This kind of typing error is very common with complicated expressions.
Now consider this expression.
(a + b) * t) / (2 * (1 - t)
This expression has three  ( and three  ) , but it still is not correct. In the middle of the
expression,
(a + b) * t) / (2 * (1 - t)
↑
there is only one  ( but two  ) , which is an error. In the middle of an expression, the count of  (
must be greater than or equal to the count of  ) , and at the end of the
expression the two counts must be the same.
Here is a simple trick to make the counting easier without using
pencil and paper. It is difficult for the brain to keep two counts
simultaneously. Keep only one count when scan ning the expres-
sion. Start with 1 at the first opening parenthesis, add 1 whenever
Common error 4.1
© John Bell/iStockphoto.
Common error 4.2
© John Bell/iStockphoto.
© Croko/iStockphoto.
4.2 arithmetic 145
you see an opening parenthesis, and subtract one whenever you see a closing parenthesis. Say
the num bers aloud as you scan the expression. If the count ever drops below zero, or is not
zero at the end, the parentheses are unbalanced. For example, when scanning the previous
expres sion, you would mutter
(a + b) * t) / (2 * (1 - t)
1 0 -1
and you would find the error.
spaces in expressions
It is easier to read
x1 = (-b + Math.sqrt(b * b - 4 * a * c)) / (2 * a);
than
x1=(-b+Math.sqrt(b*b-4*a*c))/(2*a);
Simply put spaces around all operators  + - * / % = . However, don’t put a space after a unary
minus: a – used to negate a single quantity, such as  -b . That way, it can be easily distinguished
from a binary minus, as in  a - b .
It is customary not to put a space after a method name. That is, write  Math.sqrt(x) and not
Math.sqrt (x) .
combining Assignment and Arithmetic
In Java, you can combine arithmetic and assignment. For example, the instruction
balance += amount;
is a shortcut for
balance = balance + amount;
Similarly,
total *= 2;
is another way of writing
total = total * 2;
Many programmers find this a convenient shortcut. If you like it, go ahead and use it in your
own code. For simplic ity, we won’t use it in this book, though.
instance methods and static methods
In the preceding section, you encountered the  Math class, which contains a collection of helpful
methods for carrying out mathematical computations. These methods do not operate on an
object. That is, you don’t call
double root = 2.sqrt(); // Error
In Java, numbers are not objects, so you can never invoke a method on a number. Instead,
you pass a number as an argument (explicit parameter) to a method, enclosing the number in
parentheses after the method name:
double root = Math.sqrt(2);
programming tip 4.2
© Eric Isselé/iStockphoto.
special topic 4.2
© Eric Isselé/iStockphoto.
special topic 4.3
© Eric Isselé/iStockphoto.
146 Chapter 4 Fundamental data types
Such methods are called static methods. (The term “static” is a historical holdover from the C
and C++ programming languages. It has nothing to do with the usual meaning of the word.)
Static methods do not operate on objects, but they are still declared inside classes. When
calling the method, you specify the class to which the  sqrt method belongs:
The name of the static method The name of the class
Math.sqrt(2)
In contrast, a method that is invoked on an object is called an instance method. As a rule of
thumb, you use static methods when you manipulate numbers. You will learn more about the
distinction between static and instance methods in Chapter 8.
In 1994, Intel Corporation released what
was then its most powerful processor, the
pentium. unlike previous generations of its processors, it
had a very fast floating-point unit. Intel’s goal was to com-
pete aggressively with the makers of higher-end proces sors
for engineering workstations. the pentium was a huge suc-
cess immediately.
In the summer of 1994, dr. thomas nicely of lynchburg
College in Virginia ran an extensive set of computations
to analyze the sums of reciprocals of certain sequences of
prime numbers. the results were not always what his the-
ory predicted, even after he took into account the inevita ble
roundoff errors. then dr. nicely noted that the same pro-
gram did produce the correct results when running on the
slower 486 processor that preceded the pentium in Intel’s
lineup. this should not have happened. the optimal round-
off behavior of floating-point calculations has been stan-
dardized by the Institute for electrical and electronic engi-
neers (Ieee) and Intel claimed to adhere to the Ieee standard
in both the 486 and the pentium processors. upon further
checking, dr. nicely discovered that indeed there was a very
small set of numbers for which the prod uct of two num-
bers was computed differently on the two processors. For
example,
4195835 4195835 3145727 3145727 , , , , , , , , − ( ) × ( )
is mathematically equal to 0, and it did compute as 0 on
a 486 processor. on his pentium processor the result was
256.
as it turned out, Intel had independently discovered
the bug in its testing and had started to produce chips that
fixed it. the bug was caused by an error in a table that was
used to speed up the floating-point multiplication algo rithm
of the processor. Intel determined that the problem was
exceedingly rare. they claimed that under normal use, a
typical consumer would only notice the problem once every
27,000 years. unfortunately for Intel, dr. nicely had not
been a normal user.
now Intel had a real problem on its hands. It figured that
the cost of replacing all pentium processors that it had sold
so far would cost a great deal of money. Intel already had
more orders for the chip than it could produce, and it would
be particularly galling to have to give out the scarce chips
as free replacements instead of selling them. Intel’s man-
agement decided to punt on the issue and initially offered
to replace the processors only for those customers who
could prove that their work required absolute preci sion in
mathematical calculations. naturally, that did not go over
well with the hundreds of thousands of customers who had
paid retail prices of $700 and more for a pentium chip and
did not want to live with the nagging feeling that perhaps,
one day, their income tax program would pro duce a faulty
return.
ultimately, Intel caved in to public demand and replaced
all defective chips, at a cost of about 475 million dollars.
This graph shows a set of numbers for which the original
Pentium processor obtained the wrong quotient.
Courtesy of Larry Hoyle, Institute for Policy & Social Research, University of Kansas.
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
-0.20
-0.40
-0.60
-0.80
-1.00
-1.20
-1.40
-1.60
-1.80
-2.00
1.40
1.10
0.80
0.50
0.20
-0.10
-0.40
-0.70
-1.00
-1.30
1.333680000
1.333700000
1.333720000
1.333740000
1.333760000
1.333780000
1.333800000
1.333820000
1.333840000
x/y
4195835+
3145727+
Pentium FDIV error
Computing & Society 4.1  the pentium Floating-point Bug
© MediaBakery.
4.3 Input and output 147
4.3 Input and output
In the following sections, you will see how to read user input and how to control the
appearance of the output that your programs produce.
4.3.1 reading Input
You can make your programs more flexible if you ask the program user for inputs
rather than using fixed values. Consider, for example, a program that processes prices
and quantities of soda containers. Prices and quantities are likely to fluctuate. The
program user should provide them as inputs.
When a program asks for user input, it should first print a message that tells the
user which input is expected. Such a message is called a prompt.
System.out.print("Please enter the number of bottles: "); //  Display prompt
Use the  print method, not  println , to display the prompt. You want the input to
appear after the colon, not on the following line. Also remember to leave a space after
the colon.
Because output is sent to  System.out , you might think that you use  System.in for
input. Unfortunately, it isn’t quite that simple. When Java was first designed, not
much attention was given to reading keyboard input. It was assumed that all pro-
grammers would produce graphical user interfaces with text fields and menus.
System.in was given a minimal set of features and must be combined with other classes
to be use ful.
To read keyboard input, you use a class called  Scanner . You obtain a  Scanner object
by using the following statement:
Scanner in = new Scanner(System.in);
Once you have a scanner, you use its  nextInt method to read an integer value:
System.out.print("Please enter the number of bottles: ");
int bottles = in.nextInt();
© Media Bakery.
A supermarket
scanner reads bar
codes. The Java
Scanner reads
numbers and text.
use the  Scanner class
to read keyboard
input in a
console window.
syntax 4.3  Input statement
import java.util.Scanner;
.
.
Scanner in = new Scanner(System.in);
.
.
System.out.print("Please enter the number of bottles: ");
int bottles = in.nextInt();
Display a prompt in the console window.
The program waits for user input,
then places the input into the variable.
Define a variable to hold the input value.
Don't use  println here.
Create a  Scanner object
to read keyboard input.
Include this line so you can
use the  Scanner class.
148 Chapter 4 Fundamental data types
When the  nextInt method is called, the program waits until the user types a number
and presses the Enter key. After the user supplies the input, the number is placed into
the  bottles variable, and the program continues.
To read a floating-point number, use the  nextDouble method instead:
System.out.print("Enter price: ");
double price = in.nextDouble();
The  Scanner class belongs to the package  java.util . When using the  Scan ner class,
import it by placing the following declaration at the top of your program file:
import java.util.Scanner;
4.3.2 Formatted output
When you print the result of a computation, you often want to control its appear-
ance. For exam ple, when you print an amount in dollars and cents, you usually want
it to be rounded to two significant digits. That is, you want the output to look like
Price per liter: 1.22
instead of
Price per liter: 1.215962441314554
The following command displays the price with two digits after the decimal point:
System.out.printf("%.2f", price);
You can also specify a field width:
System.out.printf("%10.2f", price);
The price is printed using ten characters: six spaces followed by the four characters  1.22 .
1 . 2 2
The construct  %10.2f is called a format specifier: it describes how a value should be for-
matted. The letter  f at the end of the format specifier indicates that we are displaying a
floating-point number. Use  d for an integer and  s for a string; see Table 6 for examples.
A format string contains format specifiers and literal characters. Any characters that
are not format specifiers are printed verbatim. For example, the command
System.out.printf("Price per liter:%10.2f", price);
prints
Price per liter: 1.22
use the  printf
method to specify
how values should
be formatted.
You use the  printf method to line
up your output in neat columns.
© Koele/iStockphoto.
4.3 Input and Output  149
Table 6  Format Specifier Examples
Format String Sample Output Comments
"%d" 24 Use  d with an integer.
"%5d" 24 Spaces are added so that the field width is 5.
"Quantity:%5d" Quantity: 24 Characters inside a format string but outside a
format specifier appear in the output.
"%f" 1.21997 Use  f with a floating-point number.
"%.2f" 1.22 Prints two digits after the decimal point.
"%7.2f" 1.22 Spaces are added so that the field width is 7.
"%s" Hello Use  s with a string.
"%d %.2f" 24 1.22 You can format multiple values at once.
You can print multiple values with a single call to the  printf method. Here is a typical
example:
System.out.printf("Quantity: %d Total: %10.2f", quantity, total);
Q u a n t i t y : 2 4 : l a t o T 1 7 . 2 9
Two digits after
the decimal point
The  printf method does not
start a new line here.
width 10
No field width was specified,
so no padding added
The  printf method, like the  print method, does not start a new line after the output. If
you want the next output to be on a separate line, you can call  System.out.println() .
Alternatively, Section 4.5.4 shows you how to add a newline character to the format
string.
Our next example program will
prompt for the price of a six-pack of soda
and a two-liter bottle, and then print out
the price per liter for both. The program
puts to work what you just learned about
reading input and formatting output.
What is the better deal? A six-pack of 12-ounce
cans or a two-liter bottle?
cans: © blackred/iStockphoto. bottle: © travismanley/iStockphoto.
150 Chapter 4 Fundamental data types
section_3/volume.java
1 import java.util.Scanner;
2
3 /**
4 This program prints the price per liter for a six-pack of cans and
5 a two-liter bottle.
6 */
7 public class Volume
8 {
9 public static void main(String[] args)
10 {
11 //  Read price per pack
12
13 Scanner in = new Scanner(System.in);
14
15 System.out.print("Please enter the price for a six-pack: ");
16 double packPrice = in.nextDouble();
17
18 //  Read price per bottle
19
20 System.out.print("Please enter the price for a two-liter bottle: ");
21 double bottlePrice = in.nextDouble();
22
23 final double CANS_PER_PACK = 6;
24 final double CAN_VOLUME = 0.355; //  12 oz. = 0.355 l
25 final double BOTTLE_VOLUME = 2;
26
27 //  Compute and print price per liter
28
29 double packPricePerLiter = packPrice / (CANS_PER_PACK * CAN_VOLUME);
30 double bottlePricePerLiter = bottlePrice / BOTTLE_VOLUME;
31
32 System.out.printf("Pack price per liter: %8.2f", packPricePerLiter);
33 System.out.println();
34
35 System.out.printf("Bottle price per liter: %8.2f", bottlePricePerLiter);
36 System.out.println();
37 }
38 }
program run
Please enter the price for a six-pack: 2.95
Please enter the price for a two-liter bottle: 2.85
Pack price per liter: 1.38
Bottle price per liter: 1.43
11.  Write statements to prompt for and read the user’s age using a  Scanner variable
named  in .
12.  What is wrong with the following statement sequence?
System.out.print("Please enter the unit price: ");
double unitPrice = in.nextDouble();
int quantity = in.nextInt();
© Nicholas Homrich/iStockphoto.
s e l F  c h e c k
4.3 Input and output 151
13.  What is problematic about the following statement sequence?
System.out.print("Please enter the unit price: ");
double unitPrice = in.nextInt();
14.  What is problematic about the following statement sequence?
System.out.print("Please enter the number of cans");
int cans = in.nextInt();
15.  What is the output of the following statement sequence?
int volume = 10;
System.out.printf("The volume is %5d", volume);
16.  Using the  printf method, print the values of the integer variables  bottles and  cans
so that the output looks like this:
Bottles: 8
Cans: 24
The numbers to the right should line up. (You may assume that the numbers
have at most 8 digits.)
practice it  Now you can try these exercises at the end of the chapter: R4.11, E4.6, E4.7.
step 1  Understand the problem: What are the inputs? What are the desired outputs?
In this problem, there are two inputs:
• The denomination of the bill that the customer inserts
• The price of the purchased item
There are two desired outputs:
• The number of dollar coins that the machine returns
• The number of quarters that the machine returns
step 2  Work out examples by hand.
This is a very important step. If you can’t compute a couple of solutions by hand, it’s unlikely
that you’ll be able to write a program that automates the computation.
Let’s assume that a customer purchased an item that cost $2.25 and inserted a $5 bill. The
customer is due $2.75, or two dollar coins and three quarters, in change.
That is easy for you to see, but how can a Java program come to the same conclusion? The
key is to work in pen nies, not dollars. The change due the customer is 275 pennies. Dividing
by 100 yields 2, the number of dollars. Dividing the remainder (75) by 25 yields 3, the number
of quarters.
© Steve Simzer/iStockphoto.
hoW to 4.1  carrying Out computations
Many programming problems require arithmetic computations. This How To shows you
how to turn a problem statement into pseudocode and, ultimately, a Java program.
problem statement  Suppose you are asked to write a program that simulates a vending
machine. A customer selects an item for purchase and inserts a bill into the vending machine.
The vending machine dispenses the purchased item and gives change. We will assume that all
item prices are multiples of 25 cents, and the machine gives all change in dollar coins and quar-
ters. Your task is to compute how many coins of each type to return.
152 Chapter 4 Fundamental data types
step 3  Write pseudocode for computing the answers.
In the previous step, you worked out a specific instance of the problem. You now need to
come up with a method that works in general.
Given an arbitrary item price and payment, how can you compute the coins due? First,
compute the change due in pennies:
change due = 100 x bill value - item price in pennies
To get the dollars, divide by 100 and discard the remainder:
dollar coins = change due / 100 (without remainder)
The remaining change due can be computed in two ways. If you are familiar with the modulus
operator, you can simply compute
change due = change due % 100
Alternatively, subtract the penny value of the dollar coins from the change due:
change due = change due - 100 x dollar coins
To get the quarters due, divide by 25:
quarters = change due / 25
step 4  Declare the variables and constants that you need, and specify their types.
Here, we have five variables:
•  billValue
•  itemPrice
•  changeDue
•  dollarCoins
•  quarters
Should we introduce constants to explain 100 and 25 as  PENNIES_PER_DOLLAR and  PENNIES_PER_
QUARTER ? Doing so will make it easier to convert the program to international markets, so we
will take this step.
It is very important that  changeDue and  PENNIES_PER_DOLLAR are of type  int because the com-
putation of  dollarCoins uses integer division. Similarly, the other variables are integers.
step 5  Turn the pseudocode into Java statements.
If you did a thorough job with the pseudocode, this step should be easy. Of course, you have
to know how to express mathematical operations (such as powers or integer division) in Java.
changeDue = PENNIES_PER_DOLLAR * billValue - itemPrice;
dollarCoins = changeDue / PENNIES_PER_DOLLAR;
changeDue = changeDue % PENNIES_PER_DOLLAR;
quarters = changeDue / PENNIES_PER_QUARTER;
step 6  Provide input and output.
Before starting the computation, we prompt the user for the bill value and item price:
System.out.print("Enter bill value (1 = $1 bill, 5 = $5 bill, etc.): ");
billValue = in.nextInt();
System.out.print("Enter item price in pennies: ");
itemPrice = in.nextInt();
When the computation is finished, we display the result. For extra credit, we use the  printf
method to make sure that the output lines up neatly.
System.out.printf("Dollar coins: %6d", dollarCoins);
System.out.printf("Quarters: %6d", quarters);
4.3 Input and Output  153
Step 7
A vending machine takes bills
and gives change in coins.
Photos.com/Jupiter Images.
Provide a class with a  main method.
Your computation needs to be placed into a class. Find an appropriate name for the class that
describes the purpose of the computation. In our example, we will choose the name  Vending-
Machine .
Inside the class, supply a  main method.
In the  main method, you need to declare constants and variables (Step 4), carry out compu-
tations (Step 5), and provide input and output (Step 6). Clearly, you will want to first get the
input, then do the computations, and finally show the output. Declare the constants at the
beginning of the method, and declare each variable just before it is needed.
Here is the complete program,  how_to_1/VendingMachine.java :
import java.util.Scanner;
/**
This program simulates a vending machine that gives change.
*/
public class VendingMachine
{
public static void main(String[] args)
{
Scanner in = new Scanner(System.in);
final int PENNIES_PER_DOLLAR = 100;
final int PENNIES_PER_QUARTER = 25;
System.out.print("Enter bill value (1 = $1 bill, 5 = $5 bill, etc.): ");
int billValue = in.nextInt();
System.out.print("Enter item price in pennies: ");
int itemPrice = in.nextInt();
//  Compute change due
int changeDue = PENNIES_PER_DOLLAR * billValue - itemPrice;
int dollarCoins = changeDue / PENNIES_PER_DOLLAR;
changeDue = changeDue % PENNIES_PER_DOLLAR;
int quarters = changeDue / PENNIES_PER_QUARTER;
//  Print change due
System.out.printf("Dollar coins: %6d", dollarCoins);
System.out.println();
154 Chapter 4 Fundamental data types
System.out.printf("Quarters: %6d", quarters);
System.out.println();
}
}
program run
Enter bill value (1 = $1 bill, 5 = $5 bill, etc.): 5
Enter item price in pennies:  225
Dollar coins: 2
Quarters: 3
4.4 problem solving: First do It By hand
A very important step for developing an algorithm is to first carry out the computa-
tions by hand. If you can’t compute a solution yourself, it’s unlikely that you’ll be
able to write a program that automates the computation.
To illustrate the use of hand calculations, consider the following problem.
A row of black and white tiles needs to be placed along a wall. For aesthetic rea-
sons, the architect has specified that the first and last tile shall be black.
Your task is to compute the number of tiles needed and the gap at each end, given
the space available and the width of each tile.
Total width
Gap
To make the problem more concrete, let’s assume the following dimensions:
• Total width: 100 inches
• Tile width: 5 inches
The obvious solution would be to fill the space with 20 tiles, but that would not
work—the last tile would be white.
Worked example 4.1  computing the volume and
surface Area of a pyramid
Learn how to design a class for computing the volume and
surface area of a pyramid. Go to  wiley.com/go/javaexamples and
download Worked Example 4.1.
© Holger Mette/iStockphoto.
pick concrete values
for a typical situation
to use in a hand
calculation.
4.4 problem solving: First do It By hand 155
Instead, look at the problem this way: The first tile must always be black, and then
we add some num ber of white/black pairs:
The first tile takes up 5 inches, leaving 95 inches to be covered by pairs. Each pair is
10 inches wide. Therefore the number of pairs is 95 / 10 = 9.5. However, we need to
discard the fractional part since we can’t have fractions of tile pairs.
Therefore, we will use 9 tile pairs or 18 tiles, plus the initial black tile. Altogether,
we require 19 tiles.
The tiles span 19 × 5 = 95 inches, leaving a total gap of 100 – 19 × 5 = 5 inches.
The gap should be evenly distributed at both ends. At each end, the gap is
(100 – 19 × 5) / 2 = 2.5 inches.
This computation gives us enough information to devise an algorithm with arbi-
trary values for the total width and tile width.
number of pairs = integer part of (total width - tile width) / (2 x tile width)
number of tiles = 1 + 2 x number of pairs
gap at each end = (total width - number of tiles x tile width) / 2
As you can see, doing a hand calculation gives enough insight into the problem that it
becomes easy to develop an algorithm.
17.  Translate the pseudocode for computing the number of tiles and the gap width
into Java.
18.  Suppose the architect specifies a pattern with black, gray, and white tiles, like
this:
Again, the first and last tile should be black. How do you need to modify the
algorithm?
19.  A robot needs to tile a floor with alternating black and white tiles. Develop
an algorithm that yields the color (0 for black, 1 for white), given the row and
column number. Start with specific values for the row and column, and then
generalize.
1 2 3 4
1
2
3
4
20.  For a particular car, repair and maintenance costs in year 1 are estimated at $100;
in year 10, at $1,500. Assuming that the repair cost increases by the same amount
every year, develop pseudocode to com pute the repair cost in year 3 and then
generalize to year  n .
21.  The shape of a bottle is approximated by two cylinders of radius r 1 and r 2 and
heights h 1 and h 2 , joined by a cone section of height h 3 .
Full cOde exAmple
Go to  wiley.com/go/
javacode to download
a program that
implements this
algorithm.
© Nicholas Homrich/iStockphoto.
s e l F  c h e c k
r 2
h 2
h 1
h 3
r 1
156 Chapter 4 Fundamental data types
Using the formulas for the volume of a cylinder,V r h = π
2
, and a cone section,
V
r r r r h
=
+ +
( )
π
1
2
1 2 2
2
3
,
develop pseudocode to compute the volume of the bottle. Using an actual bottle
with known volume as a sample, make a hand calculation of your pseudocode.
practice it  Now you can try these exercises at the end of the chapter: R4.16, R4.18, R4.19.
4.5 strings
Many programs process text, not numbers. Text
consists of characters: letters, numbers, punc-
tuation, spaces, and so on. A string is a sequence
of characters. For example, the string  "Harry" is a
sequence of five char acters.
4.5.1 the  String type
You can declare variables that hold strings.
String name = "Harry";
We distinguish between string variables (such as the variable  name declared above) and
string literals (char acter sequences enclosed in quotes, such as  "Harry" ). A string vari-
able is simply a variable that can hold a string, just as an integer variable can hold an
integer. A string literal denotes a particular string, just as a number literal (such as 2)
denotes a particular number.
The number of characters in a string is called the length of the string. For example,
the length of  "Harry" is 5. As you saw in Section 2.3, you can compute the length of a
string with the  length method.
int n = name.length();
A string of length 0 is called the empty string. It contains no characters and is written
as "".
Worked example 4.2  computing travel time
Learn how to develop a hand calculation to compute the time
that a robot requires to retrieve an item from rocky terrain. Go to
wiley.com/go/javaexamples and download Worked Example 4.2.
Courtesy NASA/JPL-Caltech.
© essxboy/iStockphoto.
strings are sequences
of characters.
the  length method
yields the number
of characters in
a string.
4.5 strings 157
4.5.2 Concatenation
Given two strings, such as  "Harry" and  "Morgan" , you can concatenate them to one
long string. The result consists of all characters in the first string, followed by all
characters in the second string. In Java, you use the  + operator to concatenate two
strings.
For example,
String fName = "Harry";
String lName = "Morgan";
String name = fName + lName;
results in the string
"HarryMorgan"
What if you’d like the first and last name separated by a space? No problem:
String name = fName + " " + lName;
This statement concatenates three strings:  fName , the string literal  " " , and  lName . The
result is
"Harry Morgan"
When the expression to the left or the right of a  + operator is a string, the other one
is automatically forced to become a string as well, and both strings are concatenated.
For example, consider this code:
String jobTitle = "Agent";
int employeeId = 7;
String bond = jobTitle + employeeId;
Because  jobTitle is a string,  employeeId is converted from the integer 7 to the string  "7" .
Then the two strings  "Agent" and  "7" are concatenated to form the string  "Agent7" .
This concatenation is very useful for reducing the number of  System.out.print
instructions. For exam ple, you can combine
System.out.print("The total is ");
System.out.println(total);
to the single call
System.out.println("The total is " + total);
The concatenation  "The total is " + total computes a single string that consists of the
string  "The total is " , followed by the string equivalent of the number  total .
4.5.3 string Input
You can read a string from the console:
System.out.print("Please enter your name: ");
String name = in.next();
When a string is read with the  next method, only one word is read. For example, sup-
pose the user types
Harry Morgan
as the response to the prompt. This input consists of two words. The call  in.next()
yields the string  "Harry" . You can use another call to  in.next() to read the second word.
use the  + operator to
concatenate strings;
that is, to put them
together to yield a
longer string.
Whenever one of
the arguments of the
+ operator is a string,
the other argument
is converted to
a string.
use the  next method
of the  Scanner class
to read a string con-
taining a single word.
158 Chapter 4 Fundamental data types
4.5.4 escape sequences
To include a quotation mark in a literal string, precede it with a backslash ( \ ), like this:
"He said \"Hello\""
The backslash is not included in the string. It indicates that the quotation mark that
follows should be a part of the string and not mark the end of the string. The sequence
\" is called an escape sequence.
To include a backslash in a string, use the escape sequence  \\ , like this:
"C:\\Temp\\Secret.txt"
Another common escape sequence is  \n , which denotes a newline character. Print-
ing a newline character causes the start of a new line on the display. For example, the
statement
System.out.print("*\n**\n***\n");
prints the characters
*
**
***
on three separate lines.
You often want to add a newline character to the end of the format string when
you use  System.out.printf :
System.out.printf("Price: %10.2f\n", price);
4.5.5 strings and Characters
Strings are sequences of Unicode characters (see Comput-
ing & Society 4.2). In Java, a character is a value of the
type  char . Characters have numeric values. You can find
the values of the characters that are used in Western Euro-
pean languages in Appendix A. For example, if you look
up the value for the charac ter  'H' , you can see that it is actu-
ally encoded as the number 72.
Character literals are delimited by single quotes, and you should not con fuse them
with strings.
•  'H' is a character, a value of type  char .
•  "H" is a string containing a single character, a value of type  String .
The  charAt method returns a  char value from a string. The first string position is
labeled 0, the second one 1, and so on.
0 1 2 3 4
H a r r y
The position number of the last character (4 for the string  "Harry" ) is always one less
than the length of the string.
© slpix/iStockphoto.
A string is a sequence of
characters.
string positions are
counted starting
with 0.
4.5 strings 159
For example, the statement
String name = "Harry";
char start = name.charAt(0);
char last = name.charAt(4);
sets  start to the value  'H' and  last to the value  'y' .
4.5.6 substrings
Once you have a string, you can extract substrings by using the  substring method.
The method call
str.substring(start, pastEnd)
returns a string that is made up of the characters in the string  str , starting at posi-
tion  start , and containing all characters up to, but not including, the position  pastEnd .
Here is an example:
String greeting = "Hello, World!";
String sub = greeting.substring(0, 5); // sub  is "Hello"
Here the  substring operation makes a string that consists of the first five characters
taken from the string  greeting .
0 1 2 3 4 5 6 7 8 9 10 11 12
H e l l o , W o r l d !
Let’s figure out how to extract the substring  "World" . Count characters starting at 0,
not 1. You find that  W has position number 7. The first character that you don’t want,
! , is the character at position 12. There fore, the appropriate substring command is
String sub2 = greeting.substring(7, 12);
0 1 2 3 4 5 6 7 8 9 10 11 12
H e l l o , W o r l d !
5
It is curious that you must specify the position of the first character that you do want
and then the first character that you don’t want. There is one advantage to this setup.
You can easily compute the length of the substring: It is  pastEnd - start . For example,
the string  "World" has length 12 – 7 = 5.
If you omit the end position when calling the  substring method, then all characters
from the starting position to the end of the string are copied. For example,
String tail = greeting.substring(7); //  Copies all characters from position 7 on
sets  tail to the string  "World!" .
Following is a simple program that puts these concepts to work. The program asks
for your name and that of your significant other. It then prints out your initials.
use the  substring
method to extract a
part of a string.
160 Chapter 4 Fundamental data types
The operation  first.substring(0, 1) makes
a string consisting of one character, taken from
the start of  first . The program does the same
for the  second . Then it concatenates the result-
ing one-character strings with the string literal
"&" to get a string of length 3, the  initials
string. (See Figure 3.)
section_5/initials.java
1 import java.util.Scanner;
2
3 /**
4 This program prints a pair of initials.
5 */
6 public class Initials
7 {
8 public static void main(String[] args)
9 {
10 Scanner in = new Scanner(System.in);
11
12 //  Get the names of the couple
13
14 System.out.print("Enter your first name: ");
15 String first = in.next();
16 System.out.print("Enter your significant other's first name: ");
17 String second = in.next();
18
19 //  Compute and display the inscription
20
21 String initials = first.substring(0, 1)
22 + "&" + second.substring(0, 1);
23 System.out.println(initials);
24 }
25 }
program run
Enter your first name: Rodolfo
Enter your significant other's first name: Sally
R&S
© Rich Legg/iStockphoto. Initials are formed from the first
letter of each name.
Figure 3  Building the  initials string
0 1 2
R & S initials =
0 1 2 3 4
S a l l y second =
0 1 2 3 4 5
R o d o l f
6
o first =
4.5 strings 161
table 7 string operations
statement result Comment
string str = "Ja";
str = str + "va";
str is set to  "Java" When applied to strings,  + denotes
concatenation.
System.out.println("Please"
+ " enter your name: ");
Prints
Please enter your name:
Use concatenation to break up strings
that don’t fit into one line.
team = 49 + "ers" team is set to  "49ers" Because  "ers" is a string, 49 is converted
to a string.
String first = in.next();
String last = in.next();
(User input: Harry Morgan)
first contains  "Harry"
last contains  "Morgan"
The  next method places the next word
into the string variable.
String greeting = "H & S";
int n = greeting.length();
n is set to 5 Each space counts as one character.
String str = "Sally";
char ch = str.charAt(1);
ch is set to  'a' This is a  char value, not a  String . Note
that the initial position is 0.
String str = "Sally";
String str2 = str.substring(1, 4);
str2 is set to  "all" Extracts the substring starting at
position 1 and ending before position 4.
String str = "Sally";
String str2 = str.substring(1);
str2 is set to  "ally" If you omit the end position, all
characters from the position until the
end of the string are included.
String str = "Sally";
String str2 = str.substring(1, 2);
str2 is set to  "a" Extracts a  String of length 1; contrast
with  str.charAt(1) .
String last = str.substring(
str.length() - 1);
last is set to the string
containing the last
character in  str
The last character has position
str.length() - 1 .
22.  What is the length of the string "Java Program" ?
23.  Consider this string variable.
String str = "Java Program";
Give a call to the  substring method that returns the substring  "gram" .
24.  Use string concatenation to turn the string variable  str from Self Check 23 into
"Java Programming" .
25.  What does the following statement sequence print?
String str = "Harry";
int n = str.length();
String mystery = str.substring(0, 1) + str.substring(n - 1, n);
System.out.println(mystery);
26.  Give an input statement to read a name of the form “John Q. Public”.
practice it  Now you can try these exercises at the end of the chapter: R4.8, R4.12, E4.14, P4.6.
© Nicholas Homrich/iStockphoto.
s e l F  c h e c k
162 Chapter 4 Fundamental data types
reading exception reports
You will often have programs that terminate and display an error message, such as
Exception in thread “main” java.lang.StringIndexOutOfBoundsException:
String index out of range: -4
at java.lang.String.substring(String.java:1444)
at Homework1.main(Homework1.java:16)
If this happens to you, don’t say “it didn’t work,” or “my program died.” Instead, read the
error message. Admittedly, the format of the exception report is not very friendly. But it is
actu ally easy to decipher it.
When you have a close look at the error message, you will notice two pieces of useful infor-
mation:
1.  The name of the exception, such as  StringIndexOutOfBoundsException
2.  The line number of the code that contained the statement that caused the exception,
such as  Homework1.java:16
The name of the exception is always in the first line of the report, and it ends in  Exception . If
you get a  StringIndex OutOfBoundsException , then there was a problem with accessing an invalid
position in a string. That is useful informa tion.
The line number of the offending code is a little harder to determine. The exception report
contains the entire stack trace—that is, the names of all methods that were pending when
the exception hit. The first line of the stack trace is the method that actually generated the
exception. The last line of the stack trace is a line in  main . Often, the exception was thrown by
a method that is in the standard library. Look for the first line in your code that appears in the
exception report. For example, skip the line that refers to
java.lang.String.substring(String.java:1444)
The next line in our example mentions a line number in your code,  Homework1.java . Once you
have the line number in your code, open up the file, go to that line, and look at it! Also look
at the name of the exception. In most cases, these two pieces of information will make it com-
pletely obvious what went wrong, and you can easily fix your error.
using dialog boxes for input and Output
Most program users find the console window rather old-fashioned. The easiest alternative is
to create a separate pop-up window for each input.
An Input Dialog Box
Call the static  showInputDialog method of the  JOptionPane class, and supply the string that
prompts the input from the user. For example,
String input = JOptionPane.showInputDialog("Enter price:");
That method returns a  String object. Of course, often you need the input as a number. Use the
Integer.parseInt and  Double.parseDouble methods to convert the string to a number:
double price = Double.parseDouble(input);
programming tip 4.3
© Eric Isselé/iStockphoto.
special topic 4.4
© Eric Isselé/iStockphoto.
Full cOde exAmple
Go to  wiley.com/go/
javacode to download
a complete program
that uses option
panes for input
and output.
© pvachier/iStockphoto.
Chapter summary 163
You can also display output in a dialog box:
JOptionPane.showMessageDialog(null, "Price: " + price);
choose appropriate types for representing numeric data.
• Java has eight primitive types, including four integer types and two
floating-point types.
• A numeric computation overflows if the result falls outside the
range for the number type.
• Rounding errors occur when an exact conversion between
numbers is not possible.
• A  final variable is a constant. Once its value has been set, it cannot
be changed.
• Use named constants to make your programs easier to read and maintain.
the  english  alpha-
bet is pretty simple:
upper- and lowercase a to z. other
european languages have accent marks
and special characters. For example,
German has three so-called umlaut
characters, ä, ö, ü, and a double-s char-
acter ß. these are not optional frills;
you couldn’t write a page of German
text without using these characters
a few times. German keyboards have
keys for these characters.
The German Keyboard Layout
many countries don’t use the roman
script at all. russian, Greek, hebrew,
arabic, and thai letters, to name just a
few, have completely diff erent shapes.
to complicate matters, hebrew and
arabic are typed from right to left. each
of these alphabets has about as many
characters as the english alphabet.
© jcarillet/iStockphoto.
Hebrew, Arabic, and English
the Chi nese languages as well as
Japanese and korean use Chinese char-
acters. each character represents an
idea or thing. Words are made up of
one or more of these ideo graphic char-
acters. over 70,000 ideo graphs are
known.
starting in 1987, a consortium of
hardware and software manufactur-
ers developed a uniform encoding
scheme called unicode that is capable
of encoding text in essentially all writ-
ten languages of the world. an early
version of unicode used 16 bits for
each character. the Java  char type cor-
responds to that encoding.
today unicode has grown to a
21-bit code, with definitions for over
100,000 characters ( www.unicode.org ).
there are even plans to add codes
for extinct languages, such as egyp-
tian hieroglyphics. unfortunately, that
means that a Java  char value does not
always correspond to a unicode char-
acter. some characters in languages
such as Chinese or ancient egyptian
occupy two char values.
© Saipg/iStockphoto.
The Chinese Script
Computing & Society 4.2  International alphabets and unicode
© MediaBakery.
C h a p t e r s u m m a ry
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164 Chapter 4 Fundamental data types
Write arithmetic expressions in Java.
• Mixing integers and floating-point values in an arithmetic expression yields a
floating-point value.
• The  ++ operator adds 1 to a variable; the  -- operator subtracts 1.
• If both arguments of  / are integers, the remainder is discarded.
• The  % operator computes the remainder of an integer division.
• The Java library declares many mathematical functions, such as  Math.sqrt (square
root) and  Math.pow (raising to a power).
• You use a cast  ( typeName ) to convert a value to a different type.
Write programs that read user input and print formatted output.
• Use the  Scanner class to read keyboard input in a console window.
• Use the  printf method to specify how values should be
formatted.
carry out hand calculations when developing an algorithm.
• Pick concrete values for a typical situation to use in a hand calculation.
Write programs that process strings.
• Strings are sequences of characters.
• The  length method yields the number of characters in
a string.
• Use the  + operator to concatenate strings; that is, to put them together to yield a
longer string.
• Whenever one of the arguments of the  + operator is a string, the other argument is
converted to a string.
• Use the  next method of the  Scanner class to read a string containing
a single word.
• String positions are counted starting with 0.
• Use the  substring method to extract a part of a string.
© Michael Flippo/iStockphoto.
© Media Bakery. © Koele/iStockphoto.
© slpix/iStockphoto.
© essxboy/iStockphoto.
© Rich Legg/iStockphoto.
java.io.PrintStream
printf
java.lang.Double
parseDouble
java.lang.Integer
MAX_VALUE
MIN_VALUE
parseInt
java.lang.Math
PI
abs
cos
exp
log
log10
max
min
pow
round
sin
sqrt
tan
toDegrees
toRadians
java.lang.String
charAt
length
substring
java.lang.System
in
java.math.BigDecimal
add
multiply
subtract
java.math.BigInteger
add
multiply
subtract
java.util.Scanner
next
nextDouble
nextInt
javax.swing.JOptionPane
showInputDialog
showMessageDialog
s ta n d a r d l I B r a ry I t e m s I n t r o d u C e d I n t h I s C h ap t e r
review Questions 165
• r4.1  Write declarations for storing the following quantities. Choose between integers and
floating-point numbers. Declare constants when appropriate.
a.  The number of days per week
b.  The number of days until the end of the semester
c.  The number of centimeters in an inch
d.  The height of the tallest person in your class, in centimeters
• r4.2  What is the value of  mystery after this sequence of statements?
int mystery = 1;
mystery = 1 - 2 * mystery;
mystery = mystery + 1;
• r4.3  What is wrong with the following sequence of statements?
int mystery = 1;
mystery = mystery + 1;
int mystery = 1 - 2 * mystery;
•• r4.4  Write the following Java expressions in mathematical notation.
a.  dm = m * (Math.sqrt(1 + v / c) / Math.sqrt(1 - v / c) - 1);
b.  volume = Math.PI * r * r * h;
c.  volume = 4 * Math.PI * Math.pow(r, 3) / 3;
d.  z = Math.sqrt(x * x + y * y);
•• r4.5  Write the following mathematical expressions in Java.
s s v t gt
G
a
p m m
= + +
=
+
= ⋅ +
0 0
2
2
3
2
1 2
1
2
4
1
π
( )
FV PV
INT
100 0
YRS






= + − c a b ab
2 2
2 cos γ
•• r4.6  What are the values of the following expressions? In each line, assume that
double x = 2.5;
double y = -1.5;
int m = 18;
int n = 4;
a.  x + n * y - (x + n) * y
b.  m / n + m % n
c.  5 * x - n / 5
d.  1 - (1 - (1 - (1 - (1 - n))))
e.  Math.sqrt(Math.sqrt(n))
r e V I e W Q u e s t I o n s
166 Chapter 4 Fundamental data types
• r4.7  What are the values of the following expressions, assuming that  n is 17 and  m is 18?
a.  n / 10 + n % 10
b.  n % 2 + m % 2
c.  (m + n) / 2
d.  (m + n) / 2.0
e.  (int) (0.5 * (m + n))
f.  (int) Math.round(0.5 * (m + n))
•• r4.8  What are the values of the following expressions? In each line, assume that
String s = "Hello";
String t = "World";
a.  s.length() + t.length()
b.  s.substring(1, 2)
c.  s.substring(s.length() / 2, s.length())
d.  s + t
e.  t + s
• r4.9  Find at least five compile-time errors in the following program.
public class HasErrors
{
public static void main();
{
System.out.print(Please enter two numbers:)
x = in.readDouble;
y = in.readDouble;
System.out.printline("The sum is " + x + y);
}
}
•• r4.10  Find three run-time errors in the following program.
public class HasErrors
{
public static void main(String[] args)
{
int x = 0;
int y = 0;
Scanner in = new Scanner("System.in");
System.out.print("Please enter an integer:");
x = in.readInt();
System.out.print("Please enter another integer: ");
x = in.readInt();
System.out.println("The sum is " + x + y);
}
}
•• r4.11  Consider the following code:
CashRegister register = new CashRegister();
register.recordPurchase(19.93);
register.receivePayment(20, 0, 0, 0, 0);
System.out.print("Change: ");
System.out.println(register.giveChange());
The code segment prints the total as  0.07000000000000028 . Explain why. Give a recom-
mendation to improve the code so that users will not be confused.
review Questions 167
• r4.12  Explain the differences between  2 ,  2.0 ,  '2' ,  "2" , and  "2.0" .
• r4.13  Explain what each of the following program segments computes.
a.  x = 2;
y = x + x;
b.  s = "2";
t = s + s;
•• r4.14  Write pseudocode for a program that reads a word and then prints the first charac ter,
the last character, and the characters in the middle. For example, if the input is  Harry ,
the program prints  H y arr .
•• r4.15  Write pseudocode for a program that reads a name (such as  Harold James Morgan ) and
then prints a monogram consisting of the initial letters of the first, middle, and last
name (such as  HJM ).
••• r4.16  Write pseudocode for a program that computes the first and last digit of a num-
ber. For example, if the input is  23456 , the program should print  2 and  6 . Hint:  % ,
Math.log10 .
• r4.17  Modify the pseudocode for the program in How To 4.1 so that the pro gram gives
change in quarters, dimes, and nickels. You can assume that the price is a multiple of
5 cents. To develop your pseudocode, first work with a couple of spe cific values.
•• r4.18  A cocktail shaker is composed of three cone sections.
Using realistic values for the radii and heights, compute the total volume, using the
formula given in Self Check 21 for a cone section. Then develop an algorithm that
works for arbitrary dimensions.
••• r4.19  You are cutting off a piece of pie like this, where c is the length of the straight part
(called the chord length) and h is the height of the piece.
There is an approximate formula for the area:
A ch
h
c
≈ +
2
3
3
2
However, h is not so easy to measure, whereas the
diameter d of a pie is usually well-known. Calculate
the area where the diameter of the pie is 12 inches and
the chord length of the segment is 10 inches. Gen-
eralize to an algorithm that yields the area for any
diameter and chord length.
•• r4.20  The following pseudocode describes how to obtain the name of a day, given the day
number (0 = Sunday, 1 = Monday, and so on.)
Declare a string called names containing "SunMonTueWedThuFriSat".
Compute the starting position as 3 x the day number.
Extract the substring of names at the starting position with length 3.
Check this pseudocode, using the day number 4. Draw a diagram of the string that is
being computed, similar to Figure 3.
••• r4.21  The following pseudocode describes how to swap two letters in a word.
We are given a string str and two positions i and j. (i comes before j)
Set first to the substring from the start of the string to the last position before i.
© Media Bakery.
h
c
d
168 Chapter 4 Fundamental data types
Set middle to the substring from positions i + 1 to j - 1.
Set last to the substring from position j + 1 to the end of the string.
Concatenate the following five strings: first, the string containing just the character at position j,
middle, the string containing just the character at position i, and last.
Check this pseudocode, using the string  "Gateway" and positions 2 and 4. Draw a
diagram of the string that is being computed, similar to Figure 3.
•• r4.22  How do you get the first character of a string? The last character? How do you
remove the first character? The last character?
••• r4.23  Write a program that prints the values
3 * 1000 * 1000 * 1000
3.0 * 1000 * 1000 * 1000
Explain the results.
• e4.1  Write a program that displays the dimensions of a letter-size (8.5 × 11 inches) sheet
of paper in millimeters. There are 25.4 millimeters per inch. Use con stants and com-
ments in your program.
• e4.2  Write a program that computes and displays the perimeter of a letter-size (8.5 × 11
inches) sheet of paper and the length of its diagonal.
• e4.3  Write a program that reads a number and displays the square, cube, and fourth
power. Use the  Math.pow method only for the fourth power.
•• e4.4  Write a program that prompts the user for two integers and then prints
• The sum
• The difference
• The product
• The average
• The distance (absolute value of the difference)
• The maximum (the larger of the two)
• The minimum (the smaller of the two)
Hint: The  max and  min functions are declared in the  Math class.
•• e4.5  Enhance the output of Exercise E4.4 so that the numbers are properly aligned:
Sum: 45
Difference: -5
Product: 500
Average: 22.50
Distance: 5
Maximum: 25
Minimum: 20
•• e4.6  Write a program that prompts the user for a measurement in meters and then con-
verts it to miles, feet, and inches.
p r a C t I C e e x e r C I s e s
practice exercises 169
• e4.7  Write a program that prompts the user for a radius and then prints
• The area and circumference of a circle with that radius
• The volume and surface area of a sphere with that radius
•• e4.8  Write a program that asks the user for the lengths of a rectangle’s sides. Then print
• The area and perimeter of the rectangle
• The length of the diagonal (use the Pythagorean theorem)
• e4.9  Improve the program discussed in How To 4.1 to allow input of quar ters in addition
to bills.
•• e4.10  Write a program that asks the user to input
• The number of gallons of gas in the tank
• The fuel efficiency in miles per gallon
• The price of gas per gallon
Then print the cost per 100 miles and how far the car can go with the gas in the tank.
• e4.11  File names and extensions. Write a program that prompts the user for the drive letter
( C ), the path ( \Windows\System ), the file name ( Readme ), and the extension ( txt ). Then
print the complete file name  C:\Windows\System\Readme.txt . (If you use UNIX or a
Macintosh, skip the drive name and use  / instead of \ to separate directories.)
••• e4.12  Write a program that reads a number between 1,000 and 999,999 from the user, where
the user enters a comma in the input. Then print the number without a comma.
Here is a sample dialog; the user input is in color:
Please enter an integer between 1,000 and 999,999: 23,456
23456
Hint: Read the input as a string. Measure the length of the string. Suppose it contains
n characters. Then extract substrings consisting of the first n – 4 characters and the
last three characters.
•• e4.13  Write a program that reads a number between 1,000 and 999,999 from the user and
prints it with a comma separating the thousands. Here is a sample dialog; the user
input is in color:
Please enter an integer between 1000 and 999999: 23456
23,456
• e4.14  Printing a grid. Write a program that prints the following grid to play tic-tac-toe.
+--+--+--+
| | | |
+--+--+--+
| | | |
+--+--+--+
| | | |
+--+--+--+
Of course, you could simply write seven statements of the form
System.out.println("+--+--+--+");
You should do it the smart way, though. Declare string variables to hold two kinds
of patterns: a comb-shaped pattern and the bottom line. Print the comb three times
and the bottom line once.
170 Chapter 4  Fundamental Data Types
•• E4.15  Write a program that reads in an integer and breaks it into a sequence of individual
digits. For example, the input 16384 is displayed as
1 6 3 8 4
You may assume that the input has no more than five digits and is not negative.
•• E4.16  Write a program that reads two times in military format (0900, 1730) and prints the
number of hours and minutes between the two times. Here is a sample run. User
input is in color.
Please enter the first time: 0900
Please enter the second time: 1730
8 hours 30 minutes
Extra credit if you can deal with the case where the first time is later than the second:
Please enter the first time: 1730
Please enter the second time: 0900
15 hours 30 minutes
••• E4.17  Writing large letters. A large letter H can be produced like this:
* *
* *
*****
* *
* *
It can be declared as a string literal like this:
final string LETTER_H = "* *\n* *\n*****\n* *\n* *\n";
(The  \n escape sequence denotes a “newline” character that causes subsequent
characters to be printed on a new line.) Do the same for the letters  E ,  L , and  O . Then
write the message
H
E
L
L
O
in large letters.
•• E4.18  Write a program that transforms numbers  1 ,  2 ,  3 , …,  12
into the corresponding month names  January ,  February ,
March , …,  December . Hint: Make a very long string  "January
February March ..." , in which you add spaces such that
each month name has the same length. Then use  sub string
to extract the month you want.
•• E4.19  Write a program that prints a Christmas tree:
/\
/ \
/ \
/ \
--------
" "
" "
" "
Remember to use escape sequences.
© José Luis Gutiérrez/iStockphoto.
programming projects 171
e4.20  Enhance the  CashRegister class by adding separate methods  enterDollars ,  enter-
Quarters ,  enterDimes ,  enterNickels , and  enterPennies .
Use this tester class:
public class CashRegisterTester
{
public static void main (String[] args)
{
CashRegister register = new CashRegister();
register.recordPurchase(20.37);
register.enterDollars(20);
register.enterQuarters(2);
System.out.println("Change: " + register.giveChange());
System.out.println("Expected: 0.13");
}
}
•• e4.21  Implement a class  IceCreamCone with methods  getSurfaceArea() and  getVolume() . In the
constructor, supply the height and radius of the cone. Be careful when looking up
the formula for the surface area—you should only include the outside area along the
side of the cone since the cone has an opening on the top to hold the ice cream.
•• e4.22  Implement a class  SodaCan whose constructor receives the height and diameter of the
soda can. Supply methods  getVolume and  getSurfaceArea . Supply a  SodaCanTester class
that tests your class.
••• e4.23  Implement a class  Balloon that models a spherical balloon that is being filled with air.
The constructor constructs an empty balloon. Supply these methods:
•  void addAir(double amount) adds the given amount of air
•  double getVolume() gets the current volume
•  double getSurfaceArea() gets the current surface area
•  double getRadius() gets the current radius
Supply a  BalloonTester class that constructs a balloon, adds 100 cm 3 of air, tests the
three accessor meth ods, adds another 100 cm 3 of air, and tests the accessor methods
again.
••• p4.1  Write a program that helps a person decide
whether to buy a hybrid car. Your pro gram’s
inputs should be:
• The cost of a new car
• The estimated miles driven per year
• The estimated gas price
• The efficiency in miles per gallon
• The estimated resale value after 5 years
Compute the total cost of owning the car for
five years. (For simplic ity, we will not take the cost of financing into account.)
© asiseeit/iStockphoto.
p r o G r a m m I n G p r o J e C t s
172 Chapter 4 Fundamental data types
Obtain realistic prices for a new and used hybrid and a com parable car from the
Web. Run your program twice, using today’s gas price and 15,000 miles per year.
Include pseudocode and the program runs with your assignment.
•• p4.2  Easter Sunday is the first Sun day after the first full moon of spring. To compute
the date, you can use this algorithm, invented by the mathe matician Carl Friedrich
Gauss in 1800:
1.  Let  y be the year (such as 1800 or 2001).
2.  Divide  y by  19 and call the remainder  a . Ignore the quotient.
3.  Divide  y by  100 to get a quotient  b and a remainder  c .
4.  Divide  b by  4 to get a quotient  d and a remainder  e .
5.  Divide  8 * b + 13 by  25 to get a quotient  g . Ignore the remainder.
6.  Divide  19 * a + b - d - g + 15 by  30 to get a remainder  h . Ignore the quotient.
7.  Divide  c by  4 to get a quotient  j and a remainder  k .
8.  Divide  a + 11 * h by  319 to get a quotient  m . Ignore the remainder.
9.  Divide  2 * e + 2 * j - k - h + m + 32 by  7 to get a remainder  r . Ignore the
quotient.
10.  Divide  h - m + r + 90 by  25 to get a quotient  n . Ignore the remainder.
11.  Divide  h - m + r + n + 19 by  32 to get a remainder  p . Ignore the quotient.
Then Easter falls on day  p of month  n . For example, if  y is  2001 :
a = 6  h = 18  n = 4
b = 20, c = 1  j = 0, k = 1  p = 15
d = 5, e = 0  m = 0
g = 6  r = 6
Therefore, in 2001, Easter Sun day fell on April 15. Write a program that prompts the
user for a year and prints out the month and day of Easter Sunday.
••• p4.3  In this project, you will perform calculations with triangles. A triangle is defined by
the x- and y-coordinates of its three corner points.
Your job is to compute the following properties of a given triangle:
• the lengths of all sides
• the angles at all corners
• the perimeter
• the area
Implement a  Triangle class with appropriate methods. Supply a program that
prompts a user for the corner point coordinates and produces a nicely formatted
table of the triangle properties.
••• p4.4  The  CashRegister class has an unfortunate limitation: It is closely tied to the coin sys-
tem in the United States and Canada. Research the system used in most of Europe.
Your goal is to produce a cash register that works with euros and cents. Rather than
designing another limited  CashRegister implementation for the European market,
you should design a separate  Coin class and a cash register that can work with coins of
all types.
•• business p4.5  The following pseudocode describes how a bookstore computes the price of an
order from the total price and the number of the books that were ordered.
programming projects 173
Read the total book price and the number of books.
Compute the tax (7.5 percent of the total book price).
Compute the shipping charge ($2 per book).
The price of the order is the sum of the total book price, the tax, and the shipping charge.
Print the price of the order.
Translate this pseudocode into a Java program.
•• business p4.6  The following pseudocode describes how to turn a string containing a ten-digit
phone number (such as  "4155551212" ) into a more readable string with parentheses
and dashes, like this:  "(415) 555-1212" .
Take the substring consisting of the first three characters and surround it with "(" and ") ". This is the
area code.
Concatenate the area code, the substring consisting of the next three characters, a hyphen, and the
substring consisting of the last four characters. This is the formatted number.
Translate this pseudocode into a Java program that reads a telephone number into a
string variable, com putes the formatted number, and prints it.
•• business p4.7  The following pseudocode describes how to extract the dollars and cents from a
price given as a floating-point value. For example, a price 2.95 yields values 2 and 95
for the dollars and cents.
Assign the price to an integer variable dollars.
Multiply the difference price - dollars by 100 and add 0.5.
Assign the result to an integer variable cents.
Translate this pseudocode into a Java program. Read a price and print the dollars and
cents. Test your program with inputs 2.95 and 4.35.
•• business p4.8  Giving change. Implement a program that directs a cashier
how to give change. The program has two inputs: the
amount due and the amount received from the customer.
Display the dollars, quarters, dimes, nickels, and pennies
that the customer should receive in return. In order to avoid
roundoff errors, the program user should supply both
amounts in pennies, for example 274 instead of 2.74.
• business p4.9  An online bank wants you to create a program that shows prospective customers
how their deposits will grow. Your program should read the initial balance and the
annual interest rate. Interest is compounded monthly. Print out the balances after the
first three months. Here is a sample run:
Initial balance: 1000
Annual interest rate in percent: 6.0
After first month: 1005.00
After second month: 1010.03
After third month: 1015.08
•• business p4.10  A video club wants to reward its best members with a discount based on the mem-
ber’s number of movie rentals and the number of new members referred by the
member. The discount is in percent and is equal to the sum of the rentals and the
referrals, but it cannot exceed 75 percent. (Hint:  Math.min .) Write a program  Discount-
Calculator to calculate the value of the discount.
© Captainflash/iStockphoto.
174 Chapter 4 Fundamental data types
Here is a sample run:
Enter the number of movie rentals: 56
Enter the number of members referred to the video club: 3
The discount is equal to: 59.00 percent.
• science p4.11  Consider the following circuit.
R 1
R 2 R 3
Write a program that reads the resistances of the three resistors and computes the
total resistance, using Ohm’s law.
•• science p4.12  The dew point temperature T d can be calculated (approximately) from the relative
humidity RH and the actual temperature T by
T
b f T RH
a f T RH
f T RH
a T
b T
RH
d
=
⋅ ( )
− ( )
( ) =
⋅
+
+ ( )
,
,
, ln
where a = 17.27 and b = 237.7° C.
Write a program that reads the relative humidity (between 0 and 1) and the tempera-
ture (in degrees C) and prints the dew point value. Use the Java function  log to
compute the natural logarithm.
••• science p4.13  The pipe clip temperature sensors shown here are robust sensors that can be clipped
directly onto copper pipes to measure the temperature of the liquids in the pipes.
Each sensor contains a device called a thermistor. Thermistors are semiconductor
devices that exhibit a temperature-dependent resistance described by:
R R e
T T
=
−






0
1 1
0
β
where R is the resistance (in Ω) at the temperature T (in °K), and R 0 is the resistance
(in Ω) at the temperature T 0 (in °K).  β is a constant that depends on the material used
programming projects 175
to make the thermistor. Thermistors are specified by providing values for R 0 , T 0 ,
and  β .
The thermistors used to make the pipe clip temperature sensors have R 0 = 1075 Ω
at T 0 = 85 °C, and  β = 3969 °K. (Notice that  β has units of °K. Recall that the tem-
perature in °K is obtained by adding 273 to the temperature in °C.) The liquid
temperature, in °C, is determined from the resistance R, in Ω, using
T
T
T
R
R
=





 +
−
β
β
0
0
0
273
ln
Write a Java program that prompts the user for the thermistor resistance R and prints
a message giving the liquid temperature in °C.
••• science p4.24  The circuit shown below illustrates some important
aspects of the connection between a power company
and one of its customers. The customer is represented
by three parameters, V t , P, and pf. V t is the voltage
accessed by plugging into a wall outlet. Customers
depend on having a dependable value of V t in order
for their appliances to work properly. Accordingly,
the power company regulates the value of V t carefully.
P describes the amount of power used by the customer and is the primary factor in
determining the customer’s electric bill. The power factor, pf, is less familiar. (The
power factor is calculated as the cosine of an angle so that its value will always be
between zero and one.) In this problem you will be asked to write a Java program to
investigate the significance of the power factor.
V s
Customer
+
–
R = 10 Ω
Power
Lines
Power
Company
R = 10 Ω
P = 260 W
pf = 0.6
V t  = 120 Vrms
+
–
In the figure, the power lines are represented, somewhat simplistically, as resistances
in Ohms. The power company is represented as an AC voltage source. The source
voltage, V s , required to provide the customer with power P at voltage V t can be
determined using the formula
V V
RP
V
RP
pfV
pf
s t
t t
= +






+






−
( )
2 2
1
2 2
2
(V s has units of Vrms.) This formula indicates that the value of V s depends on the
value of pf. Write a Java program that prompts the user for a power factor value and
then prints a message giving the corresponding value of V s , using the values for P, R,
and V t shown in the figure above.
© TebNad/iStockphoto.
176 Chapter 4 Fundamental data types
••• science p4.25  Consider the following tuning circuit connected to an antenna, where C is a variable
capacitor whose capacitance ranges from C min to C max .
L C
Antenna
The tuning circuit selects the frequency f
LC
=
2 π
. To design this circuit for a given
frequency, take C C C =
min max
and calculate the required inductance L from f and
C. Now the circuit can be tuned to any frequency in the range f
LC
min
max
=
2 π
to
f
LC
max
min
=
2 π
.
Write a Java program to design a tuning circuit for a given frequency, using a variable
capacitor with given values for C min and C max . (A typical input is f = 16.7 MHz,
C min = 14 pF, and C max = 365 pF.) The program should read in f (in Hz), C min and
C max (in F), and print the required inductance value and the range of frequencies to
which the circuit can be tuned by varying the capacitance.
• science p4.26  According to the Coulomb force law, the electric force between two charged
particles of charge Q 1 and Q 2 Coulombs, that are a distance r meters apart, is
F
Q Q
r
=
1 2
2
4 π ε
Newtons, where  ε = ×
−
8 854 10
12
. Farads/meter. Write a program
that calculates the force on a pair of charged particles, based on the user input of
Q 1  Coulombs, Q 2 Coulombs, and r meters, and then computes and displays the
electric force.
answers to self-Check Questions 177
a n s W e rs t o s e l F - C h e C k Q u e s t I o n s
1.  int and  double .
2.  The world’s most populous country, China, has
about 1.2 x 10 9 inhabitants. There fore, individ-
ual population counts could be held in an  int .
However, the world pop ulation is over 6 × 10 9 .
If you compute totals or averages of multiple
countries, you can exceed the largest  int value.
Therefore,  double is a better choice. You could
also use  long , but there is no benefit because the
exact population of a country is not known at
any point in time.
3.  The first initialization is incorrect. The right
hand side is a value of type  double , and it is not
legal to initialize an  int variable with a  double
value. The second initializa tion is correct—an
int value can always be converted to a  double .
4.  The first declaration is used inside a method,
the second inside a class.
5.  Two things: You should use a named constant,
not the “magic number” 3.14, and 3.14 is not
an accurate representation of  π .
6.  double interest = balance * percent / 100;
7.  double sideLength = Math.sqrt(area);
8.  4 * PI * Math.pow(radius, 3) / 3
or  (4.0 / 3) * PI * Math.pow(radius, 3) ,
but not  (4 / 3) * PI * Math.pow(radius, 3)
9.  17 and 29
10.  It is the second-to-last digit of  n . For example,
if  n is  1729 , then  n / 10 is  172 , and  (n / 10) % 10
is 2 .
11.  System.out.print("How old are you? ");
int age = in.nextInt();
12.  There is no prompt that alerts the program
user to enter the quantity.
13.  The second statement calls  nextInt , not  next-
Double . If the user were to enter a price such as
1.95 , the program would be terminated with an
“input mismatch exception”.
14.  There is no colon and space at the end of the
prompt. A dialog would look like this:
Please enter the number of cans6
15.  The total volume is 10
There are four spaces between  is and  10 . One
space originates from the format string (the
space between  s and  % ), and three spaces are
added before  10 to achieve a field width of 5.
16.  Here is a simple solution:
System.out.printf("Bottles: %8d\n", bottles);
System.out.printf("Cans: %8d\n", cans);
Note the spaces after  Cans: . Alternatively,
you can use format specifiers for the strings.
You can even com bine all output into a single
statement:
System.out.printf("%-9s%8d\n%-9s%8d\n",
"Bottles: ", bottles, "Cans:", cans);
17.  int pairs = (totalWidth - tileWidth)
/ (2 * tileWidth);
int tiles = 1 + 2 * pairs;
double gap = (totalWidth -
tiles * tileWidth) / 2.0;
Be sure that  pairs is declared as an  int .
18.  Now there are groups of four tiles (gray/
white/gray/black) following the initial black
tile. Therefore, the algorithm is now
number of groups = integer part of (total width - tile width) /
(4 x tile width)
number of tiles = 1 + 4 x number of groups
The formula for the gap is not changed.
19.  The answer depends only on whether the row
and column numbers are even or odd, so let’s
first take the remainder after dividing by 2.
Then we can enumerate all expected answers:
Row  % 2 Column  % 2 Color
0 0 0
0  1  1
1  0  1
1  1  0
In the first three entries of the table, the color
is simply the sum of the remainders. In the
fourth entry, the sum would be 2, but we want
a zero. We can achieve that by taking another
remainder operation:
color = ((row  % 2) + (column  % 2))  % 2
20.  In nine years, the repair costs increased by
$1,400. Therefore, the increase per year is
$1,400 / 9 ≈ $156. The repair cost in year 3
would be $100 + 2 × $156 = $412. The repair
cost in year  n is $100 +  n × $156. To avoid
accumulation of roundoff errors, it is actually
178 Chapter 4 Fundamental data types
a good idea to use the original expression that
yielded $156, that is,
Repair cost in year n = 100 + n x 1400 / 9
21.  The pseudocode follows from the equations:
bottom volume =  π x r 1 2 x h 1
top volume =  π x r 2 2 x h 2
middle volume =  π x (r 1 2 + r 1 x r 2 + r 2 2 ) x h 3 / 3
total volume = bottom volume + top volume + middle volume
Measuring a typical wine bottle yields
r 1 = 3.6, r 2 = 1.2, h 1 = 15, h 2 = 7, h 3 = 6
(all in centimeters). Therefore,
bottom volume = 610.73
top volume = 31.67
middle volume = 135.72
total volume = 778.12
The actual volume is 750 ml, which is close
enough to our computation to give confidence
that it is cor rect.
22.  The length is 12. The space counts as a
character.
23.  str.substring(8, 12) or  str.substring(8)
24.  str = str + "ming";
25.  Hy
26.  String first = in.next();
String middle = in.next();
String last = in.next();
5
C h a p t e r
179
DeCisions
to implement decisions using
if statements
to compare integers, floating-point numbers, and strings
to write statements using the Boolean data type
to develop strategies for testing your programs
to validate user input
C h a p t e r G o a l s
C h a p t e r C o n t e n t s
5.1 The if STaTemenT  180
Syntax 5.1:  if Statement 182
Programming Tip 5.1: Brace layout 184
Programming Tip 5.2: always Use Braces 184
Common Error 5.1: a semicolon after the
if Condition 184
Programming Tip 5.3: tabs 185
Special Topic 5.1: the Conditional operator 185
Programming Tip 5.4: avoid
Duplication in Branches 186
5.2 Comparing ValueS 186
Syntax 5.2: Comparisons 187
Common Error 5.2: Using == to Compare
strings 192
How To 5.1: implementing an if statement 193
Worked Example 5.1: extracting the Middle
Computing & Society 5.1: Denver’s luggage
handling system 195
5.3 mulTiple alTernaTiVeS 196
Special Topic 5.2: the switch statement 199
5.4 neSTed BranCheS 200
Programming Tip 5.5: hand-tracing 203
Common Error 5.3: the Dangling else problem 204
Special Topic 5.3: Block scope 205
Special Topic 5.4: enumeration types 206
5.5 proBlem SolVing: flowCharTS 207
5.6 proBlem SolVing: SeleCTing
TeST CaSeS 210
Programming Tip 5.6: Make a schedule and Make
time for Unexpected problems 212
Special Topic 5.5: logging 212
5.7 Boolean VariaBleS
and operaTorS 213
Common Error 5.4: Combining Multiple
relational operators 216
Common Error 5.5: Confusing && and ||
Conditions 216
Special Topic 5.6: short-Circuit evaluation of
Boolean operators 217
Special Topic 5.7: De Morgan’s law 217
5.8 appliCaTion: inpuT ValidaTion 218
Computing & Society 5.2: artificial
intelligence 221
© zennie/iStockphoto.
180
one of the essential features of computer programs is
their ability to make decisions. like a train that changes
tracks depending on how the switches are set, a program
can take different actions depending on inputs and other
circumstances.
in this chapter, you will learn how to program simple and
complex decisions. You will apply what you learn to the
task of checking user input.
5.1 the  if statement
The  if statement is used to implement a decision (see Syntax 5.1). When a condition is
fulfilled, one set of statements is executed. Otherwise, another set of statements is
executed.
Here is an example using the  if statement: In
many countries, the number 13 is considered
unlucky. Rather than offending superstitious ten
ants, building owners sometimes skip the thir
teenth floor; floor 12 is immediately followed by
floor 14. Of course, floor 13 is not usually left
empty or, as some conspiracy theorists believe,
filled with secret offices and research labs. It is
simply called floor 14. The computer that controls
the building elevators needs to compensate for
this foible and adjust all floor numbers above 13.
Let’s simulate this process in Java. We will ask
the user to type in the desired floor number and
then compute the actual floor. When the input is
above 13, then we need to decrement the input to
obtain the actual floor. For example, if the user
provides an input of 20, the program determines
the actual floor to be 19. Otherwise, it simply uses
the supplied floor number.
int actualFloor;
if (floor > 13)
{
actualFloor = floor - 1;
}
else
{
actualFloor = floor;
}
The flowchart in Figure 1 shows the branching behavior.
In our example, each branch of the  if statement contains a single statement. You
can include as many statements in each branch as you like. Sometimes, it happens that
the  if statement
allows a program to
carry out different
actions depending on
the nature of the data
to be processed.
© DrGrounds/iStockphoto.
This elevator panel “skips” the
thirteenth floor. The floor is not
actually missing—the computer
that controls the elevator adjusts
the floor numbers above 13.
photo.
5.1 the if statement 181
figure 1 
Flowchart for  if statement
floor > 13 ?
True False
actualFloor =
floor - 1
actualFloor =
floor
Condition
figure 2 
Flowchart for  if statement with no  else Branch
floor > 13 ?
True False
actualFloor--
No  else branch
there is nothing to do in the  else branch of the statement. In that case, you can omit it
entirely, such as in this example:
int actualFloor = floor;
if (floor > 13)
{
actualFloor--;
} //  No  else needed
See Figure 2 for the flowchart.
An  if statement is like a fork in
the road. Depending upon a
decision, different parts of the
program are executed.
© Media Bakery.
182 Chapter 5  Decisions
Syntax 5.1  if  Statement
Don’t put a semicolon here!
See page 184.
Lining up braces
is a good idea.
See page 184.
if (floor > 13)
{
actualFloor = floor - 1;
}
else
{
actualFloor = floor;
}
A condition that is true or false.
Often uses relational operators:
== != < <= > >= (See page 187.)
If the condition is true, the statement(s)
in this branch are executed in sequence;
if the condition is false, they are skipped.
Braces are not required
if the branch contains a
single statement, but it's
good to always use them.
See page 184.
If the condition is false, the statement(s)
in this branch are executed in sequence;
if the condition is true, they are skipped.
Omit the  else branch
if there is nothing to do.
if ( condition )
{
statements
}
if ( condition ) {  statements 1 }
else {  statements 2 }
Syntax
The following program puts the  if statement to work. This program asks for the
desired floor and then prints out the actual floor.
section_1/ElevatorSimulation.java
1 import java.util.Scanner;
2
3 /**
4 This program simulates an elevator panel that skips the 13th floor.
5 */
6 public class ElevatorSimulation
7 {
8 public static void main(String[] args)
9 {
10 Scanner in = new Scanner(System.in);
11 System.out.print("Floor: ");
12 int floor = in.nextInt();
13
14 //  Adjust floor if necessary
15
16 int actualFloor;
17 if (floor > 13)
18 {
19 actualFloor = floor - 1;
20 }
21 else
22 {
5.1 the if statement 183
23 actualFloor = floor;
24 }
25
26 System.out.println("The elevator will travel to the actual floor "
27 + actualFloor);
28 }
29 }
program run
Floor:  20
The elevator will travel to the actual floor 19
1.  In some Asian countries, the number 14 is considered unlucky. Some building
owners play it safe and skip both the thirteenth and the fourteenth floor. How
would you modify the sample program to handle such a building?
2.  Consider the following  if statement to compute a discounted price:
if (originalPrice > 100)
{
discountedPrice = originalPrice - 20;
}
else
{
discountedPrice = originalPrice - 10;
}
What is the discounted price if the original price is 95? 100? 105?
3.  Compare this  if statement with the one in Self Check 2:
if (originalPrice < 100)
{
discountedPrice = originalPrice - 10;
}
else
{
discountedPrice = originalPrice - 20;
}
Do the two statements always compute the same value? If not, when do the
values differ?
4.  Consider the following statements to compute a discounted price:
discountedPrice = originalPrice;
if (originalPrice > 100)
{
discountedPrice = originalPrice - 10;
}
What is the discounted price if the original price is 95? 100? 105?
5.  The variables  fuelAmount and  fuelCapacity hold the actual amount of fuel and the
size of the fuel tank of a vehicle. If less than 10 percent is remaining in the tank, a
status light should show a red color; otherwise it shows a green color. Simu late
this process by printing out either  "red" or  "green" .
practice it  Now you can try these exercises at the end of the chapter: R5.5, R5.6, E5.9.
© Nicholas Homrich/iStockphoto.
S e l f  C h e C k
184 Chapter 5 Decisions
Brace layout
The compiler doesn’t care where you place
braces. In this book, we follow the simple rule
of making  { and  } line up.
if (floor > 13)
{
floor--;
}
This style makes it easy to spot matching
braces. Some programmers put the opening
brace on the same line as the  if :
if (floor > 13) {
floor--;
}
This style makes it harder to match the braces, but it saves a line of code, allowing you to view
more code on the screen with out scrolling. There are passionate a dvocates of both styles.
It is important that you pick a layout style and stick with it consistently within a given
programming project. Which style you choose may depend on your personal prefer ence or a
coding style guide that you need to follow.
always use Braces
When the body of an  if statement consists of a single statement, you need not use braces. For
example, the following is legal:
if (floor > 13)
floor--;
However, it is a good idea to always include the braces:
if (floor > 13)
{
floor--;
}
The braces make your code easier to read. They also make it easier for you to maintain the
code because you won’t have to worry about adding braces when you add statements inside
an  if statement.
a Semicolon after the  if  Condition
The following code fragment has an unfortunate error:
if (floor > 13) ; //  ERROR
{
floor--;
}
There should be no semicolon after the  if condition. The compiler interprets this statement as
follows: If  floor is greater than 13, execute the statement that is denoted by a single semi colon,
that is, the donothing statement. The statement enclosed in braces is no longer a part of the  if
programming tip 5.1
© Eric Isselé/iStockphoto.
© TACrafts/iStockphoto.
Properly lining up your code makes your
programs easier to read.
programming tip 5.2
© Eric Isselé/iStockphoto.
Common error 5.1
© John Bell/iStockphoto.
5.1 the if statement 185
statement. It is always executed. In other words, even if the value of  floor is not above 13, it is
decremented.
Tabs
Blockstructured code has the property that nested statements are indented by one or more
levels:
public class ElevatorSimulation
{
| public static void main(String[] args)
| {
| | int floor;
| | . . .
| | if (floor > 13)
| | {
| | | floor--;
| | }
| | . . .
| }
| | | |
0 1 2 3 Indentation level
How do you move the cursor from the leftmost col umn to the appropriate indentation level?
A perfectly reasonable strategy is to hit the space bar a sufficient number of times. With most
editors, you can use the Tab key instead. A tab moves the cursor to the next indentation level.
Some editors even have an option to fill in the tabs automatically.
While the Tab key is nice, some editors use tab characters for alignment, which is not so
nice. Tab characters can lead to problems when you send your file to another person or a
printer. There is no univer sal agreement on the width of a tab character, and some software
will ignore tab characters altogether. It is therefore best to save your files with spaces instead of
tabs. Most editors have a setting to automatically convert all tabs to spaces. Look at the docu
mentation of your development environment to find out how to activate this useful setting.
The Conditional operator
Java has a  conditional operator of the form
condition ?  value 1 :  value 2
The value of that expression is either  value 1 if the test passes or  value 2 if it fails. For example,
we can compute the actual floor number as
actualFloor = floor > 13 ? floor - 1 : floor;
which is equivalent to
if (floor > 13) { actualFloor = floor - 1; } else { actualFloor = floor; }
You can use the conditional operator anywhere that a value is expected, for example:
System.out.println("Actual floor: " + (floor > 13 ? floor - 1 : floor));
We don’t use the conditional operator in this book, but it is a convenient construct that you
will find in many Java programs.
programming tip 5.3
© Eric Isselé/iStockphoto.
Photo by Vincent LaRussa/John Wiley & Sons, Inc.
You use
the Tab key
to move the
cursor to the next
indentation level.
special topic 5.1
© Eric Isselé/iStockphoto.
186 Chapter 5 Decisions
avoid duplication in Branches
Look to see whether you duplicate code in each branch. If so, move it out of the  if statement.
Here is an example of such duplication:
if (floor > 13)
{
actualFloor = floor - 1;
System.out.println("Actual floor: " + actualFloor);
}
else
{
actualFloor = floor;
System.out.println("Actual floor: " + actualFloor);
}
The output statement is exactly the same in both branches. This is not an error—the program
will run correctly. However, you can simplify the program by moving the duplicated state
ment, like this:
if (floor > 13)
{
actualFloor = floor - 1;
}
else
{
actualFloor = floor;
}
System.out.println("Actual floor: " + actualFloor);
Removing duplication is particularly important when programs are maintained for a long
time. When there are two sets of statements with the same effect, it can easily happen that a
programmer modifies one set but not the other.
5.2 Comparing Values
Every  if statement contains a condi tion. In many cases, the condition involves
comparing two values. In the following sections, you will learn how to implement
comparisons in Java.
5.2.1 relational operators
A relational operator tests the relation
ship between two values. An example
is the  > operator that we used in the test
floor > 13 . Java has six relational operators
(see Table 1).
In Java, you use a relational operator to check
whether one value is greater than another.
programming tip 5.4
© Eric Isselé/iStockphoto.
Use relational
operators
( < <= > >= == != )
to compare numbers.
© arturbo/iStockphoto.
relational operators
compare values. the
= = operator tests for
equality.
5.2 Comparing Values 187
table 1 relational operators
Java Math notation Description
> >  Greater than
>= ≥  Greater than or equal
< <  Less than
<= ≤  Less than or equal
== =  Equal
!= ≠  Not equal
As you can see, only two Java relational operators ( > and  < ) look as you would
expect from the mathematical notation. Computer keyboards do not have keys for ≥,
≤, or ≠, but the  >= ,  <= , and  != operators are easy to remember because they look similar.
The  == operator is initially confusing to most newcomers to Java.
In Java,  = already has a meaning, namely assignment. The  == operator denotes
equality testing:
floor = 13; //  Assign 13 to  floor
if (floor == 13) //  Test whether  floor equals 13
You must remember to use  == inside tests and to use  = outside tests.
syntax 5.2  Comparisons
floor > 13
floor == 13
String input;
if (input.equals("Y"))
double x; double y; final double EPSILON = 1E-14;
if (Math.abs(x - y) < EPSILON)
These quantities are compared.
Checks for equality.
Check that you have the right direction:
> (greater than) or  < (less than)
Use  == , not  = .
One of:  == != < <= > >= (See Table 1.)
Use  equals to compare strings. (See page 189.)
Checks that these floating-point numbers are very close.
See page 188.
Check the boundary condition:
>  (greater) or  >= (greater or equal)?
188 Chapter 5 Decisions
The relational operators in Table 1 have a lower precedence than the arithmetic
operators. That means you can write arithmetic expressions on either side of the rela
tional operator without using parentheses. For example, in the expression
floor - 1 < 13
both sides ( floor - 1 and  13 ) of the  < operator are evaluated, and the results are com
pared. Appendix B shows a table of the Java operators and their precedence.
5.2.2 Comparing Floating-point numbers
You have to be careful when comparing floatingpoint numbers in order to cope with
roundoff errors. For example, the following code multiplies the square root of 2 by
itself and then subtracts 2.
double r = Math.sqrt(2);
double d = r * r - 2;
if (d == 0)
{
System.out.println("sqrt(2) squared minus 2 is 0");
}
else
{
System.out.println("sqrt(2) squared minus 2 is not 0 but " + d);
}
Even though the laws of mathematics tell us that  2 2
2
( )
− equals 0, this program
fragment prints
sqrt(2) squared minus 2 is not 0 but 4.440892098500626E-16
Unfortunately, such roundoff errors are unavoidable. It plainly does not make sense
in most circum stances to compare floatingpoint numbers exactly. Instead, test
whether they are close enough.
To test whether a number x is close to zero, you can test whether the absolute value
|x| (that is, the number with its sign removed) is less than a very small threshold num
ber. That threshold value is often called  e (the Greek letter epsilon). It is common to
set  e to 10 –14 when testing  double numbers.
Similarly, you can test whether two numbers are approximately equal by checking
whether their dif ference is close to 0.
x y − ≤ ε
In Java, we program the test as follows:
final double EPSILON = 1E-14;
if (Math.abs(x - y) <= EPSILON)
{
// x is approximately equal to  y
}
5.2.3 Comparing strings
To test whether two strings are equal to each other, you must use the method called
equals :
if (string1.equals(string2)) . . .
When comparing
floating-point
numbers, don’t test
for equality. instead,
check whether they
are close enough.
5.2 Comparing Values 189
Do not use the  == operator to compare strings. The comparison
if (string1 == string2) //  Not useful
has an unrelated meaning. It tests whether the two strings are stored in the same
memory location. You can have strings with identical contents stored in different
locations, so this test never makes sense in actual programming; see Common Error
5.2 on page 192.
If two strings are not identical, you still may want to know the relationship
between them. The  compareTo method compares strings in lexicographic order. This
ordering is very similar to the way in which words are sorted in a dictionary. If
string1.compareTo(string2) < 0
then the string  string1 comes before the string  string2 in the dictionary. For example,
this is the case if  string1 is  "Harry" , and  string2 is  "Hello" .
Conversely, if
string1.compareTo(string2) > 0
then  string1 comes after  string2 in dictionary order.
Finally, if
string1.compareTo(string2) == 0
then  string1 and  string2 are equal.
There are a few technical differences between the ordering in a dictionary and the
lexicographic ordering in Java. In Java:
• All uppercase letters come before the lowercase letters. For example,  "Z" comes
before  "a" .
• The space character comes before all printable characters.
• Numbers come before letters.
• For the ordering of punctuation marks, see Appendix A.
When comparing two strings, you compare the first letters of each word, then the
second letters, and so on, until one of the strings ends or you find the first letter pair
that doesn’t match.
If one of the strings ends, the longer string is considered the “larger” one. For
example, compare  "car" with  "cart" . The first three letters match, and we reach the
end of the first string. Therefore  "car" comes before  "cart" in lexicographic ordering.
When you reach a mismatch, the string containing the “larger” character is consid
ered “larger”. For example, compare  "cat" with  "cart" . The first two letters match.
Because  t comes after  r , the string  "cat" comes after  "cart" in the lexicographic
ordering.
To see which of two terms comes first in the dictionary,
consider the first letter in which they differ.
Do not use the  = =
operator to compare
strings. Use the
equals method
instead.
the  compareTo
method compares
strings in
lexicographic order.
c a r t
c a r
c a t
Letters
match
r comes
before  t
Lexicographic
Ordering
Corbis Digital Stock.
190 Chapter 5 Decisions
5.2.4 Comparing objects
If you compare two object references with the  == operator, you test whether the refer
ences refer to the same object. Here is an example:
Rectangle box1 = new Rectangle(5, 10, 20, 30);
Rectangle box2 = box1;
Rectangle box3 = new Rectangle(5, 10, 20, 30);
The comparison
box1 == box2
is  true . Both object variables refer to the same object. But the comparison
box1 == box3
is  false . The two object variables refer to different objects (see Figure 3). It does not
matter that the objects have identical contents.
You can use the  equals method to test whether two rectangles have the same con
tents, that is, whether they have the same upperleft corner and the same width and
height. For example, the test
box1.equals(box3)
is true.
However, you must be careful when using the  equals method. It works correctly
only if the implemen tors of the class have supplied it. The  Rectangle class has an  equals
method that is suitable for comparing rectangles.
For your own classes, you need to supply an appropriate  equals method. You will
learn how to do that in Chapter 9. Until that point, you should not use the  equals
method to compare objects of your own classes.
5.2.5 testing for  null
An object reference can have the special value  null if it refers to no object at all. It is
common to use the  null value to indicate that a value has never been set. For example,
the = = operator
tests whether two
object references
are identical. to
compare the
contents of objects,
you need to use the
equals method.
figure 3 
Comparing object references
box1 =
box2 =
x =
Rectangle
y =
width =
height =
5
10
20
30
box3 =
x =
Rectangle
y =
width =
height =
5
10
20
30
the  null reference
refers to no object.
5.2 Comparing Values 191
String middleInitial = null; //  Not set
if ( . . . )
{
middleInitial = middleName.substring(0, 1);
}
You use the  == operator (and not  equals ) to test whether an object reference is a  null
reference:
if (middleInitial == null)
{
System.out.println(firstName + " " + lastName);
}
else
{
System.out.println(firstName + " " + middleInitial + ". " + lastName);
}
Note that the  null reference is not the same as the empty string  "" . The empty string
is a valid string of length 0, whereas a  null indicates that a string variable refers to no
string at all.
Table 2 summarizes how to compare values in Java.
table 2 relational operator examples
expression Value Comment
3 <= 4 true  3 is less than 4;  <= tests for “less than or equal”.
3 =< 4 error The “less than or equal” operator is  <= , not  =< .
The “less than” symbol comes first.
3 > 4 false  > is the opposite of  <= .
4 < 4 false The lefthand side must be strictly smaller than
the righthand side.
4 <= 4 true Both sides are equal;  <= tests for “less than or
equal”.
3 == 5 - 2 true == tests for equality.
3 != 5 - 1 true != tests for inequality. It is true that 3 is not 5 – 1.
3 = 6 / 2 error Use  == to test for equality.
1.0 / 3.0 == 0.333333333 false Although the values are very close to one another,
they are not exactly equal. See Section 5.2.2.
"10" > 5 error You cannot compare a string to a number.
"Tomato".substring(0, 3).equals("Tom") true Always use the  equals method to check whether
two strings have the same contents.
"Tomato".substring(0, 3) == ("Tom") false Never use  == to compare strings; it only checks
whether the strings are stored in the same
location. See Common Error 5.2 on page 192.
full Code example
Go to  wiley.com/
go/javacode to
download a program
that demonstrates
comparisons of
numbers and strings
192 Chapter 5 Decisions
6.  Which of the following conditions are true, provided  a is 3 and  b is 4?
a.  a + 1 <= b
b.  a + 1 >= b
c.  a + 1 != b
7.  Give the opposite of the condition
floor > 13
8.  What is the error in this statement?
if (scoreA = scoreB)
{
System.out.println("Tie");
}
9.  Supply a condition in this  if statement to test whether the user entered a Y:
System.out.println("Enter Y to quit.");
String input = in.next();
if (. . .)
{
System.out.println("Goodbye.");
}
10.  Give two ways of testing that a string  str is the empty string.
11.  What is the value of  s.length() if  s is
a.  the empty string  "" ?
b.  the string  " " containing a space?
c.  null ?
12.  Which of the following comparisons are syntactically incorrect? Which of them
are syntactically cor rect, but logically questionable?
String a = "1";
String b = "one";
double x = 1;
double y = 3 * (1.0 / 3);
a.  a == "1"
b.  a == null
c.  a.equals("")
d.  a == b
e.  a == x
f.  x == y
g.  x - y == null
h.  x.equals(y)
practice it  Now you can try these exercises at the end of the chapter: R5.4, R5.7, E5.13.
using  ==  to Compare Strings
If you write
if (nickname == "Rob")
then the test succeeds only if the variable  nickname refers to the exact same location as the string
literal  "Rob" .
© Nicholas Homrich/iStockphoto.
S e l f  C h e C k
Common error 5.2
© John Bell/iStockphoto.
5.2 Comparing Values 193
The test will pass if a string variable was initialized with the same string literal:
String nickname = "Rob";
. . .
if (nickname == "Rob") //  Test is true
However, if the string with the letters  R o b has been assembled in some other way, then the test
will fail:
String name = "Robert";
String nickname = name.substring(0, 3);
. . .
if (nickname == "Rob") //  Test is false
In this case, the  substring method produces a string in a different memory location. Even
though both strings have the same contents, the comparison fails.
You must remember never to use  == to compare strings. Always use  equals to check whether
two strings have the same contents.
Step 1  Decide upon the branching condition.
In our sample problem, the obvious choice for the
condition is:
original price < 128?
That is just fine, and we will use that condition in
our solution.
But you could equally well come up with a
cor rect solution if you choose the opposite condi
tion: Is the original price at least $128? You might
choose this condition if you put yourself into the
position of a shopper who wants to know when
the bigger discount applies.
Step 2  Give pseudocode for the work that needs to be
done when the condition is true.
In this step, you list the action or actions that are taken in the “positive” branch. The details
depend on your problem. You may want to print a message, compute values, or even exit the
program.
In our example, we need to apply an 8 percent discount:
discounted price = 0.92 x original price
© Steve Simzer/iStockphoto.
hoW to 5.1  implementing an  if  Statement
This How To walks you through the process of implementing an  if statement. We will illus
trate the steps with the following example problem.
problem Statement  The university bookstore has a Kilobyte Day sale every October 24,
giving an 8 percent discount on all computer accessory purchases if the price is less than $128,
and a 16 percent discount if the price is at least $128. Write a program that asks the cashier for
the original price and then prints the discounted price.
© MikePanic/iStockphoto.
Sales discounts are often higher for
expensive products. Use the  if state ment
to implement such a decision.
194 Chapter 5 Decisions
Step 3  Give pseudocode for the work (if any) that needs to be done when the condition is not true.
What do you want to do in the case that the condition of Step 1 is not satisfied? Sometimes,
you want to do nothing at all. In that case, use an  if statement without an  else branch.
In our example, the condition tested whether the price was less than $128. If that condi tion
is not true, the price is at least $128, so the higher discount of 16 percent applies to the sale:
discounted price = 0.84 x original price
Step 4  Doublecheck relational operators.
First, be sure that the test goes in the right direction. It is a common error to confuse  > and  < .
Next, consider whether you should use the  < operator or its close cousin, the  <= operator.
What should happen if the original price is exactly $128? Reading the problem carefully, we
find that the lower discount applies if the original price is less than $128, and the higher dis
count applies when it is at least $128. A price of $128 should therefore not fulfill our con dition,
and we must use  < , not  <= .
Step 5  Remove duplication.
Check which actions are common to both branches, and move them outside. (See Program
ming Tip 5.4 on page 186.)
In our example, we have two statements of the form
discounted price = ___ x original price
They only differ in the discount rate. It is best to just set the rate in the branches, and to do the
computation afterwards:
If original price < 128
discount rate = 0.92
Else
discount rate = 0.84
discounted price = discount rate x original price
Step 6  Test both branches.
Formulate two test cases, one that fulfills the condition of the  if statement, and one that does
not. Ask yourself what should happen in each case. Then follow the pseudocode and act each
of them out.
In our example, let us consider two scenarios for the original price: $100 and $200. We
expect that the first price is discounted by $8, the second by $32.
When the original price is 100, then the condition 100 < 128 is true, and we get
discount rate = 0.92
discounted price = 0.92 x 100 = 92
When the original price is 200, then the condition 200 < 128 is false, and
discount rate = 0.84
discounted price = 0.84 x 200 = 168
In both cases, we get the expected answer.
Step 7  Assemble the  if statement in Java.
Type the skeleton
if ()
{
}
else
{
5.2 Comparing Values 195
}
and fill it in, as shown in Syntax 5.1 on page 182. Omit the  else branch if it is not needed.
In our example, the completed statement is
if (originalPrice < 128)
{
discountRate = 0.92;
}
else
{
discountRate = 0.84;
}
discountedPrice = discountRate * originalPrice;
full Code example
Go to  wiley.com/go/
javacode to download
the complete
program for
calculating a
discounted price.
WorkeD exaMple 5.1  extracting the middle
Learn how to extract the middle character from a string, or the two middle
characters if the length of the string is even. Go to  www.wiley.com/go/
javaexamples and download Worked Example 5.1.
0 1 2 3 4
c r a t e
Making decisions is
an essential part of
any computer program. nowhere is
this more obvious than in a computer
system that helps sort luggage at an
airport. after scanning the luggage
identification codes, the system sorts
the items and routes them to differ-
ent conveyor belts. human operators
then place the items onto trucks. When
the city of Denver built a huge airport
to replace an outdated and congested
facility, the luggage system contractor
went a step further. the new system
was designed to replace the human
operators with robotic carts. Unfortu-
nately, the system plainly did not
work. it was plagued by mechanical
problems, such as luggage falling onto
the tracks and jamming carts. equally
frustrating were the software glitches.
Carts would uselessly accu mulate at
some locations when they were needed
elsewhere.
the airport had been scheduled
to open in 1993, but without a func-
tioning luggage system, the opening
was delayed for over a year while the
contractor tried to fix the problems.
the  contractor  never  succeeded,
and ultimately a manual system was
installed. the delay cost the city and
airlines close to a billion dollars, and
the contractor, once the leading lug-
gage systems vendor in the United
states, went bankrupt.
Clearly, it is very risky to build a
large system based on a technology
that has never been tried on a smaller
scale. as robots and the software that
controls them get better over time,
they will take on a larger share of lug-
gage handling in the future. But it is
likely that this will happen in an incre-
mental fashion.
Bob Daemmrich/Getty Images.
The Denver airport originally had a
fully automatic system for moving lug
gage, replacing human operators with
robotic carts. Unfortunately, the sys
tem never worked and was dismantled
before the airport was opened.
Computing & Society 5.1  Denver’s luggage handling system
© MediaBakery.
196 Chapter 5 Decisions
5.3 Multiple alternatives
In Section 5.1, you saw how to program a twoway branch with an  if statement. In
many situations, there are more than two cases. In this section, you will see how to
implement a decision with multiple alternatives.
For example, consider a program that displays the effect of an earthquake, as mea
sured by the Richter scale (see Table 3).
table 3 richter scale
Value effect
8 Most structures fall
7 Many buildings destroyed
6 Many buildings considerably
damaged, some collapse
4.5 Damage to poorly constructed
buildings
The Richter scale is a measurement of the strength of an earthquake. Every step in
the scale, for example from 6.0 to 7.0, signifies a tenfold increase in the strength of the
quake.
In this case, there are five branches: one each for the four descriptions of damage,
and one for no destruction. Figure 4 shows the flowchart for this multiplebranch
statement.
You use multiple  if statements to implement multiple alternatives, like this:
if (richter >= 8.0)
{
description = "Most structures fall";
}
else if (richter >= 7.0)
{
description = "Many buildings destroyed";
}
else if (richter >= 6.0)
{
description = "Many buildings considerably damaged, some collapse";
}
else if (richter >= 4.5)
{
description = "Damage to poorly constructed buildings";
}
else
{
description = "No destruction of buildings";
}
As soon as one of the four tests succeeds, the effect is displayed, and no further tests
are attempted. If none of the four cases applies, the final  else clause applies, and a
default message is printed.
Multiple  if
statements can be
combined to evaluate
complex decisions.
The 1989 Loma Prieta
earth quake that
damaged the Bay
Bridge in San Francisco
and destroyed many
buildings measured 7.1
on the Richter scale.
© kevinruss/iStockphoto.
Camera: © james steidl/iStockphoto.
Globe: © Alex Slobodkin/iStockphoto.
A N IM AT IO N
Multiple Alternatives
5.3 Multiple alternatives 197
figure 4 
Multiple alternatives
richter  ≥ 8.0?
richter  ≥ 7.0?
richter  ≥ 6.0?
richter  ≥ 4.5?
No destruction
of buildings
False
False
False
False
True
True
True
True
Most
structures
fall
Many
buildings
destroyed
Many buildings
considerably
damaged,
some collapse
Damage to
poorly constructed
buildings
Here you must sort the conditions and test against the largest cutoff first.
Suppose we reverse the order of tests:
if (richter >= 4.5) //  Tests in wrong order
{
description = "Damage to poorly constructed buildings";
}
else if (richter >= 6.0)
{
description = "Many buildings considerably damaged, some collapse";
}
else if (richter >= 7.0)
{
description = "Many buildings destroyed";
198 Chapter 5 Decisions
}
else if (richter >= 8.0)
{
description = "Most structures fall";
}
This does not work. Suppose the value of  richter is 7.1. That value is at least 4.5,
matching the first case. The other tests will never be attempted.
The remedy is to test the more specific conditions first. Here, the condition
richter >= 8.0 is more specific than the condition  richter >= 7.0 , and the condition
richter >= 4.5 is more general (that is, fulfilled by more values) than either of the
first two.
In this example, it is also important that we use an  if/else if/else sequence, not
just multiple independent  if statements. Consider this sequence of independent tests.
if (richter >= 8.0) //  Didn’t use  else
{
description = "Most structures fall";
}
if (richter >= 7.0)
{
description = "Many buildings destroyed";
}
if (richter >= 6.0)
{
description = "Many buildings considerably damaged, some collapse";
}
if (richter >= 4.5)
{
"Damage to poorly constructed buildings";
}
Now the alternatives are no longer exclusive. If  richter is 7.1, then the last three tests
all match. The  description variable is set to three different strings, ending up with the
wrong one.
13.  In a game program, the scores of players A and B are stored in variables  scoreA
and  scoreB . Assuming that the player with the larger score wins, write an  if/
else if/else sequence that prints out  "A won" ,  "B won" , or  "Game tied" .
14.  Write a conditional statement with three branches that sets  s to 1 if  x is positive,
to –1 if  x is negative, and to 0 if  x is zero.
15.  How could you achieve the task of Self Check 14 with only two branches?
16.  Beginners sometimes write statements such as the following:
if (price > 100)
{
discountedPrice = price - 20;
}
else if (price <= 100)
{
discountedPrice = price - 10;
}
Explain how this code can be improved.
17.  Suppose the user enters  -1 into the earthquake program. What is printed?
When using multiple
if statements, test
general conditions
after more specific
conditions.
full Code example
Go to  wiley.com/go/
javacode to download
the program for
printing earthquake
descriptions.
© Nicholas Homrich/iStockphoto.
S e l f  C h e C k
5.3 Multiple alternatives 199
18.  Suppose we want to have the earthquake program check whether the user en
tered a negative number. What branch would you add to the  if state ment, and
where?
practice it  Now you can try these exercises at the end of the chapter: R5.22, E5.10, E5.24.
The  switch  Statement
An  if/else if/else sequence that compares a value against several alternatives can be imple
mented as a  switch statement. For example,
int digit = . . .;
switch (digit)
{
case 1: digitName = "one"; break;
case 2: digitName = "two"; break;
case 3: digitName = "three"; break;
case 4: digitName = "four"; break;
case 5: digitName = "five"; break;
case 6: digitName = "six"; break;
case 7: digitName = "seven"; break;
case 8: digitName = "eight"; break;
case 9: digitName = "nine"; break;
default: digitName = ""; break;
}
This is a shortcut for
int digit = . . .;
if (digit == 1) { digitName = "one"; }
else if (digit == 2) { digitName = "two"; }
else if (digit == 3) { digitName = "three"; }
else if (digit == 4) { digitName = "four"; }
else if (digit == 5) { digitName = "five"; }
else if (digit == 6) { digitName = "six"; }
else if (digit == 7) { digitName = "seven"; }
else if (digit == 8) { digitName = "eight"; }
else if (digit == 9) { digitName = "nine"; }
else { digitName = ""; }
It isn’t much of a shortcut, but it has one advan tage—it is obvious that all branches test the
same value, namely  digit .
The  switch statement can be applied only in narrow circumstances. The values in the case
clauses must be constants. They can be integers or characters. As of Java 7, strings are permit
ted as well. You cannot use a  switch statement to branch on floatingpoint values.
Every branch of the switch should be termi nated by a  break instruction. If the  break is miss
ing, execution falls through to the next branch, and so on, until a  break or the end of the  switch
is reached. In practice, this fallthrough behavior is rarely useful, but it is a common cause
of errors. If you accidentally forget a  break statement, your program compiles but executes
unwanted code. Many programmers consider the  switch statement somewhat dangerous and
prefer the  if statement.
We leave it to you to use the  switch statement for your own code or not. At any rate, you
need to have a reading knowledge of  switch in case you find it in other programmers’ code.
special topic 5.2
© Eric Isselé/iStockphoto.
© travelpixpro/iStockphoto.
The  switch statement lets you choose
from a fixed set of alternatives.
200 Chapter 5 Decisions
5.4 nested Branches
It is often necessary to include an  if statement inside another. Such an arrangement is
called a nested set of statements.
Here is a typical example: In the United States, different tax rates are used depend
ing on the taxpayer’s marital status. There are different tax schedules for single and
for married tax payers. Married taxpayers add their income together and pay taxes
on the total. Table 4 gives the tax rate computations, using a simplification of the
schedules that were in effect for the 2008 tax year. A different tax rate applies to each
“bracket”. In this schedule, the income in the first bracket is taxed at 10 percent, and
the income in the second bracket is taxed at 25 per cent. The income limits for each
bracket depend on the marital status.
table 4 Federal tax rate schedule
if your status is single and
if the taxable income is  the tax is of the amount over
at most $32,000 10% $0
over $32,000 $3,200 + 25% $32,000
if your status is Married and
if the taxable income is the tax is of the amount over
at most $64,000 10% $0
over $64,000 $6,400 + 25% $64,000
Now compute the taxes due, given a marital status and an income figure. The key
point is that there are two levels of decision making. First, you must branch on the
marital status. Then, for each marital status, you must have another branch on income
level.
The twolevel decision process is reflected in two levels of  if statements in the pro
gram at the end of this section. (See Figure 5 for a flowchart.) In theory, nesting can go
deeper than two levels. A threelevel decision process (first by state, then by marital
status, then by income level) requires three nesting levels.
When a decision
statement is
contained inside the
branch of another
decision statement,
the statements
are nested.
nested decisions
are required for
problems that
have two levels of
decision making.
Camera: © james steidl/iStockphoto.
Globe: © Alex Slobodkin/iStockphoto.
A N IM AT IO N
Nested Branches
Computing income taxes requires
multiple levels of decisions.
© ericsphotography/iStockphoto.
5.4 nested Branches 201
section_4/
figure 5  income tax Computation
10%
bracket
25%
bracket
Single
income
≤ 32,000
10%
bracket
25%
bracket
income
≤ 64,000
False
True
True
False
True
False
Taxreturn.java
1 /**
2 A tax return of a taxpayer in 2008.
3 */
4 public class TaxReturn
5 {
6 public static final int SINGLE = 1;
7 public static final int MARRIED = 2;
8
9 private static final double RATE1 = 0.10;
10 private static final double RATE2 = 0.25;
11 private static final double RATE1_SINGLE_LIMIT = 32000;
12 private static final double RATE1_MARRIED_LIMIT = 64000;
13
14 private double income;
15 private int status;
16
17 /**
18 Constructs a  TaxReturn object for a given income and
19 marital status.
20 @param anIncome  the taxpayer income
21 @param aStatus  either  SINGLE or  MARRIED
22 */
23 public TaxReturn(double anIncome, int aStatus)
24 {
25 income = anIncome;
26 status = aStatus;
27 }
28
29 public double getTax()
30 {
202 Chapter 5 Decisions
31 double tax1 = 0;
32 double tax2 = 0;
33
34 if (status == SINGLE)
35 {
36 if (income <= RATE1_SINGLE_LIMIT)
37 {
38 tax1 = RATE1 * income;
39 }
40 else
41 {
42 tax1 = RATE1 * RATE1_SINGLE_LIMIT;
43 tax2 = RATE2 * (income - RATE1_SINGLE_LIMIT);
44 }
45 }
46 else
47 {
48 if (income <= RATE1_MARRIED_LIMIT)
49 {
50 tax1 = RATE1 * income;
51 }
52 else
53 {
54 tax1 = RATE1 * RATE1_MARRIED_LIMIT;
55 tax2 = RATE2 * (income - RATE1_MARRIED_LIMIT);
56 }
57 }
58
59 return tax1 + tax2;
60 }
61 }
section_4/TaxCalculator.java
1 import java.util.Scanner;
2
3 /**
4 This program calculates a simple tax return.
5 */
6 public class TaxCalculator
7 {
8 public static void main(String[] args)
9 {
10 Scanner in = new Scanner(System.in);
11
12 System.out.print("Please enter your income: ");
13 double income = in.nextDouble();
14
15 System.out.print("Are you married? (Y/N) ");
16 String input = in.next();
17 int status;
18 if (input.equals("Y"))
19 {
20 status = TaxReturn.MARRIED;
21 }
22 else
23 {
24 status = TaxReturn.SINGLE;
25 }
26 TaxReturn aTaxReturn = new TaxReturn(income, status);
5.4 Nested Branches  203
27 System.out.println("Tax: "
28 + aTaxReturn.getTax());
29 }
30 }
Program Run
Please enter your income: 80000
Are you married? (Y/N) Y
Tax: 10400.0
19.  What is the amount of tax that a single taxpayer pays on an income of $32,000?
20.  Would that amount change if the first nested  if statement changed from
if (income <= RATE1_SINGLE_LIMIT)
to
if (income < RATE1_SINGLE_LIMIT)
21.  Suppose Harry and Sally each make $40,000 per year. Would they save taxes if
they married?
22.  How would you modify the  TaxCalculator.java program in order to check that
the user entered a correct value for the marital status (i.e.,  Y or  N )?
23.  Some people object to higher tax rates for higher incomes, claiming that you
might end up with less money after taxes when you get a raise for working hard.
What is the flaw in this argument?
Practice It  Now you can try these exercises at the end of the chapter: R5.9, R5.21, E5.14, E5.17.
Hand-Tracing
A very useful technique for understanding whether a pro-
gram works correctly is called hand-tracing. You simulate
the program’s activity on a sheet of paper. You can use this
method with pseudocode or Java code.
Get an index card, a cocktail napkin, or whatever sheet
of paper is within reach. Make a column for each variable.
Have the program code ready. Use a marker, such as a
paper clip, to mark the current statement. In your mind,
exe cute statements one at a time. Every time the value of a
variable changes, cross out the old value and write the new
value below the old one.
For example, let’s trace the  getTax method with the data
from the program run above.
When the  TaxReturn object is constructed, the  income
instance variable is set to 80,000 and  status is set to  MARRIED . Then the  getTax method is called.
In lines 31 and 32 of  TaxReturn.java ,  tax1 and  tax2 are initialized to 0.
29  public double getTax()
30  {
31  double tax1 = 0;
32  double tax2 = 0;
33
© Nicholas Homrich/iStockphoto.
S e l f C H e C k
Programming Tip 5.5
© Eric Isselé/iStockphoto.
© thomasd007/iStockphoto.
.
Hand-tracing helps you
understand whether a
program works correctly.
income  status  tax1  tax2
80000 MARRIED  0  0
204 Chapter 5  Decisions
Because  status is not  SINGLE , we move to the  else branch of the outer if statement (line 46).
34  if (status == SINGLE)
35  {
36  if (income <= RATE1_SINGLE_LIMIT)
37  {
38  tax1 = RATE1 * income;
39  }
40  else
41  {
42  tax1 = RATE1 * RATE1_SINGLE_LIMIT;
43  tax2 = RATE2 * (income - RATE1_SINGLE_LIMIT);
44  }
45  }
46  else
47  {
Because  income is not  <= 64000 , we move to the  else branch of the inner  if statement (line 52).
48  if (income <= RATE1_MARRIED_LIMIT)
49  {
50  tax1 = RATE1 * income;
51  }
52  else
53  {
54  tax1 = RATE1 * RATE1_MARRIED_LIMIT;
55  tax2 = RATE2 * (income - RATE1_MARRIED_LIMIT);
56  }
The values of tax1 and  tax2 are updated.
53  {
54  tax1 = RATE1 * RATE1_MARRIED_LIMIT;
55  tax2 = RATE2 * (income - RATE1_MARRIED_LIMIT);
56  }
57  }
Their sum is returned and the method ends.
58
59  return tax1 + tax2;
60  }
Because the program trace shows the expected
return value ($10,400), it successfully demon-
strates that this test case works correctly.
The Dangling  else Problem
When an  if statement is nested inside another  if statement, the following error may occur.
double shippingCharge = 5.00; //  $5 inside continental U.S.
if (country.equals("USA"))
if (state.equals("HI"))
shippingCharge = 10.00; //  Hawaii is more expensive
else //  Pitfall!
shippingCharge = 20.00; //  As are foreign shipments
The indentation level seems to suggest that the  else is grouped with the test country.
equals("USA") . Unfortunately, that is not the case. The compiler ignores all indenta tion and
matches the  else with the preceding  if . That is, the code is actually
double shippingCharge = 5.00; //  $5 inside continental U.S.
if (country.equals("USA"))
if (state.equals("HI"))
shippingCharge = 10.00; //  Hawaii is more expensive
else //  Pitfall!
shippingCharge = 20.00; //  As are foreign shipments
income  status  tax1  tax2
80000 MARRIED  0  0
6400  4000
return
income  status  tax1  tax2  value
80000 MARRIED  0  0
6400  4000  10400
Common Error 5.3
© John Bell/iStockphoto.
5.4 nested Branches 205
That isn’t what you want. You want to group the  else with the first  if .
The ambiguous  else is called a dangling else. You can avoid this pitfall if you always use
braces, as recommended in Programming Tip 5.2 on page 184:
double shippingCharge = 5.00; //  $5 inside continental U.S.
if (country.equals("USA"))
{
if (state.equals("HI"))
{
shippingCharge = 10.00; //  Hawaii is more expensive
}
}
else
{
shippingCharge = 20.00; //  As are foreign shipments
}
Block Scope
A block is a sequence of statements that is enclosed in braces. For example, consider this
statement:
if (status == TAXABLE)
{
double tax = price * TAX_RATE;
price = price + tax;
}
The highlighted part is a block. You can declare a variable in a block, such as the  tax variable in
this example. Such a variable is only visible inside the block.
{
double tax = price * TAX_RATE; //  Variable declared inside a block
price = price + tax;
}
//  You can no longer access the  tax variable here
In fact, the variable is only created after the program enters the block, and it is removed as soon
as the program exits the block. Such a variable is said to have block scope. In general, the scope
of a variable is the part of the program in which the variable can be accessed. A variable with
block scope is visible only inside a block.
It is considered good design to minimize the scope of a variable. This reduces the possibil
ity of accidental modification and name conflicts. For example, as long as the  tax variable is not
special topic 5.3
© Eric Isselé/iStockphoto.
In the same way that there can be a street named “Main Street” in different cities,
a Java program can have multiple variables with the same name.
© jchamp/iStockphoto (Railway and Main); © StevenCarrieJohnson/iStockphoto (Main and N. Putnam); © jsmith/iStockphoto (Main and South St.
206 Chapter 5 Decisions
needed outside the block, it is a good idea to declare it inside the block. However, if you need
the variable outside the block, you must define it outside. For example,
double tax = 0;
if (status == TAXABLE)
{
tax = price * TAX_RATE;
}
price = price + tax;
Here, the  tax variable is used outside the block of the  if statement, and you must declare it
outside.
In Java, the scope of a local variable can never contain the declaration of another local vari
able with the same name. For example, the following is an error:
double tax = 0;
if (status == TAXABLE)
{
double tax = price * TAX_RATE;
//  Error: Cannot declare another variable with the same name
price = price + tax;
}
However, you can have local variables with identical names if their scopes do not overlap, such
as
if (Math.random() > 0.5)
{
Rectangle r = new Rectangle(5, 10, 20, 30);
. . .
} //  Scope of r ends here
else
{
int r = 5;
//  OK—it is legal to declare another  r here
. . .
}
These variables are independent from each other. You can have local variables with the same
name, as long as their scopes don’t overlap.
enumeration Types
In many programs, you use variables that can hold one of a finite number of values. For exam
ple, in the tax return class, the  status instance variable holds one of the values  SINGLE or  MARRIED .
We arbitrarily declared  SINGLE as the number 1 and  MARRIED as 2. If, due to some programming
error, the  status variable is set to another integer value (such as  -1 ,  0 , or  3 ), then the program
ming logic may produce invalid results.
In a simple program, this is not really a problem. But as programs grow over time, and more
cases are added (such as the “married filing separately” status), errors can slip in. Java version
5.0 introduces a remedy: enumeration types. An enumeration type has a finite set of values,
for example
public enum FilingStatus { SINGLE, MARRIED, MARRIED_FILING_SEPARATELY }
You can have any number of values, but you must include them all in the  enum declaration.
You can declare variables of the enumeration type:
FilingStatus status = FilingStatus.SINGLE;
special topic 5.4
© Eric Isselé/iStockphoto.
5.5 Problem Solving: Flowcharts  207
If you try to assign a value that isn’t a  FilingStatus , such as 2 or  "S" , then the compiler reports
an error.
Use the  == operator to compare enumeration values, for example:
if (status == FilingStatus.SINGLE) . . .
Place the  enum declaration inside the class that implements your program, such as
public class TaxReturn
{
public enum FilingStatus { SINGLE, MARRIED, MARRIED_FILING_SEPARATELY }
. . .
}
5.5 Problem Solving: Flowcharts
You have seen examples of flowcharts earlier in this chapter. A flowchart shows the
structure of decisions and tasks that are required to solve a problem. When you have
to solve a complex problem, it can help to draw a flowchart to visualize the flow of
control.
The basic flowchart elements are shown in Figure 6.
The basic idea is simple enough. Link tasks and input/output boxes in the sequence
in which they should be executed. Whenever you need to make a decision, draw a
diamond with two outcomes (see Figure 7).
Flow charts are made
up of elements for
tasks, input/output,
and decisions.
Figure 6  
Flowchart Elements
True
False
Condition Simple task
Input/output
Each branch of a
decision can contain
tasks and further
decisions.
Figure 7  
Flowchart with Two Outcomes
True
False
False branch True branch
Condition
208 Chapter 5 Decisions
figure 8 
Flowchart with Multiple Choices
True
False
Choice 1
“Choice 1”
branch
True
False
Choice 2
“Choice 2”
branch
True
False
Choice 3
“Choice 3”
branch
“Other”
branch
Each branch can contain a sequence of tasks and even additional decisions. If there
are multiple choices for a value, lay them out as in Figure 8.
There is one issue that you need to be aware of when drawing flow charts. Uncon
strained branching and merging can lead to “spaghetti code”, a messy network of
possible pathways through a program.
There is a simple rule for avoiding spaghetti code: Never point an arrow inside
another branch.
To understand the rule, consider this example: Shipping costs are $5 inside the
United States, except that to Hawaii and Alaska they are $10. Inter national shipping
costs are also $10.You might start out with a flowchart like the following:
False
True
Shipping
cost = $10
Inside US?
True
False
Continental US?
Shipping
cost = $5
never point an
arrow inside
another branch.
5.5 problem solving: Flowcharts 209
Now you may be tempted to reuse the “shipping cost = $10” task:
False
True
Shipping
cost = $10
Inside US?
True
False
Continental US?
Shipping
cost = $5
Don’t do that! The red arrow points inside a different branch. Instead, add another
task that sets the ship ping cost to $10, like this:
False
True
Shipping
cost = $10
Inside US?
True
False
Continental US?
Shipping
cost = $10
Shipping
cost = $5
Not only do you avoid spaghetti code, but it is also a better design. In the future it
may well happen that the cost for international shipments is different from that to
Alaska and Hawaii.
Flowcharts can be very useful for getting an intuitive understanding of the flow of
an algorithm. However, they get large rather quickly when you add more details. At
that point, it makes sense to switch from flowcharts to pseudocode.
Spaghetti code has so many pathways that
it becomes impossible to understand.
full Code example
Go to  wiley.com/go/
javacode to
download a program
that computes
shipping costs.
© Ekspansio/iStockphoto.
210 Chapter 5 Decisions  testing track
24.  Draw a flowchart for a program that reads a value  temp and prints “Frozen” if it
is less than zero.
25.  What is wrong with the flowchart at right?
26.  How do you fix the flowchart of
Self Check 25?
27.  Draw a flowchart for a program that reads a
value  x . If it is less than zero, print “Error”.
Otherwise, print its square root.
28.  Draw a flowchart for a program that reads
a value  temp . If it is less than zero, print
“Ice”. If it is greater than 100, print “Steam”.
Otherwise, print “Liquid”.
practice it  Now you can try these exercises at the end of the
chapter: R5.12, R5.13, R5.14.
5.6 problem solving: selecting test Cases
Testing the functionality of a program without consideration of its internal structure
is called black-box testing. This is an important part of testing, because, after all, the
users of a program do not know its internal structure. If a program works perfectly
on all inputs, then it surely does its job.
However, it is impossible to ensure absolutely that a program will work correctly
on all inputs just by supplying a finite number of test cases. As the famous computer
scientist Edsger Dijkstra pointed out, testing can show only the presence of bugs—
not their absence. To gain more confidence in the correct ness of a program, it is useful
to consider its internal structure. Testing strategies that look inside a pro gram are
called white-box testing. Performing unit tests of each method is a part of whitebox
testing.
You want to make sure that each part of your program is exercised at least once by
one of your test cases. This is called code coverage. If some code is never executed
by any of your test cases, you have no way of knowing whether that code would
perform correctly if it ever were executed by user input. That means that you need
to look at every  if / else branch to see that each of them is reached by some test case.
Many conditional branches are in the code only to take care of strange and abnor
mal inputs, but they still do something. It is a common phenomenon that they end
up doing something incorrectly, but those faults are never discovered during testing,
because nobody supplied the strange and abnormal inputs. The remedy is to ensure
that each part of the code is covered by some test case.
For example, in testing the  getTax method of the  TaxReturn class, you want to make
sure that every  if statement is entered for at least one test case. You should test both
single and married taxpayers, with incomes in each of the three tax brackets.
When you select test cases, you should make it a habit to include boundary test
cases: legal values that lie at the boundary of the set of acceptable inputs.
© Nicholas Homrich/iStockphoto.
S e l f  C h e C k
True
False
Input < 0?
True
False
Input > 100?
Status = “OK” Status = “Error”
Black-box testing
describes a testing
method that does not
take the structure of
the implementation
into account.
White-box testing
uses information
about the structure
of a program.
Code coverage
is a measure of
how many parts
of a program have
been tested.
testing track 5.6 problem solving: selecting test Cases 211
Here is a plan for obtaining a compre hensive set of test cases for the tax program:
• There are two possibilities for the marital status and two tax brackets for each
status, yielding four test cases.
• Test a handful of boundary conditions, such as an income that is at the boundary
between two brack ets, and a zero income.
• If you are responsible for error checking (which is discussed in Section 5.8), also
test an invalid input, such as a negative income.
Make a list of the test cases and the expected outputs:
Test Case Married Expected Output Comment
30,000 N 3,000 10% bracket
72,000 N 13,200 3,200 + 25% of 40,000
50,000 Y 5,000 10% bracket
104,000 Y 16,400 6,400 + 25% of 40,000
32,000 N 3,200 boundary case
0 0 boundary case
When you develop a set of test cases, it is helpful to have a flowchart of your program
(see Section 5.5). Check off each branch that has a test case. Include test cases for the
boundary cases of each decision. For example, if a decision checks whether an input is
less than 100, test with an input of 100.
It is always a good idea to design test cases before starting to code. Working
through the test cases gives you a better understanding of the algorithm that you are
about to implement.
29.  Using Figure 1 on page 181 as a guide, follow the process described in this section
to design a set of test cases for the  ElevatorSimulation.java program in Section 5.1.
30.  What is a boundary test case for the algorithm in How To 5.1 on page 193? What is
the expected out put?
31.  Using Figure 4 on page 197 as a guide, follow the process described in Section 5.6 to
design a set of test cases for the  Earthquake.java program in Section 5.3.
32.  Suppose you are designing a part of a program for a
medical robot that has a sensor returning an x- and
ylocation (measured in cm). You need to check
whether the sensor location is inside the circle,
outside the circle, or on the boundary (specifically,
having a distance of less than 1 mm from the bound
ary). Assume the circle has center (0, 0) and a radius of
2 cm. Give a set of test cases.
practice it  Now you can try these exercises at the end of the chapter: R5.15, R5.16.
Boundary test cases
are test cases that are
at the boundary of
acceptable inputs.
it is a good idea to
design test cases
before implementing
a program.
© Nicholas Homrich/iStockphoto.
S e l f  C h e C k
2 cm
(0, 0)
212 Chapter 5 Decisions  testing track
make a Schedule and make Time for unexpected problems
Commercial software is notorious for being delivered later than promised. For example,
Microsoft originally promised that its Windows Vista operating system would be available late
in 2003, then in 2005, then in March 2006; it finally was released in January 2007. Some of the
early promises might not have been realistic. It was in Microsoft’s interest to let pro spective
customers expect the imminent availability of the product. Had customers known the actual
delivery date, they might have switched to a different product in the meantime. Undeniably,
though, Microsoft had not anticipated the full complexity of the tasks it had set itself to solve.
Microsoft can delay the delivery of its product, but it is likely that you cannot. As a stu dent
or a programmer, you are expected to manage your time wisely and to finish your assignments
on time. You can probably do simple programming exercises the night before the due date,
but an assignment that looks twice as hard may well take four times as long, because more
things can go wrong. You should therefore make a schedule whenever you start a program
ming project.
First, estimate realistically how much time it
will take you to:
• Design the program logic.
• Develop test cases.
• Type the program in and fix syntax errors.
• Test and debug the program.
For example, for the income tax program I might
estimate an hour for the design; 30 min utes for
developing test cases; an hour for data entry and
fixing syntax errors; and an hour for testing and
debugging. That is a total of 3.5 hours. If I work
two hours a day on this project, it will take me
almost two days.
Then think of things that can go wrong. Your computer might break down. You might be
stumped by a problem with the computer system. (That is a particularly important con cern
for beginners. It is very common to lose a day over a trivial problem just because it takes time
to track down a person who knows the magic command to overcome it.) As a rule of thumb,
double the time of your estimate. That is, you should start four days, not two days, before the
due date. If nothing went wrong, great; you have the program done two days early. When the
inevitable problem occurs, you have a cushion of time that protects you from embarrassment
and failure.
logging
Sometimes you run a program and you are not sure where it spends its time. To get a printout
of the program flow, you can insert trace messages into the program, such as this one:
if (status == SINGLE)
{
System.out.println("status is SINGLE");
. . .
}
However, there is a problem with using  System.out.println for trace messages. When you are
done testing the pro gram, you need to remove all print statements that produce trace mes
sages. If you find another error, however, you need to stick the print statements back in.
programming tip 5.6
© Eric Isselé/iStockphoto.
Bananastock/Media Bakery.
Make a schedule for your programming
work and build in time for problems.
special topic 5.5
© Eric Isselé/iStockphoto.
5.7 Boolean Variables and Operators  213
To overcome this problem, you should use the  Logger class, which allows you to turn off the
trace messages with out removing them from the program.
Instead of printing directly to  System.out , use the global logger object that is returned
by the call  Logger.getGlobal() . (Prior to Java 7, you obtained the global logger as
Logger.getLogger("global") .) Then call the  info method:
Logger.getGlobal().info("status is SINGLE");
By default, the message is printed. But if you call
Logger.getGlobal().setLevel(Level.OFF);
at the beginning of the  main method of your program, all log message printing is suppressed.
Set the level to  Level.INFO to turn logging of  info messages on again. Thus, you can turn off
the log messages when your program works fine, and you can turn them back on if you find
another error. In other words, using  Logger.getGlobal().info is just like  System.out.println ,
except that you can easily activate and deactivate the logging.
The  Logger class has many other options for industrial-strength logging. Check out the API
documentation if you want to have more control over logging.
5.7 Boolean Variables and Operators
Sometimes, you need to evaluate a logical condi tion in one part of a program and use
it else where. To store a condition that can be true or false, you use a Boolean variable.
Boolean vari ables are named after the mathematician George Boole (1815–1864), a
pioneer in the study of logic.
In Java, the  boolean data type has exactly two values, denoted  false and  true . These
values are not strings or integers; they are special values, just for Boolean variables.
Here is a declaration of a Boolean variable:
boolean failed = true;
You can use the value later in your program to make a decision:
if (failed) //  Only executed if  failed has been set to  true
{
. . .
}
When you make complex decisions, you often need to combine Boolean values. An
operator that combines Boolean conditions is called a Boolean operator. In Java, the
&& operator (called and) yields  true only when both conditions are true. The  || opera-
tor (called or) yields the result  true if at least one of the conditions is true.
Logging messages can be
deactivated when testing 
is complete.
The Boolean type
boolean  has two
values,  false
and  true .
Cusp/SuperStock.
A Boolean variable
is also called a flag
because it can be
either up (true) or
down (false).
Figure 9  Boolean Truth Tables
A B A  &&  B
true true true
true false false
false true false
false false false
A B A  ||  B
true true true
true false true
false true true
false false false
A ! A
true false
false true
214 Chapter 5 Decisions
At this geyser in Iceland,
you can see ice, liquid
water, and steam.
© toos/iStockphoto.
Suppose you write a program that processes temperature values, and you want to
test whether a given temperature corresponds to liquid water. (At sea level, water
freezes at 0 degrees Celsius and boils at 100 degrees.) Water is liquid if the tempera
ture is greater than zero and less than 100:
if (temp > 0 && temp < 100) { System.out.println("Liquid"); }
The condition of the test has two parts, joined by the  && operator. Each part is a Bool
ean value that can be true or false. The combined expression is true if both individual
expressions are true. If either one of the expressions is false, then the result is also false
(see Figure 9).
The Boolean operators  && and  || have a lower precedence than the relational opera
tors. For that reason, you can write relational expressions on either side of the Bool
ean operators without using parentheses. For example, in the expression
temp > 0 && temp < 100
the expressions  temp > 0 and  temp < 100 are evaluated first. Then the  && operator com
bines the results. Appendix B shows a table of the Java operators and their
prece dence.
Conversely, let’s test whether water is not liquid at a given temperature. That is the
case when the temperature is at most 0 or at least 100.
Java has two Boolean
operators that
combine conditions:
&& (and) and  || (or).
full Code example
Go to  wiley.com/go/
javacode to
download a program
comparing numbers
using Boolean
expressions.
figure 10  Flowcharts for and and or Combinations
True True True
True
False
False
False False
Temperature
> 0?
Temperature
< 100?
Water is
liquid
Water is
not liquid
Temperature
≤ 0?
Temperature
≥ 100?
Both conditions
must be true
At least
one condition
must be true
and  or
5.7 Boolean Variables and operators 215
table 5 Boolean operator examples
expression Value Comment
0 < 200 && 200 < 100 false Only the first condition is true.
0 < 200 || 200 < 100 true The first condition is true.
0 < 200 || 100 < 200 true The  || is not a test for “eitheror”. If both
conditions are true, the result is true.
0 < x && x < 100 || x == -1  (0 < x && x < 100)
|| x == -1
The  && operator has a higher precedence than the
|| operator (see Appendix B).
0 < x < 100 error error: This expression does not test whether  x
is between 0 and 100. The expression  0 < x is a
Boolean value. You cannot compare a Boolean
value with the integer 100.
x && y > 0 error error: This expression does not test whether  x and
y are positive. The lefthand side of  && is an integer,
x , and the righthand side,  y > 0 , is a Boolean value.
You cannot use  && with an integer argument.
!(0 < 200) false  0 < 200 is  true , therefore its negation is  false .
frozen == true frozen There is no need to compare a Boolean variable
with  true .
frozen == false !frozen It is clearer to use  ! than to compare with  false .
Use the  || (or) operator to combine the expressions:
if (temp <= 0 || temp >= 100) { System.out.println("Not liquid"); }
Figure 10 shows flowcharts for these examples.
Sometimes you need to invert a condition with the not Boolean operator. The
! operator takes a single condition and evaluates to  true if that condition is false and to
false if the condition is true. In this example, output occurs if the value of the Boolean
variable  frozen is  false : .
if (!frozen) { System.out.println("Not frozen"); }
Table 5 illustrates additional examples of evaluating Boolean operators.
33.  Suppose  x and  y are two integers. How do you test whether both of them
are zero?
34.  How do you test whether at least one of them is zero?
35.  How do you test whether exactly one of them is zero?
36.  What is the value of  !!frozen ?
37.  What is the advantage of using the type  boolean rather than strings  "false"/"true"
or integers 0/1?
practice it  Now you can try these exercises at the end of the chapter: R5.29, E5.22, E5.23.
to invert a condition,
use the  ! (not)
operator.
© Nicholas Homrich/iStockphoto.
S e l f  C h e C k
216 Chapter 5 Decisions
Combining multiple relational operators
Consider the expression
if (0 <= temp <= 100) //  Error
This looks just like the mathematical test 0 ≤  temp ≤ 100. But in Java, it is a compiletime error.
Let us dissect the condition. The first half,  0 <= temp , is a test with an outcome  true or  false .
The outcome of that test ( true or  false ) is then compared against 100. This seems to make no
sense. Is  true larger than 100 or not? Can one compare truth values and numbers? In Java, you
cannot. The Java compiler rejects this statement.
Instead, use  && to combine two separate tests:
if (0 <= temp && temp <= 100) . . .
Another common error, along the same lines, is to write
if (input == 1 || 2) . . . //  Error
to test whether  input is 1 or 2. Again, the Java compiler flags this construct as an error. You
cannot apply the  || operator to numbers. You need to write two Boolean expressions and join
them with the  || operator:
if (input == 1 || input == 2) . . .
Confusing  &&  and  ||  Conditions
It is a surprisingly common error to confuse and and or conditions. A value lies between 0 and
100 if it is at least 0 and at most 100. It lies outside that range if it is less than 0 or greater than
100. There is no golden rule; you just have to think carefully.
Often the and or or is clearly stated, and then it isn’t too hard to implement it. But some
times the wording isn’t as explicit. It is quite common that the individual conditions are nicely
set apart in a bulleted list, but with little indication of how they should be combined.
Consider these instructions for filing a tax return. You can claim single filing status if any
one of the following is true:
• You were never married.
• You were legally separated or divorced on the last day of the tax year.
• You were widowed, and did not remarry.
Because the test passes if any one of the conditions is true, you must combine the conditions
with or.
Elsewhere, the same instructions state that you may use the more advantageous sta tus of
married filing jointly if all five of the following conditions are true:
• Your spouse died less than two years ago and you did not remarry.
• You have a child whom you can claim as dependent.
• That child lived in your home for all of the tax year.
• You paid over half the cost of keeping up your home for this child.
• You filed a joint return with your spouse the year he or she died.
Because all of the conditions must be true for the test to pass, you must combine them with
an and.
Common error 5.4
© John Bell/iStockphoto.
Common error 5.5
© John Bell/iStockphoto.
5.7 Boolean Variables and operators 217
Short-Circuit evaluation of Boolean operators
The  && and  || operators are computed using shortcircuit evaluation.
In other words, logical expressions are evaluated from left to right,
and evaluation stops as soon as the truth value is determined. When
an  && is evaluated and the first condition is false, the second condition
is not evaluated, because it does not matter what the outcome of the
second test is.
For example, consider the expression
quantity > 0 && price / quantity < 10
Suppose the value of  quantity is zero. Then the test  quantity > 0 fails, and the second test is not
attempted. That is just as well, because it is illegal to divide by zero.
Similarly, when the first condition of an  || expression is true, then the remainder is not
evaluated because the result must be true.
This process is called short-circuit evaluation.
In a short circuit, electricity travels along the path of least
resistance. Similarly, shortcircuit evaluation takes the fast
est path for computing the result of a Boolean expression.
de morgan’s law
Humans generally have a hard time comprehending logical conditions with not operators
applied to and/or expressions. De Morgan’s Law, named after the logician Augustus De
Morgan (1806–1871), can be used to simplify these Boolean expressions.
Suppose we want to charge a higher shipping rate if we don’t ship within the continental
United States:
if (!(country.equals("USA") && !state.equals("AK") && !state.equals("HI")))
{
shippingCharge = 20.00;
}
This test is a little bit complicated, and you have to think carefully through the logic. When it
is not true that the country is USA and the state is not Alaska and the state is not Hawaii, then
charge $20.00. Huh? It is not true that some people won’t be confused by this code.
The computer doesn’t care, but it takes human programmers to write and maintain the
code. Therefore, it is useful to know how to simplify such a condition.
De Morgan’s Law has two forms: one for the negation of an
and expression and one for the negation of an or expression:
!( A && B ) is the same as ! A || ! B
!( A || B ) is the same as ! A && ! B
Pay particular attention to the fact that the and and or operators are reversed by moving the
not inward. For example, the negation of “the state is Alaska or it is Hawaii”,
!(state.equals("AK") || state.equals("HI"))
is “the state is not Alaska and it is not Hawaii”:
!state.equals("AK") && !state.equals("HI")
special topic 5.6
© Eric Isselé/iStockphoto.
the  && and  ||
operators are
computed using
shortcircuit
evaluation: as soon
as the truth value
is determined, no
further conditions
are evaluated.
© YouraPechkin/iStockphoto.
special topic 5.7
© Eric Isselé/iStockphoto.
De Morgan’s law tells
you how to negate  &&
and  || conditions.
218 Chapter 5 Decisions
Now apply the law to our shipping charge computation:
!(country.equals("USA")
&& !state.equals("AK")
&& !state.equals("HI"))
is equivalent to
!country.equals("USA")
|| !!state.equals("AK")
|| !!state.equals("HI"))
Because two  ! cancel each other out, the result is the simpler test
!country.equals("USA")
|| state.equals("AK")
|| state.equals("HI")
In other words, higher shipping charges apply when the destination is outside the United
States or to Alaska or Hawaii.
To simplify conditions with negations of and or or expressions, it is usually a good idea to
apply De Morgan’s Law to move the negations to the innermost level.
5.8 application: input Validation
An important application for the  if statement is input validation. Whenever
your program accepts user input, you need to make sure that the usersupplied
values are valid before you use them in your computations.
Consider our elevator simulation program. Assume that the elevator panel
has buttons labeled 1 through 20 (but not 13). The following are illegal inputs:
• The number 13
• Zero or a negative number
• A number larger than 20
• An input that is not a sequence of digits, such as  five
In each of these cases, we want to give an error message and exit the program.
It is simple to guard against an input of 13:
if (floor == 13)
{
System.out.println("Error: There is no thirteenth floor.");
}
Here is how you ensure that the user doesn’t enter a number outside the valid range:
if (floor <= 0 || floor > 20)
{
System.out.println("Error: The floor must be between 1 and 20.");
}
However, dealing with an input that is not a valid integer is a more serious problem.
When the statement
floor = in.nextInt();
is executed, and the user types in an input that is not an integer (such as  five ), then
the integer variable  floor is not set. Instead, a runtime exception occurs and the pro
gram is terminated. To avoid this problem, you should first call the  hasNextInt method
Tetra Images/Media Bakery.
Like a quality control worker,
you want to make sure that
user input is correct before
processing it.
5.8 application: input Validation 219
which checks whether the next input is an integer. If that method returns  true , you
can safely call  nextInt . Otherwise, print an error message and exit the program:
if (in.hasNextInt())
{
int floor = in.nextInt();
Process the input value.
}
else
{
System.out.println("Error: Not an integer.");
}
Here is the complete elevator simulation program with input validation:
section_8/elevatorSimulation2.java
1 import java.util.Scanner;
2
3 /**
4 This program simulates an elevator panel that skips the 13th floor, checking for
5 input errors.
6 */
7 public class ElevatorSimulation2
8 {
9 public static void main(String[] args)
10 {
11 Scanner in = new Scanner(System.in);
12 System.out.print("Floor: ");
13 if (in.hasNextInt())
14 {
15 //  Now we know that the user entered an integer
16
17 int floor = in.nextInt();
18
19 if (floor == 13)
20 {
21 System.out.println("Error: There is no thirteenth floor.");
22 }
23 else if (floor <= 0 || floor > 20)
24 {
25 System.out.println("Error: The floor must be between 1 and 20.");
26 }
27 else
28 {
29 //  Now we know that the input is valid
30
31 int actualFloor = floor;
32 if (floor > 13)
33 {
34 actualFloor = floor - 1;
35 }
36
37 System.out.println("The elevator will travel to the actual floor "
38 + actualFloor);
39 }
40 }
41 else
42 {
43 System.out.println("Error: Not an integer.");
Call the  hasNextInt
or  hasNextDouble
method to ensure
that the next input is
a number.
220 Chapter 5 Decisions
44 }
45 }
46 }
program run
Floor:  13
Error: There is no thirteenth floor.
38.  In the  ElevatorSimulation2 program, what is the output when the input is
a.  100?
b.  –1?
c.  20?
d.  thirteen?
39.  Your task is to rewrite lines 19–26 of the  ElevatorSimulation2 program so that
there is a single  if statement with a complex condition. What is the condition?
if (. . .)
{
System.out.println("Error: Invalid floor number");
}
40.  In the Sherlock Holmes story “The Adventure of the Sussex Vampire”, the
inimitable detective uttered these words: “Matilda Briggs was not the name of
a young woman, Watson, … It was a ship which is associated with the giant rat
of Sumatra, a story for which the world is not yet prepared.” Over a hundred
years later, researchers found giant rats in Western New Guinea, another part of
Indonesia.
Suppose you are charged with writing a program that processes rat weights. It
contains the statements
System.out.print("Enter weight in kg: ");
double weight = in.nextDouble();
What input checks should you supply?
When processing inputs, you want to reject values that are too large. But how large is too large?
These giant rats, found in Western New Guinea, are about five times the size of a city rat.
41.  Run the following test program and supply inputs  2 and  three at the prompts.
What happens? Why?
import java.util.Scanner
public class Test
{
public static void main(String[] args)
{
Scanner in = new Scanner(System.in);
System.out.print("Enter an integer: ");
int m = in.nextInt();
System.out.print("Enter another integer: ");
int n = in.nextInt();
System.out.println(m + " " + n);
}
}
practice it  Now you can try these exercises at the end of the chapter: R5.3, R5.32, E5.12.
© Nicholas Homrich/iStockphoto.
S e l f  C h e C k
© jeanma85/iStockphoto.
5.8 application: input Validation 221
When  one  uses  a
sophisticated  com-
puter program such as a tax prepara-
tion package, one is bound to attribute
some intelligence to the computer.
the computer asks sensible questions
and makes computations that we find
a mental challenge. after all, if doing
one’s taxes were easy, we wouldn’t
need a computer to do it for us.
as programmers, however, we
know that all this apparent intelli gence
is an illusion. human program mers
have carefully “coached” the software
in all possible scenarios, and it simply
replays the actions and deci sions that
were programmed into it.
Would it be possible to write com-
puter programs that are genuinely
intelligent in some sense? From the
earliest days of computing, there was
a sense that the human brain might
be nothing but an immense computer,
and that it might well be feasible to
program computers to imitate some
processes of human thought. serious
research into artificial intelligence
began in the mid-1950s, and the first
twenty years brought some impres-
sive successes. programs that play
chess—surely an activity that appears
to require remarkable intellectual pow-
ers—have become so good that they
now routinely beat all but the best
human players. as far back as 1975,
an  expertsystem  program  called
Mycin gained fame for being better in
diagnosing meningitis in patients than
the average physician.
however, there were serious set-
backs as well. From 1982 to 1992,
the Japanese government embarked
on a massive research project, funded
at over 40 billion Japanese yen. it was
known as the FifthGeneration Project.
its goal was to develop new hardware
and software to greatly improve the
performance of expert system soft-
ware. at its outset, the project created
fear in other countries that the Japa-
nese computer industry was about to
become the undisputed leader in the
field. however, the end results were
disappointing and did little to bring
artificial intelligence applications to
market.
From the very outset, one of the
stated goals of the ai community was
to produce software that could trans-
late text from one language to another,
for example from english to russian.
that undertaking proved to be enor-
mously complicated. human language
appears to be much more subtle and
interwoven with the human experi-
ence than had originally been thought.
even the grammar-checking tools that
come with word-processing programs
today are more of a gim mick than a
useful tool, and analyzing grammar
is just the first step in trans lating
sentences.
the CYC (from encyclopedia) proj-
ect, started by Douglas lenat in 1984,
tries to codify the implicit assump-
tions that underlie human speech and
writing. the team members started
out analyzing news articles and asked
themselves what unmen tioned facts
are necessary to actually understand
the sentences. For exam ple, consider
the sentence, “last fall she enrolled in
Michigan state”. the reader automati-
cally realizes that “fall” is not related
to falling down in this context, but
refers to the season. While there is
a state of Michigan, here Michigan
state denotes the university. a priori,
a computer program has none of this
knowledge. the goal of the CYC proj-
ect is to extract and store the requi-
site facts—that is, (1) people enroll in
universities; (2) Michigan is a state; (3)
many states have universi ties named
x state University, often abbreviated
as x state; (4) most peo ple enroll in
a university in the fall. By 1995, the
project had codified about 100,000
common-sense concepts and about
a million facts of knowledge relating
them. even this massive amount of
data has not proven suffi cient for use-
ful applications.
in recent years, artificial intelli-
gence technology has seen substantial
advances. one of the most astounding
examples is the outcome of a series
of “grand challenges” for autono-
mous vehicles posed by the Defense
advanced research projects agency
(Darpa). Competitors were invited to
submit a com puter-controlled vehi-
cle that had to complete an obstacle
course without a human driver or
remote control. the first event, in
2004, was a disap pointment, with
none of the entrants finishing the
route. in 2005, five vehicles com-
pleted a grueling 212 km course in the
Mojave desert. stan ford’s stanley came
in first, with an average speed of 30
km/h. in 2007, Darpa moved the com-
petition to an “urban” environment, an
abandoned air force base. Vehicles
had to be able to
interact  with  each
other, following Cali-
fornia traffic laws. as
stanford’s  sebastian
thrun explained: “in
the last Grand Chal-
lenge, it didn’t really
mat ter  whether  an
obstacle was a rock
or a bush, because
either way you’d just
drive around it. the
current challenge is to
move from just sens-
ing the envi ronment
to understanding it.”
Vaughn Youtz/Zuma Press.
Winner of the 2007 DARPA Urban Challenge
Computing & Society 5.2  artificial intelligence
© MediaBakery.
222 Chapter 5 Decisions
use the  if  statement to implement a decision.
• The  if statement allows a program to carry out
dif ferent actions depending on the nature of the
data to be processed.
implement comparisons of numbers and objects.
• Use relational operators ( < <= > >= == != ) to compare numbers.
• Relational operators compare values. The  = = operator tests for equality.
• When comparing floatingpoint numbers, don’t test for equality. Instead, check
whether they are close enough.
• Do not use the  = = operator to compare strings. Use the  equals method instead.
• The  compareTo method compares strings in lexicographic order.
• The  = = operator tests whether two object references are
identical. To compare the contents of objects, you need to
use the  equals method.
• The  null reference refers to no object.
implement complex decisions that require multiple  if  statements.
• Multiple  if statements can be combined to evaluate complex deci sions.
• When using multiple  if statements, test general conditions after more specific
conditions.
implement decisions whose branches require further decisions.
• When a decision statement is contained inside the branch of another decision
statement, the statements are nested.
• Nested decisions are required for problems that have two levels of
decision making.
draw flowcharts for visualizing the control flow of a program.
• Flow charts are made up of elements for tasks,
input/output, and decisions.
• Each branch of a decision can contain tasks and
further decisions.
• Never point an arrow inside another branch.
C h a p t e r s U M M a rY
© Media Bakery.
© arturbo/iStockphoto.
Corbis Digital Stock.
© kevinruss/iStockphoto.
© ericsphotography/iStockphoto.
.
True
False
Condition
review Questions 223
design test cases for your programs.
• Blackbox testing describes a testing method that does not take the structure of
the implementation into account.
• Whitebox testing uses information about the structure of a program.
• Code coverage is a measure of how many parts of a program have been tested.
• Boundary test cases are test cases that are at the boundary of acceptable inputs.
• It is a good idea to design test cases before implementing a program.
• Logging messages can be deactivated when testing is complete.
use the Boolean data type to store and combine conditions that can be true or false.
• The Boolean type  boolean has two values,  false and  true .
• Java has two Boolean operators that combine conditions:  &&  (and) and  || (or).
• To invert a condition, use the  ! (not) operator.
• The  && and  || operators are computed using short-circuit evaluation: As soon as
the truth value is determined, no further conditions are evaluated.
• De Morgan’s Law tells you how to negate  && and  || conditions.
apply  if  statements to detect whether user input is valid.
• Call the  hasNextInt or  hasNextDouble method to ensure that the
next input is a number.
• r5.1  What is the value of each variable after the  if statement?
a.  int n = 1; int k = 2; int r = n;
if (k < n) { r = k; }
b.  int n = 1; int k = 2; int r;
if (n < k) { r = k; }
else { r = k + n; }
c.  int n = 1; int k = 2; int r = k;
if (r < k) { n = r; }
else { k = n; }
d.  int n = 1; int k = 2; int r = 3;
if (r < n + k) { r = 2 * n; }
else { k = 2 * r; }
Cusp/SuperStock.
.
Tetra Images/Media Bakery.
java.awt.Rectangle
equals
java.lang.String
equals
compareTo
java.util.Scanner
hasNextDouble
hasNextInt
java.util.logging.Level
INFO
OFF
java.util.logging.Logger
getGlobal
info
setLevel
s ta n D a r D l i B r a rY i t e M s i n t r o D U C e D i n t h i s C h a p t e r
r e V i e W Q U e s t i o n s
224 Chapter 5 Decisions
•• r5.2  Explain the difference between
s = 0;
if (x > 0) { s++; }
if (y > 0) { s++; }
and
s = 0;
if (x > 0) { s++; }
else if (y > 0) { s++; }
•• r5.3  Find the errors in the following  if statements.
a.  if x > 0 then System.out.print(x);
b.  if (1 + x > Math.pow(x, Math.sqrt(2)) { y = y + x; }
c.  if (x = 1) { y++; }
d.  x = in.nextInt();
if (in.hasNextInt())
{
sum = sum + x;
}
else
{
System.out.println("Bad input for x");
}
e.  String letterGrade = "F";
if (grade >= 90) { letterGrade = "A"; }
if (grade >= 80) { letterGrade = "B"; }
if (grade >= 70) { letterGrade = "C"; }
if (grade >= 60) { letterGrade = "D"; }
• r5.4  What do these code fragments print?
a.  int n = 1;
int m = -1;
if (n < -m) { System.out.print(n); }
else { System.out.print(m); }
b.  int n = 1;
int m = -1;
if (-n >= m) { System.out.print(n); }
else { System.out.print(m); }
c.  double x = 0;
double y = 1;
if (Math.abs(x - y) < 1) { System.out.print(x); }
else { System.out.print(y); }
d.  double x = Math.sqrt(2);
double y = 2;
if (x * x == y) { System.out.print(x); }
else { System.out.print(y); }
•• r5.5  Suppose  x and  y are variables of type  double . Write a code fragment that sets  y to  x if  x
is positive and to 0 otherwise.
•• r5.6  Suppose  x and  y are variables of type  double . Write a code fragment that sets  y to the
absolute value of  x without calling the  Math.abs function. Use an  if statement.
•• r5.7  Explain why it is more difficult to compare floatingpoint numbers than integers.
Write Java code to test whether an integer  n equals 10 and whether a floatingpoint
number  x is approximately equal to 10.
review Questions 225
• r5.8  It is easy to confuse the  = and  == operators. Write a test program containing the
statement
if (floor = 13)
What error message do you get? Write another test program with the statement
count == 0;
What does your compiler do when you compile the program?
•• r5.9  Each square on a chess board can be described by a letter
and number, such as  g5 in the example at right.
The following pseudocode describes an algorithm that
determines whether a square with a given letter and
number is dark (black) or light (white).
If the letter is an a, c, e, or g
If the number is odd
color = "black"
Else
color = "white"
Else
If the number is even
color = "black"
Else
color = "white"
Using the procedure in Programming Tip 5.5, trace this pseudocode with input  g5 .
•• Testing r5.10  Give a set of four test cases for the algorithm of Exercise R5.9 that covers all
branches.
•• r5.11  In a scheduling program, we want to check whether two appointments overlap. For
simplicity, appointments start at a full hour, and we use military time (with hours
0–24). The following pseudocode describes an algorithm that determines whether
the appointment with start time  start1 and end time  end1 overlaps with the appoint
ment with start time  start2 and end time  end2 .
If start1 > start2
s = start1
Else
s = start2
If end1 < end2
e = endl
Else
e = end2
If s < e
The appointments overlap.
Else
The appointments don’t overlap.
Trace this algorithm with an appointment from 10–12 and one from 11–13, then with
an appointment from 10–11 and one from 12–13.
• r5.12  Draw a flow chart for the algorithm in Exercise R5.11.
• r5.13  Draw a flow chart for the algorithm in Exercise E5.13.
1
2
4
6
8
3
5
7
1
2
4
6
8
3
5
7
a
a
b
b
d
d
f
f
h
h
c
c
e
e
g5
g
g
226 Chapter 5 Decisions
• r5.14  Draw a flow chart for the algorithm in Exercise E5.14.
•• Testing r5.15  Develop a set of test cases for the algorithm in Exercise R5.11.
•• Testing r5.16  Develop a set of test cases for the algorithm in Exercise E5.14.
•• r5.17  Write pseudocode for a program that prompts the user for a month and day and
prints out whether it is one of the following four holidays:
• New Year’s Day (January 1)
• Independence Day (July 4)
• Veterans Day (November 11)
• Christmas Day (December 25)
•• r5.18  Write pseudocode for a program that assigns letter grades for a quiz, according to the
following table:
Score  Grade
90-100  A
80-89  B
70-79  C
60-69  D
< 60  F
•• r5.19  Explain how the lexicographic ordering of strings in Java differs from the order
ing of words in a dictionary or telephone book. Hint: Consider strings such as  IBM ,
wiley.com ,  Century 21 , and  While-U-Wait .
•• r5.20  Of the following pairs of strings, which comes first in lexicographic order?
a.  "Tom", "Jerry"
b.  "Tom", "Tomato"
c.  "church", "Churchill"
d.  "car manufacturer", "carburetor"
e.  "Harry", "hairy"
f.  "Java", " Car"
g.  "Tom", "Tom"
h.  "Car", "Carl"
i.  "car", "bar"
• r5.21  Explain the difference between an  if/else if/else sequence and nested  if state ments.
Give an example of each.
•• r5.22  Give an example of an  if/else if/else sequence where the order of the tests does not
matter. Give an example where the order of the tests matters.
• r5.23  Rewrite the condition in Section 5.3 to use  < operators instead of  >= operators. What
is the impact on the order of the comparisons?
•• Testing r5.24  Give a set of test cases for the tax program in Exercise P5.2. Manually compute the
expected results.
• r5.25  Make up a Java code example that shows the dangling  else problem using the follow
ing statement: A student with a GPA of at least 1.5, but less than 2, is on probation.
With less than 1.5, the student is failing.
review Questions 227
••• r5.26  Complete the following truth table by finding the truth values of the Boolean
expressions for all combinations of the Boolean inputs  p ,  q , and  r .
p q r
(p && q) || !r !(p && (q || !r))
false false false
false false true
false true false
. . .
5 more combinations
. . .
••• r5.27  True or false? A  && B is the same as B  && A for any Boolean conditions A and B.
• r5.28  The “advanced search” feature of many search engines allows you to use Boolean
operators for complex queries, such as “(cats OR dogs) AND NOT pets”. Contrast
these search operators with the Boolean operators in Java.
•• r5.29  Suppose the value of  b is  false and the value of  x is 0. What is the value of each of the
following expressions?
a.  b && x == 0
b.  b || x == 0
c.  !b && x == 0
d.  !b || x == 0
e.  b && x != 0
f.  b || x != 0
g.  !b && x != 0
h.  !b || x != 0
•• r5.30  Simplify the following expressions. Here,  b is a variable of type  boolean .
a.  b == true
b.  b == false
c.  b != true
d.  b != false
••• r5.31  Simplify the following statements. Here,  b is a variable of type  boolean and  n is a vari
able of type  int .
a.  if (n == 0) { b = true; } else { b = false; }
(Hint: What is the value of  n == 0 ?)
b.  if (n == 0) { b = false; } else { b = true; }
c.  b = false; if (n > 1) { if (n < 2) { b = true; } }
d.  if (n < 1) { b = true; } else { b = n > 2; }
• r5.32  What is wrong with the following program?
System.out.print("Enter the number of quarters: ");
int quarters = in.nextInt();
if (in.hasNextInt())
{
total = total + quarters * 0.25;
System.out.println("Total: " + total);
228 Chapter 5 Decisions
}
else
{
System.out.println("Input error.");
}
• e5.1  Write a program that reads an integer and prints whether it is negative, zero, or
positive.
•• e5.2  Write a program that reads a floatingpoint number and prints “zero” if the number
is zero. Otherwise, print “positive” or “negative”. Add “small” if the absolute value
of the number is less than 1, or “large” if it exceeds 1,000,000.
•• e5.3  Write a program that reads an integer and prints how many digits the number has, by
checking whether the number is ≥ 10, ≥ 100, and so on. (Assume that all integers are
less than ten billion.) If the number is negative, first multiply it with –1.
•• e5.4  Write a program that reads three numbers and prints “all the same” if they are all the
same, “all different” if they are all different, and “neither” otherwise.
•• e5.5  Write a program that reads three numbers and prints “increasing” if they are in
increasing order, “decreasing” if they are in decreasing order, and “neither” other
wise. Here, “increasing” means “strictly increasing”, with each value larger than its
pre decessor. The sequence 3 4 4 would not be considered increasing.
•• e5.6  Repeat Exercise E5.5, but before reading the numbers, ask the user whether increas
ing/decreasing should be “strict” or “lenient”. In lenient mode, the sequence 3 4 4 is
increasing and the sequence 4 4 4 is both increasing and decreasing.
•• e5.7  Write a program that reads in three integers and prints “in order” if they are sorted in
ascending or descending order, or “not in order” otherwise. For example,
1 2 5 in order
1 5 2 not in order
5 2 1 in order
1 2 2 in order
•• e5.8  Write a program that reads four integers and prints “two pairs” if the input consists
of two matching pairs (in some order) and “not two pairs” otherwise. For example,
1 2 2 1 two pairs
1 2 2 3 not two pairs
2 2 2 2 two pairs
•• Business e5.9  Write a program that reads in the name and salary of an employee. Here the salary
will denote an hourly wage, such as $9.25. Then ask how many hours the employee
worked in the past week. Be sure to accept fractional hours. Compute the pay. Any
overtime work (over 40 hours per week) is paid at 150 percent of the regular wage.
Print a paycheck for the employee. In your solution, implement a class  Paycheck .
• e5.10  Write a program that reads a temperature value and the letter C for Celsius or F for
Fahrenheit. Print whether water is liquid, solid, or gaseous at the given temperature
at sea level.
p r a C t i C e e x e r C i s e s
practice exercises 229
• e5.11  The boiling point of water drops by about one degree centigrade for every 300
meters (or 1,000 feet) of altitude. Improve the program of Exercise E5.10 to allow
the user to supply the altitude in meters or feet.
• e5.12  Add error handling to Exercise E5.11. If the user does not enter a number when
expected, or provides an invalid unit for the altitude, print an error message and end
the program.
•• e5.13  When two points in time are compared, each given as hours (in military time,
rang ing from 0 and 23) and minutes, the following pseudocode determines which
comes first.
If hour1 < hour2
time1 comes first.
Else if hour1 and hour2 are the same
If minute1 < minute2
time1 comes first.
Else if minute1 and minute2 are the same
time1 and time2 are the same.
Else
time2 comes first.
Else
time2 comes first.
Write a program that prompts the user for two points in time and prints the time that
comes first, then the other time. In your program, supply a class  Time and a method
public int compareTo(Time other)
that returns –1 if the time comes before the other, 0 if both are the same, and 1
otherwise.
•• e5.14  The following algorithm yields the season (Spring, Summer, Fall, or Winter) for a
given month and day.
If month is 1, 2, or 3, season = “Winter”
Else if month is 4, 5, or 6, season = “Spring”
Else if month is 7, 8, or 9, season = “Summer”
Else if month is 10, 11, or 12, season = “Fall”
If month is divisible by 3 and day >= 21
If season is “Winter”, season = “Spring”
Else if season is “Spring”, season = “Summer”
Else if season is “Summer”, season = “Fall”
Else season = “Winter”
Write a program that prompts the user for a month and day and then prints the
season, as determined by this algorithm. Use a class  Date with a method  getSeason .
•• e5.15  Write a program that translates a letter grade into a number grade. Letter grades are
A, B, C, D, and F, possibly followed by + or –. Their numeric values are 4, 3, 2, 1, and
0. There is no F+ or F–. A + increases the numeric value by 0.3, a – decreases it by 0.3.
However, an A+ has value 4.0.
Enter a letter grade: B-
The numeric value is 2.7.
Use a class  Grade with a method  getNumericGrade .
© rotofrank/iStockphoto.
230 Chapter 5 Decisions
•• e5.16  Write a program that translates a number between 0 and 4 into the closest letter
grade. For example, the number 2.8 (which might have been the average of several
grades) would be converted to B–. Break ties in favor of the better grade; for exam ple
2.85 should be a B.
Use a class  Grade with a method  getNumericGrade .
•• e5.17  The original U.S. income tax of 1913 was quite simple. The tax was
• 1 percent on the first $50,000.
• 2 percent on the amount over $50,000 up to $75,000.
• 3 percent on the amount over $75,000 up to $100,000.
• 4 percent on the amount over $100,000 up to $250,000.
• 5 percent on the amount over $250,000 up to $500,000.
• 6 percent on the amount over $500,000.
There was no separate schedule for single or married taxpayers. Write a program that
computes the income tax according to this schedule.
•• e5.18  Write a program that takes user input describing a playing card in the following
shorthand notation:
A Ace
2 ... 10  Card values
J Jack
Q Queen
K King
D Diamonds
H Hearts
S Spades
C Clubs
Your program should print the full description of the card. For example,
Enter the card notation: QS
Queen of Spades
Implement a class  Card whose constructor takes the card notation string and whose
getDescription method returns a description of the card. If the notation string is not
in the correct format, the  getDescription method should return the string  "Unknown" .
•• e5.19  Write a program that reads in three floatingpoint numbers and prints the largest of
the three inputs. For example:
Please enter three numbers: 4 9 2.5
The largest number is 9.
•• e5.20  Write a program that reads in three strings and sorts them lexicographically.
Enter three strings: Charlie Able Baker
Able
Baker
Charlie
programming projects 231
•• e5.21  Write a program that reads in two floatingpoint numbers and tests whether they are
the same up to two decimal places. Here are two sample runs.
Enter two floating-point numbers: 2.0 1.99998
They are the same up to two decimal places.
Enter two floating-point numbers: 2.0 1.98999
They are different.
• e5.22  Write a program that prompts the user to provide a single character from the alpha
bet. Print Vowel or Consonant, depending on the user input. If the user input is not a
letter (between a and z or A and Z), or is a string of length > 1, print an error message.
•• e5.23  Write a program that asks the user to enter a month (1 for January, 2 for February,
etc.) and then prints the number of days in the month. For February, print “28 days”.
Enter a month: 5
30 days
Use a class  Month with a method
public int getLength()
Do not use a separate  if/else branch for each month. Use Boolean operators.
• Business e5.24  A supermarket awards coupons depending on how much a customer spends on
groceries. For example, if you spend $50, you will get a coupon worth eight percent
of that amount. The following table shows the percent used to calculate the coupon
awarded for different amounts spent. Write a program that calculates and prints the
value of the coupon a person can receive based on groceries purchased.
Here is a sample run:
Please enter the cost of your groceries: 14
You win a discount coupon of $ 1.12. (8% of your purchase)
Money spent Coupon percentage
Less than $10 No coupon
From $10 to $60 8%
More than $60 to $150 10%
More than $150 to $210 12%
More than $210 14%
•• p5.1  Write a program that prompts for the day and month of the user’s birthday and then
prints a horoscope. Make up fortunes for programmers, like this:
Please enter your birthday (month and day): 6 16
Gemini are experts at figuring out the behavior of complicated programs.
You feel where bugs are coming from and then stay one step ahead. Tonight,
your style wins approval from a tough critic.
Each fortune should contain the name of the astrological sign. (You will find the
names and date ranges of the signs at a distressingly large number of sites on the
Internet.) Use a class  Date with a method  getFortune .
p r o G r a M M i n G p r o J e C t s
© lillisphotography/iStockphoto.
232 Chapter 5 Decisions
•• p5.2  Write a program that computes taxes for the following schedule.
if your status is single and
if the taxable income is over but not over the tax is of the amount over
$0 $8,000 10% $0
$8,000 $32,000 $800 + 15% $8,000
$32,000 $4,400 + 25% $32,000
if your status is Married and
if the taxable income is over but not over the tax is of the amount over
$0 $16,000 10% $0
$16,000 $64,000 $1,600 + 15% $16,000
$64,000 $8,800 + 25% $64,000
••• p5.3  The  TaxReturn.java program uses a simplified version of the 2008 U.S. income tax
schedule. Look up the tax brackets and rates for the current year, for both single and
married filers, and implement a program that computes the actual income tax.
••• p5.4  Unit conversion. Write a unit conversion program that asks the users from which
unit they want to convert (fl. oz, gal, oz, lb, in, ft, mi) and to which unit they want
to convert (ml, l, g, kg, mm, cm, m, km). Reject incompatible conversions (such as
gal → km). Ask for the value to be converted, then display the result:
Convert from? gal
Convert to? ml
Value? 2.5
2.5 gal = 9462.5 ml
••• p5.5  A year with 366 days is called a leap year. Leap years are necessary to keep the cal
endar synchronized with the sun because the earth revolves around the sun once
every 365.25 days. Actually, that figure is not entirely precise, and for all dates after
1582 the Gregorian correction applies. Usually years that are divisible by 4 are leap
years, for example 1996. However, years that are divisible by 100 (for example, 1900)
are not leap years, but years that are divisible by 400 are leap years (for exam ple,
2000). Write a program that asks the user for a year and computes whether that year
is a leap year. Provide a class  Year with a method  isLeapYear . Use a single  if statement
and Boolean operators.
••• p5.6  Roman numbers. Write a program that converts a positive integer into the Roman
number system. The Roman number system has digits
I 1
V 5
X 10
L 50
C 100
D 500
M 1,000
programming projects 233
Numbers are formed according to the following rules:
a.  Only numbers up to 3,999 are represented.
b.  As in the decimal system, the thousands, hundreds, tens, and ones are
expressed separately.
c.  The numbers 1 to 9 are expressed as
I 1 VI  6
II 2 VII  7
III  3 VIII  8
IV 4 IX  9
V 5
As you can see, an I preceding a V or X is subtracted from the value, and you
can never have more than three I’s in a row.
d.  Tens and hundreds are done the same way, except that the letters X, L, C and C,
D, M are used instead of I, V, X, respectively.
Your program should take an input, such as 1978, and convert it to Roman numer als,
MCMLXXVIII.
••• p5.7  French country names are feminine when they end with the letter e, masculine other
wise, except for the following which are masculine even though they end with e:
• le Belize
• le Cambodge
• le Mexique
• le Mozambique
• le Zaïre
• le Zimbabwe
Write a program that reads the French name of a country and adds the article: le for
masculine or la for feminine, such as le Canada or la Belgique.
However, if the country name starts with a vowel, use l’; for example, l’Afghanistan.
For the following plural country names, use les:
• les EtatsUnis
• les PaysBas
••• Business p5.8  Write a program to simulate a bank transaction. There are two bank accounts: check
ing and savings. First, ask for the initial balances of the bank accounts; reject negative
balances. Then ask for the transactions; options are deposit, withdrawal, and trans
fer. Then ask for the account; options are checking and savings. Reject transactions
that overdraw an account. At the end, print the balances of both accounts.
•• Business p5.9  When you use an automated teller machine (ATM) with
your bank card, you need to use a personal identification
number (PIN) to access your account. If a user fails more
than three times when entering the PIN, the machine will
block the card. Assume that the user’s PIN is “1234” and
write a program that asks the user for the PIN no more than
three times, and does the following:
© Straitshooter/iStockphoto.
© Mark Evans/iStockphoto.
234 Chapter 5 Decisions
• If the user enters the right number, print a message saying, “Your PIN is cor
rect”, and end the program.
• If the user enters a wrong number, print a message saying, “Your PIN is incor
rect” and, if you have asked for the PIN less than three times, ask for it again.
• If the user enters a wrong number three times, print a message saying “Your
bank card is blocked” and end the program.
• Business p5.10  Calculating the tip when you go to a restaurant is not difficult, but your restaurant
wants to suggest a tip according to the service diners receive. Write a program that
calculates a tip according to the diner’s satisfaction as follows:
• Ask for the diners’ satisfaction level using these ratings: 1 = Totally satisfied,
2 = Satisfied, 3 = Dissatisfied.
• If the diner is totally satisfied, calculate a 20 percent tip.
• If the diner is satisfied, calculate a 15 percent tip.
• If the diner is dissatisfied, calculate a 10 percent tip.
• Report the satisfaction level and tip in dollars and cents.
• Science p5.11  Write a program that prompts the user for a wavelength value and prints a descrip
tion of the corresponding part of the electromagnetic spectrum, as given in the fol
lowing table.
electromagnetic spectrum
type Wavelength (m) Frequency (hz)
Radio Waves > 10 –1 < 3 × 10 9
Microwaves 10 –3 to 10 –1 3 × 10 9 to 3 × 10 11
Infrared 7 × 10 –7 to 10 –3 3 × 10 11 to 4 × 10 14
Visible light 4 × 10 –7 to 7 × 10 –7 4 × 10 14 to 7.5 × 10 14
Ultraviolet 10 –8 to 4 × 10 –7 7.5 × 10 14 to 3 × 10 16
Xrays 10 –11 to 10 –8 3 × 10 16 to 3 × 10 19
Gamma rays < 10 –11 > 3 × 10 19
• Science p5.12  Repeat Exercise P5.11, modifying the program so that it prompts for the frequency
instead.
•• Science p5.13  Repeat Exercise P5.11, modifying the program so that it first asks the user whether
the input will be a wavelength or a frequency.
••• Science p5.14  A minivan has two sliding doors. Each door can be
opened by either a dashboard switch, its inside handle,
or its outside handle. However, the inside handles do not
work if a child lock switch is activated. In order for the
sliding doors to open, the gear shift must be in park, and
the master unlock switch must be activated. (This book’s
author is the longsuffering owner of just such a vehicle.)
© drxy/iStockphoto.
© nano/iStockphoto.
programming projects 235
Your task is to simulate a portion of the control software for the vehicle. The input is
a sequence of values for the switches and the gear shift, in the following order:
• Dashboard switches for left and right sliding door, child lock, and master
unlock (0 for off or 1 for activated)
• Inside and outside handles on the left and right sliding doors (0 or 1)
• The gear shift setting (one of P N D 1 2 3 R).
A typical input would be 0 0 0 1 0 1 0 0 P .
Print “left door opens” and/or “right door opens” as appropriate. If neither door
opens, print “both doors stay closed”.
• Science p5.15  Sound level L in units of decibel (dB) is determined by
L = 20 log 10 (p/p 0 )
where p is the sound pressure of the sound (in Pascals, abbreviated Pa), and p 0 is a
reference sound pres sure equal to 20 × 10 –6 Pa (where L is 0 dB). The following table
gives descriptions for certain sound lev els.
Threshold of pain  130 dB
Possible hearing damage  120 dB
Jack hammer at 1 m  100 dB
Traffic on a busy roadway at 10 m  90 dB
Normal conversation  60 dB
Calm library  30 dB
Light leaf rustling  0 dB
Write a program that reads a value and a unit, either dB or Pa, and then prints the
closest description from the list above.
•• Science p5.16  The electric circuit shown below is designed to measure the temperature of the gas in
a chamber.
+
–
V s = 20 V
R s = 75 Ω
R V m
+
–
Voltmeter
11.43 V
The resistor R represents a temperature sensor enclosed in the chamber. The resis
tance R, in Ω, is related to the temperature T, in °C, by the equation
R R kT = +
0
In this device, assume R 0 = 100 Ω and k = 0.5. The voltmeter displays the value of the
voltage, V m , across the sensor. This voltage V m indicates the temperature, T, of the
gas according to the equation
T
R
k
R
k
R
k
V
V V
R
k
s m
s m
= − =
−
−
0 0
© Photobuff/iStockphoto.
236 Chapter 5 Decisions
Suppose the voltmeter voltage is constrained to the range V min = 12 volts ≤ V m ≤
V max = 18 volts. Write a program that accepts a value of V m and checks that it’s
between 12 and 18. The program should return the gas temperature in degrees
Celsius when V m is between 12 and 18 and an error message when it isn’t.
••• Science p5.17  Crop damage due to frost is one of the many risks confronting farmers. The figure
below shows a simple alarm circuit designed to warn of frost. The alarm circuit uses
a device called a thermistor to sound a buzzer when the temperature drops below
freezing. Thermistors are semiconductor devices that exhibit a temperature depen
dent resistance described by the equation
R R e
T T
=
−






0
1 1
0
β
where R is the resistance, in Ω, at the temperature T, in °K, and R 0 is the resistance,
in Ω, at the temperature T 0 , in°K.  β is a constant that depends on the material used to
make the thermistor.
–
+
9 V
R 3
R 4 R 2
R Thermistor
9 V
Comparator
Buzzer
The circuit is designed so that the alarm will sound when
R
R R
R
R R
2
2
4
3 4
+
<
+
The thermistor used in the alarm circuit has R 0 = 33,192 Ω at T 0 = 40 °C, and
β = 3,310 °K. (Notice that  β has units of °K. The temperature in °K is obtained by
adding 273° to the temperature in °C.) The resistors R 2 , R 3 , and R 4 have a resistance
of 156.3 kΩ = 156,300 Ω.
Write a Java program that prompts the user for a temperature in °F and prints a
message indicating whether or not the alarm will sound at that temperature.
• Science p5.18  A mass m = 2 kilograms is attached to the end of a rope of length r = 3 meters. The
mass is whirled around at high speed. The rope can withstand a maximum tension
of T = 60 Newtons. Write a program that accepts a rotation speed v and determines
whether such a speed will cause the rope to break. Hint: T mv r =
2
.
• Science p5.19  A mass m is attached to the end of a rope of length r = 3 meters. The rope can only
be whirled around at speeds of 1, 10, 20, or 40 meters per second. The rope can with
stand a maximum tension of T = 60 Newtons. Write a program where the user enters
the value of the mass m, and the program determines the greatest speed at which it
can be whirled without breaking the rope. Hint: T mv r =
2
.
© rotofrank/iStockphoto.
answers to self-Check Questions 237
•• Science p5.20  The average person can jump off the ground with a
velocity of 7 mph without fear of leaving the planet.
However, if an astronaut jumps with this velocity
while standing on Halley’s Comet, will the astro
naut ever come back down? Create a program that
allows the user to input a launch velocity (in mph)
from the surface of Halley’s Comet and determine
whether a jumper will return to the surface. If not,
the program should calculate how much more
massive the comet must be in order to return the
jumper to the surface.
Hint: Escape velocity is v
GM
R
escape
= 2 , where G N m kg = ×
−
6 67 10
11 2 2
. is
the gravitational constant, M kg = × 1 3 10 22 .
is the mass of Halley’s comet, and
R m = × 1 153 10 6 . is its radius.
Courtesy NASA/JPL-Caltech.
a n s W e rs t o s e l F - C h e C k Q U e s t i o n s
1.  Change the  if statement to
if (floor > 14)
{
actualFloor = floor - 2;
}
2.  85. 90. 85.
3.  The only difference is if  originalPrice is 100.
The statement in Self Check 2 sets  discounted-
Price to 90; this one sets it to 80.
4.  95. 100. 95.
5.  if (fuelAmount < 0.10 * fuelCapacity)
{
System.out.println("red");
}
else
{
System.out.println("green");
}
6.  (a) and (b) are both true, (c) is false.
7.  floor <= 13
8.  The values should be compared with  == , not  = .
9.  input.equals("Y")
10.  str.equals("")  or str.length() == 0
11.  (a) 0; (b) 1; (c) An exception occurs.
12.  Syntactically incorrect: e, g, h. Logically ques
tionable: a, d, f.
13.  if (scoreA > scoreB)
{
System.out.println("A won");
}
else if (scoreA < scoreB)
{
System.out.println("B won");
}
else
{
System.out.println("Game tied");
}
14.  if (x > 0) { s = 1; }
else if (x < 0) { s = -1; }
else { s = 0; }
15.  You could first set  s to one of the three values:
s = 0;
if (x > 0) { s = 1; }
else if (x < 0) { s = -1; }
16.  The  if (price <= 100) can be omitted (leaving
just  else ), making it clear that the  else branch is
the sole alternative.
17.  No destruction of buildings.
18.  Add a branch before the final  else :
else if (richter < 0)
{
System.out.println("Error: Negative input");
}
19.  3200.
238 Chapter 5 Decisions
20.  No. Then the computation is 0.10 × 32000 +
0.25 × (32000 – 32000).
21.  No. Their individual tax is $5,200 each, and if
they married, they would pay $10,400. Actu
ally, taxpayers in higher tax brackets (which
our program does not model) may pay higher
taxes when they marry, a phenomenon known
as the marriage penalty.
22.  Change  else in line 22 to
else if (maritalStatus.equals("N"))
and add another branch after line 25:
else
{
System.out.println(
"Error: Please answer Y or N.");
}
23.  The higher tax rate is only applied on the
income in the higher bracket. Suppose you are
single and make $31,900. Should you try to get
a $200 raise? Absolutely: you get to keep 90
percent of the first $100 and 75 percent of the
next $100.
24.
25.  The “True” arrow from the first decision
points into the “True” branch of the sec ond
decision, creating spaghetti code.
26.  Here is one solution. In Section 5.7, you will
see how you can combine the condi tions for a
more elegant solution.
27.
28.
True
False
temp  < 0?
Print “Frozen”
Read  temp
True
False
Input < 0?
Status = “Error”
True
False
Input > 100?
Status = “OK”
Status = “Error”
True
False
Print  Print “Error”
x < 0?
Read  x
True
False
temp  < 0?
Print “Ice”
True
False
temp  > 100?
Print “Steam”
Print “Liquid”
Read  temp
answers to self-Check Questions 239
29.
Test
Case
Expected
Output
Comment
12 12 Below 13th floor
14 13 Above 13th floor
13 ? The specification is not clear— See
Section 5.8 for a version of this
program with error handling
30.  A boundary test case is a price of $128. A 16
percent discount should apply because the
problem statement states that the larger dis
count applies if the price is at least $128. Thus,
the expected output is $107.52.
31.
Test
Case
Expected
Output
Comment
9 Most structures fall
7.5 Many buildings destroyed
6.5 Many buildings ...
5 Damage to poorly...
3 No destruction...
8.0 Most structures fall  Boundary case. In this
program, boundary cases
are not as significant
because the behavior of
an earthquake changes
gradually.
-1 The specification is not
clear—see Self Check
18 for a version of this
program with error
handling.
32.
Test Case Expected Output Comment
(0.5, 0.5)  inside
(4, 2)  outside
(0, 2) on the boundary Exactly on the boundary
(1.414, 1.414) on the boundary Close to the boundary
(0, 1.9) inside Not less than 1 mm
from the boundary
(0, 2.1) outside Not less than 1 mm
from the boundary
33.  x == 0 && y == 0
34.  x == 0 || y == 0
35.  (x == 0 && y != 0) || (y == 0 && x != 0)
36.  The same as the value of  frozen .
37.  You are guaranteed that there are no other
values. With strings or integers, you would
need to check that no values such as  "maybe"
or –1 enter your calculations.
38.  (a)  Error: The floor must be between 1 and 20.
(b)  Error: The floor must be between 1 and 20.
(c)  19  (d)  Error: Not an integer.
39.  floor == 13 || floor <= 0 || floor > 20
40.  Check for  in.hasNextDouble() , to make sure a
researcher didn’t supply an input such as  oh
my . Check for  weight <= 0 , because any rat must
surely have a positive weight. We don’t know
how giant a rat could be, but the New Guinea
rats weighed no more than 2 kg. A regular
house rat (rattus rattus) weighs up to 0.2 kg, so
we’ll say that any weight > 10 kg was surely an
input error, perhaps confusing grams and kilo
grams. Thus, the checks are
if (in.hasNextDouble())
{
double weight = in.nextDouble();
if (weight < 0)
{
System.out.println(
"Error: Weight cannot be negative.");
}
else if (weight > 10)
{
System.out.println(
"Error: Weight > 10 kg.");
}
else
{
Process valid weight.
}
}
else
}
System.out.print("Error: Not a number");
}
41.  The second input fails, and the program termi
nates without printing anything.
6
C h a p t e r
241
Loops
to implement  while ,  for , and  do loops
to hand-trace the execution of a program
to learn to use common loop algorithms
to understand nested loops
to implement programs that read and process data sets
to use a computer for simulations
to learn about the debugger
C h a p t e r G o a L s
C h a p t e r C o n t e n t s
6.1 The while loop 242
Syntax 6.1: while statement 243
Common Error 6.1: Don’t think “are We
there Yet?” 247
Common Error 6.2: Infinite Loops 248
Common Error 6.3: off-by-one errors 248
6.2 problem Solving:
hand-Tracing 249
Computing & Society 6.1: software piracy 253
6.3 The for loop 254
Syntax 6.2: for statement 254
Programming Tip 6.1: Use for Loops for their
Intended purpose only 259
Programming Tip 6.2: Choose Loop Bounds that
Match Your task 260
Programming Tip 6.3: Count Iterations 260
Special Topic 6.1: Variables Declared in a
for Loop header 261
6.4 The do loop 262
Programming Tip 6.4: Flowcharts for Loops 263
6.5 applicaTion: proceSSing
SenTinel valueS 263
Special Topic 6.2: redirection of Input
and output 266
Special Topic 6.3: the “Loop and a half”
problem 266
Special Topic 6.4: the break and continue
statements 267
6.6 problem Solving:
SToryboardS 269
6.7 common loop algoriThmS 272
How To 6.1: Writing a Loop 276
Worked Example 6.1: Credit Card processing
6.8 neSTed loopS 279
Worked Example 6.2: Manipulating the pixels
in an Image
6.9 applicaTion: random numberS 
and SimulaTionS 283
6.10 uSing a debugger 286
How To 6.2: Debugging 289
Worked Example 6.3: a sample
Debugging session
Computing & Society 6.2: the First Bug 291
© photo75/iStockphoto.
242
In a loop, a part of a program is repeated over and over,
until a specific goal is reached. Loops are important for
calculations that require repeated steps and for processing
input consisting of many data items. In this chapter, you will
learn about loop statements in Java, as well as techniques
for writing programs that process input and simulate
activities in the real world.
6.1 the  while Loop
In this section, you will learn about loop statements that
repeatedly execute instructions until a goal has been
reached.
Recall the investment problem from Chapter 1. You
put $10,000 into a bank account that earns 5 percent inter
est per year. How many years does it take for the account
balance to be double the original investment?
In Chapter 1 we developed the following algorithm for
this problem:
Start with a year value of 0, a column for the interest, and a balance of $10,000.
year interest  balance
0  $10,000
Repeat the following steps while the balance is less than $20,000.
Add 1 to the year value.
Compute the interest as balance x 0.05 (i.e., 5 percent interest).
Add the interest to the balance.
Report the final year value as the answer.
You now know how to declare and update the variables in Java. What you don’t yet
know is how to carry out “Repeat steps while the balance is less than $20,000”.
© AlterYourReality/iStockphoto.
Because the interest
earned also earns interest,
a bank balance grows
exponentially.
In a particle accelerator, subatomic particles
traverse a loop-shaped tunnel multiple times,
gaining the speed required for physical experiments.
Similarly, in computer science, statements in a
loop are executed while a condition is true.
6.1 the while Loop 243
In Java, the  while statement implements such a
repetition (see Syntax 6.1). It has the form
while ( condition )
{
statements
}
As long as the condition remains true, the statements
inside the  while statement are executed. These state
ments are called the body of the  while statement.
In our case, we want to increment the year coun
ter and add interest while the balance is less than the
target balance of $20,000:
while (balance < targetBalance)
{
year++;
double interest = balance * RATE / 100;
balance = balance + interest;
}
A  while statement is an example of a loop. If you draw a flowchart, the flow of execu
tion loops again to the point where the condition is tested (see Figure 1).
figure 1  Flowchart of a  while Loop
False
True
Calculate
interest
Add  interest
to  balance
Increment
year
balance <
targetBalance ?
a loop executes
instructions
repeatedly while a
condition is true.
syntax 6.1  while statement
Lining up braces
is a good idea.
See page 184.
double balance = 0;
.
.
.
while (balance < targetBalance)
{
double interest = balance * RATE / 100;
balance = balance + interest;
}
If the condition
never becomes false,
an infinite loop occurs.
See page 248.
These statements
are executed while
the condition is true.
Don’t put a semicolon here!
See page 184.
Beware of “off-by-one”
errors in the loop condition.
See page 248.
Braces are not required if the body contains
a single statement, but it’s good to always use them.
See page 184.
This variable is declared outside the loop
and updated in the loop.
This variable is created
in each loop iteration.
while ( condition )
{
statements
}
Syntax
244 Chapter 6 Loops
When you declare a variable inside the loop body, the variable is created for each
iteration of the loop and removed after the end of each iteration. For example, con
sider the  interest variable in this loop:
while (balance < targetBalance)
{
year++;
double interest = balance * RATE / 100; 
balance = balance + interest;
}
// interest no longer declared here
A new  interest variable
is created in each iteration.
figure 2 
execution of the
Investment Loop
while (balance < targetBalance)
{
year++;
double interest = balance * RATE / 100;
balance = balance + interest;
}
System.out.println(year);
while (balance < targetBalance)
{
year++;
double interest = balance * RATE / 100;
balance = balance + interest;
}
Check the loop condition
1 The condition is true
while (balance < targetBalance)
{
year++;
double interest = balance * RATE / 100;
balance = balance + interest;
}
Execute the statements in the loop
2
while (balance < targetBalance)
{
year++;
double interest = balance * RATE / 100;
balance = balance + interest;
}
Check the loop condition again
3 The condition is still true
while (balance < targetBalance)
{
year++;
double interest = balance * RATE / 100;
balance = balance + interest;
}
After 15 iterations
4
The condition is
no longer true
Execute the statement following the loop
5
.
.
.
year = 0
balance = 10000
year = 1
interest = 500
balance = 10500
year = 1
balance = 10500
year = 15
balance = 20789.28
year = 15
balance = 20789.28
6.1 the while Loop 245
In contrast, the  balance and  year variables were declared outside the loop body. That
way, the same vari able is used for all iterations of the loop.
Here is the program that solves the investment problem. Figure 2 illustrates the
program’s execution.
section_1/investment.java
1 /**
2 A class to monitor the growth of an investment that
3 accumulates interest at a fixed annual rate.
4 */
5 public class Investment
6 {
7 private double balance;
8 private double rate;
9 private int year;
10
11 /**
12 Constructs an  Investment object from a starting balance and
13 interest rate.
14 @param aBalance  the starting balance
15 @param aRate  the interest rate in percent
16 */
17 public Investment(double aBalance, double aRate)
18 {
19 balance = aBalance;
20 rate = aRate;
21 year = 0;
22 }
23
24 /**
25 Keeps accumulating interest until a target balance has
26 been reached.
27 @param targetBalance  the desired balance
28 */
29 public void waitForBalance(double targetBalance)
30 {
31 while (balance < targetBalance)
32 {
33 year++;
34 double interest = balance * rate / 100;
35 balance = balance + interest;
36 }
37 }
38
39 /**
40 Gets the current investment balance.
41 @return  the current balance
42 */
43 public double getBalance()
44 {
45 return balance;
46 }
47
48 /**
49 Gets the number of years this investment has accumulated
50 interest.
51 @return  the number of years since the start of the investment
246 Chapter 6 Loops
52 */
53 public int getYears()
54 {
55 return year;
56 }
57 }
section_1/investmentrunner.java
1 /**
2 This program computes how long it takes for an investment
3 to double.
4 */
5 public class InvestmentRunner
6 {
7 public static void main(String[] args)
8 {
9 final double INITIAL_BALANCE = 10000;
10 final double RATE = 5;
11 Investment invest = new Investment(INITIAL_BALANCE, RATE);
12 invest.waitForBalance(2 * INITIAL_BALANCE);
13 int years = invest.getYears();
14 System.out.println("The investment doubled after "
15 + years + " years");
16 }
17 }
program run
The investment doubled after 15 years.
1.  How many years does it take for the investment to triple? Modify the program
and run it.
2.  If the interest rate is 10 percent per year, how many years does it take for the
investment to double? Modify the program and run it.
3.  Modify the program so that the balance after each year is printed. How did you
do that?
4.  Suppose we change the program so that the condition of the  while loop is
while (balance <= targetBalance)
What is the effect on the program? Why?
5.  What does the following loop print?
int n = 1;
while (n < 100)
{
n = 2 * n;
System.out.print(n + " ");
}
practice it  Now you can try these exercises at the end of the chapter: R6.1, R6.5, E6.13.
© Nicholas Homrich/iStockphoto.
S e l f  c h e c k
6.1 the while Loop 247
table 1 while Loop examples
Loop output explanation
i = 0; sum = 0;
while (sum < 10)
{
i++; sum = sum + i;
Print i  and sum;
}
1 1
2 3
3 6
4 10
When  sum is 10, the loop condition is
false, and the loop ends.
i = 0; sum = 0;
while (sum < 10)
{
i++; sum = sum - i;
Print i  and sum;
}
1 -1
2 -3
3 -6
4 -10
. . .
Because  sum never reaches 10, this is an
“infinite loop” (see Common Error 6.2
on page 248).
i = 0; sum = 0;
while (sum < 0)
{
i++; sum = sum - i;
Print i  and sum;
}
(No output) The statement  sum < 0 is false when the
condition is first checked, and the loop
is never executed.
i = 0; sum = 0;
while (sum >= 10)
{
i++; sum = sum + i;
Print i  and sum;
}
(No output) The programmer probably thought,
“Stop when the sum is at least 10.”
However, the loop condition controls
when the loop is executed, not when it
ends (see Common Error 6.1 on page
247).
i = 0; sum = 0;
while (sum < 10) ;
{
i++; sum = sum + i;
Print i  and sum;
}
(No output, program
does not terminate)
Note the semicolon before the  { .
This loop has an empty body. It runs
forever, checking whether  sum < 10 and
doing nothing in the body.
don’t Think “are we There yet?”
When doing something repetitive, most of us want to know when
we are done. For exam ple, you may think, “I want to get at least
$20,000,” and set the loop condition to
balance >= targetBalance
But the  while loop thinks the opposite: How long am I allowed to
keep going? The correct loop condition is
while (balance < targetBalance)
In other words: “Keep at it while the balance is less than the target.”
When writing a loop condition, don’t ask, “Are we there yet?”
The condition determines how long the loop will keep going.
Common error 6.1
© John Bell/iStockphoto.
© MsSponge/iStockphoto.
248 Chapter 6 Loops
infinite loops
A very annoying loop error is an infinite loop: a loop that
runs forever and can be stopped only by killing the program
or restarting the computer. If there are output statements
in the program, then reams and reams of output flash by on
the screen. Otherwise, the program just sits there and hangs,
seeming to do nothing. On some systems, you can kill a hang
ing program by hitting Ctrl + C. On others, you can close the
window in which the program runs.
A common reason for infinite loops is forgetting to update
the variable that controls the loop:
int year = 1;
while (year <= 20)
{
double interest = balance * RATE / 100;
balance = balance + interest;
}
Here the programmer forgot to add a  year++ command in the loop. As a result, the year always
stays at 1, and the loop never comes to an end.
Another common reason for an infinite loop is accidentally incrementing a counter that
should be decremented (or vice versa). Consider this example:
int year = 20;
while (year > 0)
{
double interest = balance * RATE / 100;
balance = balance + interest;
year++;
}
The  year variable really should have been decremented, not incremented. This is a common
error because increment ing counters is so much more common than decrementing that your
fingers may type the  ++ on autopilot. As a con sequence,  year is always larger than 0, and the
loop never ends. (Actually,  year may eventually exceed the largest representable positive inte
ger and wrap around to a negative number. Then the loop ends—of course, with a com pletely
wrong result.)
off-by-one errors
Consider our computation of the number of years that are required to double an investment:
int year = 0;
while (balance < targetBalance)
{
year++;
balance = balance * (1 + RATE / 100);
}
System.out.println("The investment doubled after "
+ year + " years.");
Should  year start at 0 or at 1? Should you test for  balance < targetBalance or for  balance <= target-
Balance ? It is easy to be off by one in these expressions.
Common error 6.2
© John Bell/iStockphoto.
© ohiophoto/iStockphoto.
Like this hamster who can’t
stop running in the tread mill,
an infinite loop never ends.
Common error 6.3
© John Bell/iStockphoto.
6.2 problem solving: hand-tracing 249
Some people try to solve off-by-one errors by randomly inserting  +1 or  -1 until the pro
gram seems to work—a terrible strategy. It can take a long time to compile and test all the vari
ous possibilities. Expending a small amount of mental effort is a real time saver.
Fortunately, offbyone errors are easy to avoid, simply by
thinking through a couple of test cases and using the information
from the test cases to come up with a rationale for your decisions.
Should  year start at 0 or at 1? Look at a scenario with simple val
ues: an initial balance of $100 and an interest rate of 50 percent. After
year 1, the balance is $150, and after year 2 it is $225, or over $200. So
the investment doubled after 2 years. The loop executed two times,
incrementing  year each time. Hence  year must start at 0, not at 1.
year  balance
0  $100
1  $150
2  $225
In other words, the  balance variable denotes the balance after the end of the year. At the outset,
the  balance variable contains the balance after year 0 and not after year 1.
Next, should you use a  < or  <= comparison in the test? This is harder to figure out, because it
is rare for the balance to be exactly twice the initial balance. There is one case when this happens,
namely when the interest is 100 percent. The loop executes once. Now  year is 1, and  balance is
exactly equal to  2 * INITIAL_BALANCE . Has the investment doubled after one year? It has. There
fore, the loop should not execute again. If the test condition is  balance < targetBalance , the loop
stops, as it should. If the test condition had been  balance <= targetBalance , the loop would have
executed once more.
In other words, you keep adding interest while the balance has not yet doubled.
6.2 problem solving: hand-tracing
In Programming Tip 5.5, you learned about the method of handtracing. When you
handtrace code or pseudocode, you write the names of the variables on a sheet of
paper, mentally execute each step of the code, and update the variables.
It is best to have the code written or printed on a sheet of paper. Use a marker,
such as a paper clip, to mark the current line. Whenever a variable changes, cross out
the old value and write the new value below. When a program produces output, also
write down the output in another column.
Consider this example. What value is displayed?
int n = 1729;
int sum = 0;
while (n > 0)
{
int digit = n % 10;
sum = sum + digit;
n = n / 10;
}
System.out.println(sum);
an off-by-one error
is a common error
when programming
loops. think through
simple test cases
to avoid this type
of error.
hand-tracing is a
simulation of code
execution in which
you step through
instructions and
track the values of
the variables.
250 Chapter 6 Loops
There are three variables:  n ,  sum , and  digit .
The first two variables are initialized with 1729 and 0 before the loop is entered.
int n = 1729;
int sum = 0;
while (n > 0)
{
int digit = n % 10;
sum = sum + digit;
n = n / 10;
}
System.out.println(sum);
Because  n is greater than zero, enter the loop. The variable  digit is set to 9 (the remain
der of dividing 1729 by 10). The variable  sum is set to 0 + 9 = 9.
int n = 1729;
int sum = 0;
while (n > 0)
{
int digit = n % 10;
sum = sum + digit;
n = n / 10;
}
System.out.println(sum);
Finally in this iteration,  n becomes 172. (Recall that the remainder in the division
1729 / 10 is discarded because both argu ments are integers.)
Cross out the old values and write the new ones under the old ones.
int n = 1729;
int sum = 0;
while (n > 0)
{
int digit = n % 10;
sum = sum + digit;
n = n / 10;
}
System.out.println(sum);
Now check the loop condition again.
int n = 1729;
int sum = 0;
while (n > 0)
{
int digit = n % 10;
sum = sum + digit;
n = n / 10;
}
System.out.println(sum);
n  sum  digit
n  sum  digit
1729  0
© Yvan Dube/iStockphoto.
n  sum  digit
1729  0
9  9
© Yvan Dube/iStockphoto.
n  sum  digit
1729  0
172  9  9
© Yvan Dube/iStockphoto.© Yvan Dube/iStockphoto.
6.2 problem solving: hand-tracing 251
Because  n is still greater than zero, repeat
the loop. Now  digit becomes 2,  sum is set to
9 + 2 = 11, and  n is set to 17.
Repeat the loop once again, setting  digit to
7,  sum to 11 + 7 = 18, and  n to 1.
Enter the loop for one last time. Now  digit
is set to 1,  sum to 19, and  n becomes zero.
int n = 1729;
int sum = 0;
while (n > 0)
Because  n equals zero,
this condition is not true.
{
int digit = n % 10;
sum = sum + digit;
n = n / 10;
}
System.out.println(sum);
The condition  n > 0 is now false. Continue with the statement after the loop.
int n = 1729;
int sum = 0;
while (n > 0)
{
int digit = n % 10;
sum = sum + digit;
n = n / 10;
}
System.out.println(sum);
This statement is an output statement. The value that is output is the value of  sum ,
which is 19.
n  sum  digit
1729  0
172  9  9
17  11  2
n  sum  digit
1729  0
172  9  9
17  11  2
1  18  7
n  sum  digit
1729  0
172  9  9
17  11  2
1  18  7
0  19  1
© Yvan Dube/iStockphoto.
n  sum  digit  output
1729  0
172  9  9
17  11  2
1  18  7
0  19  1  19
© Yvan Dube/iStockphoto.
Camera: © james steidl/iStockphoto.
Globe: © Alex Slobodkin/iStockphoto.
A N IM AT IO N
Tracing a Loop
252 Chapter 6 Loops
Of course, you can get the same answer by just running the code. However, hand
tracing can give you an insight that you would not get if you simply ran the code.
Consider again what happens in each itera tion:
• We extract the last digit of  n .
• We add that digit to  sum .
• We strip the digit off  n .
In other words, the loop forms the sum of the digits in  n . You now know what the
loop does for any value of  n , not just the one in the example. (Why would anyone
want to form the sum of the digits? Operations of this kind are useful for checking
the validity of credit card numbers and other forms of ID numbers.)
Handtracing does not just help you understand code that works correctly. It is
a powerful technique for finding errors in your code. When a program behaves in a
way that you don’t expect, get out a sheet of paper and track the values of the vari
ables as you mentally step through the code.
You don’t need a working program to do handtracing. You can handtrace
pseudocode. In fact, it is an excellent idea to handtrace your pseudocode before you
go to the trouble of translating it into actual code, to confirm that it works correctly.
6.  Handtrace the following code, showing the value of  n and the output.
int n = 5;
while (n >= 0)
{
n--;
System.out.print(n);
}
7.  Handtrace the following code, showing the value of  n and the output. What
potential error do you notice?
int n = 1;
while (n <= 3)
{
System.out.print(n + ", ");
n++;
}
8.  Handtrace the following code, assuming that  a is 2 and  n is 4. Then explain what
the code does for arbitrary values of  a and  n.
int r = 1;
int i = 1;
while (i <= n)
{
r = r * a;
i++;
}
9.  Trace the following code. What error do you observe?
int n = 1;
while (n != 50)
{
System.out.println(n);
n = n + 10;
}
hand-tracing can
help you understand
how an unfamiliar
algorithm works.
hand-tracing can
show errors in code
or pseudocode.
© Nicholas Homrich/iStockphoto.
S e l f  c h e c k
6.2 Problem Solving: Hand-Tracing  253
10.  The following pseudocode is intended to count the number of digits in the
number  n :
count = 1
temp = n
while (temp > 10)
Increment count.
Divide temp by 10.0.
Trace the pseudocode for  n = 123 and  n = 100. What error do you find?
Practice It  Now you can try these exercises at the end of the chapter: R6.3, R6.6.
payment.
Or you may
have been
frustrated
by a copy
protection
device on your
music player
that made
it dif cult
for you to
listen  to 
s o n g s
that you
paid for.
A d m i t -
tedly, it can
be dif cult to have a lot of sympathy
for a musical ensemble whose pub-
lisher charges a lot of money for what
seems to have been very little effort
on their part, at least when compared
to the effort that goes into designing
and implementing a software pack-
age. Nevertheless, it seems only fair
that artists and authors receive some
compensation for their efforts. How
to pay artists, authors, and program-
mers fairly, without bur dening honest
customers, is an unsolved problem
at the time of this writing, and many
computer scientists are engaged in
research in this area.
As you read this, you
will have written a few
computer programs and experienced
firsthand how much effort it takes to
write even the hum blest of programs.
Writing a real soft ware product, such
as a financial application or a computer
game, takes a lot of time and money.
Few people, and fewer companies, are
going to spend that kind of time and
money if they don’t have a reasonable
chance to make more money from their
effort. (Actually, some companies give
away their software in the hope that
users will upgrade to more elaborate
paid versions. Other companies give
away the software that enables users to
read and use files but sell the software
needed to create those files. Finally,
there are individuals who donate their
time, out of enthusiasm, and produce
programs that you can copy freely.)
When selling software, a company
must rely on the honesty of its cus-
tomers. It is an easy matter for an
unscrupulous person to make copies
of computer programs without paying
for them. In most countries that is ille-
gal. Most governments provide legal
protection, such as copyright laws and
patents, to encourage the develop-
ment of new products. Countries that
tolerate widespread piracy have found
that they have an ample cheap supply
of foreign software, but no local man-
ufacturers willing to design good soft-
ware for their own citizens, such as
word processors in the local script or
financial programs adapted to the local
tax laws.
When a mass market for software
first appeared, vendors were enraged
by the money they lost through piracy.
They tried to fight back by var ious
schemes to ensure that only the legiti-
mate owner could use the soft ware,
such as dongles—devices that must
be attached to a printer port before
the software will run. Legitimate users
hated these mea sures. They paid for
the software, but they had to suffer
through inconve niences, such as hav-
ing multiple don gles stick out from
their computer. In the United States,
market pressures forced most vendors
to give up on these copy protection
schemes, but they are still common-
place in other parts of the world.
Because it is so easy and inexpen-
sive to pirate software, and the chance
of being found out is minimal, you
have to make a moral choice for your-
self. If a package that you would really
like to have is too expensive for your
budget, do you steal it, or do you stay
honest and get by with a more afford-
able product?
Of course, piracy is not limited to
software. The same issues arise for
other digital products as well. You may
have had the opportunity to obtain
copies of songs or movies with out
© RapidEye/iStockphoto.
Computing & Society 6.1  Software Piracy
© Media Bakery.
254 Chapter 6 Loops
6.3 the  for Loop
It often happens that you want to execute a sequence of statements a given number
of times. You can use a  while loop that is controlled by a counter, as in the following
example:
int counter = 1; //  Initialize the counter
while (counter <= 10) //  Check the counter
{
System.out.println(counter);
counter++; //  Update the counter
}
Because this loop type is so common, there is a spe
cial form for it, called the  for loop (see Syntax 6.2).
for (int counter = 1; counter <= 10; counter++)
{
System.out.println(counter);
}
Some people call this loop count-controlled. In con
trast, the  while loop of the preceding section can be
called an event-controlled loop because it executes
until an event occurs; namely that the balance reaches
the target. Another commonly used term for a
countcontrolled loop is definite. You know from
the outset that the loop body will be executed a
definite number of times; ten times in our example.
In contrast, you do not know how many iterations it
takes to accumulate a target balance. Such a loop is
called indefinite.
the  for loop is
used when a
value runs from a
starting point to an
ending point with a
constant increment
or decrement.
© Enrico Fianchini/iStockphoto.
You can visualize the  for loop as
an orderly sequence of steps.
syntax 6.2  for statement
for (int i = 5; i <= 10; i++)
{
sum = sum + i;
}
This loop executes 6 times.
See page 260.
This initialization
happens once
before the loop starts.
The condition is
checked before
each iteration.
This update is
executed after
each iteration.
The variable  i is
defined only in this  for loop.
See page 261.
These three
expressions should be related.
See page 259.
for ( initialization ;  condition ;  update )
{
statements
}
Syntax
6.3 the for Loop 255
The  for loop neatly groups the initialization, condition, and update expressions
together. However, it is important to realize that these expressions are not executed
together (see Figure 3).
• The initialization is executed once, before the loop is entered.  1
• The condition is checked before each iteration.  2 5
• The update is executed after each iteration.  4
A  for loop can count down instead of up:
for (int counter = 10; counter >= 0; counter--) . . .
The increment or decrement need not be in steps of 1:
for (int counter = 0; counter <= 10; counter = counter + 2) . . .
See Table 2 on page 258 for additional variations.
So far, we have always declared the  counter variable in the loop initialization:
for (int counter = 1; counter <= 10; counter++)
{
. . .
}
// counter  no longer declared here
Camera: © james steidl/iStockphoto.
Globe: © Alex Slobodkin/iStockphoto.
A N IM AT IO N
The  for Loop
figure 3 
execution of a
for Loop
for (int counter = 1; counter <= 10; counter++)
{
System.out.println(counter);
}
Initialize counter
1
for (int counter = 1; counter <= 10; counter++)
{
System.out.println(counter);
}
Check condition
2
for (int counter = 1; counter <= 10; counter++)
{
System.out.println(counter);
}
Execute loop body
3
for (int counter = 1; counter <= 10; counter++)
{
System.out.println(counter);
}
Update counter
4
for (int counter = 1; counter <= 10; counter++)
{
System.out.println(counter);
}
Check condition again
5
counter = 1
counter = 1
counter = 1
counter = 2
counter = 2
256 Chapter 6 Loops
Such a variable is declared for all iterations of the loop, but you cannot use it after
the loop. If you declare the  counter variable before the loop, you can continue to use
it after the loop:
int counter;
for (counter = 1; counter <= 10; counter++)
{
. . .
}
// counter  still declared here
A common use of the  for loop is to traverse all characters of a string:
for (int i = 0; i < str.length(); i++)
{
char ch = str.charAt(i);
Process ch .
}
Note that the counter variable  i starts at 0, and the loop is terminated when  i reaches
the length of the string. For example, if  str has length 5,  i takes on the values 0, 1, 2, 3,
and 4. These are the valid positions in the string.
Here is another typical use of the  for loop. We want to compute the growth of our
savings account over a period of years, as shown in this table:
Year Balance
1 10500.00
2 11025.00
3 11576.25
4 12155.06
5 12762.82
The  for loop pattern applies because the variable
year starts at 1 and then moves in constant incre
ments until it reaches the target:
for (int year = 1; year <= numberOfYears; year++)
{
Update balance.
}
Following is the complete program. Figure 4
shows the corresponding flowchart.
figure 4  Flowchart of a  for Loop
True
False
year++
year ≤
numberOfYears  ?
year = 1
Update balance
6.3 the for Loop 257
section_3/investment.java
1 /**
2 A class to monitor the growth of an investment that
3 accumulates interest at a fixed annual rate.
4 */
5 public class Investment
6 {
7 private double balance;
8 private double rate;
9 private int year;
10
11 /**
12 Constructs an  Investment object from a starting balance and
13 interest rate.
14 @param aBalance  the starting balance
15 @param aRate  the interest rate in percent
16 */
17 public Investment(double aBalance, double aRate)
18 {
19 balance = aBalance;
20 rate = aRate;
21 year = 0;
22 }
23
24 /**
25 Keeps accumulating interest until a target balance has
26 been reached.
27 @param targetBalance  the desired balance
28 */
29 public void waitForBalance(double targetBalance)
30 {
31 while (balance < targetBalance)
32 {
33 year++;
34 double interest = balance * rate / 100;
35 balance = balance + interest;
36 }
37 }
38
39 /**
40 Keeps accumulating interest for a given number of years.
41 @param numberOfYears  the number of years to wait
42 */
43 public void waitYears(int numberOfYears)
44 {
45 for (int i = 1; i <= numberOfYears; i++)
46 {
47 double interest = balance * rate / 100;
48 balance = balance + interest;
49 }
50 year = year + n;
51 }
52
53 /**
54 Gets the current investment balance.
55 @return  the current balance
56 */
57 public double getBalance()
58 {
258 Chapter 6 Loops
59 return balance;
60 }
61
62 /**
63 Gets the number of years this investment has accumulated
64 interest.
65 @return  the number of years since the start of the investment
66 */
67 public int getYears()
68 {
69 return year;
70 }
71 }
section_3/investmentrunner.java
1 /**
2 This program computes how much an investment grows in
3 a given number of years.
4 */
5 public class InvestmentRunner
6 {
7 public static void main(String[] args)
8 {
9 final double INITIAL_BALANCE = 10000;
10 final double RATE = 5;
11 final int YEARS = 20;
12 Investment invest = new Investment(INITIAL_BALANCE, RATE);
13 invest.waitYears(YEARS);
14 double balance = invest.getBalance();
15 System.out.printf("The balance after %d years is %.2f\n",
16 YEARS, balance);
17 }
18 }
program run
The balance after 20 years is 26532.98
table 2 for Loop examples
Loop Values of  i Comment
for (i = 0; i <= 5; i++)  0 1 2 3 4 5 Note that the loop is executed 6 times. (See
Programming Tip 6.3 on page 260.)
for (i = 5; i >= 0; i--)  5 4 3 2 1 0 Use  i-- for decreasing values.
for (i = 0; i < 9; i = i + 2)  0 2 4 6 8 Use  i = i + 2 for a step size of 2.
for (i = 0; i != 9; i = i + 2)  0 2 4 6 8 10 12 14 …
(infinite loop)
You can use  < or  <= instead of  != to avoid
this problem.
for (i = 1; i <= 20; i = i * 2)  1 2 4 8 16 You can specify any rule for modifying  i ,
such as doubling it in every step.
for (i = 0; i < str.length(); i++)  0 1 2 … until the last valid
index of the string  str
In the loop body, use the expression
str.charAt(i) to get the  i th character.
6.3 the for Loop 259
11.  Write the  for loop of the  Investment class as a  while loop.
12.  How many numbers does this loop print?
for (int n = 10; n >= 0; n--)
{
System.out.println(n);
}
13.  Write a  for loop that prints all even numbers between 10 and 20 (inclusive).
14.  Write a  for loop that computes the sum of the integers from 1 to  n .
15.  How would you modify the  InvestmentRunner.java program to print the balances
after 20, 40, …, 100 years?
practice it  Now you can try these exercises at the end of the chapter: R6.4, R6.10, E6.8, E6.12.
use  for  loops for Their intended purpose only
A  for loop is an idiom for a loop of a particular form. A value runs from the start to the end,
with a constant incre ment or decrement.
The compiler won’t check whether the initialization, condition, and update expressions are
related. For example, the following loop is legal:
//  Confusing—unrelated expressions
for (System.out.print("Inputs: "); in.hasNextDouble(); sum = sum + x)
{
x = in.nextDouble();
}
However, programmers reading such a  for loop will be confused because it does not match
their expectations. Use a  while loop for iterations that do not follow the  for idiom.
You should also be careful not to update the loop counter in the body of a  for loop. Con
sider the following exam ple:
for (int counter = 1; counter <= 100; counter++)
{
if (counter % 10 == 0) //  Skip values that are divisible by 10
{
counter++; //  Bad style—you should not update the counter in a  for loop
}
System.out.println(counter);
}
Updating the counter inside a  for loop is confusing because the counter is updated again at the
end of the loop itera tion. In some loop iterations,  counter is incremented once, in others twice.
This goes against the intuition of a pro grammer who sees a  for loop.
If you find yourself in this situation, you can either change from a  for loop to a  while loop,
or implement the “skipping” behavior in another way. For example:
for (int counter = 1; counter <= 100; counter++)
{
if (counter % 10 != 0) //  Skip values that are divisible by 10
{
System.out.println(counter);
}
}
© Nicholas Homrich/iStockphoto.
S e l f  c h e c k
programming tip 6.1
© Eric Isselé/iStockphoto.
260 Chapter 6 Loops
choose loop bounds That match your Task
Suppose you want to print line numbers that go from 1 to 10. Of course, you will use a loop:
for (int i = 1; i <= 10; i++)
The values for  i are bounded by the relation  1 ≤  i ≤  10 . Because there are ≤ on both bounds, the
bounds are called symmetric bounds.
When traversing the characters in a string, it is more natural to use the bounds
for (int i = 0; i < str.length(); i++)
In this loop,  i traverses all valid positions in the string. You can access the  i th character as
str.charAt(i) . The values for  i are bounded by  0 ≤  i <  str.length() , with a ≤ to the left and a <
to the right. That is appropriate, because  str.length() is not a valid position. Such bounds are
called asymmetric bounds.
In this case, it is not a good idea to use symmetric bounds:
for (int i = 0; i <= str.length() - 1; i++) //  Use  < instead
The asymmetric form is easier to understand.
count iterations
Finding the correct lower and upper bounds for an iteration can be confusing. Should you
start at 0 or at 1? Should you use  <= b or  < b as a termination condition?
Counting the number of iterations is a very useful device for better understanding a loop.
Counting is easier for loops with asymmetric bounds. The loop
for (int i = a; i < b; i++)
is executed  b - a times. For example, the loop traversing the characters in a string,
for (int i = 0; i < str.length(); i++)
runs  str.length() times. That makes perfect sense, because there are  str.length() characters in
a string.
The loop with symmetric bounds,
for (int i = a; i <= b; i++)
is executed  b - a + 1 times. That “+1” is the source of many programming errors.
For example,
for (int i = 0; i <= 10; i++)
runs 11 times. Maybe that is what you want; if not, start at 1 or use  < 10 .
One way to visualize this “+1” error is
by looking at a fence. Each section has one
fence post to the left, and there is a final post
on the right of the last section. Forgetting to
count the last value is often called a “fence
post error”.
How many posts do you need for a fence
with four sections? It is easy to be “off by one”
with problems such as this one.
programming tip 6.2
© Eric Isselé/iStockphoto.
programming tip 6.3
© Eric Isselé/iStockphoto.
© akaplummer/iStockphoto.
6.3 the for Loop 261
variables declared in a  for  loop header
As mentioned, it is legal in Java to declare a variable in the header of a  for loop. Here is the
most common form of this syntax:
for (int i = 1; i <= n; i++)
{
. . .
}
// i  no longer defined here
The scope of the variable extends to the end of the  for loop. Therefore,  i is no longer defined
after the loop ends. If you need to use the value of the variable beyond the end of the loop, then
you need to declare it outside the loop. In this loop, you don’t need the value of  i —you know
it is  n + 1  when the loop is finished. (Actually, that is not quite true—it is possible to break
out of a loop before its end; see Special Topic 6.4 on page 267). When you have two or more exit
condi tions, though, you may still need the variable. For example, consider the loop
for (i = 1; balance < targetBalance && i <= n; i++)
{
. . .
}
You want the balance to reach the target but you are willing to wait only a certain number of
years. If the balance doubles sooner, you may want to know the value of  i . Therefore, in this
case, it is not appropriate to declare the vari able in the loop header.
Note that the variables named  i in the following pair of  for loops are independent:
for (int i = 1; i <= 10; i++)
{
System.out.println(i * i);
}
for (int i = 1; i <= 10; i++) //  Declares a new variable  i
{
System.out.println(i * i * i);
}
In the loop header, you can declare multiple variables, as long as they are of the same type, and
you can include mul tiple update expressions, separated by commas:
for (int i = 0, j = 10; i <= 10; i++, j–-)
{
. . .
}
However, many people find it confusing if a  for loop controls more than one variable. I rec
ommend that you not use this form of the  for statement (see Programming Tip 6.2 on page 260).
Instead, make the  for loop control a single counter, and update the other variable explicitly:
int j = 10;
for (int i = 0; i <= 10; i++)
{
. . .
j––;
}
special topic 6.1
© Eric Isselé/iStockphoto.
262 Chapter 6 Loops
6.4 the  do Loop
Sometimes you want to execute the body of a loop at least once and perform the loop
test after the body is executed. The  do loop serves that purpose:
do
{
statements
}
while ( condition );
The body of the  do loop is executed first, then the condition is tested.
Some people call such a loop a post-test loop because
the condition is tested after completing the loop body.
In contrast,  while and  for loops are pre-test loops. In
those loop types, the condition is tested before enter
ing the loop body.
A typical example for a  do loop is input validation.
Suppose you ask a user to enter a value < 100. If the
user doesn’t pay attention and enters a larger value,
you ask again, until the value is correct. Of course, you
cannot test the value until the user has entered it. This
is a perfect fit for the  do loop (see Figure 5):
int value;
do
{
System.out.print("Enter an integer < 100: ");
value = in.nextInt();
}
while (value >= 100);
figure 5  Flowchart of a  do Loop
16.  Suppose that we want to check for inputs that are at least 0 and at most 100.
Modify the  do loop for this check.
17.  Rewrite the input check  do loop using a  while loop. What is the disadvantage of
your solution?
18.  Suppose Java didn’t have a  do loop. Could you rewrite any  do loop as a  while
loop?
19.  Write a  do loop that reads integers and computes their sum. Stop when reading
the value 0.
20.  Write a  do loop that reads integers and computes their sum. Stop when reading a
zero or the same value twice in a row. For example, if the input is 1 2 3 4 4, then
the sum is 14 and the loop stops.
practice it  Now you can try these exercises at the end of the chapter: R6.9, R6.16, R6.17.
the  do loop is
appropriate when
the loop body
must be executed
at least once.
full code example
Go to  wiley.com/go/
javacode to download
a program that
illustrates the use of
the  do loop for input
validation.
True
False
value ≥ 100?
Prompt user
to enter
a value <  100
Copy the input
to  value
© Nicholas Homrich/iStockphoto.
S e l f  c h e c k
6.5 application: processing sentinel Values 263
flowcharts for loops
In Section 5.5, you learned how to use flowcharts to visualize the flow of control in a program.
There are two types of loops that you can include in a flowchart; they correspond to a  while
loop and a  do loop in Java. They differ in the placement of the condition—either before or after
the loop body.
False
True
Loop body
Condition?
True
False
Loop body
Condition?
As described in Section 5.5, you want to avoid “spaghetti code” in your flowcharts. For loops,
that means that you never want to have an arrow that points inside a loop body.
6.5 application: processing sentinel Values
In this section, you will learn how to write loops that read and process a sequence of
input values.
Whenever you read a sequence of inputs, you need to have some method of indi
cating the end of the sequence. Sometimes you are lucky and no input value can be
zero. Then you can prompt the user to keep entering numbers, or 0 to finish the
sequence. If zero is allowed but negative numbers
are not, you can use –1 to indicate termination.
Such a value, which is not an actual input,
but serves as a signal for termination, is called a
sentinel.
Let’s put this technique to work in a program
that computes the average of a set of salary values.
In our sample program, we will use –1 as a sentinel.
An employee would surely not work for a negative
salary, but there may be volunteers who work for
free.
In the military, a sentinel guards a border or passage.
In computer science, a sentinel value denotes the end of an
input sequence or the border between input sequences.
programming tip 6.4
© Eric Isselé/iStockphoto.
© Rhoberazzi/iStockphoto.
a sentinel value
denotes the end of a
data set, but it is not
part of the data.
264 Chapter 6 Loops
Inside the loop, we read an input. If the input is not –1, we process it. In order to
compute the average, we need the total sum of all salaries, and the number of inputs.
salary = in.nextDouble();
if (salary != -1)
{
sum = sum + salary;
count++;
}
We stay in the loop while the sentinel value is not detected.
while (salary != -1)
{
. . .
}
There is just one problem: When the loop is entered for the first time, no data value
has been read. We must make sure to initialize  salary with some value other than the
sentinel:
double salary = 0;
//  Any value other than –1 will do
After the loop has finished, we compute and print the average. Here is the complete
program:
section_5/Sentineldemo.java
1 import java.util.Scanner;
2
3 /**
4 This program prints the average of salary values that are terminated with a sentinel.
5 */
6 public class SentinelDemo
7 {
8 public static void main(String[] args)
9 {
10 double sum = 0;
11 int count = 0;
12 double salary = 0;
13 System.out.print("Enter salaries, -1 to finish: ");
14 Scanner in = new Scanner(System.in);
15
16 //  Process data until the sentinel is entered
17
18 while (salary != -1)
19 {
20 salary = in.nextDouble();
21 if (salary != -1)
22 {
23 sum = sum + salary;
24 count++;
25 }
26 }
27
28 //  Compute and print the average
29
30 if (count > 0)
31 {
32 double average = sum / count;
6.5 application: processing sentinel Values 265
33 System.out.println("Average salary: " + average);
34 }
35 else
36 {
37 System.out.println("No data");
38 }
39 }
40 }
program run
Enter salaries, -1 to finish:  10 10 40 -1
Average salary: 20
Some programmers don’t like the “trick” of initializing the input variable with a value
other than the sen tinel. Another approach is to use a Boolean variable:
System.out.print("Enter salaries, -1 to finish: ");
boolean done = false;
while (!done)
{
value = in.nextDouble();
if (value == -1)
{
done = true;
}
else
{
Process value.
}
}
Special Topic 6.4 on page 267 shows an alternative mechanism for leaving such a loop.
Now consider the case in which any number (positive, negative, or zero) can be
an acceptable input. In such a situation, you must use a sentinel that is not a number
(such as the letter Q). As you have seen in Section 5.8, the condition
in.hasNextDouble()
is  false if the next input is not a floatingpoint number. Therefore, you can read and
process a set of inputs with the following loop:
System.out.print("Enter values, Q to quit: ");
while (in.hasNextDouble())
{
value = in.nextDouble();
Process value.
}
21.  What does the  SentinelDemo.java program print when the user immediately types
–1 when prompted for a value?
22.  Why does the  SentinelDemo.java program have two checks of the form
salary != -1
23.  What would happen if the declaration of the  salary variable in  SentinelDemo.java
was changed to
double salary = -1;
You can use a
Boolean variable to
control a loop. set
the variable before
entering the loop,
then set it to the
opposite to leave
the loop.
© Nicholas Homrich/iStockphoto.
S e l f  c h e c k
266 Chapter 6 Loops
24.  In the last example of this section, we prompt the user “Enter values, Q to quit: ”
What happens when the user enters a different letter?
25.  What is wrong with the following loop for reading a sequence of values?
System.out.print("Enter values, Q to quit: ");
do
{
double value = in.nextDouble();
sum = sum + value;
count++;
}
while (in.hasNextDouble());
practice it  Now you can try these exercises at the end of the chapter: R6.13, E6.17, E6.18.
redirection of input and output
Consider the  SentinelDemo program that computes the average
value of an input sequence. If you use such a program, then it is
quite likely that you already have the values in a file, and it seems
a shame that you have to type them all in again. The command
line interface of your operating system provides a way to link a
file to the input of a pro gram, as if all the characters in the file had
actually been typed by a user. If you type
java SentinelDemo < numbers.txt
the program is executed, but it no longer expects input from the keyboard. All input com
mands get their input from the file  numbers.txt . This process is called input redirection.
Input redirection is an excellent tool for testing programs. When you develop a program
and fix its bugs, it is bor ing to keep entering the same input every time you run the program.
Spend a few minutes putting the inputs into a file, and use redirection.
You can also redirect output. In this program, that is not terribly useful. If you run
java SentinelDemo < numbers.txt > output.txt
the file  output.txt contains the input prompts and the output, such as
Enter salaries, -1 to finish: Enter salaries, -1 to finish:
Enter salaries, -1 to finish: Enter salaries, -1 to finish:
Average salary: 15
However, redirecting output is obviously useful for programs that produce lots of output.
You can format or print the file con taining the output.
The “loop and a half” problem
Reading input data sometimes requires a loop such as the following, which is somewhat
unsightly:
boolean done = false;
while (!done)
{
String input = in.next();
if (input.equals("Q"))
{
done = true;
special topic 6.2
© Eric Isselé/iStockphoto.
Use input redirection to
read input from a file.
Use output redirection to
capture program output
in a file.
special topic 6.3
© Eric Isselé/iStockphoto.
6.5 application: processing sentinel Values 267
}
else
{
Process data.
}
}
The true test for loop termination is in the middle of the loop, not at the top. This is called a
“loop and a half”, because one must go halfway into the loop before knowing whether one
needs to terminate.
Some programmers dislike the introduction of an additional Boolean variable for loop
control. Two Java language features can be used to alleviate the “loop and a half” problem. I
don’t think either is a superior solution, but both approaches are fairly common, so it is worth
knowing about them when reading other people’s code.
You can combine an assignment and a test in the loop condition:
while (!(input = in.next()).equals("Q"))
{
Process data.
}
The expression
(input = in.next()).equals("Q")
means, “First call  in.next() , then assign the result to  input , then test whether it equals  "Q" ”.
This is an expression with a side effect. The primary purpose of the expression is to serve as a
test for the  while loop, but it also does some work—namely, reading the input and storing it in
the variable  input . In general, it is a bad idea to use side effects, because they make a program
hard to read and maintain. In this case, however, that practice is somewhat seductive, because
it eliminates the control variable  done , which also makes the code hard to read and maintain.
The other solution is to exit the loop from the middle, either by a  return statement or by a
break statement (see Special Topic 6.4 on page 267).
public void processInput(Scanner in)
{
while (true)
{
String input = in.next();
if (input.equals("Q"))
{
return;
}
Process data.
}
}
The  break  and  continue  Statements
You already encountered the  break statement in Special Topic 5.2, where it was used to exit a
switch statement. In addition to breaking out of a  switch statement, a  break statement can also
be used to exit a  while ,  for , or  do loop.
For example, the  break statement in the following loop terminates the loop when the end of
input is reached.
while (true)
{
special topic 6.4
© Eric Isselé/iStockphoto.
268 Chapter 6 Loops
String input = in.next();
if (input.equals("Q"))
{
break;
}
double x = Double.parseDouble(input);
data.add(x);
}
A loop with  break statements can be difficult to understand because you have to look closely
to find out how to exit the loop. However, when faced with the bother of introducing a sepa
rate loop control variable, some programmers find that  break statements are beneficial in the
“loop and a half” case. This issue is often the topic of heated (and quite unproductive) debate.
In this book, we won’t use the  break statement, and we leave it to you to decide whether you
like to use it in your own programs.
In Java, there is a second form of the  break statement that is used to break out of a nested
statement. The statement  break  label ; immediately jumps to the end of the statement that is
tagged with a label. Any statement (including  if and block statements) can be tagged with a
label—the syntax is
label :  statement
The labeled  break statement was invented to break out of a set of nested loops.
outerloop:
while ( outer loop condition )
{ . . .
while ( inner loop condition )
{ . . .
if ( something really bad happened )
{
break outerloop;
}
}
}
Jumps here if something really bad happened.
Naturally, this situation is quite rare. We recommend that you try to introduce additional
methods instead of using complicated nested loops.
Finally, there is the  continue statement, which jumps to the end of the current iteration of
the loop. Here is a pos sible use for this statement:
while (!done)
{
String input = in.next();
if (input.equals("Q"))
{
done = true;
continue; //  Jump to the end of the loop body
}
double x = Double.parseDouble(input);
data.add(x);
// continue  statement jumps here
}
By using the  continue statement, you don’t need to place the remainder of the loop code inside
an  else clause. This is a minor benefit. Few programmers use this statement.
6.6 problem solving: storyboards 269
6.6 problem solving: storyboards
When you design a program that interacts with a user, you need to make a plan for
that interaction. What information does the user provide, and in which order? What
information will your program display, and in which format? What should happen
when there is an error? When does the program quit?
This planning is similar to the development of a movie or a computer game, where
storyboards are used to plan action sequences. A storyboard is made up of panels that
show a sketch of each step. Annota tions explain what is happening and note any spe
cial situations. Storyboards are also used to develop software—see Figure 6.
Making a storyboard is very helpful when you begin designing a program. You
need to ask yourself which information you need in order to compute the answers
that the program user wants. You need to decide how to present those answers. These
are important considerations that you want to settle before you design an algorithm
for computing the answers.
Let’s look at a simple example. We want to write a program that helps users with
questions such as “How many tablespoons are in a pint?” or “How many inches are
30 centimeters?”
What information does the user provide?
• The quantity and unit to convert from
• The unit to convert to
What if there is more than one quantity? A user may have a whole table of centimeter
values that should be converted into inches.
What if the user enters units that our program doesn’t know how to handle, such
as ångström?
What if the user asks for impossible conversions, such as inches to gallons?
a storyboard
consists of annotated
sketches for each
step in an action
sequence.
Developing a
storyboard helps
you understand the
inputs and outputs
that are required for
a program.
figure 6 
storyboard for the
Design of a Web
application
Courtesy of Martin Hardee.
270 Chapter 6 Loops
Let’s get started with a storyboard panel. It is a good idea to write the user inputs in
a different color. (Underline them if you don’t have a color pen handy.)
What unit do you want to convert from? cm
What unit do you want to convert to? in
Enter values, terminated by zero
30
30 cm = 11.81 in
100
100 cm = 39.37 in
0
What unit do you want to convert from?
Format makes clear what got converted
Allows conversion of multiple values
Converting a Sequence of Values
The storyboard shows how we deal with a potential confusion. A user who wants to
know how many inches are 30 centimeters may not read the first prompt carefully
and specify inches. But then the output is “30 in = 76.2 cm”, alerting the user to the
problem.
The storyboard also raises an issue. How is the user supposed to know that “cm”
and “in” are valid units? Would “centimeter” and “inches” also work? What happens
when the user enters a wrong unit? Let’s make another storyboard to demonstrate
error handling.
What unit do you want to convert from? cm
What unit do you want to convert to? inches
Sorry, unknown unit.
What unit do you want to convert to? inch
Sorry, unknown unit.
What unit do you want to convert to? grrr
Handling Unknown Units (needs improvement)
To eliminate frustration, it is better to list the units that the user can supply.
From unit (in, ft, mi, mm, cm, m, km, oz, lb, g, kg, tsp, tbsp, pint, gal): cm
To unit: in
No need to list the units again
We switched to a shorter prompt to make room for all the unit names. Exercise R6.22
explores a different alternative.
There is another issue that we haven’t addressed yet. How does the user quit the
program? The first storyboard suggests that the program will go on forever.
We can ask the user after seeing the sentinel that terminates an input sequence.
6.6 problem solving: storyboards 271
From unit (in, ft, mi, mm, cm, m, km, oz, lb, g, kg, tsp, tbsp, pint, gal): cm
To unit: in
Enter values, terminated by zero
30
30 cm = 11.81 in
0
More conversions (y, n)? n
(Program exits)
Sentinel triggers the prompt to exit
Exiting the Program
As you can see from this case study, a storyboard is essential for developing a work
ing program. You need to know the flow of the user interaction in order to structure
your program.
26.  Provide a storyboard panel for a program that reads a number of test scores and
prints the average score. The program only needs to process one set of scores.
Don’t worry about error handling.
27.  Google has a simple interface for converting units. You just type the question,
and you get the answer.
Make storyboards for an equivalent interface in a Java program. Show a scenario
in which all goes well, and show the handling of two kinds of errors.
28.  Consider a modification of the program in Self Check 26. Suppose we want to
drop the lowest score before computing the average. Provide a storyboard for
the situation in which a user only provides one score.
29.  What is the problem with implementing the following storyboard in Java?
Enter scores: 90 80 90 100 80
The average is 88
Enter scores: 100 70 70 100 80
The average is 88
Enter scores: -1
(Program exits)
-1 is used as a sentinel to exit the program
Computing Multiple Averages
30.  Produce a storyboard for a program that compares the growth of a $10,000
investment for a given number of years under two interest rates.
practice it  Now you can try these exercises at the end of the chapter: R6.21, R6.22, R6.23.
© Nicholas Homrich/iStockphoto.
S e l f  c h e c k
272 Chapter 6 Loops
6.7 Common Loop algorithms
In the following sections, we discuss some of the most common algorithms that are
implemented as loops. You can use them as starting points for your loop designs.
6.7.1 sum and average Value
Computing the sum of a number of inputs is a very common task. Keep a running
total, a variable to which you add each input value. Of course, the total should be
initialized with 0.
double total = 0;
while (in.hasNextDouble())
{
double input = in.nextDouble();
total = total + input;
}
Note that the  total variable is declared outside the loop. We want the loop to update
a single variable. The  input variable is declared inside the loop. A separate variable is
created for each input and removed at the end of each loop iteration.
To compute an average, count how many values you have, and divide by the count.
Be sure to check that the count is not zero.
double total = 0;
int count = 0;
while (in.hasNextDouble())
{
double input = in.nextDouble();
total = total + input;
count++;
}
double average = 0;
if (count > 0)
{
average = total / count;
}
6.7.2 Counting Matches
You often want to know how many values fulfill a particular condition. For example,
you may want to count how many spaces are in a string. Keep a counter, a variable
that is initialized with 0 and incremented whenever there is a match.
int spaces = 0;
for (int i = 0; i < str.length(); i++)
{
char ch = str.charAt(i);
if (ch == ' ')
{
spaces++;
}
}
For example, if  str is  "My Fair Lady" ,  spaces is incremented twice (when  i is 2 and 7).
to compute an
average, keep a
total and a count
of all values.
to count values that
fulfill a condition,
check all values and
increment a counter
for each match.
6.7 Common Loop algorithms 273
Note that the  spaces variable is declared outside the loop. We want the loop to
update a single variable. The  ch variable is declared inside the loop. A separate variable
is created for each iteration and removed at the end of each loop iteration.
This loop can also be used for scanning inputs. The following loop reads text a
word at a time and counts the number of words with at most three letters:
int shortWords = 0;
while (in.hasNext())
{
String input = in.next();
if (input.length() <= 3)
{
shortWords++;
}
}
6.7.3 Finding the First Match
When you count the values that fulfill a condition, you need to look at all values.
However, if your task is to find a match, then you can stop as soon as the condition is
fulfilled.
Here is a loop that finds the first space in a string. Because we do not visit all ele
ments in the string, a  while loop is a better choice than a  for loop:
boolean found = false;
char ch = '?';
int position = 0;
while (!found && position < str.length())
{
ch = str.charAt(position);
if (ch == ' ') { found = true; }
else { position++; }
}
If a match was found, then  found is  true ,  ch is
the first matching character, and  position is
the index of the first match. If the loop did
not find a match, then  found remains  false
after the end of the loop.
Note that the variable  ch is declared out-
side the  while loop because you may want to
use the input after the loop has finished. If it
had been declared inside the loop body, you
would not be able to use it outside the loop.
In a loop that counts matches,
a counter is incremented
whenever a match is found.
© Hiob/iStockphoto.
If your goal is to find
a match, exit the loop
when the match
is found.
© drflet/iStockphoto.
When searching, you look at items until a
match is found.
274 Chapter 6 Loops
6.7.4 prompting Until a Match is Found
In the preceding example, we searched a string for a character that matches a condi
tion. You can apply the same process to user input. Suppose you are asking a user to
enter a positive value < 100. Keep asking until the user provides a correct input:
boolean valid = false;
double input = 0;
while (!valid)
{
System.out.print("Please enter a positive value < 100: ");
input = in.nextDouble();
if (0 < input && input < 100) { valid = true; }
else { System.out.println("Invalid input."); }
}
Note that the variable  input is declared outside the  while loop because you will want to
use the input after the loop has finished.
6.7.5 Maximum and Minimum
To compute the largest value in a sequence, keep a variable that stores the largest ele
ment that you have encountered, and update it when you find a larger one.
double largest = in.nextDouble();
while (in.hasNextDouble())
{
double input = in.nextDouble();
if (input > largest)
{
largest = input;
}
}
This algorithm requires that there is at least one input.
To compute the smallest value, simply reverse the comparison:
double smallest = in.nextDouble();
while (in.hasNextDouble())
{
double input = in.nextDouble();
if (input < smallest)
{
smallest = input;
}
}
to find the largest
value, update the
largest value seen so
far whenever you see
a larger one.
To find the height of the tallest bus rider,
remember the largest value so far, and
update it whenever you see a taller one.
© CEFutcher/iStockphoto.
6.7 Common Loop algorithms 275
6.7.6 Comparing adjacent Values
When processing a sequence of values in a loop, you sometimes need to compare a
value with the value that just preceded it. For example, suppose you want to check
whether a sequence of inputs, such as  1 7 2 9 9 4 9 , contains adjacent duplicates.
Now you face a challenge. Consider the typical loop for reading a value:
double input;
while (in.hasNextDouble())
{
input = in.nextDouble();
. . .
}
How can you compare the current input
with the preceding one? At any time,  input
contains the current input, overwriting the
previous one.
The answer is to store the previous input,
like this:
double input = 0;
while (in.hasNextDouble())
{
double previous = input;
input = in.nextDouble();
if (input == previous)
{
System.out.println("Duplicate input");
}
}
One problem remains. When the loop is entered for the first time,  input has not yet
been read. You can solve this problem with an initial input operation outside the loop:
double input = in.nextDouble();
while (in.hasNextDouble())
{
double previous = input;
input = in.nextDouble();
if (input == previous)
{
System.out.println("Duplicate input");
}
}
31.  What total is computed when no user input is provided in the algorithm in
Section 6.7.1?
32.  How do you compute the total of all positive inputs?
33.  What are the values of  position and  ch when no match is found in the algorithm
in Section 6.7.3?
34.  What is wrong with the following loop for finding the position of the first space
in a string?
boolean found = false;
for (int position = 0; !found && position < str.length(); position++)
{
to compare adjacent
inputs, store the
preceding input in
a variable.
© tingberg/iStockphoto.
When comparing adjacent values, store
the previous value in a variable.
full code example
Go to  wiley.com/go/
javacode to download
a program that
uses common loop
algorithms.
© Nicholas Homrich/iStockphoto.
S e l f  c h e c k
276 Chapter 6 Loops
char ch = str.charAt(position);
if (ch == ' ') { found = true; }
}
35.  How do you find the position of the last space in a string?
36.  What happens with the algorithm in Section 6.7.6 when no input is provided at
all? How can you overcome that problem?
practice it  Now you can try these exercises at the end of the chapter: E6.5, E6.9, E6.10.
Step 1  Decide what work must be done inside the loop.
Every loop needs to do some kind of repetitive work, such as
• Reading another item.
• Updating a value (such as a bank balance or total).
• Incrementing a counter.
If you can’t figure out what needs to go inside the loop, start by writing down the steps that
you would take if you solved the problem by hand. For example, with the temperature reading
problem, you might write
Read first value.
Read second value.
If second value is higher than the first, set highest temperature to that value, highest month to 2.
Read next value.
If value is higher than the first and second, set highest temperature to that value, highest month to 3.
Read next value.
If value is higher than the highest temperature seen so far, set highest temperature to that value,
highest month to 4.
. . .
Now look at these steps and reduce them to a set of uniform actions that can be placed into the
loop body. The first action is easy:
Read next value.
The next action is trickier. In our description, we used tests “higher than the first”, “higher
than the first and sec ond”, “higher than the highest temperature seen so far”. We need to settle
on one test that works for all iterations. The last formulation is the most general.
© Steve Simzer/iStockphoto.
hoW to 6.1  writing a loop
This How To walks you through the process of implementing a
loop statement. We will illustrate the steps with the following
example problem.
problem Statement  Read twelve temperature values (one for
each month) and display the number of the month with the high
est temperature. For example, according to  worldclimate.com , the
average maximum temperatures for Death Valley are (in order by
month, in degrees Celsius):
18.2 22.6 26.4 31.1 36.6 42.2 45.7 44.5 40.2 33.1 24.2 17.6
In this case, the month with the highest temperature (45.7 degrees
Celsius) is July, and the program should display 7.
© Stevegeer/iStockphoto.
6.7 Common Loop algorithms 277
Similarly, we must find a general way of setting the highest month. We need a variable that
stores the current month, running from 1 to 12. Then we can formulate the second loop action:
If value is higher than the highest temperature, set highest temperature to that value,
highest month to current month.
Altogether our loop is
Repeat
Read next value.
If value is higher than the highest temperature,
set highest temperature to that value,
set highest month to current month.
Increment current month.
Step 2  Specify the loop condition.
What goal do you want to reach in your loop? Typical examples are
• Has a counter reached its final value?
• Have you read the last input value?
• Has a value reached a given threshold?
In our example, we simply want the current month to reach 12.
Step 3  Determine the loop type.
We distinguish between two major loop types. A count-controlled loop is executed a defi
nite number of times. In an event-controlled loop, the number of iterations is not known in
advance—the loop is executed until some event happens.
Countcontrolled loops can be implemented as  for statements. For other loops, consider
the loop condition. Do you need to complete one iteration of the loop body before you can
tell when to terminate the loop? In that case, choose a  do loop. Otherwise, use a  while loop.
Sometimes, the condition for terminating a loop changes in the middle of the loop body. In
that case, you can use a Boolean variable that specifies when you are ready to leave the loop.
Follow this pattern:
boolean done = false;
while (!done)
{
Do some work.
If all work has been completed
{
done = true;
}
else
{
Do more work.
}
}
Such a variable is called a flag.
In summary,
• If you know in advance how many times a loop is repeated, use a  for loop.
• If the loop body must be executed at least once, use a  do loop.
• Otherwise, use a  while loop.
In our example, we read 12 temperature values. Therefore, we choose a  for loop.
Step 4  Set up variables for entering the loop for the first time.
List all variables that are used and updated in the loop, and determine how to initialize them.
Commonly, counters are initialized with 0 or 1, totals with 0.
278 Chapter 6 Loops
In our example, the variables are
current month
highest value
highest month
We need to be careful how we set up the highest temperature value. We can’t simply set it to
0. After all, our program needs to work with temperature values from Antarctica, all of which
may be negative.
A good option is to set the highest temperature value to the first input value. Of course,
then we need to remember to read in only 11 more values, with the current month starting at 2.
We also need to initialize the highest month with 1. After all, in an Australian city, we may
never find a month that is warmer than January.
Step 5  Process the result after the loop has finished.
In many cases, the desired result is simply a variable that was updated in the loop body. For
example, in our temper ature program, the result is the highest month. Sometimes, the loop
computes values that contribute to the final result. For example, suppose you are asked to
average the temperatures. Then the loop should compute the sum, not the average. After the
loop has completed, you are ready to compute the average: divide the sum by the number of
inputs.
Here is our complete loop.
Read first value; store as highest value.
highest month = 1
For current month from 2 to 12
Read next value.
If value is higher than the highest value
Set highest value to that value.
Set highest month to current month.
Step 6  Trace the loop with typical examples.
Hand trace your loop code, as described in Section 6.2. Choose example values that are not too
complex—execut ing the loop 3–5 times is enough to check for the most common errors. Pay
special attention when entering the loop for the first and last time.
Sometimes, you want to make a slight modification to make tracing feasible. For example,
when handtracing the investment doubling problem, use an interest rate of 20 percent rather
than 5 percent. When handtracing the tem perature loop, use 4 data values, not 12.
Let’s say the data are 22.6 36.6 44.5 24.2. Here is the walkthrough:
current month  current value  highest month  highest value
1  22.6
2  36.6  2  36.6
3  44.5  3  44.5
4  24.2
The trace demonstrates that highest month and highest value are properly set.
Step 7  Implement the loop in Java.
Here’s the loop for our example. Exercise E6.4 asks you to complete the program.
double highestValue;
highestValue = in.nextDouble();
int highestMonth = 1;
6.8 nested Loops 279
for (int currentMonth = 2; currentMonth <= 12; currentMonth++)
{
double nextValue = in.nextDouble();
if (nextValue > highestValue)
{
highestValue = nextValue;
highestMonth = currentMonth;
}
}
System.out.println(highestMonth);
6.8 nested Loops
In Section 5.4, you saw how to nest two  if statements. Similarly, complex iterations
sometimes require a nested loop: a loop inside another loop statement. When pro
cessing tables, nested loops occur naturally. An outer loop iterates over all rows of the
table. An inner loop deals with the columns in the current row.
In this section you will see how to print a table. For simplicity, we will simply print
the powers of x, x n , as in the table at right.
Here is the pseudocode for printing the table:
Print table header.
For x from 1 to 10
Print table row.
Print new line.
How do you print a table row? You need to print a
value for each expo nent. This requires a second loop.
For n from 1 to 4
Print x n .
This loop must be placed inside the preceding loop. We say that the inner loop is
nested inside the outer loop.
WorkeD exaMpLe 6.1  credit card processing
Learn how to use a loop to remove spaces from a credit card
number. Go to  wiley.com/go/javaexamples and download
Worked Example 6.1.
© MorePixels/iStockphoto.
When the body of
a loop contains
another loop, the
loops are nested. a
typical use of nested
loops is printing a
table with rows
and columns.
x 1 x 2 x 3 x 4
1 1 1 1
2 4 8 16
3 9 27 81
… … … …
10 100 1000 10000
The hour and minute displays in a digital clock are an
exam ple of nested loops. The hours loop 12 times, and
for each hour, the minutes loop 60 times.
© davejkahn/iStockphoto.
280 Chapter 6 Loops
figure 7 
Flowchart of a nested Loop
True
False
x ≤ 10 ?
x = 1
True
False
n ≤ 4 ?
n = 1
n++
Print x n
x++
Print new line
This loop is nested
in the outer loop.
There are 10 rows in the outer loop. For each x, the program prints four columns
in the inner loop (see Figure 7). Thus, a total of 10 × 4 = 40 values are printed.
Following is the complete program. Note that we also use two loops to print the
table header. However, those loops are not nested.
section_8/powerTable.java
1 /**
2 This program prints a table of powers of x.
3 */
4 public class PowerTable
5 {
6 public static void main(String[] args)
7 {
8 final int NMAX = 4;
9 final double XMAX = 10;
10
11 //  Print table header
12
13 for (int n = 1; n <= NMAX; n++)
14 {
15 System.out.printf("%10d", n);
16 }
17 System.out.println();
6.8 nested Loops 281
18 for (int n = 1; n <= NMAX; n++)
19 {
20 System.out.printf("%10s", "x ");
21 }
22 System.out.println();
23
24 //  Print table body
25
26 for (double x = 1; x <= XMAX; x++)
27 {
28 //  Print table row
29
30 for (int n = 1; n <= NMAX; n++)
31 {
32 System.out.printf("%10.0f", Math.pow(x, n));
33 }
34 System.out.println();
35 }
36 }
37 }
program run
1 2 3 4
x x x x
1 1 1 1
2 4 8 16
3 9 27 81
4 16 64 256
5 25 125 625
6 36 216 1296
7 49 343 2401
8 64 512 4096
9 81 729 6561
10 100 1000 10000
37.  Why is there a statement  System.out.println(); in the outer loop but not in the
inner loop?
38.  How would you change the program to display all powers from x 0 to x 5 ?
39.  If you make the change in Self Check 38, how many values are displayed?
40.  What do the following nested loops display?
for (int i = 0; i < 3; i++)
{
for (int j = 0; j < 4; j++)
{
System.out.print(i + j);
}
System.out.println();
}
41.  Write nested loops that make the following pattern of brackets:
[][][][]
[][][][]
[][][][]
practice it  Now you can try these exercises at the end of the chapter: R6.27, E6.14, E6.16.
© Nicholas Homrich/iStockphoto.
S e l f  c h e c k
282 Chapter 6 Loops
table 3 nested Loop examples
nested Loops output explanation
for (i = 1; i <= 3; i++)
{
for (j = 1; j <= 4; j++) {  Print "*" }
System.out.println();
}
****
****
****
Prints 3 rows of 4
asterisks each.
for (i = 1; i <= 4; i++)
{
for (j = 1; j <= 3; j++) {  Print "*" }
System.out.println();
}
***
***
***
***
Prints 4 rows of 3
asterisks each.
for (i = 1; i <= 4; i++)
{
for (j = 1; j <= i; j++) {  Print "*" }
System.out.println();
}
*
**
***
****
Prints 4 rows of
lengths 1, 2, 3, and 4.
for (i = 1; i <= 3; i++)
{
for (j = 1; j <= 5; j++)
{
if (j % 2 == 0) {  Print "*" }
else {  Print "-" }
}
System.out.println();
}
-*-*-
-*-*-
-*-*-
Prints asterisks
in even columns,
dashes in odd
columns.
for (i = 1; i <= 3; i++)
{
for (j = 1; j <= 5; j++)
{
if (i % 2 == j % 2) {  Print "*" }
else {  Print " " }
}
System.out.println();
}
* * *
* *
* * *
Prints a
checkerboard
pattern.
WorkeD exaMpLe 6.2  manipulating the pixels in an image
Learn how to use nested loops for manipulating the
pix els in an image. The outer loop traverses the rows of
the image, and the inner loop accesses each pixel of a row.
Go to  wiley.com/go/javaexamples and download Worked
Example 6.2.
Cay Horstmann.
6.9 application: random numbers and simulations 283
6.9 application: random numbers
and simulations
A simulation program uses the computer to simulate an activity in the real world (or
an imaginary one). Simulations are commonly used for predicting climate change,
analyzing traffic, picking stocks, and many other applications in science and busi
ness. In many simulations, one or more loops are used to modify the state of a system
and observe the changes. You will see examples in the following sections.
6.9.1 Generating random numbers
Many events in the real world are difficult to predict with absolute precision, yet we
can sometimes know the average behavior quite well. For example, a store may know
from experience that a customer arrives every five minutes. Of course, that is an aver
age—customers don’t arrive in five minute intervals. To accurately model customer
traffic, you want to take that random fluctuation into account. Now, how can you
run such a simulation in the computer?
The  Random class of the Java library implements a random number generator that
produces numbers that appear to be completely random. To generate random num
bers, you con struct an object of the  Random class, and then apply one of the following
methods:
Method returns
nextInt(n)  A random integer between the integers 0 (inclusive) and  n (exclusive)
nextDouble()  A random floatingpoint number between 0 (inclusive) and 1 (exclusive)
For example, you can simulate the cast of a die as follows:
Random generator = new Random();
int d = 1 + generator.nextInt(6);
The call  generator.nextInt(6) gives you a random number between 0 and 5 (inclusive).
Add 1 to obtain a number between 1 and 6.
To give you a feeling for the random numbers, run the following program a few
times.
section_9_1/die.java
1 import java.util.Random;
2
3 /**
4 This class models a die that, when cast, lands on a
5 random face.
6 */
7 public class Die
8 {
9 private Random generator;
10 private int sides;
In a simulation, you
use the computer to
simulate an activity.
You can introduce
randomness by
calling the random
number generator.
© ktsimage/iStockphoto.
284 Chapter 6 Loops
11
12 /**
13 Constructs a die with a given number of sides.
14 @param s  the number of sides, e.g., 6 for a normal die
15 */
16 public Die(int s)
17 {
18 sides = s;
19 generator = new Random();
20 }
21
22 /**
23 Simulates a throw of the die.
24 @return  the face of the die
25 */
26 public int cast()
27 {
28 return 1 + generator.nextInt(sides);
29 }
30 }
section_9_1/dieSimulator.java
1 /**
2 This program simulates casting a die ten times.
3 */
4 public class DieSimulator
5 {
6 public static void main(String[] args)
7 {
8 Die d = new Die(6);
9 final int TRIES = 10;
10 for (int i = 1; i <= TRIES; i++)
11 {
12 int n = d.cast();
13 System.out.print(n + " ");
14 }
15 System.out.println();
16 }
17 }
Typical program run
6 5 6 3 2 6 3 4 4 1
Typical program run (Second run)
3 2 2 1 6 5 3 4 1 2
As you can see, this program produces a different stream of simulated die casts every
time it is run.
Actually, the numbers are not completely random. They are drawn from very long
sequences of num bers that don’t repeat for a long time. These sequences are com
puted from fairly simple formulas; they just behave like random numbers. For that
reason, they are often called pseudorandom numbers. Gener ating good sequences
of numbers that behave like truly random sequences is an important and wellstud ied
problem in computer science. We won’t investigate this issue further, though; we’ll
just use the random numbers produced by the  Random class.
6.9 application: random numbers and simulations 285
6.9.2 the Monte Carlo Method
The Monte Carlo method is an
ingenious  method  for  finding
approximate solutions to problems
that cannot be precisely solved.
(The method is named after the
famous casino in Monte Carlo.)
Here is a typical exam ple. It is dif
ficult to compute the number  p , but
you can approximate it quite well
with the following simulation.
Simulate shooting a dart into a square surrounding a circle of radius 1. That is easy:
generate random x- and y- coordinates between –1 and 1.
If the generated point lies inside the circle, we count
it as a hit. That is the case when x 2 + y 2 ≤ 1. Because our
shots are entirely random, we expect that the ratio of
hits / tries is approximately equal to the ratio of the areas
of the cir cle and the square, that is,  p / 4. Therefore, our
estimate for  p is 4 × hits / tries. This method yields an
estimate for  p , using nothing but simple arithmetic.
To generate a random floatingpoint value between –1
and 1, you compute:
double r = generator.nextDouble(); //  0  ≤ r < 1
double x = -1 + 2 * r; // –1  ≤ x <  1
As  r ranges from 0 (inclusive) to 1 (exclusive),  x ranges from –1 + 2 × 0 = –1 (inclusive)
to –1 + 2 × 1 = 1 (exclusive). In our application, it does not matter that  x never reaches
1. The points that fulfill the equation x = 1 lie on a line with area 0.
Here is the program that carries out the simulation:
section_9_2/montecarlo.java
1 import java.util.Random;
2
3 /**
4 This program computes an estimate of pi by simulating dart throws onto a square.
5 */
6 public class MonteCarlo
7 {
8 public static void main(String[] args)
9 {
10 final int TRIES = 10000;
11 Random generator = new Random();
12
13 int hits = 0;
14 for (int i = 1; i <= TRIES; i++)
15 {
16 //  Generate two random numbers between –1 and 1
17
18 double r = generator.nextDouble();
19 double x = -1 + 2 * r; //  Between –1 and 1
20 r = generator.nextDouble();
21 double y = -1 + 2 * r;
22
© timstarkey/iStockphoto.
x
y
1 –1
1
–1
286 Chapter 6 Loops  testing track
23 //  Check whether the point lies in the unit circle
24
25 if (x * x + y * y <= 1) { hits++; }
26 }
27
28 /*
29 The ratio hits / tries is approximately the same as the ratio
30 circle area / square area = pi / 4
31 */
32
33 double piEstimate = 4.0 * hits / TRIES;
34 System.out.println("Estimate for pi: " + piEstimate);
35 }
36 }
program run
Estimate for pi: 3.1504
42.  How do you simulate a coin toss with the  Random class?
43.  How do you simulate the picking of a random playing card?
44.  How would you modify the  DieSimulator program to simulate tossing a pair
of dice?
45.  In many games, you throw a pair of dice to get a value between 2 and 12. What is
wrong with this simulated throw of a pair of dice?
int sum = 2 + generator.nextInt(11);
46.  How do you generate a random floatingpoint number ≥ 0 and < 100?
practice it  Now you can try these exercises at the end of the chapter: R6.28, E6.7, E6.19.
6.10 Using a Debugger
As you have undoubtedly realized by now, computer programs rarely run perfectly
the first time. At times, it can be quite frustrating to find the bugs. Of course, you can
insert print commands, run the pro gram, and try to analyze the printout. If the print
out does not clearly point to the problem, you may need to add and remove print
commands and run the program again. That can be a timeconsuming process.
Modern development environments contain special programs, called debuggers,
that help you locate bugs by letting you follow the execution of a program. You can
stop and restart your program and see the contents of variables whenever your pro
gram is temporarily stopped. At each stop, you have the choice of what variables to
inspect and how many program steps to run until the next stop.
Some people feel that debuggers are just a tool to make programmers lazy. Admit
tedly some people write sloppy programs and then fix them up with a debugger, but
the majority of programmers make an honest effort to write the best program they
can before trying to run it through a debugger. These pro grammers realize that a
debugger, while more convenient than print commands, is not costfree. It does take
time to set up and carry out an effective debugging session.
In actual practice, you cannot avoid using a debugger. The larger your programs get,
the harder it is to debug them simply by inserting print commands. The time invested
in learning about a debugger will be amply repaid in your programming career.
© Nicholas Homrich/iStockphoto.
S e l f  c h e c k
a debugger is a
program that you
can use to execute
another program
and analyze its
run-time behavior.
testing track 6.10 Using a Debugger 287
Like compilers, debuggers vary widely from one system to another. Some are quite
primitive and require you to memorize a small set of arcane commands; others have
an intuitive window interface. Figure 8 shows the debugger in the Eclipse develop
ment envi ronment, downloadable for free from the Eclipse Foundation ( eclipse.org ).
Other integrated environments, such as BlueJ, also include debuggers. A free stand
alone debugger called JSwat is available from  code.google.com/p/jswat .
You will have to find out how to prepare a program for debugging and how to start
a debugger on your system. If you use an integrated development environment (with
an editor, compiler, and debugger), this step is usually easy. You build the program in
the usual way and pick a command to start debugging. On some systems, you must
manually build a debug version of your pro gram and invoke the debugger.
Once you have started the debugger, you can go a long way with just three debug
ging commands: “set breakpoint”, “single step”, and “inspect variable”. The names
and keystrokes or mouse clicks for these commands differ widely, but all debuggers
support these basic commands. You can find out how, either from the documentation
or a lab manual, or by asking someone who has used the debugger before.
When you start the debugger, it runs at full speed until it reaches a breakpoint.
Then execution stops, and the breakpoint that causes the stop is displayed (Figure 8).
You can now inspect variables and step through the program one line at a time, or
continue running the program at full speed until it reaches the next breakpoint. When
the program terminates, the debugger stops as well.
You can make
effective use of
a debugger by
mastering just
three concepts:
breakpoints, single-
stepping, and
inspecting variables.
When a debugger
executes a program,
the execution is
suspended when-
ever a breakpoint
is reached.
figure 8 
stopping at a Breakpoint
288 Chapter 6 Loops  testing track
figure 9  Inspecting Variables
Breakpoints stay active until you remove them, so you should periodically clear
the breakpoints that you no longer need.
Once the program has stopped, you can look at the current values of variables.
Again, the method for selecting the variables differs among debuggers. Some debug
gers always show you a window with the current local variables. On other debuggers
you issue a command such as “inspect variable” and type in or click on the variable.
The debugger then displays the contents of the variable. If all variables contain what
you expected, you can run the program until the next point where you want to stop.
When inspecting objects, you often need to give a command to “open up” the
object, for example by clicking on a tree node. Once the object is opened up, you see
its instance variables (see Figure 9).
Running to a breakpoint gets you there speedily, but you don’t know how the
program got there. You can also step through the program one line at a time. Then
you know how the program flows, but it can take a long time to step through it. The
single-step command executes the current line and stops at the next program line.
Most debuggers have two singlestep commands, one called step into, which steps
inside method calls, and one called step over, which skips over method calls.
For example, suppose the current line is
String input = in.next();
Word w = new Word(input);
int syllables = w.countSyllables();
System.out.println("Syllables in " + input + ": " + syllables);
When you step over method calls, you get to the next line:
String input = in.next();
Word w = new Word(input);
int syllables = w.countSyllables();
System.out.println("Syllables in " + input + ": " + syllables);
However, if you step into method calls, you enter the first line of the  countSyllables
method.
public int countSyllables()
{
int count = 0;
the single-step
command executes
the program one line
at a time.
testing track 6.10 Using a Debugger 289
int end = text.length() - 1;
. . .
}
You should step into a method to check whether it carries out its job correctly. You
should step over a method if you know it works correctly.
Finally, when the program has finished running, the debug session is also finished.
To debug the program again, you must restart it in the debugger.
A debugger can be an effective tool for finding and removing bugs in your pro
gram. However, it is no substitute for good design and careful programming. If the
debugger does not find any errors, it does not mean that your program is bugfree.
Testing and debugging can only show the presence of bugs, not their absence.
47.  In the debugger, you are reaching a call to  System.out.println . Should you step
into the method or step over it?
48.  In the debugger, you are reaching the beginning of a method with a couple of
loops inside. You want to find out the return value that is computed at the end
of the method. Should you set a breakpoint, or should you step through the
method?
49.  When using the debugger, you find that a variable has an unexpected value. How
can you go backwards to see when the variable changed?
50.  When using a debugger, should you insert statements to print the values of
variables?
51.  Instead of using a debugger, could you simply trace a program by hand?
practice it  Now you can try these exercises at the end of the chapter: R6.30, R6.31, R6.32.
Step 1  Reproduce the error.
As you test your program, you notice that it sometimes does something wrong. It gives the
wrong output, it seems to print something random, it goes in an infinite loop, or it crashes.
Find out exactly how to reproduce that behavior. What numbers did you enter? Where did
you click with the mouse?
Run the program again; type in exactly the same numbers, and click with the mouse on the
same spots (or as close as you can get). Does the program exhibit the same behavior? If so, then
it makes sense to fire up a debugger to study this particular problem. Debuggers are good for
analyzing particular failures. They aren’t terribly useful for studying a program in general.
Step 2  Simplify the error.
Before you start up a debugger, it makes sense to spend a few minutes trying to come up with a
simpler input that also produces an error. Can you use shorter words or simpler numbers and
still have the program misbehave? If so, use those values during your debugging session.
© Nicholas Homrich/iStockphoto.
S e l f  c h e c k
© Steve Simzer/iStockphoto.
hoW to 6.2  debugging
Knowing all about the mechanics of debugging may still leave you helpless when you fire up a
debugger to look at a sick program. This How To presents a number of strategies that you can
use to recognize bugs and their causes.
290 Chapter 6 Loops  testing track
Step 3  Divide and conquer.
Now that you have a particular failure, you want to get as close to the failure as possible. The
key point of debugging is to locate the code that produces the failure. Just as with real insect
pests, finding the bug can be hard, but once you find it, squashing it is usually the easy part.
Suppose your program dies with a division by 0. Because there are many division operations
in a typical program, it is often not feasible to set breakpoints to all of them. Instead, use a
tech nique of divide and conquer. Step over the methods in  main , but don’t step inside them.
Eventually, the failure will happen again. Now you know which method contains the bug: It is
the last method that was called from  main before the program died. Restart the debugger and go
back to that line in  main , then step inside that method. Repeat the process.
Eventually, you will have pinpointed the line that contains the bad division. Maybe it is
obvious from the code why the denominator is not correct. If not, you need to find the loca
tion where it is computed. Unfortu nately, you can’t go back in the debugger. You need to
restart the program and move to the point where the denomi nator computation happens.
Step 4  Know what your program should do.
A debugger shows you what the program does. You must know what the program should do,
or you will not be able to find bugs. Before you trace through a loop, ask yourself how many
iterations you expect the program to make. Before you inspect a variable, ask yourself what
you expect to see. If you have no clue, set aside some time and think first. Have a calculator
handy to make independent computations. When you know what the value should be, inspect
the variable. If the value is what you expected, you must look further for the bug. If the value is
different, you may be on to something. Doublecheck your computation. If you are sure your
value is correct, find out why your program comes up with a different value.
In many cases, program bugs are the result of simple errors such as loop termination condi
tions that are off by one. Quite often, however, programs make computational errors. Maybe
they are supposed to add two numbers, but by accident the code was written to subtract them.
Programs don’t make a special effort to ensure that everything is a simple integer (and neither
do realworld problems). You will need to make some calculations with large integers or nasty
floatingpoint numbers. Sometimes these calculations can be avoided if you just ask yourself,
“Should this quantity be positive? Should it be larger than that value?” Then inspect variables
to verify those theories.
Step 5  Look at all details.
When you debug a program, you often have a theory about what the problem is. Nevertheless,
keep an open mind and look at all details. What strange messages are displayed? Why does the
program take another unexpected action? These details count. When you run a debugging ses
sion, you really are a detective who needs to look at every clue available.
If you notice another failure on the way to the problem that you are about to pin down,
don’t just say, “I’ll come back to it later”. That very failure may be the original cause for your
current problem. It is better to make a note of the current problem, fix what you just found,
and then return to the original mission.
Step 6  Make sure you understand each bug before you fix it.
Once you find that a loop makes too many iterations, it is very tempting to apply a “Band
Aid” solution and sub tract 1 from a variable so that the particular problem doesn’t appear
again. Such a quick fix has an overwhelming probability of creating trouble elsewhere. You
really need to have a thorough understanding of how the program should be written before
you apply a fix.
It does occasionally happen that you find bug after bug and apply fix after fix, and the
problem just moves around. That usually is a symptom of a larger problem with the program
logic. There is little you can do with the debugger. You must rethink the program design and
reorganize it.
Use the divide-and-
conquer technique
to locate the point of
failure of a program.
During debugging,
compare the actual
contents of variables
against the values
you know they
should have.
testing track Chapter summary 291
explain the flow of execution in a loop.
• A loop executes instructions repeatedly while a
condition is true.
• An offbyone error is a common error when
programming loops. Think through simple test
cases to avoid this type of error.
use the technique of hand-tracing to analyze the behavior of a pro gram.
• Handtracing is a simulation of code execution in which you step
through instructions and track the values of the variables.
• Handtracing can help you understand how an unfamiliar
algorithm works.
• Handtracing can show errors in code or pseudocode.
WorkeD exaMpLe 6.3  a Sample debugging Session
Learn how to find bugs in an algorithm for counting the
syllables of a word. Go to  wiley.com/go/javaexamples and
download Worked Example 6.3.
© Mark Poprocki/iStockphoto.
according to legend,
the  first  bug  was
found in the Mark II, a huge electro me-
chanical computer at harvard Univer-
sity. It really was caused by a bug — a
moth was trapped in a relay switch.
actually, from the note that the
operator left in the log book next to
the moth (see the photo), it appears as
if the term “bug” had already been in
active use at the time.
The First Bug
Courtesy of the Naval Surface Warfare Center, Dahlgren, VA., 1988. NHHC Collection.
the pioneering computer scientist
Maurice Wilkes wrote, “somehow, at
the Moore school and afterwards, one
had always assumed there would be
no particular difculty in getting pro-
grams right. I can remember the exact
instant in time at which it dawned on
me that a great part of my future life
would be spent finding mistakes in my
own programs.”
Computing & Society 6.2  the First Bug
© MediaBakery.
C h a p t e r s U M M a rY
© mmac72/iStockphoto.
© thomasd007/iStockphoto.
292 Chapter 6 Loops
 use  for  loops for implementing count-controlled loops.
• The  for loop is used when a value runs from a starting point to an ending point
with a constant increment or decrement.
choose between the  while  loop and the  do  loop.
• The  do loop is appropriate when the loop body must be executed at least once.
implement loops that read sequences of input data.
• A sentinel value denotes the end of a data set, but it is not part of
the data.
• You can use a Boolean variable to control a loop. Set the
variable to  true before entering the loop, then set it to  false to
leave the loop.
• Use input redirection to read input from a file. Use output
redirection to capture program output in a file.
use the technique of storyboarding for planning user interactions.
• A storyboard consists of annotated sketches for each step in an action sequence.
• Developing a storyboard helps you understand the inputs and outputs that are
required for a program.
know the most common loop algorithms.
• To compute an average, keep a total and a count of all values.
• To count values that fulfill a condition, check all values and increment a counter
for each match.
• If your goal is to find a match, exit the loop when the match is found.
• To find the largest value, update the largest value seen so far whenever you see a
larger one.
• To compare adjacent inputs, store the preceding input in a variable.
use nested loops to implement multiple levels of iteration.
• When the body of a loop contains another loop, the loops are nested. A typical
use of nested loops is printing a table with rows and columns.
apply loops to the implementation of simulations.
• In a simulation, you use the computer to simulate an activity.
• You can introduce randomness by calling the random number
generator.
© Enrico Fianchini/iStockphoto.
© Rhoberazzi/iStockphoto.
© Hiob/iStockphoto.
© davejkahn/iStockphoto.
© ktsimage/iStockphoto.
review Questions 293
use a debugger to analyze your programs.
• A debugger is a program that you can use to execute another program
and analyze its runtime behavior.
• You can make effective use of a debugger by mastering just three concepts: break
points, singlestepping, and inspecting variables.
• When a debugger executes a program, the execution is suspended whenever a
breakpoint is reached.
• The singlestep command executes the program one line at a time.
• Use the divideandconquer technique to locate the point of failure of a program.
• During debugging, compare the actual contents of variables against the values
you know they should have.
• r6.1  Write a  while loop that prints
a.  All squares less than  n . For example, if  n is 100, print 0 1 4 9 16 25 36 49 64 81.
b.  All positive numbers that are divisible by 10 and less than  n . For example, if  n is
100, print 10 20 30 40 50 60 70 80 90
c.  All powers of two less than  n . For example, if  n is 100, print 1 2 4 8 16 32 64.
•• r6.2  Write a loop that computes
a.  The sum of all even numbers between 2 and 100 (inclusive).
b.  The sum of all squares between 1 and 100 (inclusive).
c.  The sum of all odd numbers between  a and  b (inclusive).
d.  The sum of all odd digits of  n . (For example, if  n is 32677, the sum would
be 3 + 7 + 7 = 17.)
• r6.3  Provide trace tables for these loops.
a.  int i = 0; int j = 10; int n = 0;
while (i < j) { i++; j--; n++; }
b.  int i = 0; int j = 0; int n = 0;
while (i < 10) { i++; n = n + i + j; j++; }
c.  int i = 10; int j = 0; int n = 0;
while (i > 0) { i--; j++; n = n + i - j; }
d.  int i = 0; int j = 10; int n = 0;
while (i != j) { i = i + 2; j = j - 2; n++; }
java.util.Random
nextDouble
nextInt
s ta n D a r D L I B r a rY I t e M s I n t r o D U C e D I n t h I s C h ap t e r
r e V I e W Q U e s t I o n s
294 Chapter 6 Loops
• r6.4  What do these loops print?
a.  for (int i = 1; i < 10; i++) { System.out.print(i + " "); }
b.  for (int i = 1; i < 10; i += 2) { System.out.print(i + " "); }
c.  for (int i = 10; i > 1; i--) { System.out.print(i + " "); }
d.  for (int i = 0; i < 10; i++) { System.out.print(i + " "); }
e.  for (int i = 1; i < 10; i = i * 2) { System.out.print(i + " "); }
f.  for (int i = 1; i < 10; i++) { if (i % 2 == 0) { System.out.print(i + " "); } }
• r6.5  What is an infinite loop? On your computer, how can you terminate a program that
executes an infinite loop?
• r6.6  Write a program trace for the pseudocode in Exercise E6.6, assuming the input val
ues are 4 7 –2 –5 0.
•• r6.7  W hat is an “offbyone” error? Give an example from your own programming
experience.
• r6.8  What is a sentinel value? Give a simple rule when it is appropriate to use a numeric
sentinel value.
• r6.9  Which loop statements does Java support? Give simple rules for when to use each
loop type.
• r6.10  How many iterations do the following loops carry out? Assume that  i is not
changed in the loop body.
a.  for (int i = 1; i <= 10; i++) . . .
b.  for (int i = 0; i < 10; i++) . . .
c.  for (int i = 10; i > 0; i--) . . .
d.  for (int i = -10; i <= 10; i++) . . .
e.  for (int i = 10; i >= 0; i++) . . .
f.  for (int i = -10; i <= 10; i = i + 2) . . .
g.  for (int i = -10; i <= 10; i = i + 3) . . .
•• r6.11  Write pseudocode for a program that prints a calendar such as the following.
Su M T W Th F Sa
1 2 3 4
5 6 7 8 9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29 30 31
• r6.12  Write pseudocode for a program that prints a Celsius/Fahrenheit conversion table
such as the following.
Celsius | Fahrenheit
--------+-----------
0 | 32
10 | 50
20 | 68
. . . . . .
100 | 212
• r6.13  Write pseudocode for a program that reads a student record, consisting of the stu
dent’s first and last name, followed by a sequence of test scores and a sentinel of –1.
review Questions 295
The program should print the student’s average score. Then provide a trace table for
this sample input:
Harry Morgan 94 71 86 95 -1
•• r6.14  Write pseudocode for a program that reads a sequence of student records and prints
the total score for each student. Each record has the student’s first and last name,
followed by a sequence of test scores and a sentinel of –1. The sequence is termi nated
by the word  END . Here is a sample sequence:
Harry Morgan 94 71 86 95 -1
Sally Lin 99 98 100 95 90 -1
END
Provide a trace table for this sample input.
• r6.15  Rewrite the following  for loop into a  while loop.
int s = 0;
for (int i = 1; i <= 10; i++)
{
s = s + i;
}
• r6.16  Rewrite the following  do loop into a  while loop.
int n = in.nextInt();
double x = 0;
double s;
do
{
s = 1.0 / (1 + n * n);
n++;
x = x + s;
}
while (s > 0.01);
• r6.17  Provide trace tables of the following loops.
a.  int s = 1;
int n = 1;
while (s < 10) { s = s + n; }
n++;
b.  int s = 1;
for (int n = 1; n < 5; n++) { s = s + n; }
c.  int s = 1;
int n = 1;
do
{
s = s + n;
n++;
}
while (s < 10 * n);
• r6.18  What do the following loops print? Work out the answer by tracing the code, not by
using the computer.
a.  int s = 1;
for (int n = 1; n <= 5; n++)
{
s = s + n;
System.out.print(s + " ");
}
296 Chapter 6 Loops
b.  int s = 1;
for (int n = 1; s <= 10; System.out.print(s + " "))
{
n = n + 2;
s = s + n;
}
c.  int s = 1;
int n;
for (n = 1; n <= 5; n++)
{
s = s + n;
n++;
}
System.out.print(s + " " + n);
• r6.19  What do the following program segments print? Find the answers by tracing the
code, not by using the computer.
a.  int n = 1;
for (int i = 2; i < 5; i++) { n = n + i; }
System.out.print(n);
b.  int i;
double n = 1 / 2;
for (i = 2; i <= 5; i++) { n = n + 1.0 / i; }
System.out.print(i);
c.  double x = 1;
double y = 1;
int i = 0;
do
{
y = y / 2;
x = x + y;
i++;
}
while (x < 1.8);
System.out.print(i);
d.  double x = 1;
double y = 1;
int i = 0;
while (y >= 1.5)
{
x = x / 2;
y = x + y;
i++;
}
System.out.print(i);
•• r6.20  Give an example of a  for loop where symmetric bounds are more natural. Give an
example of a  for loop where asymmetric bounds are more natural.
• r6.21  Add a storyboard panel for the conversion program in Section 6.6 on page 269 that
shows a sce nario where a user enters incompatible units.
• r6.22  In Section 6.6, we decided to show users a list of all valid units in the prompt. If the
program supports many more units, this approach is unworkable. Give a story board
panel that illustrates an alternate approach: If the user enters an unknown unit, a list
of all known units is shown.
practice exercises 297
• r6.23  Change the storyboards in Section 6.6 to support a menu that asks users whether
they want to convert units, see program help, or quit the program. The menu should
be displayed at the beginning of the program, when a sequence of values has been
converted, and when an error is displayed.
• r6.24  Draw a flow chart for a program that carries out unit conversions as described in
Section 6.6.
•• r6.25  In Section 6.7.5, the code for finding the largest and smallest input initializes the
largest and  smallest variables with an input value. Why can’t you initialize them
with zero?
• r6.26  What are nested loops? Give an example where a nested loop is typically used.
•• r6.27  The nested loops
for (int i = 1; i <= height; i++)
{
for (int j = 1; j <= width; j++) { System.out.print("*"); }
System.out.println();
}
display a rectangle of a given width and height, such as
****
****
****
Write a single  for loop that displays the same rectangle.
•• r6.28  Suppose you design an educational game to teach children how to read a clock. How
do you generate random values for the hours and minutes?
••• r6.29  In a travel simulation, Harry will visit one of his friends that are located in three
states. He has ten friends in California, three in Nevada, and two in Utah. How do
you produce a random number between 1 and 3, denoting the destination state, with
a probability that is proportional to the number of friends in each state?
• Testing r6.30  Explain the differences between these debugger operations:
• Stepping into a method
• Stepping over a method
•• Testing r6.31  Explain in detail how to inspect the string stored in a  String object in your debugger.
•• Testing r6.32  Explain in detail how to inspect the information stored in a  Rectangle object in your
debugger.
•• Testing r6.33  Explain in detail how to use your debugger to inspect the balance stored in a  Bank–
Account object.
•• Testing r6.34  Explain the divideandconquer strategy to get close to a bug in a debugger.
• e6.1  Write programs with loops that compute
a.  The sum of all even numbers between 2 and 100 (inclusive).
b.  The sum of all squares between 1 and 100 (inclusive).
p r a C t I C e e x e r C I s e s
298 Chapter 6 Loops
c.  All powers of 2 from 2 0 up to 2 20 .
d.  The sum of all odd numbers between  a and  b (inclusive), where  a and  b are
inputs.
e.  The sum of all odd digits of an input. (For example, if the input is 32677, the
sum would be 3 + 7 + 7 = 17.)
•• e6.2  Write programs that read a sequence of integer inputs and print
a.  The smallest and largest of the inputs.
b.  The number of even and odd inputs.
c.  Cumulative totals. For example, if the input is 1 7 2 9, the program should print
1 8 10 19.
d.  All adjacent duplicates. For example, if the input is 1 3 3 4 5 5 6 6 6 2, the
program should print 3 5 6.
•• e6.3  Write programs that read a line of input as a string and print
a.  Only the uppercase letters in the string.
b.  Every second letter of the string.
c.  The string, with all vowels replaced by an underscore.
d.  The number of vowels in the string.
e.  The positions of all vowels in the string.
•• e6.4  Complete the program in How To 6.1 on page 276. Your program should read twelve
temperature values and print the month with the highest temperature.
•• e6.5  Write a program that reads a set of floatingpoint values. Ask the user to enter the
values (prompting only a single time for the values), then print
• the average of the values.
• the smallest of the values.
• the largest of the values.
• the range, that is the difference between the smallest and largest.
Your program should use a class  DataSet . That class should have a method
public void add(double value)
and methods  getAverage ,  getSmallest ,  getLargest , and  getRange .
• e6.6  Translate the following pseudocode for finding the minimum value from a set of
inputs into a Java program.
Set a Boolean variable "first" to true.
While another value has been read successfully
If first is true
Set the minimum to the value.
Set first to false.
Else if the value is less than the minimum
Set the minimum to the value.
Print the minimum.
practice exercises 299
••• e6.7  Translate the following pseudocode for randomly permuting the characters in a
string into a Java program.
Read a word.
Repeat word.length() times
Pick a random position i in the word, but not the last position.
Pick a random position j > i in the word.
Swap the letters at positions j and i.
Print the word.
To swap the letters, construct substrings as follows:
first middle last i j
Then replace the string with
first + word.charAt(j) + middle + word.charAt(i) + last
• e6.8  Write a program that reads a word and prints each character of the word on a sepa
rate line. For example, if the user provides the input  "Harry" , the program prints
H
a
r
r
y
•• e6.9  Write a program that reads a word and prints the word in reverse. For example, if the
user provides the input  "Harry" , the program prints
yrraH
• e6.10  Write a program that reads a word and prints the number of vowels in the word. For
this exercise, assume that  a e i o u y are vowels. For example, if the user pro vides the
input  "Harry" , the program prints  2 vowels .
••• e6.11  Write a program that reads a word and prints all substrings, sorted by length. For
example, if the user provides the input  "rum" , the program prints
r
u
m
ru
um
rum
• e6.12  Write a program that prints all powers of 2 from 2 0 up to 2 20 .
•• e6.13  Write a program that reads a number and prints all of its binary digits: Print the
remainder  number % 2 , then replace the number with  number / 2 . Keep going until the
number is 0. For example, if the user provides the input 13, the output should be
1
0
1
1
© Anthony Rosenberg/iStockphoto.
300 Chapter 6 Loops
• e6.14  Write a program that prints a multiplication table, like this:
1 2 3 4 5 6 7 8 9 10
2 4 6 8 10 12 14 16 18 20
3 6 9 12 15 18 21 24 27 30
. . .
10 20 30 40 50 60 70 80 90 100
•• e6.15  Write a program that reads an integer and displays, using asterisks, a filled and hol
low square, placed next to each other. For example if the side length is 5, the pro gram
should display
***** *****
***** * *
***** * *
***** * *
***** *****
•• e6.16  Write a program that reads an integer and displays, using asterisks, a filled diamond
of the given side length. For example, if the side length is 4, the program should
dis play
*
***
*****
*******
*****
***
*
•• business e6.17  Currency conversion. Write a program
that first asks the user to type today’s
price for one dollar in Japanese yen,
then reads U.S. dollar values and
converts each to yen. Use 0 as a sentinel.
•• business e6.18  Write a program that first asks the user
to type in today’s price of one dollar
in Jap anese yen, then reads U.S. dollar
values and converts each to Japanese
yen. Use 0 as the sentinel value to denote the end of dollar inputs. Then the program
reads a sequence of yen amounts and converts them to dollars. The second sequence
is ter minated by another zero value.
•• e6.19  The Monty Hall Paradox. Marilyn vos Savant described the following problem
(loosely based on a game show hosted by Monty Hall) in a popular magazine:
“Suppose you’re on a game show, and you’re given the choice of three doors: Behind
one door is a car; behind the others, goats. You pick a door, say No. 1, and the host,
who knows what’s behind the doors, opens another door, say No. 3, which has a
goat. He then says to you, “Do you want to pick door No. 2?” Is it to your advan
tage to switch your choice?”
Ms. vos Savant proved that it is to your advantage, but many of her readers, includ
ing some mathematics professors, disagreed, arguing that the probability would not
change because another door was opened.
Your task is to simulate this game show. In each iteration, ran domly pick a door
number between 1 and 3 for placing the car. Randomly have the player pick a door.
© hatman12/iStockphoto.
programming projects 301
Randomly have the game show host pick a door having a goat (but not the door that
the player picked). Increment a counter for strategy 1 if the player wins by switching
to the host’s choice, and increment a counter for strategy 2 if the player wins by
sticking with the orig inal choice. Run 1,000 iterations and print both counters.
•• p6.1  Mean and standard deviation. Write a program that reads a set of floatingpoint data
values. Choose an appropriate mechanism for prompting for the end of the data set.
When all values have been read, print out the count of the values, the aver age, and
the standard deviation. The average of a data set {x 1 , . . ., x n } is x x n
i
= ∑ , where
∑ = + + x x x
i n 1
… is the sum of the input values. The standard deviation is
s
x x
n
i
=
− ( )
−
∑
2
1
However, this formula is not suitable for the task. By the time the program has
computed x , the individ ual x i are long gone. Until you know how to save these
values, use the numerically less stable formula
s
x x
n
i
n
i
=
−
( )
−
∑ ∑
2
1
2
1
You can compute this quantity by keeping track of the count, the sum, and the sum
of squares as you process the input values.
Your program should use a class  DataSet . That class should have a method
public void add(double value)
and methods  getAverage and  getStandardDeviation .
•• p6.2  The Fibonacci numbers are defined by the sequence
f
f
f f f
n n n
1
2
1 2
1
1
=
=
= +
− −
Reformulate that as
fold1 = 1;
fold2 = 1;
fnew = fold1 + fold2;
After that, discard  fold2 , which is no longer needed, and set  fold2 to  fold1 and  fold1 to
fnew . Repeat an appropriate number of times.
Implement a program that prompts the user for an integer n and prints the nth
Fibonacci number, using the above algorithm.
p r o G r a M M I n G p r o J e C t s
© GlobalP/iStockphoto.
Fibonacci numbers describe the
growth of a rabbit population.
302 Chapter 6 Loops
••• p6.3  Factoring of integers. Write a program that asks the user for an integer and then
prints out all its factors. For example, when the user enters 150, the program should
print
2
3
5
5
Use a class  FactorGenerator with a constructor  FactorGenerator(int numberToFactor) and
methods  nextFactor and  hasMoreFactors . Supply a class  FactorPrinter whose  main
method reads a user input, constructs a  Factor Generator object, and prints the factors.
••• p6.4  Prime numbers. Write a program that prompts the user for an integer and then prints
out all prime numbers up to that integer. For example, when the user enters 20, the
program should print
2
3
5
7
11
13
17
19
Recall that a number is a prime number if it is not divisible by any number except 1
and itself.
Use a class  PrimeGenerator with methods  nextPrime and  isPrime . Supply a class  Prime-
Printer whose  main method reads a user input, constructs a  Prime Generator object, and
prints the primes.
••• p6.5  The game of Nim. This is a wellknown game with a number of variants. The fol
lowing variant has an interesting winning strategy. Two players alternately take
marbles from a pile. In each move, a player chooses how many marbles to take. The
player must take at least one but at most half of the marbles. Then the other player
takes a turn. The player who takes the last marble loses.
Write a program in which the computer plays against a human opponent. Generate a
random integer between 10 and 100 to denote the initial size of the pile. Generate a
random integer between 0 and 1 to decide whether the computer or the human takes
the first turn. Generate a random integer between 0 and 1 to decide whether the
computer plays smart or stupid. In stupid mode the computer simply takes a random
legal value (between 1 and n/2) from the pile whenever it has a turn. In smart mode
the computer takes off enough marbles to make the size of the pile a power of two
minus 1—that is, 3, 7, 15, 31, or 63. That is always a legal move, except when the size
of the pile is currently one less than a power of two. In that case, the computer makes
a random legal move.
You will note that the computer cannot be beaten in smart mode when it has the first
move, unless the pile size happens to be 15, 31, or 63. Of course, a human player who
has the first turn and knows the win ning strategy can win against the computer.
•• p6.6  The Drunkard’s Walk. A drunkard in a grid of streets randomly picks one of four
directions and stumbles to the next intersection, then again randomly picks one of
four directions, and so on. You might think that on average the drunkard doesn’t
move very far because the choices cancel each other out, but that is not the case.
programming projects 303
Represent locations as integer pairs (x, y). Implement the drunkard’s walk over 100
intersections, starting at (0, 0), and print the ending location.
• p6.7  A simple random generator is obtained by the formula
r a r b m
new old
= ⋅ + ( ) %
and then setting r old to r new . If m is chosen as 2 32 , then you can compute
r a r b
new old
= ⋅ +
because the truncation of an overflowing result to the  int type is equivalent to
computing the remainder.
Write a program that asks the user to enter a value for r old . (Such a value is often
called a seed). Then print the first 100 random integers generated by this formula,
using a = 32310901 and b = 1729.
•• p6.8  The Buffon Needle Experiment. The follow ing experiment was devised by Comte
GeorgesLouis Leclerc de Buffon (1707–1788), a French naturalist. A needle of
length 1 inch is dropped onto paper that is ruled with lines 2 inches apart. If the
needle drops onto a line, we count it as a hit. (See Figure 10.) Buffon discovered that
the quotient tries/hits approximates  p .
For the Buffon needle experiment, you must generate two random numbers: one to
describe the starting position and one to describe the angle of the needle with the
xaxis. Then you need to test whether the needle touches a grid line.
Generate the lower point of the needle. Its xcoordinate is irrelevant, and you may
assume its ycoordi nate y low to be any random number between 0 and 2. The angle  a
between the needle and the xaxis can be any value between 0 degrees and 180
degrees ( p radians). The upper end of the needle has ycoordinate
y y
high low
= + sin α
The needle is a hit if y high is at least 2, as shown in Figure 11. Stop after 10,000 tries
and print the quotient tries/hits. (This program is not suitable for computing the
value of  p . You need  p  in the computation of the angle.)
figure 10 
the Buffon needle experiment
figure 11 
a hit in the Buffon needle experiment
2
0
y high
y low α
304 Chapter 6 Loops
•• business p6.9  Your company has shares of stock it would like to sell when their value exceeds a
certain target price. Write a program that reads the target price and then reads the
current stock price until it is at least the target price. Your program should use a
Scanner to read a sequence of  double values from standard input. Once the minimum
is reached, the program should report that the stock price exceeds the target price.
•• business p6.10  Write an application to presell a limited number of cinema tickets. Each buyer can
buy as many as 4 tickets. No more than 100 tickets can be sold. Implement a pro
gram called  TicketSeller that prompts the user for the desired number of tickets and
then displays the number of remaining tickets. Repeat until all tickets have been
sold, and then display the total number of buyers.
•• business p6.11  You need to control the number of people who can be in an oyster bar at the same
time. Groups of people can always leave the bar, but a group cannot enter the bar
if they would make the number of people in the bar exceed the maximum of 100
occupants. Write a program that reads the sizes of the groups that arrive or depart.
Use negative numbers for departures. After each input, display the current number
of occupants. As soon as the bar holds the maximum number of people, report that
the bar is full and exit the program.
•• Science p6.12  In a predatorprey simulation, you compute the populations of predators and prey,
using the following equations:
prey prey A B pred
pred pred C D
n n n
n n
+
+
= × + − × ( )
= × − + ×
1
1
1
1 p prey n ( )
Here, A is the rate at which prey birth exceeds natural
death, B is the rate of predation, C is the rate at which
predator deaths exceed births without food, and D repre
sents predator increase in the presence of food.
Write a program that prompts users for these rates, the
initial population sizes, and the number of peri ods. Then
print the populations for the given number of periods. As
inputs, try A = 0.1, B = C = 0.01, and D = 0.00002 with
initial prey and predator populations of 1,000 and 20.
•• Science p6.13  Projectile flight. Suppose a cannonball is propelled straight into the air with a start ing
velocity v 0 . Any calculus book will state that the position of the ball after t sec onds is
s t gt v t ( ) = − +
1
2
2
0
, where  = g 9.81 m s 2 is the gravi tational force of the earth. No
calculus textbook ever mentions why someone would want to carry out such an
obviously dangerous experiment, so we will do it in the safety of the computer.
In fact, we will confirm the theorem
from calculus by a simulation. In our
simulation, we will consider how the
ball moves in very short time intervals
Δt. In a short time interval the velocity v
is nearly con stant, and we can compute
the distance the ball moves as Δs = vΔt.
In our program, we will simply set
const double DELTA_T = 0.01;
© Charles Gibson/iStockphoto.
© MOF/iStockphoto.
programming projects 305
and update the position by
s = s + v * DELTA_T;
The velocity changes constantly—in fact, it is reduced by the gravitational force of
the earth. In a short time interval, Δv = –gΔt, we must keep the velocity updated as
v = v - g * DELTA_T;
In the next iteration the new velocity is used to update the distance.
Now run the simulation until the cannonball falls back to the earth. Get the initial
velocity as an input (100 m̸s is a good value). Update the position and velocity 100
times per second, but print out the posi tion only every full second. Also printout the
values from the exact formulas t gt v t ( ) = − +
1
2
2
0
for com parison.
Note: You may wonder whether there is a benefit to this simulation when an exact
formula is available. Well, the formula from the calculus book is not exact. Actually,
the gravitational force diminishes the farther the cannonball is away from the surface
of the earth. This complicates the algebra sufficiently that it is not possible to give an
exact formula for the actual motion, but the computer simulation can simply be
extended to apply a variable gravitational force. For cannonballs, the calculusbook
formula is actually good enough, but computers are necessary to compute accurate
trajectories for higherflying objects such as ballistic mis siles.
••• Science p6.14  A simple model for the hull of a ship is given by
y
B x
L
z
T
= −














−













 2
1
2
1
2 2
where B is the beam, L is the length, and T is the draft. (Note: There are two values of
y for each x and z because the hull is symmetric from starboard to port.)
Solving Problems with Algorithms, James P. Holloway (John Wiley & Sons, Inc., 2004) Reprinted with permission of Jo
The crosssectional area at a point x is called the “section” in nauti
cal parlance. To compute it, let z go from 0 to –T in n increments,
each of size T n. For each value of z, compute the value for y.
Then sum the areas of trapezoidal strips. At right are the strips
where n = 4.
Write a program that reads in values for B, L, T, x, and n and then
prints out the crosssectional area at x.
Solving Problems with Algorithms, Jam
306 Chapter 6 Loops
• Science p6.15  Radioactive decay of radioactive materials can be
modeled by the equation A = A 0 e -t(log 2̸h) , where A is
the amount of the material at time t, A 0 is the amount
at time 0, and h is the halflife.
Technetium99 is a radioisotope that is used in imaging
of the brain. It has a halflife of 6 hours. Your program
should display the relative amount A ̸ A 0 in a patient
body every hour for 24 hours after receiving a dose.
••• Science p6.16  The photo at left shows an electric device called a “transformer”. Transformers are
often constructed by wrapping coils of wire around a ferrite core. The figure below
illustrates a situation that occurs in various audio devices such as cell phones and
music players. In this circuit, a transformer is used to connect a speaker to the output
of an audio amplifier.
V s = 40 V
Speaker
+
–
R 0 = 20 Ω
R s = 8 Ω
Transformer Amplifier
1 : n
The symbol used to represent the transformer is intended to suggest two coils of
wire. The parameter n of the transformer is called the “turns ratio” of the trans
former. (The number of times that a wire is wrapped around the core to form a coil is
called the number of turns in the coil. The turns ratio is literally the ratio of the
number of turns in the two coils of wire.)
When designing the circuit, we are concerned primarily with the value of the power
delivered to the speakers—that power causes the speakers to produce the sounds we
want to hear. Suppose we were to connect the speakers directly to the amplifier
without using the transformer. Some fraction of the power available from the
amplifier would get to the speakers. The rest of the available power would be lost in
the amplifier itself. The transformer is added to the circuit to increase the fraction of
the amplifier power that is delivered to the speakers.
The power, P s , delivered to the speakers is calculated using the formula
P R
nV
n R R
s s
s
s
=
+








2
0
2
Write a program that models the circuit shown and varies the turns ratio from 0.01 to
2 in 0.01 increments, then determines the value of the turns ratio that maximizes the
power delivered to the speakers.
• graphics p6.17  Write a graphical application that displays a checkerboard with 64 squares, alternat
ing white and black.
••• graphics p6.18  Write a graphical application that draws a spiral, such as this one:
© Snowleopard1/iStockphoto.
© zig4photo/iStockphoto.
answers to self-Check Questions 307
•• graphics p6.19  It is easy and fun to draw graphs of curves with the Java graphics library. Simply
draw 100 line segments joining the points (x, f(x)) and (x + d, f(x + d)), where x
ranges from x min to x max and d x x = − ( )
max min
100.
Draw the curve f x x x x ( ) . . = − + + 0 00005 0 03 4 200
3 2
, where x ranges from 0 to
400 in this fashion.
••• graphics p6.20  Draw a picture of the “fourleaved rose” whose equation in polar coordinates is
r = cos( ) 2 θ . Let  q go from 0 to 2 p in 100 steps. Each time, compute r and then com
pute the (x, y) coordinates from the polar coordinates by using the formula
x r y r = ⋅ = ⋅ cos( ) sin( ) θ θ ,
a n s W e rs t o s e L F - C h e C k Q U e s t I o n s
1.  23 years.
2.  8 years.
3.  Add a statement
System.out.println(balance);
as the last statement in the  while loop.
4.  The program prints the same output. This is
because the balance after 14 years is slightly
below $20,000, and after 15 years, it is slightly
above $20,000.
5.  2 4 8 16 32 64 128
Note that the value 128 is printed even though
it is larger than 100.
6.  n output
5
4 4
3 3
2 2
1 1
0 0
-1 -1
7.  n output
1 1,
2 1, 2,
3 1, 2, 3,
4
There is a comma after the last value. Usually,
commas are between values only.
8.  a n r i
2 4 1 1
2 2
4 3
8 4
16 5
The code computes  a n .
9.  n output
1 1
11 11
21 21
31 31
41 41
51 51
61 61
...
This is an infinite loop.  n is never equal to 50.
10.  count temp
1 123
2 12.3
3 1.23
This yields the correct answer. The number
123 has 3 digits.
count temp
1 100
2 10.0
This yields the wrong answer. The number 100
also has 3 digits. The loop condition should
have been  while (temp >= 10).
11.  int year = 1;
while (year <= numberOfYears)
{
double interest = balance * RATE / 100;
balance = balance + interest;
year++;
}
12.  11 numbers: 10 9 8 7 6 5 4 3 2 1 0
13.  for (int i = 10; i <= 20; i = i + 2)
{
System.out.println(i);
}
308 Chapter 6 Loops
14.  int sum = 0;
for (int i = 1; i <= n; i++)
{
sum = sum + i;
}
15.  final int PERIODS = 5;
for (int i = 1; i <= PERIODS; i++)
{
invest.waitYears(YEARS);
System.out.printf(
"The balance after %d years is %.2f\n",
invest.getYears(), invest.getBalance());
}
16.  do
{
System.out.print(
"Enter a value between 0 and 100: ");
value = in.nextInt();
}
while (value < 0 || value > 100);
17.  int value = 100;
while (value >= 100)
{
System.out.print("Enter a value < 100: ");
value = in.nextInt();
}
Here, the variable  value had to be initialized
with an artificial value to ensure that the loop
is entered at least once.
18.  Yes. The  do loop
do {  body } while ( condition );
is equivalent to this  while loop:
boolean first = true;
while (first ||  condition )
{
body ;
first = false;
}
19.  int x;
int sum = 0;
do
{
x = in.nextInt();
sum = sum + x;
}
while (x != 0);
20.  int x = 0;
int previous;
do
{
previous = x;
x = in.nextInt();
sum = sum + x;
}
while (x != 0 && previous != x);
21.  No data
22.  The first check ends the loop after the sentinel
has been read. The second check ensures that
the sentinel is not processed as an input value.
23.  The  while loop would never be entered. The
user would never be prompted for input.
Because  count stays 0, the program would then
print  "No data" .
24.  The  nextDouble method also returns  false .
A more accurate prompt would have been:
“Enter values, a key other than a digit to
quit:” But that might be more con fusing to
the program user who would need to ponder
which key to choose.
25.  If the user doesn’t provide any numeric input,
the first call to  in.nextDouble() will fail.
26.  Computing the aver age
27.  Simple conversion
Unknown unit
Program doesn’t understand question syntax
28.  One score is not enough
29.  It would not be possible to implement this
interface using the Java features we have
covered up to this point. There is no way
for the program to know when the first set
of inputs ends. (When you read numbers
with  value = in.nextDouble() , it is your choice
Enter scores, Q to quit: 90 80 90 100 80 Q
The average is 88
(Program exits)
Your conversion question: How many in are 30 cm
30 cm = 11.81 in
(Program exits)
Run program again for another question
Only one value can be converted
Your conversion question: How many inches are 30 cm?
Unknown unit: inches
Known units are in, ft, mi, mm, cm, m, km, oz, lb, g, kg, tsp, tbsp, pint, gal
(Program exits)
Your conversion question: What is an ångström?
Please formulate your question as “How many (unit) are (value) (unit)?”
(Program exits)
Enter scores, Q to quit: 90 Q
Error: At least two scores are required.
(Program exits)
answers to self-Check Questions 309
whether to put them on a single line or mul
tiple lines.)
30.  Comparing two inter est rates
31.  The total is zero.
32.  double total = 0;
while (in.hasNextDouble())
{
double input = in.nextDouble();
if (input > 0) { total = total + input; }
}
33.  position  is  str.length() and  ch is unchanged
from its initial value,  '?' . Note that  ch must
be initialized with some value—otherwise
the compiler will complain about a possibly
uninitialized variable.
34.  The loop will stop when a match is found, but
you cannot access the match because neither
position nor  ch are defined outside the loop.
35.  Start the loop at the end of string:
boolean found = false;
int i = str.length() - 1;
while (!found && i >= 0)
{
char ch = str.charAt(i);
if (ch == ' ') { found = true; }
else { i--; }
}
36.  The initial call to  in.nextDouble() fails, termi
nating the program. One solution is to do all
input in the loop and introduce a Boolean vari
able that checks whether the loop is entered for
the first time.
double input = 0;
boolean first = true;
while (in.hasNextDouble())
{
double previous = input;
input = in.nextDouble();
if (first) { first = false; }
else if (input == previous)
{
System.out.println("Duplicate input");
}
}
37.  All values in the inner loop should be dis
played on the same line.
38.  Change lines 13, 18, and 30 to  for (int n = 0;
n <= NMAX; n++) . Change  NMAX to 5.
39.  60: The outer loop is executed 10 times, and
the inner loop 6 times.
40.  0123
1234
2345
41.  for (int i = 1; i <= 3; i++)
{
for (int j = 1; j <= 4; j++)
{
System.out.print("[]");
}
System.out.println();
}
42.  Compute  generator.nextInt(2) , and use 0 for
heads, 1 for tails, or the other way around.
43.  Compute  generator.nextInt(4) and associate
the numbers 0 . . . 3 with the four suits. Then
compute  generator.nextInt(13) and associate
the numbers 0 . . . 12 with Jack, Ace, 2 . . . 10,
Queen, and King.
44.  Construct two  Die objects:
Die d1 = new Die(6);
Die d2 = new Die(6);
Then cast and print both of them:
System.out.println(
d1.cast() + " " + d2.cast());
45.  The call will produce a value between 2 and
12, but all values have the same proba bility.
When throwing a pair of dice, the number 7 is
six times as likely as the num ber 2. The correct
formula is
int sum = generator.nextInt(6)
+ generator.nextInt(6) + 2;
46.  generator.nextDouble() * 100.0
47.  You should step over it because you are not
interested in debugging the internals of the
println method.
48.  You should set a breakpoint. Stepping through
loops can be tedious.
49.  Unfortunately, most debuggers do not support
going backwards. Instead, you must restart the
First interest rate in percent: 5
Second interest rate in percent: 10
Years: 5
Year 5% 10%
0  10000.00  10000.00
1  10500.00  11000.00
2  11025.00  12100.00
3  11576.25  13310.00
4  12155.06  14641.00
5  12762.82  16105.10
This row clarifies that 1 means
the end of the first year
310 Chapter 6 Loops
program. Try setting breakpoints at the lines in
which the variable is changed.
50.  No, there is no need. You can just inspect the
variables in the debugger.
51.  For short programs, you certainly could. But
when programs get longer, it would be very
timeconsuming to trace them manually.
7
C h a p t e r
311
arrays and
array Lists
to collect elements using arrays
and array lists
to use the enhanced  for loop for traversing arrays and array lists
to learn common algorithms for processing arrays and array lists
to work with two-dimensional arrays
to understand the concept of regression testing
C h a p t e r G o a L s
C h a p t e r C o n t e n t s
7.1 ArrAys 312
Syntax 7.1: arrays 313
Common Error 7.1: Bounds errors 318
Common Error 7.2: Uninitialized and
Unfilled arrays 318
Programming Tip 7.1: Use arrays for sequences
of related items 318
Programming Tip 7.2: Make parallel arrays into
arrays of objects 318
Special Topic 7.1: Methods with a Variable
number of arguments 319
Computing & Society 7.1: Computer Viruses 320
7.2 The enhAnced for Loop 321
Syntax 7.2: the enhanced for Loop 322
7.3 common ArrAy ALgoriThms 322
Common Error 7.3: Underestimating the size
of a data set 331
Special Topic 7.2: sorting with the
Java Library 331
7.4 probLem soLving: AdApTing
ALgoriThms 331
How To 7.1: Working with arrays 334
Worked Example 7.1: rolling the dice
7.5 probLem soLving: discovering
ALgoriThms by mAnipuLATing
physicAL objecTs 336
7.6 Two-dimensionAL ArrAys 340
Syntax 7.3: two-dimensional array
declaration 341
Worked Example 7.2: a World
population table
Special Topic 7.3: two-dimensional arrays with
Variable row Lengths 345
Special Topic 7.4: Multidimensional arrays 347
7.7 ArrAy LisTs 347
Syntax 7.4: array Lists 347
Common Error 7.4: Length and size 356
Special Topic 7.5: the diamond syntax
in Java 7 356
7.8 regression TesTing 356
Programming Tip 7.3: Batch Files and
shell scripts 358
Computing & Society 7.2: the therac-25
incidents 359
© traveler1116/iStockphoto.
312
in many programs, you need to collect large numbers of
values. in Java, you use the array and array list constructs
for this purpose. arrays have a more concise syntax,
whereas array lists can automatically grow to any desired
size. in this chapter, you will learn about arrays, array lists,
and common algorithms for processing them.
7.1 arrays
We start this chapter by introducing the array data type. Arrays are the fundamental
mechanism in Java for collecting multiple values. In the following sections, you will
learn how to declare arrays and how to access array elements.
7.1.1 declaring and Using arrays
Suppose you write a program that reads a sequence of values and prints out the
sequence, marking the largest value, like this:
32
54
67.5
29
35
80
115 <= largest value
44.5
100
65
You do not know which value to mark as the largest one until you have seen them all.
After all, the last value might be the largest one. Therefore, the program must first
store all values before it can print them.
Could you simply store each value in a separate variable? If you know that there
are ten values, then you could store the values in ten variables  value1 ,  value2 ,  value3 , …,
value10 . However, such a sequence of vari ables is not very practical to use. You would
have to write quite a bit of code ten times, once for each of the variables. In Java, an
array is a much better choice for storing a sequence of values of the same type.
Here we create an array that can hold ten values of type  double :
new double[10]
The number of elements (here, 10) is called the length of the array.
The  new operator constructs the array. You will want to store the array in a variable
so that you can access it later.
The type of an array variable is the type of the element to be stored, followed by  [] .
In this example, the type is  double[] , because the element type is  double .
Here is the declaration of an array variable of type  double[] (see Figure 1):
double[] values;  1
When you declare an array variable, it is not yet initialized. You need to initialize the
variable with the array:
double[] values = new double[10];  2
an array collects a
sequence of values of
the same type.
7.1 arrays 313
figure 1  an array of size 10
1
Declare the array variable
values =
2
double[]
0
0
0
0
0
0
0
0
0
0
values =
3
double[]
35
0
0
0
0
0
0
0
0
0
values =
[0]
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
Initialize it with an array Access an array element
Now  values is initialized with an array of 10 numbers. By default, each number in the
array is 0.
When you declare an array, you can specify the initial values. For example,
double[] moreValues = { 32, 54, 67.5, 29, 35, 80, 115, 44.5, 100, 65 };
When you supply initial values, you don’t use the  new operator. The compiler deter-
mines the length of the array by counting the initial values.
To access a value in an array, you specify which “slot” you want to use. That is
done with the  [] operator:
values[4] = 35;  3
Now the number 4 slot of  values is filled with 35 (see Figure 1). This “slot number” is
called an index. Each slot in an array contains an element.
Because  values is an array of  double values, each element  values[i] can be used like
any variable of type  dou ble . For example, you can display the element with index 4
with the following command:
System.out.println(values[4]);
individual elements
in an array are
accessed by an
integer index  i ,
using the notation
array [i] .
an array element
can be used like
any variable.
syntax 7.1  arrays
double[] values = new double[10];
double[] moreValues = { 32, 54, 67.5, 29, 35 };
Type of array variable
List of initial values
Name of array variable
Use brackets to access an element.
values[i] = 0;
The index must be ≥ 0 and < the length of the array.
See page 318.
Length
Element
type
To construct an array: new  typeName [ length ]
To access an element: arrayReference [ index ]
Syntax
314 Chapter 7 arrays and array Lists
Before continuing, we must take care of an
important detail of Java arrays. If you look
carefully at Figure 1, you will find that the
fifth element was filled when we changed
values[4] . In Java, the elements of arrays
are numbered starting at 0. That is, the
legal elements for the  values array are
values[0] , the first element
values[1] , the second element
values[2] , the third element
values[3] , the fourth element
values[4] , the fifth element
. . .
values[9] , the tenth element
In other words, the declaration
double[] values = new double[10];
creates an array with ten elements. In this array, an index can be any integer ranging
from 0 to 9.
You have to be careful that the index stays within the valid range. Trying to access
an element that does not exist in the array is a serious error. For example, if  values has
ten elements, you are not allowed to access  values[20] . Attempting to access an ele-
ment whose index is not within the valid index range is called a bounds error. The
com piler does not catch this type of error. When a bounds error occurs at run time, it
causes a run-time excep tion.
Here is a very common bounds error:
double[] values = new double[10];
values[10] = value;
There is no  values[10] in an array with ten elements—the index can range from 0 to 9.
To avoid bounds errors, you will want to know how many elements are in an array.
The expression  values.length yields the length of the  values array. Note that there are
no parentheses following  length .
table 1 declaring arrays
int[] numbers = new int[10]; An array of ten integers. All elements are
initialized with zero.
final int LENGTH = 10;
int[] numbers = new int[LENGTH];
It is a good idea to use a named constant
instead of a “magic number”.
int length = in.nextInt();
double[] data = new double[length];
The length need not be a constant.
int[] squares = { 0, 1, 4, 9, 16 }; An array of five integers, with initial values.
String[] friends = { "Emily", "Bob", "Cindy" }; An array of three strings.
double[] data = new int[10]; error: You cannot initialize a  double[]
variable with an array of type  int[] .
© Luckie8/iStockphoto.
Like a mailbox that is identified by a box
number, an array element is identified by
an index.
an array index must
be at least zero and
less than the size of
the array.
a bounds error,
which occurs if you
supply an invalid
array index, can
cause your program
to terminate.
7.1 arrays 315
The following code ensures that you only access the array when the index variable
i is within the legal bounds:
if (0 <= i && i < values.length) { values[i] = value; }
Arrays suffer from a significant limitation: their length is fixed. If you start out with
an array of 10 ele ments and later decide that you need to add additional elements,
then you need to make a new array and copy all elements of the existing array into the
new array. We will discuss this process in detail in Section 7.3.9.
To visit all elements of an array, use a variable for the index. Suppose  values has ten
elements and the inte ger variable  i is set to 0, 1, 2, and so on, up to 9. Then the expres-
sion  values[i] yields each element in turn. For example, this loop displays all elements
in the  values array:
for (int i = 0; i < 10; i++)
{
System.out.println(values[i]);
}
Note that in the loop condition the index is less than 10 because there is no element
corresponding to  values[10] .
7.1.2 array references
If you look closely at Figure 1, you will note that the variable  values does not store
any numbers. Instead, the array is stored elsewhere and the  values variable holds a
reference to the array. (The reference denotes the location of the array in memory.)
You have already seen this behavior with objects in Section 2.8. When you access an
object or array, you need not be concerned about the fact that Java uses references.
This only becomes important when you copy a reference.
When you copy an array variable into another, both variables refer to the same
array (see Figure 2).
int[] scores = { 10, 9, 7, 4, 5 };
int[] values = scores; //  Copying array reference
You can modify the array through either of the variables:
scores[3] = 10;
System.out.println(values[3]); //  Prints 10
Section 7.3.9 shows how you can make a copy of the contents of the array.
Use the expression
array .length to
find the number of
elements in an array.
an array reference
specifies the location
of an array. Copying
the reference yields a
second reference to
the same array.
figure 2 
two array Variables referencing the same array
int[]
scores =
values =
10
9
7
4
5
316 Chapter 7 arrays and array Lists
7.1.3 Using arrays with Methods
Arrays can be method arguments and return values, just like any other values.
When you define a method with an array argument, you provide a parameter vari-
able for the array. For example, the following method adds scores to a student object:
public void addScores(int[] values)
{
for (int i = 0; i < values.length; i++)
{
totalScore = totalScore + values[i];
}
}
To call this method, you have to provide an array:
int[] scores = { 10, 9, 7, 10 };
fred.addScores(scores);
Conversely, a method can return an array. For example, a  Student class can have a
method
public int[] getScores()
that returns an array with all of the student’s scores.
7.1.4 partially Filled arrays
An array cannot change size at run time. This is a problem when you don’t know in
advance how many elements you need. In that situation, you must come up with a
good guess on the maximum number of elements that you need to store. For exam-
ple, we may decide that we sometimes want to store more than ten elements, but
never more than 100:
final int LENGTH = 100;
double[] values = new double[LENGTH];
In a typical program run, only a part of the array will be occupied by actual elements.
We call such an array a partially filled array. You must keep a companion variable
that counts how many elements are actually used. In Figure 3 we call the companion
variable  currentSize .
The following loop collects inputs and fills up the  values array:
int currentSize = 0;
Scanner in = new Scanner(System.in);
while (in.hasNextDouble())
{
if (currentSize < values.length)
{
values[currentSize] = in.nextDouble();
currentSize++;
}
}
At the end of this loop,  currentSize contains the actual number of elements in the
array. Note that you have to stop accepting inputs if the  currentSize companion vari-
able reaches the array length.
arrays can occur as
method arguments
and return values.
© AlterYourReality/iStockphoto.
With a partially filled
array, you need to
remember how many
elements are filled.
With a partially
filled array, keep a
companion variable
for the current size.
7.1 arrays 317
figure 3  a partially Filled array
double[]
values =
29
67.5
54
32
values.length
.
.
.
Not currently used
currentSize
To process the gathered array elements, you again use the companion variable, not
the array length. This loop prints the partially filled array:
for (int i = 0; i < currentSize; i++)
{
System.out.println(values[i]);
}
1.  Declare an array of integers containing the first five prime numbers.
2.  Assume the array  primes has been initialized as described in Self Check 1. What
does it contain after executing the following loop?
for (int i = 0; i < 2; i++)
{
primes[4 - i] = primes[i];
}
3.  Assume the array  primes has been initialized as described in Self Check 1. What
does it contain after executing the following loop?
for (int i = 0; i < 5; i++)
{
primes[i]++;
}
4.  Given the declaration
int[] values = new int[10];
write statements to put the integer 10 into the elements of the array  values with
the lowest and the highest valid index.
5.  Declare an array called  words that can hold ten elements of type  String .
6.  Declare an array containing two strings,  "Yes" , and  "No" .
7.  Can you produce the output on page 312 without storing the inputs in an array,
by using an algorithm similar to the algorithm for finding the maximum in
Section 6.7.5?
8.  Declare a method of a class  Lottery that returns a combination of  n numbers. You
don’t need to implement the method.
practice it  Now you can try these exercises at the end of the chapter: R7.1, R7.2, R7.6, E7.1.
fuLL code exAmpLe
Go to  wiley.com/go/
javacode to download
a program that
demonstrates array
operations.
© Nicholas Homrich/iStockphoto.
s e L f  c h e c k
318 Chapter 7 arrays and array Lists
bounds errors
Perhaps the most common error in using arrays is accessing a nonexistent element.
double[] values = new double[10];
values[10] = 5.4;
//  Error— values has 10 elements, and the index can range from 0 to 9
If your program accesses an array through an out-of-bounds index, there is no compiler error
message. Instead, the program will generate an exception at run time.
uninitialized and unfilled Arrays
A common error is to allocate an array variable, but not an actual array.
double[] values;
values[0] = 29.95; //  Error— values not initialized
Array variables work exactly like object variables—they are only references to the actual array.
To construct the actual array, you must use the  new operator:
double[] values = new double[10];
Another common error is to allocate an array of objects and expect it to be filled with objects.
BankAccount[] accounts = new BankAccount[10]; //  Contains ten  null references
This array contains  null references, not default bank accounts. You need to remember to fill
the array, for example:
for (int i = 0; i < 10; i++)
{
accounts[i] = new BankAccount();
}
use Arrays for sequences of related items
Arrays are intended for storing sequences of values with the same meaning. For example, an
array of test scores makes perfect sense:
int[] scores = new int[NUMBER_OF_SCORES];
But an array
int[] personalData = new int[3];
that holds a person’s age, bank balance, and shoe size in positions 0, 1, and 2 is bad design.
It would be tedious for the programmer to remember which of these data values is stored in
which array location. In this situation, it is far better to use three separate variables.
make parallel Arrays into Arrays of objects
Programmers who are familiar with arrays, but unfamiliar with object-oriented program-
ming, sometimes distribute information across separate arrays. Here is a typical example: A
program needs to manage bank data, consisting of account numbers and balances. Don’t store
the account numbers and balances in separate arrays.
//  Don’t do this
int[] accountNumbers;
double[] balances;
Common error 7.1
© John Bell/iStockphoto.
Common error 7.2
© John Bell/iStockphoto.
programming tip 7.1
© Eric Isselé/iStockphoto.
programming tip 7.2
© Eric Isselé/iStockphoto.
7.1 arrays 319
figure 4  avoid parallel arrays
int[]
accountNumbers =
double[]
balances =
Arrays such as these are called parallel arrays (see Figure 4). The  i th slice ( accountNumbers[i]
and  balances[i] ) contains data that need to be processed together.
If you find yourself using two arrays that have the same length, ask yourself whether you
couldn’t replace them with a single array of a class type. Look at a
slice and find the concept that it represents. Then make the concept
into a class. In our example each slice contains an account number
and a balance, describing a bank account. Therefore, it is an easy mat-
ter to use a single array of objects
BankAccount[] accounts;
(See Figure 5.)
Why is this beneficial? Think ahead. Maybe your program will change and you will need to
store the owner of the bank account as well. It is a simple matter to update the  BankAccount
class. It may well be quite complicated to add a new array and make sure that all methods that
accessed the original two arrays now also correctly access the third one.
methods with a variable number of Arguments
It is possible to declare methods that receive a variable number of arguments. For example, we
can write a method that can add an arbitrary number of scores to a student:
fred.addScores(10, 7); //  This method call has two arguments
fred.addScores(1, 7, 2, 9); //  Another call to the same method, now with four arguments
The method must be declared as
public void addScores(int... values)
The  int... type indicates that the method can receive any number of  int arguments. The  values
parameter variable is actually an  int[] array that contains all arguments that were passed to the
method.
avoid parallel arrays
by changing them
into arrays of objects.
figure 5  reorganizing parallel arrays into an array of objects
BankAccount[]
accounts =
accountNumber =
balance =
BankAccount
special topic 7.1
© Eric Isselé/iStockphoto.
320 Chapter 7 arrays and array Lists
The method implementation traverses the  values array and processes the elements:
public void addScores(int... values)
{
for (int i = 0; i < values.length; i++) // values is an  int[]
{
totalScore = totalScore + values[i];
}
}
in november 1988,
robert Morris, a stu-
dent at Cornell University, launched a
so-called virus program that infected a
significant fraction of computers con-
nected to the internet (which was much
smaller then than it is now).
in order to attack a computer, a virus
has to find a way to get its instructions
executed. this particular program car-
ried out a “buffer overrun” attack, pro-
viding an unexpectedly large input to a
program on another machine. that pro-
gram allocated an array of 512 charac-
ters, under the assumption that nobody
would ever provide such a long input.
Unfortunately, that program was writ-
ten in the C programming language.
C, unlike Java, does not check that an
array index is less than the length of the
array. if you write into an array using an
index that is too large, you simply over-
write memory locations that belong to
some other objects. C programmers are
supposed to provide safety checks, but
that had not happened in the program
under attack. the virus program pur-
posefully filled the 512-character array
with 536 bytes. the excess 24 bytes
overwrote a return address, which the
attacker knew was stored just after the
array. When the method that read the
input was finished, it didn’t return to its
caller but to code supplied by the virus
(see the figure). the virus was thus
able to execute its code on a remote
machine and infect it.
in Java, as in C, all programmers
must be very careful not to overrun
array boundaries. however, in Java,
this error causes a run-time excep-
tion, and it never corrupts memory
outside the array. this is one of the
safety features of Java. one may well
speculate what would possess the
virus author to spend weeks designing
a program that disabled thousands of
computers. it appears that the break-in
was fully intended by the author, but
the disabling of the computers was a
bug caused by continuous reinfection.
Morris was sentenced to 3 years proba-
tion, 400 hours of community service,
and a $10,000 fine.
in recent years, computer attacks
have intensified and the motives have
become more sinister. instead of dis-
abling computers, viruses often take
permanent residence in the attacked
computers. Criminal enterprises rent
out the processing power of millions of
hijacked computers for sending spam
e-mail. other viruses monitor every
keystroke and send those that look like
credit card numbers or banking pass-
words to their master.
typically, a machine gets infected
because a user executes code down-
loaded from the internet, clicking on an
icon or link that purports to be a game
or video clip. antivirus programs check
all downloaded programs against an
ever-growing list of known viruses.
When you use a computer for man-
aging your finances, you need to be
aware of the risk of infection. if a virus
reads your banking password and
empties your account, you will have
a hard time convincing your financial
institution that it wasn’t your act, and
you will most likely lose your money.
Keep your operating system and anti-
virus program up to date, and don't
click on suspicious links on a web page
or your e-mail inbox. Use banks that
require “two-factor authentication” for
major transactions, such as a callback
on your cell phone.
Viruses are even used for military
purposes. in 2010, a virus, dubbed
stuxnet, spread through Microsoft
Windows and infected UsB sticks.
the virus looked for siemens indus-
trial computers and reprogrammed
them in subtle ways. it appears that
the virus was designed to damage
the centrifuges of the iranian nuclear
enrichment operation. the computers
controlling the centrifuges were not
connected to the internet, but they
were configured with UsB sticks, some
of which were infected. it is rumored
that the virus was developed by U.s.
and israeli intelligence agencies, and
that it was successful in slowing down
the iranian nuclear program.
Return address
Buffer for input
(512 bytes)
1 Before the attack
2 After the attack
Return address
Overrun buffer
(536 bytes)
Malicious
code
A “Buffer Overrun” Attack
Computing & Society 7.1  Computer Viruses
© MediaBakery.
7.2 the enhanced for Loop 321
7.2 the enhanced  for Loop
Often, you need to visit all elements of an array. The enhanced  for loop makes this
process particularly easy to program.
Here is how you use the enhanced  for loop to total up all elements in an array
named  values :
double[] values = . . .;
double total = 0;
for (double element : values)
{
total = total + element;
}
The loop body is executed for each element in the array  values . At the beginning of
each loop iteration, the next element is assigned to the variable  element . Then the loop
body is executed. You should read this loop as “for each  element in  values ”.
This loop is equivalent to the following  for loop and an explicit index variable:
for (int i = 0; i < values.length; i++)
{
double element = values[i];
total = total + element;
}
Note an important difference between the enhanced  for loop and the basic  for loop.
In the enhanced  for loop, the element variable is assigned  values[0] ,  values[1] , and so
on. In the basic  for loop, the index variable  i is assigned 0, 1, and so on.
Keep in mind that the enhanced  for loop has a very specific purpose: getting the
elements of a collection, from the beginning to the end. It is not suitable for all array
algorithms. In particular, the enhanced  for loop does not allow you to modify the
contents of an array. The following loop does not fill an array with zeroes:
for (double element : values)
{
element = 0; //  ERROR: this assignment does not modify array elements
}
When the loop is executed, the variable  element is set to  values[0] . Then  element is set to
0, then to  values[1] , then to 0, and so on. The  values array is not modified. The remedy
is simple: Use a basic  for loop:
for (int i = 0; i < values.length; i++)
{
values[i] = 0; //  OK
}
The enhanced  for loop is a convenient mechanism for
traversing all elements in a collection.
you can use the
enhanced  for loop
to visit all elements
of an array.
Use the enhanced
for loop if you do
not need the index
values in the
loop body.
fuLL code exAmpLe
Go to  wiley.com/go/
javacode to download
a program that
demonstrates the
enhanced  for loop.
© nullplus/iStockphoto.
322 Chapter 7 arrays and array Lists
9.  What does this enhanced  for loop do?
syntax 7.2  the enhanced  for Loop
for (double element : values)
{
sum = sum + element;
}
An array
These statements
are executed for each
element.
This variable is set in each loop iteration.
It is only defined inside the loop.
The variable
contains an element,
not an index.
for ( typeName variable :  collection )
{
statements
}
Syntax
int counter = 0;
for (double element : values)
{
if (element == 0) { counter++; }
}
10.  Write an enhanced  for loop that prints all elements in the array  values .
11.  Write an enhanced  for loop that multiplies all elements in a  double[] array named
factors , accumulating the result in a variable named  product .
12.  Why is the enhanced  for loop not an appropriate shortcut for the following basic
for loop?
for (int i = 0; i < values.length; i++) { values[i] = i * i; }
practice it  Now you can try these exercises at the end of the chapter: R7.7, R7.8, R7.9.
7.3 Common array algorithms
In the following sections, we discuss some of the most common algorithms for work-
ing with arrays. If you use a partially filled array, remember to replace  values.length
with the companion variable that repre sents the current size of the array.
7.3.1 Filling
This loop fills an array with squares (0, 1, 4, 9, 16, ...). Note that the element with
index 0 contains 0 2 , the element with index 1 contains 1 2 , and so on.
for (int i = 0; i < values.length; i++)
{
values[i] = i * i;
}
© Nicholas Homrich/iStockphoto.
s e L f  c h e c k
7.3 Common array algorithms 323
7.3.2 sum and average Value
You have already encountered this algorithm in Section 6.7.1. When the values are
located in an array, the code looks much simpler:
double total = 0;
for (double element : values)
{
total = total + element;
}
double average = 0;
if (values.length > 0) { average = total / values.length; }
7.3.3 Maximum and Minimum
Use the algorithm from Section 6.7.5 that keeps a variable for the largest element
already encountered. Here is the implementation of that algorithm for an array:
double largest = values[0];
for (int i = 1; i < values.length; i++)
{
if (values[i] > largest)
{
largest = values[i];
}
}
Note that the loop starts at 1 because we initialize  largest with  values[0] .
To compute the smallest element, reverse the comparison.
These algorithms require that the array contain at least one element.
7.3.4 element separators
When you display the elements of an array, you usually want to separate them, often
with commas or vertical lines, like this:
32 | 54 | 67.5 | 29 | 35
Note that there is one fewer separator than there are numbers. Print the separator
before each element in the sequence except the initial one (with index 0) like this:
for (int i = 0; i < values.length; i++)
{
if (i > 0)
{
System.out.print(" | ");
}
System.out.print(values[i]);
}
If you want comma separators, you can use the  Arrays.toString method. (You’ll need
to import  java.util.Arrays .) The expression
Arrays.toString(values)
returns a string describing the contents of the array  values in the form
[32, 54, 67.5, 29, 35]
© CEFutcher/iStockphoto.
When separating
elements, don’t place
a separator before
the first element.
© trutenka/iStockphoto.
To print five
elements, you need
four separators.
324 Chapter 7 arrays and array Lists
The elements are surrounded by a pair of brackets and separated by commas. This
method can be convenient for debugging:
System.out.println("values=" + Arrays.toString(values));
7.3.5 Linear search
You often need to search for the position of a specific element in an array so that you
can replace or remove it. Visit all elements until you have found a match or you have
come to the end of the array. Here we search for the position of the first element in an
array that is equal to 100:
int searchedValue = 100;
int pos = 0;
boolean found = false;
while (pos < values.length && !found)
{
if (values[pos] == searchedValue)
{
found = true;
}
else
{
pos++;
}
}
if (found) { System.out.println("Found at position: " + pos); }
else { System.out.println("Not found"); }
This algorithm is called linear search or sequential search because you inspect the
elements in sequence. If the array is sorted, you can use the more efficient binary
search algorithm. We discuss binary search in Chapter 14.
7.3.6 removing an element
Suppose you want to remove the element with index  pos from the array  values . As
explained in Section 7.1.4, you need a companion variable for tracking the number of
elements in the array. In this example, we use a companion variable called  currentSize .
If the elements in the array are not in any particular order, simply overwrite the
element to be removed with the last element of the array, then decrement the  current-
Size variable. (See Figure 6.)
© yekorzh/iStockphoto.
To search for a
specific element,
visit the elements
and stop when you
encounter the match.
a linear search
inspects elements
in sequence until a
match is found.
figure 6 
removing an element in an Unordered array
[0]
[1]
[2]
.
.
.
[pos]
[currentSize - 1]
Decrement after
moving element
currentSize
32
54
67.5
29
34.5
80
115
44.5
100
65
figure 7 
removing an element in an ordered array
[0]
[1]
[2]
.
.
.
[pos]
[currentSize - 1]
1
2
3
4
5
Decrement after
moving elements
32
54
67.5
29
80
115
44.5
100
65
65
7.3 Common array algorithms 325
values[pos] = values[currentSize - 1];
currentSize--;
The situation is more complex if the order of the elements matters. Then you must
move all elements fol lowing the element to be removed to a lower index, and then
decrement the variable holding the size of the array. (See Figure 7.)
for (int i = pos + 1; i < currentSize; i++)
{
values[i - 1] = values[i];
}
currentSize--;
7.3.7 inserting an element
In this section, you will see how to insert an element into an array. Note that you
need a companion vari able for tracking the array size, as explained in Section 7.1.4.
If the order of the elements does not matter, you can simply insert new elements at
the end, incrementing the variable tracking the size.
if (currentSize < values.length)
{
currentSize++;
values[currentSize - 1] = newElement;
}
It is more work to insert an element at a particular position in the middle of an array.
First, move all ele ments after the insertion location to a higher index. Then insert the
new element (see Figure 9).
Note the order of the movement: When you remove an element, you first move
the next element to a lower index, then the one after that, until you finally get to the
end of the array. When you insert an ele ment, you start at the end of the array, move
that element to a higher index, then move the one before that, and so on until you
finally get to the insertion location.
if (currentSize < values.length)
{
currentSize++;
for (int i = currentSize - 1; i > pos; i--)
{
values[i] = values[i - 1];
}
values[pos] = newElement;
}
Camera: © james steidl/iStockphoto.
Globe: © Alex Slobodkin/iStockphoto.
A N IM AT IO N
Removing from
an Array
Camera: © james steidl/iStockphoto.
Globe: © Alex Slobodkin/iStockphoto.
A N IM AT IO N
Inserting into
an Array
Before inserting
an element, move
elements to the end
of the array starting
with the last one.
figure 8 
inserting an element in an Unordered array
[0]
[1]
[2]
.
.
.
[currentSize - 1]
Incremented before
inserting element
Insert new element here
currentSize
32
54
67.5
29
34.5
80
115
44.5
100
figure 9 
inserting an element in an ordered array
[0]
[1]
[2]
.
.
.
[pos]
[currentSize - 1]
5
4
3
2
1
Incremented before
moving elements
Insert new element here
32
54
67.5
29
42
34.5
80
115
44.5
100
326 Chapter 7 arrays and array Lists
7.3.8 swapping elements
You often need to swap elements of an array. For example,
you can sort an array by repeatedly swapping elements
that are not in order.
Consider the task of swapping the elements at posi-
tions  i and  j of an array  values . We’d like to set  values[i]
to  values[j] . But that overwrites the value that is currently
stored in  values[i] , so we want to save that first:
double temp = values[i];
values[i] = values[j];
Now we can set  values[j] to the saved value.
values[j] = temp;
Figure 10 shows the process.
To swap two elements, you
need a temporary variable.
Use a temporary
variable when
swapping two
elements.
figure 10  swapping array elements
[0]
[1]
[2]
[3]
[4]
[i]
[j]
34.5
29
67.5
54
32
1
[i]
[j]
34.5
29
67.5
54
32
2
temp =  54
[i]
[j]
34.5
29
67.5
29
32
3
temp =  54
[i]
[j]
34.5
54
67.5
29
32
4
temp =  54
Values to be swapped
values =
values =
values =
values =
7.3 Common array algorithms 327
7.3.9 Copying arrays
Array variables do not themselves hold array elements. They hold a reference to the
actual array. If you copy the reference, you get another reference to the same array
(see Figure 11):
double[] values = new double[6];
. . . //  Fill array
double[] prices = values;  1
If you want to make a true copy of an array, call the  Arrays.copyOf method (as shown
in Figure 11).
double[] prices = Arrays.copyOf(values, values.length);  2
The call  Arrays.copyOf(values, n) allocates an array of length  n , copies the first  n elements
of  values (or the entire  values array if  n > values.length ) into it, and returns the new array.
In order to use the  Arrays class, you need to add the following statement to the top of
your program:
import java.util.Arrays;
Another use for  Arrays.copyOf is to grow an array that has run out of space. The fol-
lowing statements have the effect of doubling the length of an array (see Figure 12):
double[] newValues = Arrays.copyOf(values, 2 * values.length);  1
values = newValues;  2
The  copyOf method was added in Java 6. If you use Java 5, replace
double[] newValues = Arrays.copyOf(values, n)
with
Use the  Arrays.
copyOf method to
copy the elements of
an array into a
new array.
figure 11  Copying an array reference versus Copying an array
1 2
double[]
values =
prices =
32
54
67.5
29
35
47.5
double[]
values =
double[]
prices =
32
54
67.5
29
35
47.5
32
54
67.5
29
35
47.5
After the assignment  prices = values After calling  Arrays.copyOf
328 Chapter 7 arrays and array Lists
figure 12  Growing an array
double[] double[]
values =
double[]
newValues =
values =
double[]
newValues =
1 2
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Move elements to a larger array Store the reference to the larger array in  values
double[] newValues = new double[n];
for (int i = 0; i < n && i < values.length; i++)
{
newValues[i] = values[i];
}
7.3.10 reading input
If you know how many inputs the user will supply, it is simple to place them into an
array:
double[] inputs = new double[NUMBER_OF_INPUTS];
for (i = 0; i < inputs.length; i++)
{
inputs[i] = in.nextDouble();
}
However, this technique does not work if you need to read a sequence of arbitrary
length. In that case, add the inputs to an array until the end of the input has been
reached.
int currentSize = 0;
while (in.hasNextDouble() && currentSize < inputs.length)
{
inputs[currentSize] = in.nextDouble();
currentSize++;
}
7.3 Common array algorithms 329
Now  inputs is a partially filled array, and the companion variable  currentSize is set to
the number of inputs.
However, this loop silently throws away inputs that don’t fit into the array. A bet-
ter approach is to grow the array to hold all inputs.
double[] inputs = new double[INITIAL_SIZE];
int currentSize = 0;
while (in.hasNextDouble())
{
//  Grow the array if it has been completely filled
if (currentSize >= inputs.length)
{
inputs = Arrays.copyOf(inputs, 2 * inputs.length); //  Grow the  inputs  array
}
inputs[currentSize] = in.nextDouble();
currentSize++;
}
When you are done, you can discard any excess (unfilled) elements:
inputs = Arrays.copyOf(inputs, currentSize);
The following program puts these algorithms to work, solving the task that we set our-
selves at the begin ning of this chapter: to mark the largest value in an input sequence.
section_3/LargestinArray.java
1 import java.util.Scanner;
2
3 /**
4 This program reads a sequence of values and prints them, marking the largest value.
5 */
6 public class LargestInArray
7 {
8 public static void main(String[] args)
9 {
10 final int LENGTH = 100;
11 double[] values = new double[LENGTH];
12 int currentSize = 0;
13
14 //  Read inputs
15
16 System.out.println("Please enter values, Q to quit:");
17 Scanner in = new Scanner(System.in);
18 while (in.hasNextDouble() && currentSize < values.length)
19 {
20 values[currentSize] = in.nextDouble();
21 currentSize++;
22 }
23
24 //  Find the largest value
25
26 double largest = values[0];
27 for (int i = 1; i < currentSize; i++)
28 {
29 if (values[i] > largest)
30 {
31 largest = values[i];
32 }
33 }
330 Chapter 7 arrays and array Lists
34
35 //  Print all values, marking the largest
36
37 for (int i = 0; i < currentSize; i++)
38 {
39 System.out.print(values[i]);
40 if (values[i] == largest)
41 {
42 System.out.print(" <== largest value");
43 }
44 System.out.println();
45 }
46 }
47 }
program run
Please enter values, Q to quit:
34.5 80 115 44.5 Q
34.5
80
115 <== largest value
44.5
13.  Given these inputs, what is the output of the  LargestInArray program?
20 10 20 Q
14.  Write a loop that counts how many elements in an array are equal to zero.
15.  Consider the algorithm to find the largest element in an array. Why don’t we
initialize  largest and  i with zero, like this?
double largest = 0;
for (int i = 0; i < values.length; i++)
{
if (values[i] > largest)
{
largest = values[i];
}
}
16.  When printing separators, we skipped the separator before the initial element.
Rewrite the loop so that the separator is printed after each element, except for
the last element.
17.  What is wrong with these statements for printing an array with separators?
System.out.print(values[0]);
for (int i = 1; i < values.length; i++)
{
System.out.print(", " + values[i]);
}
18.  When finding the position of a match, we used a  while loop, not a  for loop. What
is wrong with using this loop instead?
for (pos = 0; pos < values.length && !found; pos++)
{
if (values[pos] > 100)
{
found = true;
}
© Nicholas Homrich/iStockphoto.
s e L f  c h e c k
7.4 problem solving: adapting algorithms 331
}
19.  When inserting an element into an array, we moved the elements with larger
index values, starting at the end of the array. Why is it wrong to start at the inser-
tion location, like this?
for (int i = pos; i < currentSize - 1; i++)
{
values[i + 1] = values[i];
}
practice it  Now you can try these exercises at the end of the chapter: R7.15, R7.18, E7.8.
underestimating the size of a data set
Programmers commonly underestimate the amount of input data that a user will pour into an
unsuspecting program. Suppose you write a program to search for text in a file. You store each
line in a string, and keep an array of strings. How big do you make the array? Surely nobody
is going to challenge your program with an input that is more than 100 lines. Really? It is very
easy to feed in the entire text of Alice in Wonderland or War and Peace (which are avail able on
the Internet). All of a sudden, your program has to deal with tens or hundreds of thousands of
lines. You either need to allow for large inputs or politely reject the excess input.
sorting with the java Library
Sorting an array efficiently is not an easy task. You will
learn in Chapter 14 how to implement efficient sorting
algorithms. Fortunately, the Java library provides an effi-
cient  sort method.
To sort an array  values , call
Arrays.sort(values);
If the array is partially filled, call
Arrays.sort(values, 0, currentSize);
7.4 problem solving: adapting algorithms
In Section 7.3, you were introduced to a number of fundamental array algorithms.
These algorithms form the building blocks for many programs that process arrays.
In general, it is a good problem-solving strat egy to have a repertoire of fundamental
algorithms that you can combine and adapt.
Consider this example problem: You are given the quiz scores of a student. You are
to compute the final quiz score, which is the sum of all scores after dropping the low-
est one. For example, if the scores are
8 7 8.5 9.5 7 4 10
then the final score is 50.
Common error 7.3
© John Bell/iStockphoto.
special topic 7.2
© Eric Isselé/iStockphoto.
© ProstoVova/iStockphoto.
By combining
fundamental
algorithms, you can
solve complex
programming tasks.
332 Chapter 7 arrays and array Lists
We do not have a ready-made algorithm for this situation. Instead, consider which
algorithms may be related. These include:
• Calculating the sum (Section 7.3.2)
• Finding the minimum value (Section 7.3.3)
• Removing an element (Section 7.3.6)
We can formulate a plan of attack that combines these algorithms:
Find the minimum.
Remove it from the array.
Calculate the sum.
Let’s try it out with our example. The minimum of
8
[0]
7
[1]
8.5
[2]
9.5
[3]
7
[4]
4
[5]
10
[6]
is 4. How do we remove it?
Now we have a problem. The removal algorithm in Section 7.3.6 locates the ele-
ment to be removed by using the position of the element, not the value.
But we have another algorithm for that:
• Linear search (Section 7.3.5)
We need to fix our plan of attack:
Find the minimum value.
Find its position.
Remove that position from the array.
Calculate the sum.
Will it work? Let’s continue with our example.
We found a minimum value of 4. Linear search tells us that the value 4 occurs at
position 5.
8
[0]
7
[1]
8.5
[2]
9.5
[3]
7
[4]
4
[5]
10
[6]
We remove it:
8
[0]
7
[1]
8.5
[2]
9.5
[3] [4]
7
[5]
10
Finally, we compute the sum: 8 + 7 + 8.5 + 9.5 + 7 + 10 = 50.
This walkthrough demonstrates that our strategy works.
Can we do better? It seems a bit inefficient to find the minimum and then make
another pass through the array to obtain its position.
We can adapt the algorithm for finding the minimum to yield the position of the
minimum. Here is the original algorithm:
double smallest = values[0];
for (int i = 1; i < values.length; i++)
{
if (values[i] < smallest)
{
smallest = values[i];
}
you should be
familiar with the
implementation
of fundamental
algorithms so that
you can adapt them.
7.4 problem solving: adapting algorithms 333
}
When we find the smallest value, we also want to update the position:
if (values[i] < smallest)
{
smallest = values[i];
smallestPosition = i;
}
In fact, then there is no reason to keep track of the smallest value any longer. It is sim-
ply  val ues[smallestPosition] . With this insight, we can adapt the algorithm as follows:
int smallestPosition = 0;
for (int i = 1; i < values.length; i++)
{
if (values[i] < values[smallestPosition])
{
smallestPosition = i;
}
}
With this adaptation, our problem is solved with the following strategy:
Find the position of the minimum.
Remove it from the array.
Calculate the sum.
The next section shows you a technique for discovering a new algorithm when none
of the fundamen tal algorithms can be adapted to a task.
20.  Section 7.3.6 has two algorithms for removing an element. Which of the two
should be used to solve the task described in this section?
21.  It isn’t actually necessary to remove the minimum in order to compute the total
score. Describe an alternative.
22.  How can you print the number of positive and negative values in a given array,
using one or more of the algorithms in Section 6.7?
23.  How can you print all positive values in an array, separated by commas?
24.  Consider the following algorithm for collecting all matches in an array:
int matchesSize = 0;
for (int i = 0; i < values.length; i++)
{
if (values[i]  fulfills the condition )
{
matches[matchesSize] = values[i];
matchesSize++;
}
}
How can this algorithm help you with Self Check 23?
practice it  Now you can try these exercises at the end of the chapter: R7.24, R7.25.
fuLL code exAmpLe
Go to  wiley.com/go/
javacode to download
a program that
computes the score
using the adapted
algorithm for finding
the minimum.
© Nicholas Homrich/iStockphoto.
s e L f  c h e c k
334 Chapter 7 arrays and array Lists
step 1  Decompose your task into steps.
You will usually want to break down your task into multiple steps, such as
• Reading the data into an array.
• Processing the data in one or more steps.
• Displaying the results.
When deciding how to process the data, you should be familiar with the array algorithms in
Section 7.3. Most processing tasks can be solved by using one or more of these algorithms.
In our sample problem, we will want to read the data. Then we will remove the minimum
and compute the total. For example, if the input is  8 7 8.5 9.5 7 5 10 , we will remove the mini-
mum of  5 , yielding  8 7 8.5 9.5 7 10 . The sum of those values is the final score of  50 .
Thus, we have identified three steps:
Read inputs.
Remove the minimum.
Calculate the sum.
step 2  Determine which algorithm(s) you need.
Sometimes, a step corresponds to exactly one of the basic array algorithms in Section 7.3. That
is the case with calcu lating the sum (Section 7.3.2) and reading the inputs (Section 7.3.10). At
other times, you need to combine several algorithms. To remove the minimum value, you can
find the minimum value (Section 7.3.3), find its position (Section 7.3.5), and remove the ele-
ment at that position (Section 7.3.6).
We have now refined our plan as follows:
Read inputs.
Find the minimum.
Find its position.
Remove the minimum.
Calculate the sum.
This plan will work—see Section 7.4. But here is an alternate approach. It is easy to compute
the sum and subtract the minimum. Then we don’t have to find its position. The revised plan is
Read inputs.
Find the minimum.
Calculate the sum.
Subtract the minimum.
© Steve Simzer/iStockphoto.
hoW to 7.1  working with Arrays
In many data processing situations, you need
to process a sequence of values. This How To
walks you through the steps for storing input
values in an array and carrying out computa-
tions with the array elements.
problem statement  Consider again the
problem from Section 7.4: A final quiz score is
computed by adding all the scores, except for
the lowest one. For example, if the scores are
8 7 8.5 9.5 7 5 10
then the final score is 50.
Thierry Dosogne/The Image Bank/Getty Images, Inc.
7.4 problem solving: adapting algorithms 335
step 3  Use classes and methods to structure the program.
Even though it may be possible to put all steps into the  main method, this is rarely a good idea.
It is better to carry out each processing step in a separate method. It is also a good idea to come
up with a class that is responsible for collect ing and processing the data.
In our example, let’s provide a class  Student . A student has an array of scores.
public class Student
{
private double[] scores;
private double scoresSize;
. . .
public Student(int capacity) { . . . }
public boolean addScore(double score) { . . . }
public double finalScore() { . . . }
}
A second class,  ScoreAnalyzer , is responsible for reading the user input and displaying the
result. Its  main method sim ply calls the  Student methods:
Student fred = new Student(100);
System.out.println("Please enter values, Q to quit:");
while (in.hasNextDouble())
{
if (!fred.addScore(in.nextDouble()))
{
System.out.println("Too many scores.");
return;
}
}
System.out.println("Final score: " + fred.finalScore());
Now the  finalScore method must do the heavy lifting. It too should not have to do all the
work. Instead, we will supply helper methods
public double sum()
public double minimum()
These methods simply implement the algorithms in Sections 7.3.2 and 7.3.3.
Then the  finalScore method becomes
public double finalScore()
{
if (scoresSize == 0)
{
return 0;
}
else if (scores.size() == 1)
{
return scores[0];
}
else
{
return sum() - minimum();
}
}
step 4  Assemble and test the program.
Place your methods into a class. Review your code and check that you handle both normal
and exceptional situa tions. What happens with an empty array? One that contains a single ele-
ment? When no match is found? When there are multiple matches? Consider these boundary
conditions and make sure that your program works correctly.
336 Chapter 7 arrays and array Lists
In our example, it is impossible to compute the minimum if the array is empty. In that case,
we should terminate the program with an error message before attempting to call the  minimum
method.
What if the minimum value occurs more than once? That means that a student had more
than one test with the same low score. We subtract only one of the occurrences of that low
score, and that is the desired behavior.
The following table shows test cases and their expected output:
test Case expected output Comment
8 7 8.5 9.5 7 5 10 50 See Step 1.
8 7 7 9 24 Only one instance of the low score should be removed.
8 0 After removing the low score, no score remains.
(no inputs) error That is not a legal input.
The complete program is in the  how_to_1 folder of your companion code.
7.5 problem solving: discovering algorithms by
Manipulating physical objects
In Section 7.4, you saw how to solve a problem by com-
bining and adapting known algorithms. But what do
you do when none of the standard algorithms is suf-
ficient for your task? In this section, you will learn a
technique for discovering algorithms by manipulating
physical objects.
Consider the following task: You are given an array
whose size is an even number, and you are to switch the
first and the second half. For example, if the array con-
tains the eight numbers
9 13 21 4 11 7 1 3
then you should change it to
9 13 21 4 11 7 1 3
WorKed exaMpLe 7.1  rolling the dice
Learn how to analyze a set of die tosses to see whether the die
is “fair”. Go to  wiley.com/go/javaexamples and download the
file for Worked Example 7.1.
© ktsimage/iStockphoto.
© JenCon/iStockphoto.
Manipulating physical objects
can give you ideas for
discovering algorithms.
7.5 Problem Solving: Discovering Algorithms by Manipulating Physical Objects  337
Many students find it quite challenging to come up with an algorithm. They may
know that a loop is required, and they may realize that elements should be inserted
(Section 7.3.7) or swapped (Section 7.3.8), but they do not have sufficient intuition to
draw diagrams, describe an algorithm, or write down pseudocode.
One useful technique for discovering an algorithm is to manipulate physical
objects. Start by lining up some objects to denote an array. Coins, playing cards, or
small toys are good choices.
Here we arrange eight coins:
coins: © jamesbenet/iStockphoto; dollar coins: © JordiDelgado/iStockphoto.
Now let’s step back and see what we can do to change the order of the coins.
We can remove a coin (Section 7.3.6):
We can insert a coin (Section 7.3.7):
Or we can swap two coins (Section 7.3.8).
Go ahead—line up some coins and try out these three operations right now so that
you get a feel for them.
Use a sequence of
coins, playing cards,
or toys to visualize
an array of values.
Visualizing the
removal of an
array element
coins: © jamesbenet/iStockphoto; dollar coins: © JordiDelgado/iStockphoto.
Visualizing the
insertion of an
array element
coins: © jamesbenet/iStockphoto; dollar coins: © JordiDelgado/iStockphoto.
Visualizing the
swapping of two
array elements
coins: © jamesbenet/iStockphoto; dollar coins: © JordiDelgado/iStockphoto.
338 Chapter 7  Arrays and Array lists
Now how does that help us with our problem, switching the first and the second
half of the array?
Let’s put the first coin into place, by swapping it with the fifth coin. However, as
Java programmers, we will say that we swap the coins in positions 0 and 4:
coins: © jamesbenet/iStockphoto; dollar coins: © JordiDelgado/iStockphoto.
Next, we swap the coins in positions 1 and 5:
coins: © jamesbenet/iStockphoto; dollar coins: © JordiDelgado/iStockphoto.
Two more swaps, and we are done:
coins: © jamesbenet/iStockphoto; dollar coins: © JordiDelgado/iStockphoto.
Now an algorithm is becoming apparent:
i = 0
j = ... (we’ll think about that in a minute)
While (don’t know yet)
Swap elements at positions i and j
i++
j++
Where does the variable  j start? When we have eight coins, the coin at position zero is
moved to position 4. In general, it is moved to the middle of the array, or to position
size / 2 .
And how many iterations do we make? We need to swap all coins in the first half.
That is, we need to swap  size / 2  coins.
7.5 Problem Solving: Discovering Algorithms by Manipulating Physical Objects  339
The pseudocode is
i = 0
j = size / 2
While (i < size / 2)
Swap elements at positions i and j
i++
j++
It is a good idea to make a walkthrough of the pseudocode (see Section 6.2). You can
use paper clips to denote the positions of the variables  i and  j . If the walkthrough is
suc cessful, then we know that there was no “off-by-one” error in the pseudocode.
Self Check 25 asks you to carry out the walkthrough, and Exercise E7.9 asks you to
translate the pseudocode to Java. Exercise R7.26 suggests a different algorithm for
switching the two halves of an array, by repeatedly removing and inserting coins.
Many people find that the manipulation of physical objects is less intimidating
than drawing diagrams or mentally envisioning algorithms. Give it a try when you
need to design a new algorithm!
25.  Walk through the algorithm that we developed in this section, using two paper
clips to indicate the positions for  i and  j . Explain why there are no bounds errors
in the pseudocode.
26.  Take out some coins and simulate the following pseudocode, using two paper
clips to indicate the positions for  i and  j .
i = 0
j = size - 1
While (i < j)
Swap elements at positions i and j
i++
j--
What does the algorithm do?
27.  Consider the task of rearranging all elements in an array so that the even num-
bers come first. Other wise, the order doesn’t matter. For example, the array
1 4 14 2 1 3 5 6 23
could be rearranged to
4 2 14 6 1 5 3 23 1
Using coins and paperclips, discover an algorithm that solves this task by
swapping elements, then describe it in pseudocode.
28.  Discover an algorithm for the task of Self Check 27 that uses removal and
insertion of elements instead of swapping.
29.  Consider the algorithm in Section 6.7.5 that finds the
largest element in a sequence of inputs—not the largest
element in an array. Why is this algorithm better visual-
ized by picking playing cards from a deck rather than
arranging toy soldiers in a sequence?
Practice It  Now you can try these exercises at the end of the chapter: R7.26, R7.27, E7.9.
Full Code examPle
Go to  wiley.com/go/
javacode  to download
a program that
implements the
algorithm that
switches the first
and second halves
of an array.
You can use paper
clips as position
markers or counters.
© Nicholas Homrich/iStockphoto.
S e l F C h e C k
© claudio.arnese/iStockphoto.
340 Chapter 7 arrays and array Lists
7.6 two-dimensional arrays
It often happens that you want to store
collections of values that have a two-
dimensional layout. Such data sets com-
monly occur in financial and scientific
applications. An arrangement consisting
of rows and columns of values is called a
two-dimensional array, or a matrix.
Let’s explore how to store the example
data shown in Figure 13: the medal counts
of the figure skating competitions at the
2010 Winter Olympics.
Gold silver Bronze
Canada 1 0 1
China 1 1 0
Germany 0 0 1
Korea 1 0 0
Japan 0 1 1
Russia 0 1 1
United States 1 1 0
figure 13  Figure skating Medal Counts
7.6.1 declaring two-dimensional arrays
In Java, you obtain a two-dimensional array by supplying the number of rows and
columns. For exam ple,  new int[7][3] is an array with seven rows and three columns.
You store a reference to such an array in a variable of type  int[][] . Here is a complete
declaration of a two-dimensional array, suitable for holding our medal count data:
final int COUNTRIES = 7;
final int MEDALS = 3;
int[][] counts = new int[COUNTRIES][MEDALS];
Alternatively, you can declare and initialize the array by grouping each row:
int[][] counts =
{
{ 1, 0, 1 },
{ 1, 1, 0 },
{ 0, 0, 1 },
{ 1, 0, 0 },
{ 0, 1, 1 },
{ 0, 1, 1 },
{ 1, 1, 0 }
};
© Trub/iStockphoto.
© technotr/iStockphoto.
Use a two-
dimensional array to
store tabular data.
7.6 two-dimensional arrays 341
syntax 7.3  two-dimensional array declaration
int[][] data = {
{ 16, 3, 2, 13 },
{ 5, 10, 11, 8 },
{ 9, 6, 7, 12 },
{ 4, 15, 14, 1 },
};
Name
List of initial values
double[][] tableEntries = new double[7][3];
Name Element type
Number of rows
Numberof columns
All values are initialized with 0.
As with one-dimensional arrays, you cannot change the size of a two-dimensional
array once it has been declared.
7.6.2 accessing elements
To access a particular element in the two-dimensional array, you need to specify
two index values in sep arate brackets to select the row and column, respectively (see
Figure 14):
int medalCount = counts[3][1];
To access all elements in a two-dimensional array, you use nested loops. For example,
the following loop prints all elements of  counts :
for (int i = 0; i < COUNTRIES; i++)
{
//  Process the  i th row
for (int j = 0; j < MEDALS; j++)
{
//  Process the  j th column in the  i th row
System.out.printf("%8d", counts[i][j]);
}
System.out.println(); //  Start a new line at the end of the row
}
individual elements
in a two-dimensional
array are accessed
by using two index
values,  array[i][j] .
figure 14 
accessing an element in a
two-dimensional array
[0]
[1]
[2]
[3]
[4]
[5]
[6]
[0][1][2]
counts[3][1]
Column index
Row index
342 Chapter 7 arrays and array Lists
In these loops, the number of rows and colums were given as constants. Alterna-
tively, you can use the following expressions:
•  counts.length is the number of rows.
•  counts[0].length is the number of columns. (See Special Topic 7.3 for an explana-
tion of this expression.)
With these expressions, the nested loops become
for (int i = 0; i < counts.length; i++)
{
for (int j = 0; j < counts[0].length; j++)
{
System.out.printf("%8d", counts[i][j]);
}
System.out.println();
}
7.6.3 Locating neighboring elements
Some programs that work with two-dimensional arrays need to locate the elements
that are adjacent to an element. This task is particularly common in games. Figure 15
shows how to compute the index values of the neighbors of an element.
For example, the neighbors of  counts[3][1] to the left and right are  counts[3][0]  and
counts[3][2] . The neighbors to the top and bottom are  counts[2][1] and  counts[4][1] .
You need to be careful about computing neighbors at the boundary of the array.
For example,  counts[0][1] has no neighbor to the top. Consider the task of computing
the sum of the neighbors to the top and bottom of the element  count[i][j] . You need
to check whether the element is located at the top or bottom of the array:
int total = 0;
if (i > 0) { total = total + counts[i - 1][j]; }
if (i < ROWS - 1) { total = total + counts[i + 1][j]; }
7.6.4 accessing rows and Columns
You often need to access all elements in a row or column, for example to compute the
sum of the elements or the largest element in a row or column.
figure 15 
neighboring Locations in a
two-dimensional array
[i - 1][j - 1] [i - 1][j] [i - 1][j + 1]
[i][j - 1] [i][j] [i][j + 1]
[i + 1][j - 1] [i + 1][j] [i + 1][j + 1]
7.6 two-dimensional arrays 343
In our sample array, the row totals give us the total num ber of medals won by a par-
ticular country.
Finding the correct index values is a bit tricky, and it is a good idea to make a quick
sketch. To compute the total of row  i , we need to visit the following elements:
[i][0] [i][1] [i][2] row i
0 MEDALS - 1
As you can see, we need to compute the sum of  counts[i][j] , where  j ranges from  0 to
MEDALS - 1 . The fol lowing loop computes the total:
int total = 0;
for (int j = 0; j < MEDALS; j++)
{
total = total + counts[i][j];
}
Computing column totals is similar. Form the sum of  counts[i][j] , where  i ranges
from  0  to  COUNTRIES - 1 .
int total = 0;
for (int i = 0; i < COUNTRIES; i++)
{
total = total + counts[i][j];
}
[0][j]
[1][j]
[2][j]
[3][j]
[4][j]
[5][j]
[6][j]
column j
COUNTRIES - 1
0
Working with two-dimensional arrays is illustrated in the following program. The
program prints out the medal counts and the row totals.
Camera: © james steidl/iStockphoto.
Globe: © Alex Slobodkin/iStockphoto.
A N IM AT IO N
Tracing a Nested
Loop in a 2D Array
344 Chapter 7 arrays and array Lists
section_6/medals.java
1 /**
2 This program prints a table of medal winner counts with row totals.
3 */
4 public class Medals
5 {
6 public static void main(String[] args)
7 {
8 final int COUNTRIES = 7;
9 final int MEDALS = 3;
10
11 String[] countries =
12 {
13 "Canada",
14 "China",
15 "Germany",
16 "Korea",
17 "Japan",
18 "Russia",
19 "United States"
20 };
21
22 int[][] counts =
23 {
24 { 1, 0, 1 },
25 { 1, 1, 0 },
26 { 0, 0, 1 },
27 { 1, 0, 0 },
28 { 0, 1, 1 },
29 { 0, 1, 1 },
30 { 1, 1, 0 }
31 };
32
33 System.out.println(" Country Gold Silver Bronze Total");
34
35 //  Print countries, counts, and row totals
36 for (int i = 0; i < COUNTRIES; i++)
37 {
38 //  Process the  i th row
39 System.out.printf("%15s", countries[i]);
40
41 int total = 0;
42
43 //  Print each row element and update the row total
44 for (int j = 0; j < MEDALS; j++)
45 {
46 System.out.printf("%8d", counts[i][j]);
47 total = total + counts[i][j];
48 }
49
50 //  Display the row total and print a new line
51 System.out.printf("%8d\n", total);
52 }
53 }
54 }
7.6 two-dimensional arrays 345
program run
Country Gold Silver Bronze Total
Canada 1 0 1 2
China 1 1 0 2
Germany 0 0 1 1
Korea 1 0 0 1
Japan 0 1 1 2
Russia 0 1 1 2
United States 1 1 0 2
30.  What results do you get if you total the columns in our sample data?
31.  Consider an 8 × 8 array for a board game:
int[][] board = new int[8][8];
Using two nested loops, initialize the board so that zeroes and ones alternate, as
on a checkerboard:
0 1 0 1 0 1 0 1
1 0 1 0 1 0 1 0
0 1 0 1 0 1 0 1
. . .
1 0 1 0 1 0 1 0
Hint: Check whether  i + j is even.
32.  Declare a two-dimensional array for representing a tic-tac-toe board. The board
has three rows and columns and contains strings  "x" ,  "o" , and  " " .
33.  Write an assignment statement to place an  "x" in the upper-right corner of the
tic-tac-toe board in Self Check 32.
34.  Which elements are on the diagonal joining the upper-left and the lower-right
corners of the tic-tac-toe board in Self Check 32?
practice it  Now you can try these exercises at the end of the chapter: R7.28, E7.15, E7.16.
Two-dimensional Arrays with variable row Lengths
When you declare a two-dimensional array with the command
int[][] a = new int[3][3];
you get a 3 × 3 matrix that can store 9 elements:
a[0][0] a[0][1] a[0][2]
a[1][0] a[1][1] a[1][2]
a[2][0] a[2][1] a[2][2]
In this matrix, all rows have the same length.
© Nicholas Homrich/iStockphoto.
s e L f  c h e c k
WorKed exaMpLe 7.2  A world population Table
Learn how to print world population data in a table with row and column headers, and with
totals for each of the data columns. Go to  wiley.com/go/javaexamples and download the file for
Worked Example 7.2.
special topic 7.3
© Eric Isselé/iStockphoto.
346 Chapter 7 arrays and array Lists
In Java it is possible to declare arrays in which the row length varies. For example, you can
store an array that has a triangular shape, such as:
b[0][0]
b[1][0] b[1][1]
b[2][0] b[2][1] b[2][2]
To allocate such an array, you must work harder. First, you allocate space to hold three rows.
Indicate that you will manually set each row by leaving the second array index empty:
double[][] b = new double[3] [] ;
Then allocate each row separately (see Figure 16):
for (int i = 0; i < b.length; i++)
{
b[i] = new double[i + 1];
}
You can access each array element as  b[i][j] . The expression  b[i] selects the  i th row, and the
[j] operator selects the  j th element in that row.
Note that the number of rows is  b.length , and the length of the  i th row is  b[i].length . For
example, the following pair of loops prints a ragged array:
for (int i = 0; i < b.length; i++)
{
for (int j = 0; j < b[i].length; j++)
{
System.out.print(b[i][j]);
}
System.out.println();
}
Alternatively, you can use two enhanced  for loops:
for (double[] row : b)
{
for (double element : row)
{
System.out.print(element);
}
System.out.println();
}
Naturally, such “ragged” arrays are not very common.
Java implements plain two-dimensional arrays in exactly the same way as ragged arrays: as
arrays of one-dimen sional arrays. The expression  new int[3][3] automatically allocates an
array of three rows, and three arrays for the rows’ contents.
figure 16  a triangular array
double[]
b =
[0]
[1]
[2]
[3]
double[] [0]
double[] [0]
[1]
double[] [0]
[1] [2]
double[] [0]
[1] [2] [3]
7.7 array Lists 347
multidimensional Arrays
You can declare arrays with more than two dimensions. For example, here is a three-
dimensional array:
int[][][] rubiksCube = new int[3][3][3];
Each array element is specified by three index values:
rubiksCube[i][j][k]
7.7 array Lists
When you write a program that collects inputs, you
don’t always know how many inputs you will have.
In such a situation, an array list offers two significant
advantages:
• Array lists can grow and shrink as needed.
• The  ArrayList class supplies methods for common
tasks, such as inserting and removing elements.
In the following sections, you will learn how to work
with array lists.
An array list expands to hold as many elements as needed.
special topic 7.4
© Eric Isselé/iStockphoto.
© digital94086/iStockphoto.
an array list stores
a sequence of
values whose
size can change.
syntax 7.4  array Lists
ArrayList<String> friends = new ArrayList<String>();
The index must be ≥ 0 and <  friends.size() .
An array list object of size 0
Use the
get and  set methods
to access an element.
friends.add("Cindy");
String name = friends.get(i);
friends.set(i, "Harry");
Variable type Variable name
The  add method
appends an element to the array list,
increasing its size.
To construct an array list: new ArrayList< typeName >()
To access an element: arraylistReference .get(index)
arraylistReference .set(index, value)
Syntax
348 Chapter 7 arrays and array Lists
7.7.1 declaring and Using array Lists
The following statement declares an array list of strings:
ArrayList<String> names = new ArrayList<String>();
The  ArrayList class is contained in the  java.util package. In order to use array lists in
your program, you need to use the statement  import java.util.ArrayList .
The type  ArrayList<String> denotes an array list of  String elements. The angle
brackets around the  String type tell you that  String is a type parameter. You can
replace  String with any other class and get a differ ent array list type. For that reason,
ArrayList is called a generic class. However, you cannot use primitive types as type
parameters—there is no  ArrayList<int> or  ArrayList<double> . Section 7.7.4 shows how
you can collect numbers in an array list.
It is a common error to forget the initialization:
ArrayList<String> names;
names.add("Harry"); //  Error— names not initialized
Here is the proper initialization:
ArrayList<String> names = new ArrayList<String>();
Note the  () after  new ArrayList<String> on the right-hand side of the initialization. It
indicates that the con structor of the  ArrayList<String> class is being called.
When the  ArrayList<String> is first constructed, it has size 0. You use the  add method
to add an element to the end of the array list.
names.add("Emily"); //  Now  names has size 1 and element  "Emily"
names.add("Bob"); //  Now  names has size 2 and elements  "Emily" ,  "Bob"
names.add("Cindy"); // names has size 3 and elements  "Emily" ,  "Bob" , and  "Cindy"
The size increases after each call to  add (see Figure 17). The  size method yields the
current size of the array list.
To obtain an array list element, use the  get method, not the  [] operator. As with
arrays, index values start at 0. For example,  names.get(2) retrieves the name with index
2, the third element in the array list:
String name = names.get(2);
As with arrays, it is an error to access a nonexistent element. A very common bounds
error is to use the following:
int i = names.size();
name = names.get(i); //  Error
The last valid index is  names.size() - 1 .
To set an array list element to a new value, use the  set method:
names.set(2, "Carolyn");
the  ArrayList class
is a generic class:
ArrayList< Type >
collects elements of
the specified type.
Use the  size method
to obtain the current
size of an array list.
Use the  get and  set
methods to access an
array list element at a
given index.
figure 17  adding an array List element with  add
1 Before  add 2 After  add
2
ArrayList<String>
names =
"Bob"
"Emily"
3
Size increased
New element
added at end
ArrayList<String>
names =
"Cindy"
"Bob"
"Emily"
7.7 array Lists 349
figure 18 
adding and
removing
elements in the
Middle of an
array List
1 Before  add
ArrayList<String>
names =
"Carolyn"
"Bob"
"Emily"
2 After  names.add(1, "Ann")
ArrayList<String>
names =
"Carolyn"
"Bob"
"Emily"
"Ann"
Moved from index 1 to 2
New element
added at index 1
Moved from index 2 to 3
3 After  names.remove(1)
ArrayList<String>
names =
"Carolyn"
"Bob"
"Emily" Moved from index 2 to 1
Moved from index 3 to 2
This call sets position 2 of the  names array list to  "Carolyn" , overwriting whatever value
was there before.
The  set method overwrites existing values. It is different from the  add method,
which adds a new ele ment to the array list.
You can insert an element in the middle of an array list. For example, the call
names.add(1, "Ann") adds a new element at position 1 and moves all elements with
index 1 or larger by one position. After each call to the  add method, the size of the
array list increases by 1 (see Figure 18).
Conversely, the  remove method removes the element at a given position, moves all
elements after the removed element down by one position, and reduces the size of the
array list by 1. Part 3 of Figure 18 illustrates the result of  names.remove(1) .
With an array list, it is very easy to get a quick printout. Simply pass the array list
to the  println method:
System.out.println(names); //  Prints  [Emily, Bob, Carolyn]
7.7.2 Using the enhanced  for Loop with array Lists
You can use the enhanced  for loop to visit all elements of an array list. For example,
the following loop prints all names:
ArrayList<String> names = . . . ;
for (String name : names)
{
System.out.println(name);
}
This loop is equivalent to the following basic  for loop:
for (int i = 0; i < names.size(); i++)
{
© Danijelm/iStockphoto.
An array list has
methods for adding
and removing ele
ments in the middle.
Use the  add and
remove methods to
add and remove
array list elements.
350 Chapter 7 arrays and array Lists
String name = names.get(i);
System.out.println(name);
}
7.7.3 Copying array Lists
As with arrays, you need to remember that array list variables hold references. Copy-
ing the reference yields two references to the same array list (see Figure 19).
ArrayList<String> friends = names;
friends.add("Harry");
Now both  names and  friends reference the same array list to which the string  "Harry"
was added.
If you want to make a copy of an array list, construct the copy and pass the original
list into the con structor:
ArrayList<String> newNames = new ArrayList<String>(names);
table 2 Working with array Lists
ArrayList<String> names = new ArrayList<String>(); Constructs an empty array list that can hold strings.
names.add("Ann");
names.add("Cindy");
Adds elements to the end.
System.out.println(names); Prints  [Ann, Cindy] .
names.add(1, "Bob"); Inserts an element at index 1.  names is now
[Ann, Bob, Cindy] .
names.remove(0); Removes the element at index 0.  names is now
[Bob, Cindy] .
names.set(0, "Bill"); Replaces an element with a different value.  names is
now  [Bill, Cindy] .
String name = names.get(i); Gets an element.
String last = names.get(names.size() - 1); Gets the last element.
ArrayList<Integer> squares = new ArrayList<Integer>();
for (int i = 0; i < 10; i++)
{
squares.add(i * i);
}
Constructs an array list holding the first ten
squares.
figure 19 
Copying an array List
reference
ArrayList<String>
"Emily"
"Bob"
"Carolyn"
"Harry"
names =
friends =
7.7 array Lists 351
7.7.4 Wrappers and auto-boxing
In Java, you cannot directly insert primitive type values—numbers, characters, or
boolean values—into array lists. For example, you cannot form an  ArrayList<double> .
Instead, you must use one of the wrapper classes shown in the following table.
primitive type Wrapper Class
byte  Byte
boolean  Boolean
char  Character
double  Double
float  Float
int  Integer
long  Long
short  Short
For example, to collect  double values in an array list, you use an  ArrayList<Double> .
Note that the wrapper class names start with uppercase letters, and that two of them
differ from the names of the corresponding primitive type:  Integer and  Character .
Conversion between primitive types and the corresponding wrapper classes is
automatic. This process is called auto-boxing (even though auto-wrapping would
have been more consistent).
For example, if you assign a  double value to a  Double variable, the number is auto-
matically “put into a box” (see Figure 20).
Double wrapper = 29.95;
Conversely, wrapper values are automatically “unboxed” to primitive types:
double x = wrapper;
Because boxing and unboxing is automatic, you don’t need to think about it. Simply
remember to use the wrapper type when you declare array lists of numbers. From
then on, use the primitive type and rely on auto-boxing.
ArrayList<Double> values = new ArrayList<Double>();
values.add(29.95);
double x = values.get(0);
© sandoclr/iStockphoto.
Like truffles that
must be in a wrapper
to be sold, a number
must be placed in a
wrapper to be stored
in an array list.
to collect numbers in
array lists, you must
use wrapper classes.
figure 20  a Wrapper Class Variable
wrapper =
value =
Double
29.95
352 Chapter 7 arrays and array Lists
7.7.5 Using array algorithms with array Lists
The array algorithms in Section 7.3 can be converted to array lists simply by using the
array list methods instead of the array syntax (see Table 3 on page 354). For example, this
code snippet finds the largest element in an array:
double largest = values[0];
for (int i = 1; i < values.length; i++)
{
if (values[i] > largest)
{
largest = values[i];
}
}
Here is the same algorithm, now using an array list:
double largest = values.get(0);
for (int i = 1; i < values.size(); i++)
{
if (values.get(i) > largest)
{
largest = values.get(i);
}
}
7.7.6 storing input Values in an array List
When you collect an unknown number of inputs, array lists are much easier to use
than arrays. Simply read inputs and add them to an array list:
ArrayList<Double> inputs = new ArrayList<Double>();
while (in.hasNextDouble())
{
inputs.add(in.nextDouble());
}
7.7.7 removing Matches
It is easy to remove elements from an array list, by calling the  remove method. A com-
mon processing task is to remove all elements that match a particular condition. Sup-
pose, for example, that we want to remove all strings of length < 4 from an array list.
Of course, you traverse the array list and look for matching elements:
ArrayList<String> words = ...;
for (int i = 0; i < words.size(); i++)
{
String word = words.get(i);
if (word.length() < 4)
{
Remove the element at index i.
}
}
But there is a subtle problem. After you remove the element, the  for loop increments
i , skipping past the next element.
7.7 array Lists 353
Consider this concrete example, where  words contains the strings  "Welcome" ,  "to" ,
"the" ,  "island!" . When  i is 1, we remove the word  "to" at index 1. Then  i is incre-
mented to 2, and the word  "the" , which is now at position 1, is never examined.
i  words
0  "Welcome", "to", "the", "island"
1  "Welcome", "the", "island"
2
We should not increment the index when removing a word. The appropriate pseudo-
code is
If the element at index i matches the condition
Remove the element.
Else
Increment i.
Because we don’t always increment the index, a  for loop is not appropriate for this
algorithm. Instead, use a  while loop:
int i = 0;
while (i < words.size())
{
String word = words.get(i);
if (word.length() < 4)
{
words.remove(i);
}
else
{
i++;
}
}
7.7.8 Choosing Between array Lists and arrays
For most programming tasks, array lists are easier to use than arrays. Array lists can
grow and shrink. On the other hand, arrays have a nicer syntax for element access and
initialization.
Which of the two should you choose? Here are some recommendations.
• If the size of a collection never changes, use an array.
• If you collect a long sequence of primitive type values and you are concerned
about efficiency, use an array.
• Otherwise, use an array list.
The following program shows how to mark the largest value in a sequence of values.
This program uses an array list. Note how the program is an improvement over the
array version on page 329. This program can process input sequences of arbitrary length.
fuLL code exAmpLe
Go to  wiley.com/go/
javacode to download
a version of the
Student class using
an array list.
354 Chapter 7 arrays and array Lists
table 3 Comparing array and array List operations
operation arrays array Lists
Get an element. x = values[4]; x = values.get(4)
Replace an element. values[4] = 35; values.set(4, 35);
Number of elements. values.length values.size()
Number of filled elements. currentSize
(companion variable, see
Section 7.1.4)
values.size()
Remove an element. See Section 7.3.6 values.remove(4);
Add an element, growing
the collection.
See Section 7.3.7 values.add(35);
Initializing a collection. int[] values = { 1, 4, 9 }; No initializer list syntax;
call  add three times.
section_7/LargestinArrayList.java
1 import java.util.ArrayList;
2 import java.util.Scanner;
3
4 /**
5 This program reads a sequence of values and prints them, marking the largest value.
6 */
7 public class LargestInArrayList
8 {
9 public static void main(String[] args)
10 {
11 ArrayList<Double> values = new ArrayList<Double>();
12
13 //  Read inputs
14
15 System.out.println("Please enter values, Q to quit:");
16 Scanner in = new Scanner(System.in);
17 while (in.hasNextDouble())
18 {
19 values.add(in.nextDouble());
20 }
21
22 //  Find the largest value
23
24 double largest = values.get(0);
25 for (int i = 1; i < values.size(); i++)
26 {
27 if (values.get(i) > largest)
28 {
29 largest = values.get(i);
30 }
31 }
32
33 //  Print all values, marking the largest
34
7.7 array Lists 355
35 for (double element : values)
36 {
37 System.out.print(element);
38 if (element == largest)
39 {
40 System.out.print(" <== largest value");
41 }
42 System.out.println();
43 }
44 }
45 }
program run
Please enter values, Q to quit:
35 80 115 44.5 Q
35
80
115 <== largest value
44.5
35.  Declare an array list  primes of integers that contains the first five prime numbers
(2, 3, 5, 7, and 11).
36.  Given the array list  primes declared in Self Check 35, write a loop to print its ele-
ments in reverse order, starting with the last element.
37.  What does the array list  names contain after the following statements?
ArrayList<String> names = new ArrayList<String>;
names.add("Bob");
names.add(0, "Ann");
names.remove(1);
names.add("Cal");
38.  What is wrong with this code snippet?
ArrayList<String> names;
names.add(Bob);
39.  Consider this method that appends the elements of one array list to another:
public void append(ArrayList<String> target, ArrayList<String> source)
{
for (int i = 0; i < source.size(); i++)
{
target.add(source.get(i));
}
}
What are the contents of  names1 and  names2 after these statements?
ArrayList<String> names1 = new ArrayList<String>();
names1.add("Emily");
names1.add("Bob");
names1.add("Cindy");
ArrayList<String> names2 = new ArrayList<String>();
names2.add("Dave");
append(names1, names2);
40.  Suppose you want to store the names of the weekdays. Should you use an array
list or an array of seven strings?
© Nicholas Homrich/iStockphoto.
s e L f  c h e c k
356 Chapter 7 arrays and array Lists testing track
41.  The ch07/ section_7 directory of your source code contains an alternate imple-
mentation of the problem solution in How To 7.1 on page 334. Compare the array
and array list implementations. What is the primary advantage of the latter?
practice it  Now you can try these exercises at the end of the chapter: R7.10, R7.32, E7.17, E7.19.
Length and size
Unfortunately, the Java syntax for determining the number of elements in an array, an array
list, and a string is not at all consistent. It is a common error to confuse these. You just have to
remember the correct syntax for every data type.
data type number of elements
Array  a.length
Array list  a.size()
String  a.length()
The diamond syntax in java 7
Java 7 introduces a convenient syntax enhancement for declaring array lists and other generic
classes. In a statement that declares and constructs an array list, you need not repeat the type
parameter in the constructor. That is, you can write
ArrayList<String> names = new ArrayList<>();
instead of
ArrayList<String> names = new ArrayList<String>();
This shortcut is called the “diamond syntax” because the empty brackets  <> look like a dia-
mond shape.
7.8 regression testing
It is a common and useful practice to make a new test whenever you find a program
bug. You can use that test to verify that your bug fix really works. Don’t throw the
test away; feed it to the next version after that and all subsequent versions. Such a col-
lection of test cases is called a test suite.
You will be surprised how often a bug that you fixed will reappear in a future ver-
sion. This is a phe nomenon known as cycling. Sometimes you don’t quite understand
the reason for a bug and apply a quick fix that appears to work. Later, you apply a
different quick fix that solves a second problem but makes the first problem appear
again. Of course, it is always best to think through what really causes a bug and fix the
root cause instead of doing a sequence of “Band-Aid” solutions. If you don’t succeed
in doing that, however, you at least want to have an honest appraisal of how well the
program works. By keeping all old test cases around and testing them against every
Common error 7.4
© John Bell/iStockphoto.
special topic 7.5
© Eric Isselé/iStockphoto.
a test suite is a
set of tests for
repeated testing.
testing track 7.8 regression testing 357
new version, you get that feedback. The process of checking each version of a pro-
gram against a test suite is called regression testing.
How do you organize a suite of tests? An easy technique is to produce multiple
tester classes, such as  ScoreTester1 ,  ScoreTester2 , and so on, where each program runs
with a separate set of test data. For example, here is a tester for the  Student class:
public class ScoreTester1
{
public static void main(String[] args)
{
Scanner in = new Scanner(System.in);
Student fred = new Student(100);
fred.addScore(10);
fred.addScore(20);
fred.addScore(5);
System.out.println("Final score: " + fred.finalScore());
System.out.println("Expected: 30");
}
}
Another useful approach is to provide a generic tester, and feed it inputs from mul-
tiple files, as in the following.
section_8/scoreTester.java
1 import java.util.Scanner;
2
3 public class ScoreTester
4 {
5 public static void main(String[] args)
6 {
7 Scanner in = new Scanner(System.in);
8 double expected = in.nextDouble();
9 Student fred = new Student(100);
10 while (in.hasNextDouble())
11 {
12 if (!fred.addScore(in.nextDouble()))
13 {
14 System.out.println("Too many scores.");
15 return;
16 }
17 }
18 System.out.println("Final score: " + fred.finalScore());
19 System.out.println("Expected: " + expected);
20 }
21 }
The program reads the expected result and the scores. By running the program with
different inputs, we can test dif ferent scenarios.
Of course, it would be tedious to type in the input values by hand every time the
test is executed. It is much better to save the inputs in a file, such as the following:
section_8/input1.txt
30
10
20
5
regression testing
involves repeating
previously run
tests to ensure that
known failures of
prior versions do
not appear in
new versions of
the software.
358 Chapter 7 arrays and array Lists testing track
When running the program from a shell window, one can link the input file to the
input of a pro gram, as if all the characters in the file had actually been typed by a user.
Type the following command into a shell window:
java ScoreTester < input1.txt
The program is executed, but it no longer reads input from the keyboard. Instead, the
System.in object (and the  Scanner that reads from  System.in ) gets the input from the file
input1.txt . We discussed this process, called input redirection, in Special Topic 6.2.
The output is still displayed in the console window:
program run
Final score: 30
Expected: 30
You can also redirect output. To capture the program’s output in a file, use the
command
java ScoreTester < input1.txt > output1.txt
This is useful for archiving test cases.
42.  Suppose you modified the code for a method. Why do you want to repeat tests
that already passed with the previous version of the code?
43.  Suppose a customer of your program finds an error. What action should you
take beyond fixing the error?
44.  Why doesn’t the  ScoreTester program contain prompts for the inputs?
practice it  Now you can try these exercises at the end of the chapter: R7.34, R7.35.
batch files and shell scripts
If you need to perform the same tasks repeatedly on the command line, then it is worth learn-
ing about the automa tion features offered by your operating system.
Under Windows, you use batch files to execute a number of commands automatically. For
example, suppose you need to test a program by running three testers:
java ScoreTester1
java ScoreTester < input1.txt
java ScoreTester < input2.txt
Then you find a bug, fix it, and run the tests again. Now you need to type the three commands
once more. There has to be a better way. Under Windows, put the commands in a text file and
call it  test.bat :
file test.bat
1 java ScoreTester1
2 java ScoreTester < input1.txt
3 java ScoreTester < input2.txt
Then you just type
test.bat
and the three commands in the batch file execute automatically.
© Nicholas Homrich/iStockphoto.
s e L f  c h e c k
programming tip 7.3
© Eric Isselé/iStockphoto.
testing track 7.8 regression testing 359
Batch files are a feature of the operating system, not of Java. On Linux, Mac OS, and UNIX,
shell scripts are used for the same purpose. In this simple example, you can execute the com-
mands by typing
sh test.bat
There are many uses for batch files and shell scripts, and it is well worth it to learn more about
their advanced fea tures, such as parameters and loops.
the therac-25 is a
computerized device
to deliver radiation treatment to can-
cer patients (see the figure). Between
June 1985 and January 1987, several of
these machines delivered serious over-
doses to at least six patients, killing
some of them and seriously maiming
the others.
the machines were controlled by
a computer program. Bugs in the pro-
gram were directly responsible for
the overdoses. according to Leveson
and turner (“an investigation of the
therac-25 accidents,” IEEE Computer,
July 1993, pp. 18–41), the program
was written by a single programmer,
who had since left the manufacturing
company producing the device and
could not be located. none of the com-
pany employees interviewed could say
anything about the educational level or
qualifications of the programmer.
the investigation by the federal
Food and drug administration (Fda)
found that the program was poorly
docu mented and that there was neither
a specification document nor a formal
test plan. (this should make you think.
do you have a formal test plan for your
programs?)
the overdoses were caused by an
amateurish design of the software
that had to control different devices
concur rently, namely the keyboard,
the display, the printer, and of course
the radiation device itself. synchroni-
zation and data sharing between the
tasks were done in an ad hoc way, even
though safe multitasking techniques
were known at the time. had the pro-
grammer enjoyed a formal education
that involved these techniques, or
taken the effort to study the literature,
a safer machine could have been built.
such a machine would have probably
involved a commer cial multitasking
system, which might have required a
more expensive computer.
the same flaws were present in
the software controlling the prede-
cessor model, the therac-20, but that
machine had hardware interlocks that
mechanically  prevented  overdoses.
the hardware safety devices were
removed in the therac-25 and replaced
by checks in the software, presumably
to save cost.
Frank houston of the Fda wrote in
1985: “a significant amount of soft-
ware for life-critical systems comes
from small firms, especially in the
medical device industry; firms that fit
the profile of those resistant to or unin-
formed of the principles of either sys-
tem safety or software engineering.”
Who is to blame? the programmer?
the manager who not only failed to
ensure that the programmer was up to
the task but also didn’t insist on com-
prehensive testing? the hospitals that
installed the device, or the Fda, for not
reviewing the design process? Unfortu-
nately, even today there are no firm
standards of what constitutes a safe
soft ware design process.
Therac-25 unit
Treatment table
Motion
power switch
Therapy room
intercom
Room
emergency
switch
Door
interlock
switch
Beam
on/off light
Motion enable
switch (footswitch)
Display
terminal
TV monitor Printer
Control
console
Turntable
position
monitor
Room
emergency
switches TV
camera
Typical Therac25 Facility
Computing & Society 7.2  the therac-25 incidents
© MediaBakery.
360 Chapter 7 arrays and array Lists
use arrays for collecting values.
• An array collects a sequence of values of the same type.
• Individual elements in an array are accessed by an integer index  i , using the
notation array [i] .
• An array element can be used like any variable.
• An array index must be at least zero and less than the size of the array.
• A bounds error, which occurs if you supply an invalid array index, can cause your
program to terminate.
• Use the expression array. length to find the number of elements in
an array.
• An array reference specifies the location of an array. Copying the
reference yields a second reference to the same array.
• Arrays can occur as method arguments and return values.
• With a partially filled array, keep a companion variable for the
current size.
• Avoid parallel arrays by changing them into arrays of objects.
know when to use the enhanced  for  loop.
• You can use the enhanced  for loop to visit all elements of an array.
• Use the enhanced  for loop if you do not need the index values in the loop body.
know and use common array algorithms.
• When separating elements, don’t place a separator before the first element.
• A linear search inspects elements in sequence until a match is found.
• Before inserting an element, move elements to the end of the array starting with
the last one .
• Use a temporary variable when swapping two elements.
• Use the  Arrays.copyOf method to copy the elements of an array into a new array.
combine and adapt algorithms for solving a programming problem.
• By combining fundamental algorithms, you can solve complex
programming tasks.
• You should be familiar with the implementation of fundamental algorithms so
that you can adapt them.
discover algorithms by manipulating physical objects.
• Use a sequence of coins, playing cards, or toys to visualize an array of values.
• You can use paper clips as position markers or counters.
C h a p t e r s U M M a ry
© Luckie8/iStockphoto.
© AlterYourReality/iStockphoto.
© yekorzh/iStockphoto.
© JenCon/iStockphoto.
review Questions 361
use two-dimensional arrays for data that is arranged in rows and columns.
• Use a two-dimensional array to store tabular data.
• Individual elements in a two-dimensional array are
accessed by using two index values, array [i][j] .
use array lists for managing collections whose size can change.
• An array list stores a sequence of values whose size can change.
• The  ArrayList class is a generic class:  ArrayList< Type >
collects elements of the specified type.
• Use the  size method to obtain the current size of an
array list.
• Use the  get and  set methods to access an array list element
at a given index.
• Use the  add and  remove methods to add and remove array
list elements.
• To collect numbers in array lists, you must use wrapper classes.
describe the process of regression testing.
• A test suite is a set of tests for repeated testing.
• Regression testing involves repeating previously run tests to ensure that known
failures of prior versions do not appear in new versions of the software.
•• r7.1  Write code that fills an array  values with each set of numbers below.
a.  1  2  3  4  5  6  7  8  9  10
b.  0  2  4  6  8  10  12  14  16  18  20
c.  1  4  9  16  25  36  49  64  81  100
d.  0  0  0  0  0  0  0  0  0  0
e.  1  4  9  16  9  7  4  9  11
f.  0  1  0 1 0 1 0 1 0 1
g.  0 1 2 3 4 0 1 2 3 4
© Trub/iStockphoto.
© digital94086/iStockphoto.
© Danijelm/iStockphoto.
© sandoclr/iStockphoto.
java.lang.Boolean
java.lang.Double
java.lang.Integer
java.util.Arrays
copyOf
toString
java.util.ArrayList<E>
add
get
remove
set
size
s ta n d a r d L i B r a ry i t e M s i n t r o d U C e d i n t h i s C h a p t e r
r e V i e W Q U e s t i o n s
362 Chapter 7 arrays and array Lists
•• r7.2  Consider the following array:
int[] a = { 1, 2, 3, 4, 5, 4, 3, 2, 1, 0 };
What is the value of  total after the following loops complete?
a.  int total = 0;
for (int i = 0; i < 10; i++) { total = total + a[i]; }
b.  int total = 0;
for (int i = 0; i < 10; i = i + 2) { total = total + a[i]; }
c.  int total = 0;
for (int i = 1; i < 10; i = i + 2) { total = total + a[i]; }
d.  int total = 0;
for (int i = 2; i <= 10; i++) { total = total + a[i]; }
e.  int total = 0;
for (int i = 1; i < 10; i = 2 * i) { total = total + a[i]; }
f.  int total = 0;
for (int i = 9; i >= 0; i--) { total = total + a[i]; }
g.  int total = 0;
for (int i = 9; i >= 0; i = i - 2) { total = total + a[i]; }
h.  int total = 0;
for (int i = 0; i < 10; i++) { total = a[i] - total; }
•• r7.3  Consider the following array:
int[] a = { 1, 2, 3, 4, 5, 4, 3, 2, 1, 0 };
What are the contents of the array  a after the following loops complete?
a.  for (int i = 1; i < 10; i++) { a[i] = a[i - 1]; }
b.  for (int i = 9; i > 0; i--) { a[i] = a[i - 1]; }
c.  for (int i = 0; i < 9; i++) { a[i] = a[i + 1]; }
d.  for (int i = 8; i >= 0; i--) { a[i] = a[i + 1]; }
e.  for (int i = 1; i < 10; i++) { a[i] = a[i] + a[i - 1]; }
f.  for (int i = 1; i < 10; i = i + 2) { a[i] = 0; }
g.  for (int i = 0; i < 5; i++) { a[i + 5] = a[i]; }
h.  for (int i = 1; i < 5; i++) { a[i] = a[9 - i]; }
••• r7.4  Write a loop that fills an array  values with ten random numbers between 1 and 100.
Write code for two nested loops that fill  values with ten different random numbers
between 1 and 100.
•• r7.5  Write Java code for a loop that simultaneously computes both the maximum and
minimum of an array.
• r7.6  What is wrong with each of the following code segments?
a.  int[] values = new int[10];
for (int i = 1; i <= 10; i++)
{
values[i] = i * i;
}
b.  int[] values;
for (int i = 0; i < values.length; i++)
{
values[i] = i * i;
}
review Questions 363
•• r7.7  Write enhanced  for loops for the following tasks.
a.  Printing all elements of an array in a single row, separated by spaces.
b.  Computing the maximum of all elements in an array.
c.  Counting how many elements in an array are negative.
•• r7.8  Rewrite the following loops without using the enhanced  for loop construct. Here,
values is an array of floating-point numbers.
a.  for (double x : values) { total = total + x; }
b.  for (double x : values) { if (x == target) { return true; } }
c.  int i = 0;
for (double x : values) { values[i] = 2 * x; i++; }
•• r7.9  Rewrite the following loops, using the enhanced  for loop construct. Here,  values is
an array of floating-point numbers.
a.  for (int i = 0; i < values.length; i++) { total = total + values[i]; }
b.  for (int i = 1; i < values.length; i++) { total = total + values[i]; }
c.  for (int i = 0; i < values.length; i++)
{
if (values[i] == target) { return i; }
}
• r7.10  What is wrong with each of the following code segments?
a.  ArrayList<int> values = new ArrayList<int>();
b.  ArrayList<Integer> values = new ArrayList();
c.  ArrayList<Integer> values = new ArrayList<Integer>;
d.  ArrayList<Integer> values = new ArrayList<Integer>();
for (int i = 1; i <= 10; i++)
{
values.set(i - 1, i * i);
}
e.  ArrayList<Integer> values;
for (int i = 1; i <= 10; i++)
{
values.add(i * i);
}
• r7.11  What is an index of an array? What are the legal index values? What is a bounds
error?
• r7.12  Write a program that contains a bounds error. Run the program. What happens on
your computer?
• r7.13  Write a loop that reads ten numbers and a second loop that displays them in the
opposite order from which they were entered.
•• r7.14  For the operations on partially filled arrays below, provide the header of a method.
Do not implement the methods.
a.  Sort the elements in decreasing order.
b.  Print all elements, separated by a given string.
c.  Count how many elements are less than a given value.
d.  Remove all elements that are less than a given value.
e.  Place all elements that are less than a given value in another array.
364 Chapter 7 arrays and array Lists
• r7.15  Trace the flow of the loop in Section 7.3.4 with the given example. Show two col-
umns, one with the value of  i and one with the output.
• r7.16  Consider the following loop for collecting all elements that match a condition; in
this case, that the element is larger than 100.
ArrayList<Double> matches = new ArrayList<Double>();
for (double element : values)
{
if (element > 100)
{
matches.add(element);
}
}
Trace the flow of the loop, where  values contains the elements 110 90 100 120 80.
Show two columns, for  element and  matches .
• r7.17  Trace the flow of the loop in Section 7.3.5, where  values contains the elements 80
90 100 120 110. Show two columns, for  pos and  found . Repeat the trace when  values
contains the elements 80 90 120 70.
•• r7.18  Trace the algorithm for removing an element described in Section 7.3.6. Use an array
values with elements 110 90 100 120 80, and remove the element at index 2.
•• r7.19  Give pseudocode for an algorithm that rotates the elements of an array by one posi-
tion, moving the initial element to the end of the array, like this:
3 5 7 11 13 2
2 3 5 7 11 13
•• r7.20  Give pseudocode for an algorithm that removes all negative values from an array,
preserving the order of the remaining elements.
•• r7.21  Suppose  values is a sorted array of integers. Give pseudocode that describes
how a new value can be inserted in its proper position so that the resulting
array stays sorted.
••• r7.22  A run is a sequence of adjacent repeated values. Give pseudocode for computing the
length of the longest run in an array. For example, the longest run in the array with
elements
1 2 5 5 3 1 2 4 3 2 2 2 2 3 6 5 5 6 3 1
has length 4.
••• r7.23  What is wrong with the following method that aims to fill an array with random
numbers?
public void makeCombination(int[] values, int n)
{
Random generator = new Random();
int[] numbers = new int[values.length];
for (int i = 0; i < numbers.length; i++)
{
numbers[i] = generator.nextInt(n);
}
values = numbers;
}
review Questions 365
•• r7.24  You are given two arrays denoting x- and y-coordinates of a set
of points in the plane. For plotting the point set, we need to know
the x- and y-coordinates of the smallest rectangle containing the
points.
How can you obtain these values from the fundamental algorithms
in Section 7.3?
• r7.25  Solve the problem described in Section 7.4 by sorting the array first. How do you
need to modify the algorithm for computing the total?
•• r7.26  Solve the task described in Section 7.5 using an algorithm that removes and inserts
elements instead of switching them. Write the pseudocode for the algorithm, assum-
ing that methods for removal and insertion exist. Act out the algorithm with a
sequence of coins and explain why it is less efficient than the swapping algorithm
developed in Section 7.5.
•• r7.27  Develop an algorithm for finding the most frequently occurring value in an array of
numbers. Use a sequence of coins. Place paper clips below each coin that count how
many other coins of the same value are in the sequence. Give the pseudocode for an
algorithm that yields the correct answer, and describe how using the coins and paper
clips helped you find the algorithm.
•• r7.28  Write Java statements for performing the following tasks with an array declared as
int[][] values = new int[ROWS][COLUMNS];
• Fill all entries with 0.
• Fill elements alternately with 0s and 1s in a checkerboard pattern.
• Fill only the elements in the top and bottom rows with zeroes.
• Compute the sum of all elements.
• Print the array in tabular form.
•• r7.29  Write pseudocode for an algorithm that fills the first and last columns as well as the
first and last rows of a two-dimensional array of integers with –1.
• r7.30  Section 7.7.7 shows that you must be careful about updating the index value when
you remove elements from an array list. Show how you can avoid this problem by
traversing the array list backwards.
•• r7.31  True or false?
a.  All elements of an array are of the same type.
b.  Arrays cannot contain strings as elements.
c.  Two-dimensional arrays always have the same number of rows and columns.
d.  Elements of different columns in a two-dimensional array can have
different types.
e.  A method cannot return a two-dimensional array.
f.  A method cannot change the length of an array argument.
g.  A method cannot change the number of columns of an argument that is a
two-dimensional array.
y
x
366 Chapter 7 arrays and array Lists
•• r7.32  How do you perform the following tasks with array lists in Java?
a.  Test that two array lists contain the same elements in the same order.
b.  Copy one array list to another.
c.  Fill an array list with zeroes, overwriting all elements in it.
d.  Remove all elements from an array list.
• r7.33  True or false?
a.  All elements of an array list are of the same type.
b.  Array list index values must be integers.
c.  Array lists cannot contain strings as elements.
d.  Array lists can change their size, getting larger or smaller.
e.  A method cannot return an array list.
f.  A method cannot change the size of an array list argument.
• Testing r7.34  Define the terms regression testing and test suite.
•• Testing r7.35  What is the debugging phenomenon known as cycling? What can you do to avoid it?
•• e7.1  Write a program that initializes an array with ten random integers and then prints
four lines of output, containing
• Every element at an even index.
• Every even element.
• All elements in reverse order.
• Only the first and last element.
•• e7.2  Write array methods that carry out the following tasks for an array of integers by
completing the  ArrayMethods class below. For each method, provide a test program.
public class ArrayMethods
{
private int[] values;
public ArrayMethods(int[] initialValues) { values = initialValues; }
public void swapFirstAndLast() { ... }
public void shiftRight() { ... }
...
}
a.  Swap the first and last elements in the array.
b.  Shift all elements by one to the right and move the last element into the first
position. For example, 1 4 9 16 25 would be transformed into 25 1 4 9 16.
c.  Replace all even elements with 0.
d.  Replace each element except the first and last by the larger of its two neighbors.
e.  Remove the middle element if the array length is odd, or the middle two
elements if the length is even.
f.  Move all even elements to the front, otherwise preserving the order of the
elements.
p r a C t i C e e x e r C i s e s
practice exercises 367
g.  Return the second-largest element in the array.
h.  Return true if the array is currently sorted in increasing order.
i.  Return true if the array contains two adjacent duplicate elements.
j.  Return true if the array contains duplicate elements (which need not be
adjacent).
• e7.3  Modify the  LargestInArray.java program in Section 7.3 to mark both the smallest and
the largest elements.
•• e7.4  Write a method  sumWithoutSmallest that computes the sum of an array of values,
except for the smallest one, in a single loop. In the loop, update the sum and the
smallest value. After the loop, return the difference.
• e7.5  Add a method  removeMin to the  Student class of Section 7.4 that removes the minimum
score without calling other methods.
•• e7.6  Compute the alternating sum of all elements in an array. For example, if your pro-
gram reads the input
1 4 9 16 9 7 4 9 11
then it computes
1 – 4 + 9 – 16 + 9 – 7 + 4 – 9 + 11 = –2
• e7.7  Write a method that reverses the sequence of elements in an array. For example, if
you call the method with the array
1 4 9 16 9 7 4 9 11
then the array is changed to
11 9 4 7 9 16 9 4 1
••• e7.8  Write a program that produces ten random permutations of the numbers 1 to 10. To
generate a random permutation, you need to fill an array with the numbers 1 to 10
so that no two entries of the array have the same contents. You could do it by brute
force, by generating random values until you have a value that is not yet in the array.
But that is inefficient. Instead, follow this algorithm.
Make a second array and fill it with the numbers 1 to 10.
Repeat 10 times
Pick a random element from the second array.
Remove it and append it to the permutation array.
• e7.9  Write a method that implements the algorithm developed in Section 7.5.
•• e7.10  Consider the following class:
public class Sequence
{
private int[] values;
public Sequence(int size) { values = new int[size]; }
public void set(int i, int n) { values[i] = n; }
}
Add a method
public boolean equals(Sequence other)
that checks whether the two sequences have the same values in the same order.
368 Chapter 7 arrays and array Lists
•• e7.11  Add a method
public boolean sameValues(Sequence other)
to the  Sequence class of Exercise E7.10 that checks whether two sequences have the
same values in some order, ignoring duplicates. For example, the two sequences
1 4 9 16 9 7 4 9 11
and
11 11 7 9 16 4 1
would be considered identical. You will probably need one or more helper methods.
••• e7.12  Add a method
public boolean isPermutationOf(Sequence other)
to the  Sequence class of Exercise E7.10 that checks whether two sequences have the
same values in some order, with the same multiplicities. For example,
1 4 9 16 9 7 4 9 11
is a permutation of
11 1 4 9 16 9 7 4 9
but
1 4 9 16 9 7 4 9 11
is not a permutation of
11 11 7 9 16 4 1 4 9
You will probably need one or more helper methods.
•• e7.13  Write a program that generates a sequence of 20 random values between 0 and 99 in
an array, prints the sequence, sorts it, and prints the sorted sequence. Use the  sort
method from the standard Java library.
•• e7.14  Consider the following class:
public class Table
{
private int[][] values;
public Table(int rows, int columns) { values = new int[rows][columns]; }
public void set(int i, int j, int n) { values[i][j] = n; }
}
Add a method that computes the average of the neighbors of a table element in the
eight directions shown in Figure 15.
public double neighborAverage(int row, int column)
However, if the element is located at the boundary of the array, only include the
neighbors that are in the table. For example, if  row and  column are both 0, there are
only three neighbors.
••• e7.15  Magic squares. An n × n matrix that is filled with the numbers
1, 2, 3, . . ., n 2 is a magic square if the sum of the elements in each row,
in each column, and in the two diagonals is the same value.
Write a program that reads in 16 values from the keyboard and tests
whether they form a magic square when put into a 4 × 4 array.
4 15 14 1
9 6 7 12
5 10 11 8
16 3 2 13
practice exercises 369
You need to test two features:
1.  Does each of the numbers 1, 2, ..., 16 occur in the user input?
2.  When the numbers are put into a square, are the sums of the rows, columns,
and diagonals equal to each other?
••• e7.16  Implement the following algorithm to construct magic n × n squares; it works only if
n is odd.
Set row = n - 1, column = n / 2.
For k = 1 ... n * n
Place k at [row][column].
Increment row and column.
If the row or column is n, replace it with 0.
If the element at [row][column] has already been filled
Set row and column to their previous values.
Decrement row.
Here is the 5 × 5 square that you get if you follow this method:
Write a program whose input is the number n and whose output is
the magic square of order n if n is odd.
•• e7.17  Write a program that reads a sequence of input values and displays a bar chart of the
values, using asterisks, like this:
**********************
****************************************
****************************
**************************
**************
You may assume that all values are positive. First figure out the maximum value.
That value’s bar should be drawn with 40 asterisks. Shorter bars should use propor-
tionally fewer asterisks.
••• e7.18  Improve the program of Exercise E7.17 to work correctly when the data set contains
nega tive values.
•• e7.19  Improve the program of Exercise E7.17 by adding captions for each bar. Prompt the
user for the captions and data values. The output should look like this:
Egypt **********************
France ****************************************
Japan ****************************
Uruguay **************************
Switzerland **************
• e7.20  Consider the following class:
public class Sequence
{
private ArrayList<Integer> values;
public Sequence() { values = new ArrayList<Integer>(); }
public void add(int n) { values.add(n); }
public String toString() { return values.toString(); }
}
17 24 1 8
23 5 7 14
4 6 13 20
10 12 19 21
15
16
22
3
11 18 25 2 9
370 Chapter 7 arrays and array Lists
Add a method
public Sequence append(Sequence other)
that creates a new sequence, appending this and the other sequence, without modify-
ing either sequence. For example, if  a is
1 4 9 16
and  b is the sequence
9 7 4 9 11
then the call  a.append(b) returns the sequence
1 4 9 16 9 7 4 9 11
without modifying  a or  b .
•• e7.21  Add a method
public Sequence merge(Sequence other)
to the  Sequence class of Exercise E7.20 that merges two sequences, alternating ele-
ments from both sequences. If one sequence is shorter than the other, then alternate
as long as you can and then append the remaining elements from the longer
sequence. For example, if  a is
1 4 9 16
and  b is
9 7 4 9 11
then  a.merge(b) returns the sequence
1 9 4 7 9 4 16 9 11
without modifying  a or  b .
•• e7.22  Add a method
public Sequence mergeSorted(Sequence other)
to the  Sequence class of Exercise E7.20 that merges two sorted sequences, producing a
new sorted sequence. Keep an index into each sequence, indicating how much of it
has been processed already. Each time, append the smallest unprocessed value from
either sequence, then advance the index. For example, if  a is
1 4 9 16
and  b is
4 7 9 9 11
then  a.mergeSorted(b) returns the sequence
1 4 4 7 9 9 9 11 16
If  a or  b is not sorted, merge the longest prefixes of  a and  b that are sorted.
•• p7.1  A run is a sequence of adjacent repeated values. Write a program that generates a
sequence of 20 random die tosses in an array and that prints the die values, marking
the runs by including them in parentheses, like this:
1 2 (5 5) 3 1 2 4 3 (2 2 2 2) 3 6 (5 5) 6 3 1
p r o G r a M M i n G p r o J e C t s
programming projects 371
Use the following pseudocode:
Set a boolean variable inRun to false.
For each valid index i in the array
If inRun
If values[i] is different from the preceding value
Print ).
inRun = false.
If not inRun
If values[i] is the same as the following value
Print (.
inRun = true.
Print values[i].
If inRun, print ).
•• p7.2  Write a program that generates a sequence of 20 random die tosses in an array and
that prints the die values, marking only the longest run, like this:
1 2 5 5 3 1 2 4 3 (2 2 2 2) 3 6 5 5 6 3 1
If there is more than one run of maximum length, mark the first one.
•• p7.3  It is a well-researched fact that men in a restroom generally prefer to maximize
their distance from already occupied stalls, by occupying the middle of the longest
sequence of unoccupied places.
For example, consider the situation where ten stalls are empty.
_ _ _ _ _ _ _ _ _ _
The first visitor will occupy a middle position:
_ _ _ _ _ X _ _ _ _
The next visitor will be in the middle of the empty area at the left.
_ _ X _ _ X _ _ _ _
Write a program that reads the number of stalls and then prints out diagrams in the
format given above when the stalls become filled, one at a time. Hint: Use an array of
boolean values to indicate whether a stall is occupied.
••• p7.4  In this assignment, you will model the game of Bulgarian Solitaire. The game starts
with 45 cards. (They need not be playing cards. Unmarked index cards work just as
well.) Randomly divide them into some number of piles of random size. For exam-
ple, you might start with piles of size 20, 5, 1, 9, and 10. In each round, you take one
card from each pile, forming a new pile with these cards. For example, the sample
starting configuration would be transformed into piles of size 19, 4, 8, 9, and 5. The
solitaire is over when the piles have size 1, 2, 3, 4, 5, 6, 7, 8, and 9, in some order. (It
can be shown that you always end up with such a configuration.)
In your program, produce a random starting configuration and print it. Then keep
applying the soli taire step and print the result. Stop when the solitaire final configu-
ration is reached.
372 Chapter 7 arrays and array Lists
•• p7.5  A theater seating chart is implemented as a two-dimensional array of ticket prices,
like this:
10 10 10 10 10 10 10 10 10 10
10 10 10 10 10 10 10 10 10 10
10 10 10 10 10 10 10 10 10 10
10 10 20 20 20 20 20 20 10 10
10 10 20 20 20 20 20 20 10 10
10 10 20 20 20 20 20 20 10 10
20 20 30 30 40 40 30 30 20 20
20 30 30 40 50 50 40 30 30 20
30 40 50 50 50 50 50 50 40 30
Write a program that prompts users to pick either a seat or a price. Mark sold seats
by changing the price to 0. When a user specifies a seat, make sure it is available.
When a user specifies a price, find any seat with that price.
••• p7.6  Write a program that plays tic-tac-toe. The tic-tac-toe
game is played on a 3 × 3 grid as in the photo at right. The
game is played by two players, who take turns. The first
player marks moves with a circle, the second with a cross.
The player who has formed a horizontal, vertical, or diag-
onal sequence of three marks wins. Your program should
draw the game board, ask the user for the coordinates of
the next mark, change the players after every successful
move, and pronounce the winner.
••• p7.7  In this assignment, you will implement a simulation of a popular casino game usually
called video poker. The card deck contains 52 cards, 13 of each suit. At the beginning
of the game, the deck is shuffled. You need to devise a fair method for shuffling. (It
does not have to be efficient.) The player pays a token for each game. Then the top
five cards of the deck are presented to the player. The player can reject none, some,
or all of the cards. The rejected cards are replaced from the top of the deck. Now the
hand is scored. Your program should pronounce it to be one of the following:
• No pair—The lowest hand, containing five separate cards that do not match up
to create any of the hands below.
• One pair—Two cards of the same value, for example two queens. Payout: 1
• Two pairs—Two pairs, for example two queens and two 5’s. Payout: 2
• Three of a kind—Three cards of the same value, for example three queens.
Payout: 3
• Straight—Five cards with consecutive values, not necessarily of the same
suit, such as 4, 5, 6, 7, and 8. The ace can either precede a 2 or follow a king.
Payout: 4
• Flush—Five cards, not necessarily in order, of the same suit. Payout: 5
• Full House—Three of a kind and a pair, for example three queens and two 5’s.
Payout: 6
• Four of a Kind—Four cards of the same value, such as four queens. Payout: 25
• Straight Flush—A straight and a flush: Five cards with consecutive values of
the same suit. Payout: 50
• Royal Flush—The best possible hand in poker. A 10, jack, queen, king, and ace,
all of the same suit. Payout: 250
© lepas2004/iStockphoto.
© KathyMuller/iStockphoto.
programming projects 373
••• p7.8  The Game of Life is a well-known mathematical game that gives rise to amazingly
complex behavior, although it can be specified by a few simple rules. (It is not
actu ally a game in the traditional sense, with players competing for a win.) Here are
the rules. The game is played on a rectangular board. Each square can be either
empty or occupied. At the beginning, you can specify empty and occupied cells in
some way; then the game runs automatically. In each generation, the next generation
is computed. A new cell is born on an empty square if it
is surrounded by exactly three occupied neighbor cells.
A cell dies of overcrowding if it is surrounded by four
or more neighbors, and it dies of loneliness if it is
surrounded by zero or one neighbor. A neighbor is an
occupant of an adjacent square to the left, right, top, or
bottom or in a diagonal direction. Figure 21 shows a cell
and its neighbor cells.
Many configurations show interesting behavior when subjected to these rules.
Figure 22 shows a glider, observed over five generations. After four generations, it is
transformed into the identical shape, but located one square to the right and below.
One of the more amazing configurations is the glider gun: a complex collection of
cells that, after 30 moves, turns back into itself and a glider (see Figure 23).
Program the game to eliminate the drudgery of computing successive generations by
hand. Use a two-dimensional array to store the rectangular configuration. Write a
program that shows successive genera tions of the game. Ask the user to specify the
original configuration, by typing in a configuration of spaces and  o characters.
figure 21 
neighborhood of a Cell
Cell
Neighbors
figure 22  Glider
Generation 0 Generation 1 Generation 2 Generation 3 Generation 4
figure 23  Glider Gun
Generation 0 Generation 30 Generation 60 Generation 90 Generation 120 Generation 150
374 Chapter 7 arrays and array Lists
•• business p7.9  A pet shop wants to give a discount to its
clients if they buy one or more pets and at
least five other items. The discount is
equal to 20 percent of the cost of the other
items, but not the pets.
Use a class  Item to describe an item, with
any needed methods and a constructor
public Item(double price, boolean isPet, int quantity)
An invoice holds a collection of  Item objects; use an array or array list to store them.
In the  Invoice class, implement methods
public void add(Item anItem)
public double getDiscount()
Write a program that prompts a cashier to enter each price and quantity, and then a  Y
for a pet or  N for another item. Use a price of –1 as a sentinel. In the loop, call the  add
method; after the loop, call the  getDiscount method and display the returned value.
•• business p7.10  A supermarket wants to reward its best customer of each day, showing the cus-
tomer’s name on a screen in the supermarket. For that purpose, the store keeps an
ArrayList<Customer> . In the  Store class, implement methods
public void addSale(String customerName, double amount)
public String nameOfBestCustomer()
to record the sale and return the name of the customer with the largest sale.
Write a program that prompts the cashier to enter all prices and names, adds them to
a  Store object, and displays the best customer’s name. Use a price of 0 as a sentinel.
••• business p7.11  Improve the program of Exercise P7.10 so that it displays the top customers, that
is, the  topN customers with the largest sales, where  topN is a value that the user of the
program supplies. Implement a method
public ArrayList<String> nameOfBestCustomers(int topN)
If there were fewer than  topN customers, include all of them.
•• science p7.12  Sounds can be represented by an array of “sample
val ues” that describe the intensity of the sound at
a point in time. The program in  ch07/sound of your
companion code reads a sound file (in WAV format),
processes the sample values, and shows the result.
Your task is to process the sound by introducing an
echo. For each sound value, add the value from 0.2
seconds ago. Scale the result so that no value is larger
than 32767.
••• science p7.13  You are given a two-dimensional array of values that give the height of a terrain at
different points in a square. Write a constructor
public Terrain(double[][] heights)
and a method
public void printFloodMap(double waterLevel)
that prints out a flood map, showing which of the points in the terrain would be
flooded if the water level was the given value.
© joshblake/iStockphoto.
© GordonHeeley/iStockphoto.
programming projects 375
In the flood map, print a  * for each flooded point and a space for each point that is
not flooded.
Here is a sample map:
* * * * * *
* * * * * * * *
* * * * * *
* * * * * *
* * * * * * * *
* * * * * * * * * *
* * * * *
* * * * * *
* *
* * *
Then write a program that reads one hundred terrain height values and shows how
the terrain gets flooded when the water level increases in ten steps from the lowest
point in the terrain to the highest.
•• science p7.14  Sample values from an experiment often need to be smoothed out. One simple
approach is to replace each value in an array with the average of the value and its two
neighboring values (or one neighboring value if it is at either end of the array). Given
a class  Data with instance fields
private double[] values;
private double valuesSize;
implement a method
public void smooth()
that carries out this operation. You should not create another array in your solution.
••• science p7.15  Write a program that models the movement of an object with mass m that is attached
to an oscillating spring. When a spring is displaced from its equilibrium position by
an amount x, Hooke’s law states that the restoring force is
F = –kx
where k is a constant that depends on the spring. (Use
10 N̸m for this simulation.)
Start with a given displacement x (say, 0.5 meter). Set the
initial velocity v to 0. Compute the acceleration a from
Newton’s law (F = ma) and Hooke’s law, using a mass of
1 kg. Use a small time interval Δt = 0.01 second. Update the
velocity––it changes by aΔt. Update the displacement––it
changes by vΔt.
Every ten iterations, plot the spring displacement as a
bar, where 1 pixel represents 1 cm, as shown here.
•• graphics p7.16  Generate the image of a checkerboard.
© nicolamargaret/iStockphoto.
x
F
Unstretched
spring
376 Chapter 7 arrays and array Lists
• graphics p7.17  Generate the image of a sine wave. Draw a line of pixels for every five degrees.
• graphics p7.18  Implement a class  Cloud that contains an array list of  Point2D.Double objects. Support
methods
public void add(Point2D.Double aPoint)
public void draw(Graphics2D g2)
Draw each point as a tiny circle. Write a graphical application that draws a cloud of
100 random points.
•• graphics p7.19  Implement a class  Polygon that contains an array list of  Point2D.Double objects. Sup port
methods
public void add(Point2D.Double aPoint)
public void draw(Graphics2D g2)
Draw the polygon by joining adjacent points with a line, and then closing it up by
joining the end and start points. Write a graphical application that draws a square
and a pentagon using two  Polygon objects.
• graphics p7.20  Write a class  Chart with methods
public void add(int value)
public void draw(Graphics2D g2)
that displays a stick chart of the added values, like this:
You may assume that the values are pixel positions.
•• graphics p7.21  Write a class  BarChart with methods
public void add(double value)
public void draw(Graphics2D g2)
that displays a bar chart of the added values. You may assume that all added values
are positive. Stretch the bars so that they fill the entire area of the screen. You must
figure out the maximum of the values, then scale each bar.
••• graphics p7.22  Improve the  BarChart class of Exercise P7.21 to work correctly when the data con-
tains negative values.
•• graphics p7.23  Write a class  PieChart with methods
public void add(double value)
public void draw(Graphics2D g2)
that displays a pie chart of the added values. Assume that all data values are positive.
Answers to Self-Check Questions 377
A n S w e rS t o S e l f - C h eC k Q u e S t i o n S
1.  int[] primes = { 2, 3, 5, 7, 11 };
2.  2, 3, 5, 3, 2
3.  3, 4, 6, 8, 12
4.  values[0] = 10;
values[9] = 10; or better:
values[values.length - 1] = 10;
5.  String[] words = new String[10];
6.  String[] words = { "Yes", "No" };
7.  No. Because you don’t store the values, you
need to print them when you read them. But
you don’t know where to add the  <= until you
have seen all values.
8.  public class Lottery
{
public int[] getCombination(int n) { . . . }
. . .
}
9.  It counts how many elements of  values are
zero.
10.  for (double x : values)
{
System.out.println(x);
}
11.  double product = 1;
for (double f : factors)
{
product = product * f;
}
12.  The loop writes a value into  values[i] . The
enhanced  for loop does not have the index
variable  i .
13.  20 <== largest value
10
20 <== largest value
14.  int count = 0;
for (double x : values)
{
if (x == 0) { count++; }
}
15.  If all elements of  values are negative, then the
result is incorrectly computed as  0 .
16.  for (int i = 0; i < values.length; i++)
{
System.out.print(values[i]);
if (i < values.length - 1)
{
System.out.print(" | ");
}
}
Now you know why we set up the loop the
other way.
17.  If the array has no elements, then the program
terminates with an exception.
18.  If there is a match, then  pos is incremented
before the loop exits.
19.  This loop sets all elements to  values[pos] .
20.  Use the first algorithm. The order of elements
does not matter when computing the sum.
21.  Find the minimum value.
Calculate the sum.
Subtract the minimum value.
22.  Use the algorithm for counting matches
(Section 6.7.2) twice, once for counting the
positive values and once for counting the
negative values.
23.  You need to modify the algorithm in
Section 7.3.4.
boolean first = true;
for (int i = 0; i < values.length; i++)
{
if (values[i] > 0))
{
if (first) { first = false; }
else { System.out.print(", "); }
}
System.out.print(values[i]);
}
Note that you can no longer use  i > 0 as the
criterion for printing a separator.
24.  Use the algorithm to collect all positive ele-
ments in an array, then use the algorithm in
Section 7.3.4 to print the array of matches.
25.  The paperclip for  i assumes positions 0, 1, 2,
3. When  i is incremented to 4, the con dition
i < size / 2 becomes false, and the loop ends.
Similarly, the paperclip for  j assumes positions
4, 5, 6, 7, which are the valid positions for the
second half of the array.
coins: © jamesbenet/iStockphoto; dollar coins: © JordiDelgado/iStockphoto; paperclips: ©
26.  It reverses the elements in the array.
378 Chapter 7 arrays and array Lists
27.  Here is one solution. The basic idea is to move
all odd elements to the end. Put one paper clip
at the beginning of the array and one at the
end. If the element at the first paper clip is odd,
swap it with the one at the other paper clip and
move that paper clip to the left. Otherwise,
move the first paper clip to the right. Stop
when the two paper clips meet. Here is the
pseudocode:
i = 0
j = size - 1
While (i < j)
If (a[i] is odd)
Swap elements at positions i and j.
j--
Else
i++
28.  Here is one solution. The idea is to remove
all odd elements and move them to the end.
The trick is to know when to stop. Nothing is
gained by moving odd elements into the area
that already contains moved elements, so we
want to mark that area with another paper clip.
i = 0
moved = size
While (i < moved)
If (a[i] is odd)
Remove the element at position i and add it
at the end.
moved--
29.  When you read inputs, you get to see values
one at a time, and you can’t peek ahead. Pick-
ing cards one at a time from a deck of cards
simulates this process better than looking at a
sequence of items, all of which are revealed.
30.  You get the total number of gold, silver, and
bronze medals in the competition. In our
example, there are four of each.
31.  for (int i = 0; i < 8; i++)
{
for (int j = 0; j < 8; j++)
{
board[i][j] = (i + j) % 2;
}
}
32.  String[][] board = new String[3][3];
33.  board[0][2] = "x";
34.  board[0][0] ,  board[1][1] ,  board[2][2]
35.  ArrayList<Integer> primes =
new ArrayList<Integer>();
primes.add(2);
primes.add(3);
primes.add(5);
primes.add(7);
primes.add(11);
36.  for (int i = primes.size() - 1; i >= 0; i--)
{
System.out.println(primes.get(i));
}
37.  "Ann", "Cal"
38.  The  names variable has not been initialized.
39.  names1 contains  "Emily" ,  "Bob" ,  "Cindy" ,  "Dave" ;
names2 contains  "Dave"
40.  Because the number of weekdays doesn’t
change, there is no disadvantage to using an
array, and it is easier to initialize:
String[] weekdayNames = { "Monday", "Tuesday",
"Wednesday", "Thursday", “Friday”,
"Saturday", "Sunday" };
41.  Reading inputs into an array list is much easier.
42.  It is possible to introduce errors when modify-
ing code.
43.  Add a test case to the test suite that verifies that
the error is fixed.
44.  There is no human user who would see the
prompts because input is provided from a file.
8
C h a p t e r
379
Designing
Classes
to learn how to choose appropriate
classes for a given problem
to understand the concept of cohesion
to minimize dependencies and
side effects
to learn how to find a data representation for a class
to understand static methods and variables
to learn about packages
to learn about unit testing frameworks
C h a p t e r g o a l s
C h a p t e r C o n t e n t s
8.1 Discovering classes 380
8.2 Designing gooD MethoDs 381
Programming Tip 8.1: Consistency 385
Special Topic 8.1: Call by Value and Call
by reference 386
8.3 ProbleM solving: Patterns for
object Data 390
8.4 static variables
anD MethoDs 395
Programming Tip 8.2: Minimize the Use of
static Methods 397
Common Error 8.1: trying to access instance
Variables in static Methods 398
Special Topic 8.2: static imports 398
Special Topic 8.3: alternative Forms of instance
and static Variable initialization 399
8.5 Packages 400
Syntax 8.1: package specification 402
Common Error 8.2: Confusing Dots 403
Special Topic 8.4: package access 404
How To 8.1: programming with packages 404
Computing & Society 8.1: personal Computing 406
8.6 Unit test fraMeworks 407
© Ivan Stevanovic/iStockphoto.
380
good design should be both functional and attractive.
When designing classes, each class should be dedicated
to a particular purpose, and classes should work well
together. in this chapter, you will learn how to discover
classes, design good methods, and choose appropriate
data representations. You will also learn how to design
features that belong to the class as a whole, not individual
objects, by using static methods and variables. You will see
how to use packages to organize your classes. Finally, we
introduce the JUnit testing framework that lets you verify
the functionality of your classes.
8.1 Discovering Classes
You have used a good number of classes in the preceding chapters and probably
designed a few classes yourself as part of your programming assignments. Design-
ing a class can be a challenge—it is not always easy to tell how to start or whether the
result is of good quality.
What makes a good class? Most importantly, a class should represent a single con-
cept from a problem domain. Some of the classes that you have seen represent con-
cepts from mathematics:
•  Point
•  Rectangle
•  Ellipse
Other classes are abstractions of real-life entities:
•  BankAccount
•  CashRegister
For these classes, the properties of a typical object are easy to understand. A  Rectangle
object has a width and height. Given a  BankAccount object, you can deposit and with-
draw money. Generally, concepts from a domain related to the program’s purpose,
such as science, business, or gaming, make good classes. The name for such a class
should be a noun that describes the concept. In fact, a simple rule of thumb for getting
started with class design is to look for nouns in the problem description.
One useful category of classes can be described as actors. Objects of an actor class
carry out certain tasks for you. Examples of actors are the  Scanner class of Chapter 4
and the  Random class in Chapter 6. A  Scanner object scans a stream for numbers and
strings. A  Random object generates random numbers. It is a good idea to choose class
names for actors that end in “-er” or “-or”. (A better name for the  Random class might
be  RandomNumberGenerator .)
Very occasionally, a class has no objects, but it contains a collection of related static
methods and con stants. The  Math class is an example. Such a class is called a utility class.
Finally, you have seen classes with only a  main method. Their sole purpose is to
start a program. From a design perspective, these are somewhat degenerate examples
of classes.
What might not be a good class? If you can’t tell from the class name what an
object of the class is sup posed to do, then you are probably not on the right track. For
a class should
represent a single
concept from a
problem domain,
such as business,
science, or
mathematics.
Ivan Stevanovic/iStockphoto.
8.2 Designing good Methods 381
example, your homework assignment might ask you to write a program that prints
paychecks. Suppose you start by trying to design a class  Pay checkProgram . What would
an object of this class do? An object of this class would have to do everything that the
homework needs to do. That doesn’t simplify anything. A better class would be  Pay
check . Then your program can manipulate one or more  Paycheck objects.
Another common mistake is to turn a single operation into a class. For example,
if your homework assignment is to compute a paycheck, you may consider writing
a class  ComputePaycheck . But can you visu alize a “ComputePaycheck” object? The fact
that “ComputePaycheck” isn’t a noun tips you off that you are on the wrong track.
On the other hand, a  Paycheck class makes intuitive sense. The word “paycheck” is a
noun. You can visualize a paycheck object. You can then think about useful methods
of the  Paycheck class, such as  computeTaxes , that help you solve the assignment.
1.  What is a simple rule of thumb for finding classes?
2.  Your job is to write a program that plays chess. Might  ChessBoard be an appropri-
ate class? How about  MovePiece ?
Practice it  Now you can try these exercises at the end of the chapter: R8.1, R8.2, R8.3.
8.2 Designing good Methods
In the following sections, you will learn several useful criteria for analyzing and
improving the public interface of a class.
8.2.1 providing a Cohesive public interface
A class should represent a single concept. All
interface features should be closely related to
the single concept that the class represents.
Such a public interface is said to be cohesive.
The members of a cohesive team
have a common goal.
If you find that the public interface of a class refers to multiple concepts, then that
is a good sign that it may be time to use separate classes instead. Consider, for exam-
ple, the public interface of the  CashRegister class in Chapter 4:
publicclassCashRegister
{
publicstaticfinaldoubleQUARTER_VALUE=0.25;
publicstaticfinaldoubleDIME_VALUE=0.1;
publicstaticfinaldoubleNICKEL_VALUE=0.05;
...
publicvoidreceivePayment(intdollars,intquarters,
intdimes,intnickels,intpennies)
...
}
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
© Sergey Ivanov/iStockphoto.
the public interface
of a class is cohesive
if all of its features
are related to the
concept that the
class represents.
382 Chapter 8 Designing Classes
There are really two concepts here: a cash register that holds coins and computes
their total, and the val ues of individual coins. (For simplicity, we assume that the cash
register only holds coins, not bills. Exercise E8.3 discusses a more general solution.)
It makes sense to have a separate  Coin class and have coins responsible for knowing
their values.
publicclassCoin
{
...
publicCoin(doubleaValue,StringaName){...}
publicdoublegetValue(){...}
...
}
Then the  CashRegister class can be simplified:
publicclassCashRegister
{
...
publicvoidreceivePayment(intcoinCount,CoincoinType){...}
{
payment=payment+coinCount*coinType.getValue();
}
...
}
Now the  CashRegister class no longer needs to know anything about coin values. The
same class can equally well handle euros or zorkmids!
This is clearly a better solution, because it separates the responsibilities of the cash
register and the coins. The only reason we didn’t follow this approach in Chapter 4
was to keep the  CashRegister example simple.
8.2.2 Minimizing Dependencies
Many methods need other classes in order to do their jobs. For example, the  receive
Payment method of the restructured  CashRegister class now uses the  Coin class. We say
that the  CashRegister class depends on the  Coin class.
To visualize relationships between classes, such as dependence, programmers draw
class diagrams. In this book, we use the UML (“Unified Modeling Language”) nota-
tion for objects and classes. UML is a notation for object-oriented analysis and design
invented by Grady Booch, Ivar Jacobson, and James Rumbaugh, three leading
researchers in object-oriented softare development. (Appendix H has a summary of
the UML notation used in this book.) The UML notation dis tinguishes between
object diagrams and class diagrams. In an object diagram the class names are under-
lined; in a class diagram the class names are not underlined. In a class diagram, you
denote dependency by a dashed line with a -shaped open arrow tip that points to
the dependent class. Figure 1 shows a class diagram indicating that the  Cash Register
class depends on the  Coin class.
Note that the  Coin class does not depend on the  CashRegister class. All  Coin methods
can carry out their work without ever calling any method in the  CashRegister class.
Conceptually, coins have no idea that they are being collected in cash registers.
Here is an example of minimizing dependencies. Consider how we have always
printed a bank balance:
System.out.println("Thebalanceisnow$"+momsSavings.getBalance());
a class depends on
another class if its
methods use that
class in any way.
8.2 Designing Good Methods  383
Figure 1  
Dependency Relationship 
Between the  CashRegister 
and  Coin  Classes
CashRegister
Coin
Why don’t we simply have a  printBalance method?
public void printBalance() //  Not recommended
{
System.out.println("The balance is now $" + balance);
}
The method depends on  System.out . Not every computing environment has  System.out .
For example, an automatic teller machine doesn’t display console messages. In other
words, this design violates the rule of minimizing dependencies. The  printBalance
method couples the  BankAccount class with the  System and  PrintStream classes.
It is best to place the code for producing output or consuming input in a separate
class. That way, you decouple input/output from the actual work of your classes.
8.2.3 Separating Accessors and Mutators
A mutator method changes the state of an object. Conversely, an accessor method
asks an object to compute a result, without changing the state.
Some classes have been designed to have only accessor methods and no mutator
methods at all. Such classes are called immutable. An example is the  String class.
Once a string has been constructed, its con tent never changes. No method in the
String class can modify the contents of a string. For example, the  toUpperCase method
does not change characters from the original string. Instead, it constructs a new string
that contains the uppercase characters:
String name = "John Q. Public";
String uppercased = name.toUpperCase(); // name is not changed
An immutable class has a major advantage: It is safe to give out references to its
objects freely. If no method can change the object’s value, then no code can modify
the object at an unexpected time.
Not every class should be immutable. Immutability makes most sense for classes
that represent values, such as strings, dates, currency amounts, colors, and so on.
In mutable classes, it is still a good idea to cleanly separate accessors and mutators,
in order to avoid accidental mutation. As a rule of thumb, a method that returns a
value should not be a mutator. For example, one would not expect that calling  get
Balance on a  BankAccount object would change the balance. (You would be pretty upset
if your bank charged you a “balance inquiry fee”.) If you follow this rule, then all
mutators of your class have return type  void .
An immutable 
class has no 
mutator methods.
References to 
objects of an
immutable class can
be safely shared.
384 Chapter 8 Designing Classes
Sometimes, this rule is bent a bit, and mutator methods return an informational
value. For example, the  ArrayList class has a  remove method to remove an object.
ArrayList<String>names=...;
booleansuccess=names.remove("Romeo");
That method returns  true if the removal was successful; that is, if the list contained the
object. Returning this value might be bad design if there was no other way to check
whether an object exists in the list. However, there is such a method––the  contains
method. It is acceptable for a mutator to return a value if there is also an accessor that
computes it.
The situation is less happy with the  Scanner class. The  next method is a mutator
that returns a value. (The  next method really is a mutator. If you call  next twice in a
row, it can return different results, so it must have mutated
something inside the  Scanner object.) Unfortunately, there
is no accessor that returns the same value. This sometimes
makes it awkward to use a  Scanner . You must carefully hang
on to the value that the  next method returns because you
have no second chance to ask for it. It would have been bet-
ter if there was another method, say  peek , that yields the next
input without consuming it.
To check the temperature of the water in the bottle, you could take a sip,
but that would be the equivalent of a mutator method.
8.2.4 Minimizing side effects
A side effect of a method is any kind of modification of data that is observable out-
side the method. Muta tor methods have a side effect, namely the modification of the
implicit parameter.
There is another kind of side effect that you should avoid. A method should gener-
ally not modify its parameter variables. Consider this example:
/**
 Computes the total balance of the given accounts.
@paramaccounts a list of bank accounts
*/
publicdoublegetTotalBalance(ArrayList<String>accounts)
{
doublesum=0;
while(studentNames.size()>0)
{
BankAccountaccount=accounts.remove(0);// Not recommended
sum=sum+account.getBalance();
}
returnsum;
}
}
This method removes all names from the  accounts parameter variable. After a call
doubletotal=getTotalBalance(allAccounts);
allAccounts is empty! Such a side effect would not be what most programmers expect.
It is better if the method visits the elements from the list without removing them.
© manley099/iStockphoto.
a side effect of
a method is any
externally observable
data modification.
8.2 Designing good Methods 385
Another example of a side effect is output. Consider again the  printBalance method
that we discussed in Section 8.2.2:
publicvoidprintBalance()// Not recommended
{
System.out.println("Thebalanceisnow$"+balance);
}
This method mutates the  System.out object,
which is not a part of the  BankAccount
object. That is a side effect.
To avoid this side effect, keep most of
your classes free from input and output
operations, and concentrate input and out-
put in one place, such as the main method
of your program.
This taxi has an undesirable side effect,
spraying bystanders with muddy water.
3.  Why is the  CashRegister class from Chapter 4 not cohesive?
4.  Why does the  Coin class not depend on the  CashRegister class?
5.  Why is it a good idea to minimize dependencies between classes?
6.  Is the  substring method of the  String class an accessor or a mutator?
7.  Is the  Rectangle class immutable?
8.  If  a refers to a bank account, then the call  a.deposit(100) modifies the bank
account object. Is that a side effect?
9.  Consider the  Student class of Chapter 7. Suppose we add a method
voidread(Scannerin)
{
while(in.hasNextDouble())
{
addScore(in.nextDouble());
}
}
Does this method have a side effect other than mutating the scores?
Practice it  Now you can try these exercises at the end of the chapter: R8.4, R8.5, R8.9.
consistency
In this section you learned of two criteria for analyzing the quality of the public interface of a
class. You should max imize cohesion and remove unnecessary dependencies. There is another
criterion that we would like you to pay attention to—consistency. When you have a set of
methods, follow a consistent scheme for their names and parameter variables. This is simply a
sign of good craftsmanship.
Sadly, you can find any number of inconsistencies in the standard library. Here is an exam-
ple: To show an input dialog box, you call
JOptionPane.showInputDialog(promptString)
When designing
methods, minimize
side effects.
AP Photo/Frank Franklin II.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
programming tip 8.1
© Eric Isselé/iStockphoto.
386 Chapter 8  Designing Classes
To show a message dialog box, you call
JOptionPane.showMessageDialog(null, messageString)
What’s the  null argument? It turns out that the  showMessage-
Dialog method needs an argument to specify the parent win-
dow, or  null if no parent window is required. But the  show-
InputDialog method requires no parent window. Why the
inconsistency? There is no reason. It would have been an easy
matter to supply a  show Message Dialog method that exactly mir-
rors the  showInputDialog method.
Inconsistencies such as these are not fatal flaws, but they
are an annoyance, particularly because they can be so easily
avoided.
Call by Value and Call by Reference
In Section 8.2.4, we recommended that you don’t invoke a mutator method on a parameter
variable. In this Special Topic, we discuss a related issue—what happens when you assign a
new value to a parameter variable. Consider this method:
public class BankAccount
{
. . .
/**
Transfers money from this account and tries to add it to a balance.
@param amount  the amount of money to transfer
@param otherBalance  balance to add the amount to
*/
public void transfer(double amount, double otherBalance)  2
{
balance = balance - amount;
otherBalance = otherBalance + amount;
//  Won’t update the argument
}  3
}
Now let’s see what happens when we call the method:
double savingsBalance = 1000;
harrysChecking.transfer(500, savingsBalance);  1
System.out.println(savingsBalance);  4
You might expect that after the call, the  savingsBalance variable has been incremented to 1500.
However, that is not the case. As the method starts, the parameter variable  otherBalance is set
to the same value as  savingsBalance (see Figure 2). Then the variable is set to a different value.
That modification has no effect on  savingsBalance , because  otherBalance is a separate variable.
When the method terminates, the  other Balance variable is removed, and  savingsBalance isn’t
increased.
In Java, parameter variables are initialized with the values of
the argument expressions. When the method exits, the param-
eter variables are removed. Computer scientists refer to this call
mechanism as “call by value”.
For that reason, a Java method can never change the contents of a variable that is passed as
an argument––the method manipulates a different variable.
Other programming languages such as C++ support a mechanism, called “call by refer-
ence”, that can change the arguments of a method call. You will sometimes read in Java books
Frank Rosenstein/Digital Vision/Getty Images, Inc.
While it is possible to eat
with mismatched silverware,
consistency is more pleasant.
Special Topic 8.1
© Eric Isselé/iStockphoto.
In Java, a method can
never change the contents
of a variable that is passed
to a method.
8.2 Designing Good Methods  387
Figure 2  Modifying a Parameter Variable of a Primitive Type Has No Effect on Caller
2
3
4
Before method call
Initializing
parameter variables
After method call
About to return
to the caller
savingsBalance =
harrysChecking =
balance =
BankAccount
2500
1000
savingsBalance =
harrysChecking =
balance =
BankAccount
2500
this =
amount = 500
1000
otherBalance = 1000
savingsBalance =
harrysChecking =
balance =
BankAccount
2000
1000
savingsBalance =
harrysChecking =
balance =
BankAccount
2000
this =
amount = 500
1000
otherBalance = 1500
1
Modification has
no effect on
savingsBalance
that “numbers are passed by value, objects are passed by reference”. That is technically not
quite correct. In Java, objects themselves are never passed as arguments; instead, both numbers
and object references are passed by value.
The confusion arises because a Java method can mutate an object when it receives an object
reference as an argument (see Figure 3).
public class BankAccount
{
. . .
/**
Transfers money from this account to another.
@param amount  the amount of money to transfer
@param otherAccount  account to add the amount to
Camera: © james steidl/iStockphoto.
Globe: © Alex Slobodkin/iStockphoto.
A N IM AT IO N
A Method Cannot
Modify a Numeric
Parameter
388 Chapter 8 Designing Classes
*/
publicvoidtransfer(doubleamount,BankAccountotherAccount) 2
{
balance=balanceamount;
otherAccount.deposit(amount);
}
3
}
figure 3  Methods Can Mutate any objects to Which they hold references
2
Before method call
Initializing
parameter variables
harrysChecking =
balance =
BankAccount
2500
1
These variables
will be removed
harrysSavings =
balance =
BankAccount
1000
harrysChecking =
balance =
BankAccount
2500
harrysSavings =
balance =
BankAccount
1000
amount =
otherAccount =
500
harrysChecking =
balance =
BankAccount
2000
harrysSavings =
balance =
BankAccount
1500
amount =
otherAccount =
500
3 About to return
to the caller
The balance has
been updated
8.2 Designing Good Methods  389
Now we pass an object reference to the  transfer method:
BankAccount harrysSavings = new BankAccount(1000);
harrysChecking.transfer(500, harrysSavings);  1
System.out.println(harrysSavings.getBalance());
This example works as expected. The parameter variable  otherAccount contains a copy of the
object reference  harrysSavings . You saw in Section 2.8 what is means to make a copy of an
object reference––you get another reference to the same object. Through that reference, the
method is able to modify the object.
However, a method cannot replace an object reference that is passed as an argument. To
appreciate this subtle difference, consider this method that tries to set the  otherAccount param-
eter variable to a new object:
public class BankAccount
{
. . .
public void transfer(double amount, BankAccount otherAccount)
{
balance = balance - amount;
double newBalance = otherAccount.balance + amount;
otherAccount = new BankAccount(newBalance); //  Won’t work
}
}
I n this situation, we are not trying to change the state of the object
to which the parameter variable  otherAccount refers; instead, we
are trying to replace the object with a different one (see Figure 4).
Now the reference stored in parame ter variable  otherAccount is
replaced with a reference to a new account. But if you call the
method with
harrysChecking.transfer(500, savingsAccount);
then that change does not affect the  savingsAccount variable that is supplied in the call. This
example demonstrates that objects are not passed by reference.
Figure 4  
Replacing the Object Reference 
in a Parameter Variable 
Has No Effect on the Caller
In Java, a method can
change the state of
an object reference
argument, but it cannot
replace the object
reference with another.
savingsAccount =
harrysChecking =
balance =
BankAccount
2500
balance =
BankAccount
1000
balance =
BankAccount
1500
this =
amount = 500
otherAccount =
Modification has
no effect on
savingsAccount
390 Chapter 8 Designing Classes
To summarize:
• A Java method can’t change the contents of any variable passed as an argument.
• A Java method can mutate an object when it receives a reference to it as an argument.
8.3 problem solving: patterns for object Data
When you design a class, you first consider the needs of the programmers who use
the class. You provide the methods that the users of your class will call when they
manipulate objects. When you implement the class, you need to come up with the
instance variables for the class. It is not always obvious how to do this. Fortunately,
there is a small set of recurring patterns that you can adapt when you design your
own classes. We introduce these patterns in the following sections.
8.3.1 Keeping a total
Many classes need to keep track of a quantity that can go up or down as certain meth-
ods are called. Examples:
• A bank account has a balance that is increased by a deposit, decreased by a
withdrawal.
• A cash register has a total that is increased when an item is added to the sale,
cleared after the end of the sale.
• A car has gas in the tank, which is increased when fuel is added and decreased
when the car drives.
In all of these cases, the implementation strategy is similar. Keep an instance variable
that represents the current total. For example, for the cash register:
privatedoublepurchase;
Locate the methods that affect the total. There is usually a method to increase it by a
given amount:
publicvoidrecordPurchase(doubleamount)
{
purchase=purchase+amount;
}
Depending on the nature of the class, there may be a method that reduces or clears the
total. In the case of the cash register, one can provide a  clear method:
publicvoidclear()
{
purchase=0;
}
There is usually a method that yields the current total. It is easy to implement:
publicdoublegetAmountDue()
{
returnpurchase;
}
All classes that manage a total follow the same basic pattern. Find the methods that
affect the total and provide the appropriate code for increasing or decreasing it. Find
an instance variable
for the total is
updated in methods
that increase or
decrease the total
amount.
8.3 problem solving: patterns for object Data 391
the methods that report or use the total, and have those methods read the current
total.
8.3.2 Counting events
You often need to count how many times certain events occur in the life of an object.
For example:
• In a cash register, you may want to know how many items have been added
in a sale.
• A bank account charges a fee for each transaction; you need to count them.
Keep a counter, such as
privateintitemCount;
Increment the counter in those methods that correspond to the events that you want
to count:
publicvoidrecordPurchase(doubleamount)
{
purchase=purchase+amount;
itemCount++;
}
You may need to clear the counter, for example at the end of a sale or a statement
period:
publicvoidclear()
{
purchase=0;
itemCount=0;
}
There may or may not be a method that reports the count to the class user. The count
may only be used to compute a fee or an average. Find out which methods in your
class make use of the count, and read the current value in those methods.
8.3.3 Collecting Values
Some objects collect numbers, strings, or other objects. For example, each multiple-
choice question has a number of choices. A cash register may need to store all prices
of the current sale.
Use an array list or an array to store the values.
(An array list is usually simpler because you won’t
need to track the number of values.) For example,
publicclassQuestion
{
privateArrayList<String>choices;
...
}
A shopping cart object needs to
manage a collection of items.
a counter that
counts events
is incremented
in methods that
correspond to
the events.
© paul prescott/iStockphoto.
an object can collect
other objects in an
array or array list.
392 Chapter 8 Designing Classes
In the constructor, initialize the instance variable to an empty collection:
publicQuestion()
{
choices=newArrayList<String>();
}
You need to supply some mechanism for adding values. It is common to provide a
method for appending a value to the collection:
publicvoidadd(Stringoption)
{
choices.add(option);
}
The user of a  Question object can call this method multiple times to add the choices.
8.3.4 Managing properties of an object
A property is a value of an object that an object user can set and retrieve. For example,
a  Student object may have a name and an ID. Provide an instance variable to store the
property’s value and methods to get and set it.
publicclassStudent
{
privateStringname;
...
publicStringgetName(){returnname;}
publicvoidsetName(StringnewName){name=newName;}
...
}
It is common to add error checking to the setter method. For example, we may want
to reject a blank name:
publicvoidsetName(StringnewName)
{
if(newName.length()>0){name=newName;}
}
Some properties should not change after they have been set in the constructor. For
example, a student’s ID may be fixed (unlike the student’s name, which may change).
In that case, don’t supply a setter method.
publicclassStudent
{
privateintid;
...
publicStudent(intanId){id=anId;}
publicStringgetId(){returnid;}
// No setId method
...
}
8.3.5 Modeling objects with Distinct states
Some objects have behavior that varies depending on what has happened in the past.
For example, a  Fish object may look for food when it is hungry and ignore food after
it has eaten. Such an object would need to remember whether it has recently eaten.
an object property
can be accessed
with a getter method
and changed with
a setter method.
8.3 problem solving: patterns for object Data 393
If a fish is in a hungry state, its behavior changes.
Supply an instance variable that models the state,
together with some constants for the state values:
publicclassFish
{
privateinthungry;
publicstaticfinalintNOT_HUNGRY=0;
publicstaticfinalintSOMEWHAT_HUNGRY=1;
publicstaticfinalintVERY_HUNGRY=2;
...
}
(Alternatively, you can use an enumeration––see Special Topic 5.4.)
Determine which methods change the state. In this example, a fish that has just
eaten won’t be hungry. But as the fish moves, it will get hungrier:
publicvoideat()
{
hungry=NOT_HUNGRY;
...
}
publicvoidmove()
{
...
if(hungry<VERY_HUNGRY){hungry++;}
}
Finally, determine where the state affects behavior. A fish that is very hungry will
want to look for food first:
publicvoidmove()
{
if(hungry==VERY_HUNGRY)
{
 Look for food.
}
...
}
8.3.6 Describing the position of an object
Some objects move around during their lifetime, and they remember their current
position. For example,
• A train drives along a track and keeps track of the distance from the terminus.
• A simulated bug living on a grid crawls from one grid location to the next, or
makes 90 degree turns to the left or right.
• A cannonball is shot into the air, then descends as it is pulled by the gravitational
force.
Such objects need to store their position. Depending on the nature of their move-
ment, they may also need to store their orientation or velocity.
© John Alexander/iStockphoto.
if your object can
have one of several
states that affect the
behavior, supply an
instance variable
for the current state.
to model a moving
object, you need to
store and update
its position.
394 Chapter 8 Designing Classes
A bug in a grid needs to store its row,
column, and direction.
If the object moves along a line, you can represent the position as a distance from a
fixed point:
privatedoubledistanceFromTerminus;
If the object moves in a grid, remember its current location and direction in the grid:
privateintrow;
privateintcolumn;
privateintdirection;// 0 = North, 1 = East, 2 = South, 3 = West
When you model a physical object such as a cannonball, you need to track both the
position and the velocity, possibly in two or three dimensions. Here we model a can-
nonball that is shot upward into the air:
privatedoublezPosition;
privatedoublezVelocity;
There will be methods that update the position. In the simplest case, you may be told
by how much the object moves:
publicvoidmove(doubledistanceMoved)
{
distanceFromTerminus=distanceFromTerminus+distanceMoved;
}
If the movement happens in a grid, you need to update the row or column, depending
on the current orientation.
publicvoidmoveOneUnit()
{
if(direction==NORTH){row;}
elseif(direction==EAST){column++;}
elseif(direction==SOUTH){row++;}
elseif(direction==WEST){column––;}
}
Exercise P8.6 shows you how to update the position of a physical object with known
velocity.
Whenever you have a moving object, keep in mind that your program will simu-
late the actual movement in some way. Find out the rules of that simulation, such as
movement along a line or in a grid with integer coordinates. Those rules determine
how to represent the current position. Then locate the methods that move the object,
and update the positions according to the rules of the simulation.
10.  Suppose we want to count the number of transactions in a bank account in a
statement period, and we add a counter to the  BankAccount class:
publicclassBankAccount
{
privateinttransactionCount;
...
fUll coDe exaMPle
go to  wiley.com/go/
javacode to download
classes that use
these patterns for
object data.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
8.4 static Variables and Methods 395
}
In which methods does this counter need to be updated?
11.  In How To 3.1, the  CashRegister class does not have a  getTotalPurchase method.
Instead, you have to call  receivePayment and then  giveChange . Which recommenda-
tion of Section 8.2.4 does this design violate? What is a better alternative?
12.  In the example in Section 8.3.3, why is the  add method required? That is, why
can’t the user of a  Question object just call the  add method of the  ArrayList<String>
class?
13.  Suppose we want to enhance the  CashRegister class in How To 3.1 to track the
prices of all purchased items for printing a receipt. Which instance variable
should you provide? Which methods should you modify?
14.  Consider an  Employee class with properties for tax ID number and salary. Which
of these properties should have only a getter method, and which should have
getter and setter methods?
15.  Suppose the  setName method in Section 8.3.4 is changed so that it returns true if
the new name is set, false if not. Is this a good idea?
16.  Look at the  direction instance variable in the bug example in Section 8.3.6. This
is an example of which pattern?
Practice it  Now you can try these exercises at the end of the chapter: E8.21, E8.22, E8.23.
8.4 static Variables and Methods
Sometimes, a value properly belongs to a class, not
to any object of the class. You use a static variable
for this purpose. Here is a typical example: We want
to assign bank account numbers sequentially. That
is, we want the bank account constructor to con-
struct the first account with number 1001, the next
with num ber 1002, and so on. To solve this prob-
lem, we need to have a single value of  lastAssigned
Number that is a property of the class, not any object
of the class. Such a variable is called a static variable
because you declare it using the  static reserved
word.
publicclassBankAccount
{
privatedoublebalance;
privateintaccountNumber;
privatestaticintlastAssignedNumber=1000;
publicBankAccount()
{
lastAssignedNumber++;
accountNumber=lastAssignedNumber;
}
...
}
© Diane Diederich/iStockphoto.
The reserved word  static is a
holdover from the C++ language.
Its use in Java has no relationship
to the normal use of the term.
a static variable
belongs to the class,
not to any object of
the class.
396 Chapter 8 Designing Classes
Every  BankAccount object has its own  balance and  accountNumber instance variables, but
all objects share a single copy of the  lastAssignedNumber variable (see Figure 5). That
variable is stored in a separate location, out side any  BankAccount objects.
Like instance variables, static variables should always be declared as  private to
ensure that methods of other classes do not change their values. However, static con-
stants may be either private or public.
For example, the  Bank Account class can define a public constant value, such as
publicclassBankAccount
{
publicstaticfinaldoubleOVERDRAFT_FEE=29.95;
...
}
Methods from any class can refer to such a constant as  BankAccount.OVERDRAFT_FEE .
Sometimes a class defines methods that are not invoked on an object. Such a
method is called a static method. A typical example of a static method is the  sqrt
method in the  Math class. Because numbers aren’t objects, you can’t invoke methods
on them. For example, if  x is a number, then the call  x.sqrt() is not legal in Java. There-
fore, the  Math class provides a static method that is invoked as  Math.sqrt(x) . No object
of the  Math class is constructed. The  Math qualifier simply tells the compiler where to
find the  sqrt method.
a static method
is not invoked on
an object.
figure 5  a static Variable and instance Variables
collegeFund =
balance =
accountNumber =
BankAccount
10000
1001
momsSavings =
balance =
accountNumber =
BankAccount
8000
1002
harrysChecking =
balance =
accountNumber =
BankAccount
0
1003
Each
BankAccount
object has its own
accountNumber
instance variable.
BankAccount.lastAssignedNumber = 1003
There is a single
lastAssignedNumber
static variable for the
BankAccount
class.
8.4 static Variables and Methods 397
You can define your own static methods for use in other classes. Here is an example:
publicclassFinancial
{
/**
 Computes a percentage of an amount. 
@parampercentage the percentage to apply
@paramamount the amount to which the percentage is applied
@return the requested percentage of the amount 
*/
publicstaticdoublepercentOf(doublepercentage,doubleamount)
{
return(percentage/100)*amount;
}
}
When calling this method, supply the name of the class containing it:
doubletax=Financial.percentOf(taxRate,total);
In object-oriented programming, static methods are not very common. Nevertheless,
the  main method is always static. When the program starts, there aren’t any objects.
Therefore, the first method of a program must be a static method.
17.  Name two static variables of the  System class.
18.  Name a static constant of the  Math class.
19.  The following method computes the average of an array of numbers:
publicstaticdoubleaverage(double[]values)
Why should it not be defined as an instance method?
20.  Harry tells you that he has found a great way to avoid those pesky objects: Put
all code into a single class and declare all methods and variables  static . Then  main
can call the other static methods, and all of them can access the static variables.
Will Harry’s plan work? Is it a good idea?
Practice it  Now you can try these exercises at the end of the chapter: R8.22, E8.5, E8.6.
Minimize the Use of static Methods
It is possible to solve programming problems by using classes with only static methods. In
fact, before object-ori ented programming was invented, that approach was quite common.
However, it usually leads to a design that is not object-oriented and makes it hard to evolve a
program.
Consider the task of How To 7.1. A program reads scores for a student and prints the final
score, which is obtained by dropping the lowest one. We solved the problem by implementing
a  Student class that stores student scores. Of course, we could have simply written a program
with a few static methods:
publicclassScoreAnalyzer
{
publicstaticdouble[]readInputs(){...}
publicstaticdoublesum(double[]values){...}
publicstaticdoubleminimum(double[]values){...}
fUll coDe exaMPle
go to  wiley.com/go/
javacode to download
a program with
static methods and
variables.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
programming tip 8.2
© Eric Isselé/iStockphoto.
398 Chapter 8 Designing Classes
publicstaticdoublefinalScore(double[]values)
{
if(values.length==0){return0;}
elseif(values.length==1){returnvalues[0];}
else{returnsum(values)minimum(values);}
}
publicstaticvoidmain(String[]args)
{
System.out.println(finalScore(readInputs()));
}
}
That solution is fine if one’s sole objective is to solve a simple homework problem. But suppose
you need to modify the program so that it deals with multiple students. An object-oriented
program can evolve the  Student class to store grades for many students. In contrast, adding
more functionality to static methods gets messy quickly (see Exercise E8.7).
trying to access instance variables in static Methods
A static method does not operate on an object. In other words, it has no implicit parameter, and
you cannot directly access any instance variables. For example, the following code is wrong:
publicclassSavingsAccount
{
privatedoublebalance;
privatedoubleinterestRate;
publicstaticdoubleinterest(doubleamount)
{
return(interestRate/100)*amount;
// ERROR: Static method accesses instance variable
}
}
Because different savings accounts can have different interest rates, the  interest method
should not be a static method.
static imports
Starting with Java version 5.0, there is a variant of the  import directive that lets you use static
methods and variables without class prefixes. For example,
importstaticjava.lang.System.*;
importstaticjava.lang.Math.*;
publicclassRootTester
{
publicstaticvoidmain(String[]args)
{
doubler=sqrt(PI); //  Instead of  Math.sqrt(Math.PI)
out.println(r); //  Instead of  System.out
}
Common error 8.1
© John Bell/iStockphoto.
special topic 8.2
© Eric Isselé/iStockphoto.
8.4 static Variables and Methods 399
}
Static imports can make programs easier to read, particularly if they use many mathematical
functions.
alternative forms of instance and static variable initialization
As you have seen, instance variables are initialized with a default value (0,  false , or  null ,
depending on their type). You can then set them to any desired value in a constructor, and that
is the style that we prefer in this book.
However, there are two other mechanisms to specify an initial value. Just as with local vari-
ables, you can specify initialization values for instance variables. For example,
publicclassCoin
{
privatedoublevalue=1;
privateStringname="Dollar";
...
}
These default values are used for every object that is being constructed.
There is also another, much less common, syntax. You can place one or more initializa-
tion blocks inside the class declaration. All statements in that block are executed whenever an
object is being constructed. Here is an example:
publicclassCoin
{
privatedoublevalue;
privateStringname;
{
value=1;
name="Dollar";
}
...
}
For static variables, you use a static initialization block:
publicclassBankAccount
{
privatestaticintlastAssignedNumber;
static
{
 lastAssignedNumber=1000;
}
...
}
All statements in the static initialization block are executed once when the class is loaded. Ini-
tialization blocks are rarely used in practice.
When an object is constructed, the initializers and initialization blocks are executed in the
order in which they appear. Then the code in the constructor is executed. Because the rules
for the alternative initialization mechanisms are somewhat complex, we recommend that you
simply use constructors to do the job of construction.
special topic 8.3
© Eric Isselé/iStockphoto.
400 Chapter 8 Designing Classes
8.5 packages
A Java program consists of a collection of classes. So far, most of your programs have
consisted of a small number of classes. As programs get larger, however, simply dis-
tributing the classes over multiple files isn’t enough. An additional structuring mech-
anism is needed.
In Java, packages provide this structuring mechanism. A Java package is a set of
related classes. For example, the Java library consists of several hundred packages,
some of which are listed in Table 1.
table 1 important packages in the Java library
package purpose sample Class
java.lang Language support Math
java.util Utilities Random
java.io Input and output PrintStream
java.awt Abstract Windowing Toolkit Color
java.applet Applets Applet
java.net Networking Socket
java.sql Database access through Structured Query Language ResultSet
javax.swing Swing user interface JButton
org.w3c.dom Document Object Model for XML documents Document
8.5.1 organizing related Classes into pack ages
To put one of your classes in a package, you must place a line
package packageName ;
as the first instruction in the source file containing the class. A package name consists
of one or more identifiers separated by periods. (See Section 8.5.3 for tips on con-
structing package names.)
For example, let’s put the  Financial class introduced in this chapter into a package
named  com.horst mann.bigjava . The  Financial.java file must start as follows:
packagecom.horstmann.bigjava;
publicclassFinancial
{
...
}
In addition to the named packages (such as  java.util or  com.horstmann.bigjava ), there is
a special package, called the default package, which has no name. If you did not
a package is a set of
related classes.
8.5 packages 401
In Java, related classes
are grouped into packages.
© Don Wilkie/iStockphoto.
include any  package statement at the top of your source file, its classes are placed in the
default package.
8.5.2 importing packages
If you want to use a class from a package, you can refer to it by its full name (package
name plus class name). For example,  java.util.Scanner refers to the  Scanner class in the
java.util package:
java.util.Scannerin=newjava.util.Scanner(System.in);
Naturally, that is somewhat inconvenient. For that reason, you usually import a name
with an  import statement:
importjava.util.Scanner;
Then you can refer to the class as  Scanner without the package prefix.
You can import all classes of a package with an  import statement that ends in  .* . For
example, you can use the statement
importjava.util.*;
to import all classes from the  java.util package. That statement lets you refer to
classes like  Scanner or  Ran dom without a  java.util prefix.
However, you never need to import the classes in the  java.lang package explicitly.
That is the package containing the most basic Java classes, such as  Math and  Object .
These classes are always available to you. In effect, an automatic  importjava.lang.*;
statement has been placed into every source file.
Finally, you don’t need to import other classes in the same package. For exam-
ple, when you implement the class  homework1.Tester , you don’t need to import the
class  homework1.Bank . The compiler will find the  Bank class without an  import statement
because it is located in the same package,  homework1 .
8.5.3 package names
Placing related classes into a package is clearly a convenient mechanism to organize
classes. However, there is a more important reason for packages: to avoid name
clashes. In a large project, it is inevitable that two people will come up with the same
name for the same concept. This even happens in the stan dard Java class library
(which has now grown to thousands of classes). There is a class  Timer in the  java.util
the  import directive
lets you refer to a
class of a package
by its class name,
without the
package prefix.
402 Chapter 8 Designing Classes
syntax 8.1  package specification
package  packageName ; Syntax
package com.horstmann.bigjava;
The classes in this file
belong to this package. A good choice for a package name
is a domain name in reverse.
package and another class called  Timer in the  javax.swing package. You can still tell the
Java com piler exactly which  Timer class you need, simply by referring to them as  java.
util.Timer and  javax.swing.Timer .
Of course, for the package-naming convention to work, there must be some way
to ensure that pack age names are unique. It wouldn’t be good if the car maker BMW
placed all its Java code into the package  bmw , and some other programmer (perhaps
Britney M. Walters) had the same bright idea. To avoid this problem, the inventors of
Java recommend that you use a package-naming scheme that takes advantage of the
uniqueness of Internet domain names.
For example, I have a domain name  horstmann.com , and there is nobody else on the
planet with the same domain name. (I was lucky that the domain name  horstmann.com
had not been taken by anyone else when I applied. If your name is Walters, you will
sadly find that someone else beat you to  walters.com .) To get a package name, turn the
domain name around to produce a package name prefix, such as  com.horstmann .
If you don’t have your own domain name, you can still create a package name that
has a high probabil ity of being unique by writing your e-mail address backwards. For
example, if Britney Walters has an e-mail address  walters@cs.sjsu.edu , then she can use
a package name  edu.sjsu.cs.walters for her own classes.
Some instructors will want you to place each of your assignments into a separate
package, such as  homework1 ,  homework2 , and so on. The reason is again to avoid name
collision. You can have two classes,  homework1.Bank and  homework2.Bank , with slightly
different properties.
8.5.4 packages and source Files
A source file must be located in a subdirectory that matches the package name. The
parts of the name between periods represent successively nested directories. For
example, the source files for classes in the package  com.horstmann.bigjava would be
placed in a subdirectory  com/horstmann/bigjava . You place the sub directory inside the
base directory holding your program’s files. For example, if you do your homework
assignment in a directory  /home/britney/hw8/problem1 , then you can place the class
files for the  com.horst mann.bigjava package into the directory  /home/britney/hw8/prob
lem1/com/horstmann/bigjava , as shown in Figure 6. (Here, we are using UNIX-style file
names. Under Windows, you might use  c:\Users\Brit ney\hw8\problem1\com\horstmann\
bigjava .)
Use a domain
name in reverse
to construct an
unambiguous
package name.
the path of a class
file must match its
package name.
8.5 packages 403
21.
figure 6  Base Directories and subdirectories for packages
Path matches
package name
Base directory
Which of the following are packages?
a.  java
b.  java.lang
c.  java.util
d.  java.lang.Math
22.  Is a Java program without  import statements limited to using the default and
java.lang packages?
23.  Suppose your homework assignments are located in the directory  /home/me/
cs101 ( c:\Users\Me\cs101 on Windows). Your instructor tells you to place your
homework into packages. In which directory do you place the class  hw1.problem1.
TicTacToe Tester ?
Practice it  Now you can try these exercises at the end of the chapter: R8.25, E8.15, E8.16.
confusing Dots
In Java, the dot symbol ( . ) is used as a separator in the following situations:
• Between package names ( java.util )
• Between package and class names ( homework1.Bank )
• Between class and inner class names ( Ellipse2D.Double )
• Between class and instance variable names ( Math.PI )
• Between objects and methods ( account.getBalance() )
When you see a long chain of dot-separated names, it can be a challenge to find out which part
is the package name, which part is the class name, which part is an instance variable name, and
which part is a method name. Consider
java.lang.System.out.println(x);
Because  println is followed by an opening parenthesis, it must be a method name. Therefore,
out must be either an object or a class with a static  println method. (Of course, we know that
out is an object reference of type  PrintStream .) Again, it is not at all clear, without context,
whether  System is another object, with a public variable  out , or a class with a  static variable.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
Common error 8.2
© John Bell/iStockphoto.
404 Chapter 8 Designing Classes
Judging from the number of pages that the Java language specification devotes to this issue,
even the compiler has trouble interpreting these dot-separated sequences of strings.
To avoid problems, it is helpful to adopt a strict coding style. If class names always start
with an uppercase letter, and variable, method, and package names always start with a lower-
case letter, then confusion can be avoided.
Package access
If a class, instance variable, or method has no  public or  private
modifier, then all methods of classes in the same package can
access the feature. For example, if a class is declared as  public ,
then all other classes in all packages can use it. But if a class is
declared without an access modifier, then only the other classes in
the same package can use it. Package access is a reasonable default
for classes, but it is extremely unfortunate for instance variables.
It is a common error to forget the reserved word  private , thereby opening up a potential
security hole. For exam ple, at the time of this writing, the  Window class in the  java.awt package
contained the following declaration:
publicclassWindowextendsContainer
{
StringwarningString;
...
}
There actually was no good reason to grant package access to the  warningString instance vari-
able—no other class accesses it.
Package access for instance variables is rarely useful and always a potential security risk.
Most instance variables are given package access by accident because the programmer sim-
ply forgot the  private reserved word. It is a good idea to get into the habit of scanning your
instance variable declarations for missing  private modifiers.
step 1  Come up with a package name.
Your instructor may give you a package name to use, such as  homework1.problem2 . Or, perhaps
you want to use a pack age name that is unique to you. Start with your e-mail address, written
backwards. For example,  walters@cs.sjsu.edu becomes  edu.sjsu.cs.walters . Then add a sub-
package that describes your project, such as  edu.sjsu.cs.walters.cs1project .
special topic 8.4
© Eric Isselé/iStockphoto.
an instance variable
or method that is not
declared as  public or
private can be accessed
by all classes in the
same package, which is
usually not desirable.
© Steve Simzer/iStockphoto.
hoW to 8.1  Programming with Packages
This How To explains in detail how to place your programs into
packages.
Problem statement  Place each homework assignment
into a separate package. That way, you can have classes with the
same name but different implementations in separate packages
(such as  homework1.problem1.Bank and  homework1.problem2.Bank ).
© Don Wilkie/iStockphoto.
8.5 packages 405
step 2  Pick a base directory.
The base directory is the directory that contains the directories for your various packages, for
example,  /home/britney or  c:\Users\Britney .
step 3  Make a subdirectory from the base directory that matches your package name.
The subdirectory must be contained in your base directory. Each segment must match a seg-
ment of the package name. For example,
mkdirp/home/britney/homework1/problem2 (in UNIX)
or
mkdir/sc:\Users\Britney\homework1\problem2 (in Windows)
step 4  Place your source files into the package subdirectory.
For example, if your homework consists of the files  Tester.java and  Bank.java , then you place
them into
/home/britney/homework1/problem2/Tester.java
/home/britney/homework1/problem2/Bank.java
or
c:\Users\Britney\homework1\problem2\Tester.java
c:\Users\Britney\homework1\problem2\Bank.java
step 5  Use the  package statement in each source file.
The first noncomment line of each file must be a package statement that lists the name of the
package, such as
packagehomework1.problem2;
step 6  Compile your source files from the base directory.
Change to the base directory (from Step 2) to compile your files. For example,
cd/home/britney
javachomework1/problem2/Tester.java
or
c:
cd\Users\Britney
javachomework1\problem2\Tester.java
Note that the Java compiler needs the source file name and not the class name. That is, you
need to supply file separa tors ( / on UNIX,  \ on Windows) and a file extension ( .java ).
step 7  Run your program from the base directory.
Unlike the Java compiler, the Java interpreter needs the class name (and not a file name) of the
class containing the  main method. That is, use periods as package separators, and don’t use a file
extension. For example,
cd/home/britney
javahomework1.problem2.Tester
or
c:
cd\Users\Britney
javahomework1.problem2.Tester
406 Chapter 8  Designing Classes
In 1971, Marcian E.
“Ted” Hoff, an engi
neer at Intel Corpo ration, was work
ing on a chip for a manufacturer of
electronic calculators. He realized that
it would be a better idea to develop a
general-purpose chip that could be
programmed to interface with the keys
and display of a calculator, rather than
to do yet another custom design. Thus,
the microprocessor was born. At the
time, its primary application was as
a con troller for calculators, washing
machines, and the like. It took years for
the computer industry to notice that
a genuine central processing unit was
now available as a single chip.
Hobbyists were the first to catch
on. In 1974 the first computer kit, the
Altair 8800, was available from MITS
Electronics for about $350. The kit
consisted of the microprocessor, a cir
cuit board, a very small amount of
memory, toggle switches, and a row of
display lights. Purchasers had to sol
der and assemble it, then program it in
machine language through the toggle
switches. It was not a big hit.
The first big hit was the Apple II. It
was a real computer with a keyboard,
a monitor, and a floppy disk drive.
When it was first released, users had a
$3,000 machine that could play Space
Invaders, run a primitive bookkeep
ing program, or let users program it
in BASIC. The original Apple II did not
even support lowercase letters, mak
ing it worthless for word processing.
The breakthrough came in 1979 with
a new spreadsheet program, VisiCalc.
In a spreadsheet, you enter financial
data and their relationships into a grid
of rows and columns (see the figure).
Then you modify some of the data
and watch in real time how the others
change. For example, you can see how
changing the mix of widgets in a manu
facturing plant might affect estimated
costs and profits. Corporate managers
snapped up VisiCalc and the computer
that was needed to run it. For them, the
computer was a spreadsheet machine.
More importantly, it was a personal
device. The managers were free to do
the calculations that they wanted to
do, not just the ones that the “high
priests” in the data center provided.
Personal computers have been with
us ever since, and countless users
have tinkered with their hardware
and software, sometimes establishing
highly successful companies or creat
ing free software for millions of users.
This “freedom to tinker” is an impor
tant part of personal computing. On
a personal device, you should be able
to install the software that you want to
install to make you more productive
or creative, even if that’s not the same
software that most people use. You
should be able to add peripheral equip
ment of your choice. For the first thirty
years of personal computing, this free
dom was largely taken for granted.
We are now entering an era where
smartphones, tablets, and smart TV
sets are replacing functions that were
traditionally fulfilled by personal com
puters. While it is amazing to carry
more computing power in your cell
phone than in the best personal com
puters of the 1990s, it is disturbing
that we lose a degree of personal con
trol. With some phone or tablet brands,
you can only install those applications
that the manufacturer publishes on
the “app store”. For example, Apple
does not allow children to learn the
Scratch language on the iPad. You’d
think it would be in Apple’s interest to
encourage the next generation to be
enthusiastic about programming, but
they have a general policy of denying
programmability on “their” devices, in
order to thwart competitive environ
ments such as Flash or Java.
When you select a device for mak
ing phone calls or watching movies, it
is worth asking who is in control. Are
you purchasing a personal device that
you can use in any way you choose, or
are you being tethered to a flow of data
that is controlled by somebody else?
Reprinted Courtesy of International Business Machines Corporation, © International Business Machines Corporation.
The Visicalc Spreadsheet Running on an Apple II
Computing & Society 8.1  Personal Computing
Media Bakery.
testing track 8.6 Unit test Frameworks 407
8.6 Unit test Frameworks
Up to now, we have used a very simple approach to testing. We provided tester classes
whose  main method computes values and prints actual and expected values. How-
ever, that approach has limitations. The  main method gets messy if it contains many
tests. And if an exception occurs during one of the tests, the remaining tests are not
executed.
Unit testing frameworks were designed to quickly execute and evaluate test suites
and to make it easy to incrementally add test cases. One of the most popular test-
ing frameworks is JUnit. It is freely available at  http://junit.org , and it is also built
into a number of development environments, including BlueJ and Eclipse. Here we
describe JUnit 4, the most current version of the library as this book is written.
When you use JUnit, you design a companion test class for each class that you
develop. You provide a method for each test case that you want to have executed. You
use “annotations” to mark the test meth ods. An annotation is an advanced Java fea-
ture that places a marker into the code that is interpreted by another tool. In the case
of JUnit, the  @Test annotation is used to mark test methods.
In each test case, you make some computations and then compute some condition
that you believe to be true. You then pass the result to a method that communicates
a test result to the framework, most commonly the  assertEquals method. The  assert
Equals method takes as arguments the expected and actual values and, for floating-
point numbers, a tolerance value.
It is also customary (but not required) that the name of the test class ends in  Test ,
such as  CashRegister Test . Here is a typical example:
importorg.junit.Test;
importorg.junit.Assert;
publicclassCashRegisterTest
{
@TestpublicvoidtwoPurchases()
{
CashRegisterregister=newCashRegister();
register.recordPurchase(0.75);
register.recordPurchase(1.50);
register.receivePayment(2,0,5,0,0);
doubleexpected=0.25;
Assert.assertEquals(expected,register.giveChange(),EPSILON);
}
// More test cases 
...
}
If all test cases pass, the JUnit tool shows a green bar (see Figure 7). If any of the test
cases fail, the JUnit tool shows a red bar and an error message.
Your test class can also have other methods (whose names should not be annotated
with  @Test ). These methods typically carry out steps that you want to share among
test methods.
The JUnit philosophy is simple. Whenever you implement a class, also make a
companion test class. You design the tests as you design the program, one test method
at a time. The test cases just keep accu mulating in the test class. Whenever you have
detected an actual failure, add a test case that flushes it out, so that you can be sure
Unit test frameworks
simplify the task of
writing classes that
contain many
test cases.
the JUnit philosophy
is to run all tests
whenever you
change your code.
408 Chapter 8 Designing Classes testing track
figure 7 
Unit testing with JUnit
that you won’t introduce that particular bug again. Whenever you modify your class,
simply run the tests again.
If all tests pass, the user interface shows a green bar and you can relax. Otherwise,
there is a red bar, but that’s also good. It is much easier to fix a bug in isolation than
inside a complex program.
24.  Provide a JUnit test class with one test case for the  Earthquake class in Chapter 5.
25.  What is the significance of the  EPSILON argument in the  assertEquals method?
Practice it  Now you can try these exercises at the end of the chapter: R8.27, E8.17, E8.18.
find classes that are appropriate for solving a programming problem.
• A class should represent a single concept from a problem domain, such as busi-
ness, science, or mathematics.
Design methods that are cohesive, consistent, and minimize side effects.
• The public interface of a class is cohesive if all of its features
are related to the concept that the class represents.
• A class depends on another class if its methods use that
class in any way.
• An immutable class has no mutator methods.
• References to objects of an immutable class can be safely shared.
• A side effect of a method is any externally observable data modification.
• When designing methods, minimize side effects.
• In Java, a method can never change the contents of a variable that is passed to
a method.
• In Java, a method can change the state of an object reference argument, but it can-
not replace the object reference with another.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
C h a p t e r s U M M a rY
© Sergey Ivanov/iStockphoto.
AP Photo/Frank Franklin II.
Review Questions 409
Use patterns to design the data representation of an object.
• An instance variable for the total is updated in methods that increase or decrease
the total amount.
• A counter that counts events is incremented in methods that correspond to
the events.
• An object can collect other objects in an array or array list.
• An object property can be accessed” with a getter method and changed with a
setter method.
• If your object can have one of several states that affect the
behavior, supply an instance variable for the current state.
• To model a moving object, you need to store and update its
position.
Understand the behavior of static variables and static methods.
• A static variable belongs to the class, not to any object of the class.
• A static method is not invoked on an object.
Use packages to organize sets of related classes.
• A package is a set of related classes.
• The  import directive lets you refer to a class of a package by
its class name, without the package prefix.
• Use a domain name in reverse to construct an unambigu-
ous package name.
• The path of a class file must match its package name.
• An instance variable or method that is not declared as  public or  private can be
accessed by all classes in the same package, which is usually not desirable.
Use JUnit for writing unit tests.
• Unit test frameworks simplify the task of writing classes that contain many
test cases.
• The JUnit philosophy is to run all tests whenever you change your code.
•• R8.1  Your task is to write a program that simulates a vending machine. Users select a
product and provide payment. If the payment is sufficient to cover the purchase
price of the product, the product is dispensed and change is given. Otherwise, the
payment is returned to the user. Name an appropriate class for implementing this
program. Name two classes that would not be appropriate and explain why.
•• R8.2  Your task is to write a program that reads a customer’s name and address, followed
by a sequence of purchased items and their prices, and prints an invoice.
© paul prescott/iStockphoto.
© John Alexander/iStockphoto.
© Diane Diederich/iStockphoto.
© Don Wilkie/iStockphoto.
R e v i e w Q u e s t i o n s
410 Chapter 8 Designing Classes
Discuss which of the following would be good classes for implementing this
program:
a.  Invoice
b.  InvoicePrinter
c.  PrintInvoice
d.  InvoiceProgram
••• r8.3  Your task is to write a program that computes paychecks. Employees are paid an
hourly rate for each hour worked; how ever, if they worked more than 40 hours per
week, they are paid at 150 percent of the regular rate for those overtime hours. Name
an actor class that would be appropriate for implementing this program. Then name
a class that isn’t an actor class that would be an appropriate alternative. How does
the choice between these alternatives affect the program structure?
•• r8.4  Look at the public interface of the  java.lang.System class and discuss whether or not it
is cohesive.
•• r8.5  Suppose an  Invoice object contains descriptions of the products ordered, and the
billing and shipping addresses of the customer. Draw a UML diagram showing the
dependencies between the classes  Invoice ,  Address ,  Customer , and  Product .
•• r8.6  Suppose a vending machine contains products, and users insert coins into the vend-
ing machine to purchase products. Draw a UML diagram showing the dependen cies
between the classes  VendingMachine ,  Coin , and  Product .
•• r8.7  On which classes does the class  Integer in the standard library depend?
•• r8.8  On which classes does the class  Rectangle in the standard library depend?
• r8.9  Classify the methods of the class  Scanner that are used in this book as accessors
and mutators.
• r8.10  Classify the methods of the class  Rectangle as accessors and mutators.
• r8.11  Is the  Resistor class in Exercise P8.8 a mutable or immutable class? Why?
• r8.12  Which of the following classes are immutable?
a.  Rectangle
b.  String
c.  Random
• r8.13  Which of the following classes are immutable?
a.  PrintStream
b.  Date
c.  Integer
r8.14  Consider a method
publicclassDataSet
{
/**
 Reads all numbers from a scanner and adds them to this data set.
@paramin a  Scanner
*/
publicvoidread(Scannerin){...}
...
review Questions 411
}
Describe the side effects of the  read method. Which of them are not recommended,
according to Section 8.2.4? Which redesign eliminates the unwanted side effect?
What is the effect of the redesign on coupling?
•• r8.15  What side effect, if any, do the following three methods have?
publicclassCoin
{
...
publicvoidprint()
{
System.out.println(name+""+value);
}
publicvoidprint(PrintStreamstream)
{
stream.println(name+""+value);
}
publicStringtoString()
{
returnname+""+value;
}
}
••• r8.16  Ideally, a method should have no side effects. Can you write a program in which no
method has a side effect? Would such a program be useful?
•• r8.17  Consider the following method that is intended to swap the values of two integers:
publicstaticvoidfalseSwap(inta,intb)
{
inttemp=a;
a=b;
b=temp;
}
publicstaticvoidmain(String[]args)
{
intx=3;
inty=4;
falseSwap(x,y);
System.out.println(x+""+y);
}
Why doesn’t the method swap the contents of  x and  y ?
••• r8.18  How can you write a method that swaps two floating-point numbers?
Hint:  java.awt.Point .
•• r8.19  Draw a memory diagram that shows why the following method can’t swap two
BankAccount objects:
publicstaticvoidfalseSwap(BankAccounta,BankAccountb)
{
BankAccounttemp=a;
a=b;
b=temp;
}
412 Chapter 8 Designing Classes
• r8.20  Consider an enhancement of the  Die class of Chapter 6 with a static variable
publicclassDie
{
privateintsides;
privatestaticRandomgenerator=newRandom();
publicDie(ints){...}
publicintcast(){...}
}
Draw a memory diagram that shows three dice:
Died4=newDie(4);
Died6=newDie(6);
Died8=newDie(8);
Be sure to indicate the values of the  sides and  generator variables.
• r8.21  Try compiling the following program. Explain the error message that you get.
publicclassPrint13
{
publicvoidprint(intx)
{
System.out.println(x);
}
publicstaticvoidmain(String[]args)
{
intn=13;
print(n);
}
}
• r8.22  Look at the methods in the  Integer class. Which are static? Why?
•• r8.23  Look at the methods in the  String class (but ignore the ones that take an argument of
type  char[] ). Which are static? Why?
•• r8.24  The  in and  out variables of the  System class are public static variables of the  System
class. Is that good design? If not, how could you improve on it?
•• r8.25  Every Java program can be rewritten to avoid  import statements. Explain how, and
rewrite  RectangleComponent.java from Section 2.9.3 to avoid  import statements.
• r8.26  What is the default package? Have you used it before this chapter in your
programming?
•• testing r8.27  What does JUnit do when a test method throws an exception? Try it out and report
your findings.
•• e8.1  Implement the  Coin class described in Section 8.2. Modify the  CashRegister class so
that coins can be added to the cash register, by supplying a method
voidreceivePayment(intcoinCount,CoincoinType)
The caller needs to invoke this method multiple times, once for each type of coin
that is present in the payment.
p r a C t i C e e x e r C i s e s
practice exercises 413
•• e8.2  Modify the  giveChange method of the  CashRegister class so that it returns the number
of coins of a particular type to return:
intgiveChange(CoincoinType)
The caller needs to invoke this method for each coin type, in decreasing value.
• e8.3  Real cash registers can handle both bills and coins. Design a single class that
expresses the commonality of these concepts. Redesign the  CashRegister class and
provide a method for entering payments that are described by your class. Your pri-
mary challenge is to come up with a good name for this class.
e8.4  Reimplement the  BankAccount class so that it is immutable. The  deposit and  withdraw
methods need to return new  BankAccount objects with the appropriate balance.
•• e8.5  Write static methods
•  publicstaticdoublecubeVolume(doubleh)
•  publicstaticdoublecubeSurface(doubleh)
•  publicstaticdoublesphereVolume(doubler)
•  publicstaticdoublesphereSurface(doubler)
•  publicstaticdoublecylinderVolume(doubler,doubleh)
•  publicstaticdoublecylinderSurface(doubler,doubleh)
•  publicstaticdoubleconeVolume(doubler,doubleh)
•  publicstaticdoubleconeSurface(doubler,doubleh)
that compute the volume and surface area of a cube with height  h , sphere with
radius  r , a cylinder with circular base with radius  r and height  h , and a cone with
circular base with radius  r and height  h . Place them into a class  Geom etry . Then write a
program that prompts the user for the values of  r and  h , calls the six methods, and
prints the results.
•• e8.6  Solve Exercise E8.5 by implementing classes  Cube ,  Sphere ,  Cylinder , and  Cone . Which
approach is more object-oriented?
••• e8.7  Modify the application of How To 7.1 so that it can deal with multiple students.
First, ask the user for all student names. Then read in the scores for all quizzes,
prompting for the score of each student. Finally, print the names of all stu dents and
their final scores. Use a single class and only static methods.
••• e8.8  Repeat Exercise E8.7, using multiple classes. Provide a  GradeBook class that col lects
objects of type  Student .
••• e8.9  Write methods
publicstaticdoubleperimeter(Ellipse2D.Doublee);
publicstaticdoublearea(Ellipse2D.Doublee);
that compute the area and the perimeter of the ellipse  e . Add these methods to a class
Geometry . The chal lenging part of this assignment is to find and implement an accurate
formula for the perimeter. Why does it make sense to use a static method in this case?
•• e8.10  Write methods
publicstaticdoubleangle(Point2D.Doublep,Point2D.Doubleq)
publicstaticdoubleslope(Point2D.Doublep,Point2D.Doubleq)
© DNY59/iStockphoto.
414 Chapter 8 Designing Classes
that compute the angle between the x-axis and the line joining two points, measured
in degrees, and the slope of that line. Add the methods to the class  Geometry . Supply
suitable preconditions. Why does it make sense to use a static method in this case?
••• e8.11  Write methods
publicstaticbooleanisInside(Point2D.Doublep,Ellipse2D.Doublee)
publicstaticbooleanisOnBoundary(Point2D.Doublep,Ellipse2D.Doublee)
that test whether a point is inside or on the boundary of an ellipse. Add the methods
to the class  Geometry .
• e8.12  Write a method
publicstaticintreadInt(
Scannerin,Stringprompt,Stringerror,intmin,intmax)
that displays the prompt string, reads an integer, and tests whether it is between the
minimum and maxi mum. If not, print an error message and repeat reading the input.
Add the method to a class  Input .
•• e8.13  Consider the following algorithm for computing x n for an integer n. If n < 0, x n is
1/x –n . If n is positive and even, then x n = (x n/2 ) 2 . If n is positive and odd, then
x n = x n–1 × x. Implement a static method  doubleintPower(doublex,intn) that uses
this algorithm. Add it to a class called  Numeric .
•• e8.14  Improve the  Die class of Chapter 6. Turn the  generator variable into a static variable
so that all needles share a single random number generator.
•• e8.15  Implement  Coin and  CashRegister classes as described in Exercise E8.1. Place
the classes into a package called  money . Keep the  CashRegisterTester class in the
default package.
• e8.16  Place a  BankAccount class in a package whose name is derived from your e-mail
address, as described in Section 8.5. Keep the  BankAccountTester class in the
default package.
•• testing e8.17  Provide a JUnit test class  StudentTest with three test methods, each of which tests a
dif ferent method of the  Student class in How To 7.1.
•• testing e8.18  Provide JUnit test class  TaxReturnTest with three test methods that test different tax
situations for the  TaxReturn class in Chapter 5.
• graphics e8.19  Write methods
•  publicstaticvoiddrawH(Graphics2Dg2,Point2D.Doublep);
•  publicstaticvoiddrawE(Graphics2Dg2,Point2D.Doublep);
•  publicstaticvoiddrawL(Graphics2Dg2,Point2D.Doublep);
•  publicstaticvoiddrawO(Graphics2Dg2,Point2D.Doublep);
that show the letters H, E, L, O in the graphics window, where the point  p is the
top-left corner of the letter. Then call the methods to draw the words “HELLO”
and “HOLE” on the graphics display. Draw lines and ellipses. Do not use the
drawString method. Do not use  System.out .
•• graphics e8.20  Repeat Exercise E8.19 by designing classes  LetterH ,  LetterE ,  LetterL , and  LetterO , each
with a constructor that takes a  Point2D.Double parameter (the top-left corner) and a
method  draw(Graphics2Dg2) .Which solution is more object-oriented?
programming projects 415
e8.21  Add a method  ArrayList<Double>getStatement() to the  BankAccount class that returns a
list of all deposits and withdrawals as positive or negative values. Also add a method
voidclearStatement() that resets the statement.
e8.22  Implement a class  LoginForm that simulates a login form that you find on many web
pages. Supply methods
publicvoidinput(Stringtext)
publicvoidclick(Stringbutton)
publicbooleanloggedIn()
The first input is the user name, the second input is the password. The  click method
can be called with arguments  "Submit" and  "Reset" . Once a user has been successfully
logged in, by supplying the user name, password, and clicking on the submit button,
the  loggedIn method returns true and further input has no effect. When a user tries to
log in with an invalid user name and password, the form is reset.
Supply a constructor with the expected user name and password.
e8.23  Implement a class  Robot that simulates a robot wandering on an infinite plane. The
robot is located at a point with integer coordinates and faces north, east, south, or
west. Supply methods
publicvoidturnLeft()
publicvoidturnRight()
publicvoidmove()
publicPointgetLocation()
publicStringgetDirection()
The  turnLeft and  turnRight methods change the direction but not the location. The
move method moves the robot by one unit in the direction it is facing. The  get
Direction method returns a string  "N" ,  "E" ,  "S" , or  "W" .
••• P8.1  Declare a class  ComboLock that works like the combination lock in a gym locker, as
shown here. The lock is constructed with a combina tion—three
numbers between 0 and 39. The  reset method resets the dial so
that it points to 0. The  turnLeft and  turnRight methods turn the dial
by a given number of ticks to the left or right. The  open method
attempts to open the lock. The lock opens if the user first turned it
right to the first number in the combination, then left to the
second, and then right to the third.
publicclassComboLock
{
...
publicComboLock(intsecret1,intsecret2,intsecret3){...}
publicvoidreset(){...}
publicvoidturnLeft(intticks){...}
publicvoidturnRight(intticks){...}
publicbooleanopen(){...}
}
••• business P8.2  Implement a program that prints paychecks for a group of student assistants. Deduct
federal and Social Security taxes. (You may want to use the tax computation used
p r o g r a M M i n g p r o J e C t s
© pixhook/iStockphoto.
416 Chapter 8 Designing Classes
in Chapter 5. Find out about Social Security taxes on the Internet.) Your program
should prompt for the names, hourly wages, and hours worked of each student.
••• P8.3  For faster sorting of letters, the United States Postal Service encourages companies
that send large volumes of mail to use a bar code denoting the ZIP code (see
Figure 8).
The encoding scheme for a five-digit ZIP
code is shown in Figure 8. There are
full-height frame bars on each side. The five
encoded digits are followed by a check digit,
which is com puted as follows: Add up all
digits, and choose the check digit to make the
sum a multiple of 10. For example, the sum of
the digits in the ZIP code 95014 is 19, so the
check digit is 1 to make the sum equal to 20.
Each digit of the ZIP code, and the check
digit, is encoded accord ing to the table at
right, where 0 denotes a half bar and 1 a full
bar. Note that they represent all combina-
tions of two full and three half bars. The digit
can be computed easily from the bar code
using the column weights 7, 4, 2, 1, 0. For
example, 01100 is
0 × 7 + 1 × 4 + 1 × 2 + 0 × 1 + 0 × 0 = 6
The only exception is 0, which would yield
11 according to the weight formula.
Write a program that asks the user for a ZIP code and prints the bar code. Use  : for
half bars,  | for full bars. For example, 95014 becomes
||:|:::|:|:||::::::||:|::|:::|||
(Alternatively, write a graphical application that draws real bars.)
Your program should also be able to carry out the opposite con version: Translate
bars into their ZIP code, reporting any errors in the input format or a mismatch of
the digits.
•• business P8.4  Design a  Customer class to handle a customer loyalty marketing campaign. After
accumulating $100 in purchases, the customer receives a $10 discount on the next
purchase.
Digit
Weight
7 4 2 1 0
1 0 0 0 1 1
2 0 0 1 0 1
3 0 0 1 1 1
4 0 1 0 0 0
5 0 1 0 1 1
6 0 1 1 0 0
7 1 0 0 0 0
8 1 0 0 1 1
9 1 0 1 0 0
0 1 1 0 0 0
figure 8  a postal Bar Code
***************
ECRLOT
**
CO57
CODE C671RTS2
JOHN DOE CO57
1009 FRANKLIN BLVD
SUNNYVALE CA 95014 – 5143
figure 9  encoding for FiveDigit Bar Codes
Frame bars
Digit 1 Digit 2 Digit 3 Digit 4 Digit 5 Check
Digit
programming projects 417
Provide methods
•  voidmakePurchase(doubleamount)
•  booleandiscountReached()
Provide a test program and test a scenario in which a customer has earned a discount
and then made over $90, but less than $100 in purchases. This should not result in a
second discount. Then add another purchase that results in the second discount.
••• business P8.5  The Downtown Marketing Association wants
to promote downtown shopping with a loyalty
program similar to the one in Exercise P8.4.
Shops are identified by a number between 1 and
20. Add a new parameter variable to the  makePur
chase method that indicates the shop. The dis-
count is awarded if a customer makes purchases
in at least three different shops, spending a total
of $100 or more.
••• science P8.6  Design a class  Cannonball to model a cannonball that is fired into the air. A ball has
• An x- and a y-position.
• An x- and a y-velocity.
Supply the following methods:
• A constructor with an x-position (the y-position is initially 0)
• A method  move(doubledeltaSec) that moves the ball to the next position First
compute the distance trav eled in  deltaSec seconds, using the current veloci-
ties, then update the x- and y-positions; then update the y-velocity by taking
into account the gravitational acceleration of –9.81 m/s 2 ; the x-velocity is
unchanged.
• A method  Point getLocation() that gets the current location of the cannonball,
rounded to integer coordinates
• A method  ArrayList<Point> shoot(doublealpha,doublev,doubledeltaSec) whose
arguments are the angle  a and initial velocity v (Compute the x-velocity as v
cos  a and the y-velocity as v sin  a ; then keep calling  move with the given time
interval until the y-position is 0; return an array list of locations after each call
to move.
Use this class in a program that prompts the user for the starting angle and the initial
velocity. Then call  shoot and print the locations.
• graphics P8.7  Continue Exercise P8.6, and draw the trajectory of the cannonball.
•• science P8.8  The colored bands on the top-most
resistor shown in the photo at right
indicate a resistance of 6.2 kΩ ±5 per-
cent. The resistor tolerance of ±5 per-
cent indicates the acceptable variation
in the resistance. A 6.2 kΩ ±5 percent
resistor could have a resistance as small
as 5.89 kΩ or as large as 6.51 kΩ. We say
that 6.2 kΩ is the nominal value of the
© ThreeJays/iStockphoto.
© Maria Toutoudaki/iStockphoto.
418 Chapter 8 Designing Classes
resistance and that the actual value of the resistance can be any value between 5.89
kΩ and 6.51 kΩ.
Write a program that represents a resistor as a class. Provide a single constructor that
accepts values for the nominal resistance and tolerance and then determines the
actual value randomly. The class should provide public methods to get the nominal
resistance, tolerance, and the actual resistance.
Write a  main method for the program that demonstrates that the class works properly
by displaying actual resistances for ten 330 Ω ±10 percent resistors.
•• science P8.9  In the  Resistor class from Exercise P8.8, supply a method
that returns a description of the “color bands” for the
resistance and tolerance. A resistor has four color bands:
• The first band is the first significant digit of the
resistance value.
• The second band is the second significant digit of the resistance value.
• The third band is the decimal multiplier.
• The fourth band indicates the tolerance.
Color Digit Multiplier tolerance
Black 0 ×10 0 —
Brown 1 ×10 1 ±1%
Red 2 ×10 2 ±2%
Orange 3 ×10 3 —
Yellow 4 ×10 4 —
Green 5 ×10 5 ±0.5%
Blue 6 ×10 6 ±0.25%
Violet 7 ×10 7 ±0.1%
Gray 8 ×10 8 ±0.05%
White 9 ×10 9 —
Gold — ×10 –1 ±5%
Silver — ×10 –2  ±10%
None — — ±20%
For example (using the values from the table as a key), a resistor with red, violet,
green, and gold bands (left to right) will have 2 as the first digit, 7 as the second digit,
a multiplier of 10 5 , and a tolerance of ±5 percent, for a resistance of 2,700 kΩ, plus or
minus 5 percent.
•• science P8.10  The figure below shows a frequently used electric circuit called a “voltage divider”.
The input to the circuit is the voltage v i . The output is the voltage v o . The output of
First band
Second band Multiplier
Tolerance
answers to selfCheck Questions 419
a voltage divider is proportional to the input, and the constant of proportionality is
called the “gain” of the circuit. The voltage divider is represented by the equation
G
v
v
R
R R
o
i
= =
+
2
1 2
where G is the gain and R 1 and R 2 are the resistances of the two resistors that com-
prise the voltage divider.
+
–
v i
R 1
v o
+
–
R 2
Manufacturing variations cause the actual resistance values to deviate from the
nominal values, as described in Exercise P8.8. In turn, variations in the resistance val-
ues cause variations in the values of the gain of the voltage divider. We calculate the
nominal value of the gain using the nominal resistance values and the actual value of
the gain using actual resistance values.
Write a program that contains two classes,  VoltageDivider and  Resistor . The  Resistor
class is described in Exercise P8.8. The  VoltageDivider class should have two instance
variables that are objects of the  Resistor class. Provide a single constructor that
accepts two  Resistor objects, nominal values for their resistances, and the resistor
tolerance. The class should provide public methods to get the nominal and actual
values of the voltage divider’s gain.
Write a  main method for the program that demonstrates that the class works properly
by displaying nominal and actual gain for ten voltage dividers each consisting of 5
percent resistors having nominal values R 1 = 250 Ω and R 2 = 750 Ω.
a n s W e rs t o s e l F  C h e C K Q U e s t i o n s
1.  Look for nouns in the problem description.
2.  Yes ( ChessBoard ) and no ( MovePiece ).
3.  Some of its features deal with payments, others
with coin values.
4.  None of the coin operations require the
CashRegister class.
5.  If a class doesn’t depend on another, it is not
affected by interface changes in the other class.
6.  It is an accessor—calling  substring doesn’t
modify the string on which the method is
invoked. In fact, all methods of the  String class
are accessors.
7.  No— translate is a mutator method.
8.  It is a side effect; this kind of side effect is com-
mon in object-oriented programming.
9.  Yes—the method affects the state of the  Scanner
argument.
10.  It needs to be incremented in the  deposit and
withdraw methods. There also needs to be some
method to reset it after the end of a statement
period.
11.  The  giveChange method is a mutator that returns
a value that cannot be determined any other
way. Here is a better design. The  receivePayment
method could decrease the  purchase instance
variable. Then the program user would call
receivePayment , determine the change by calling
getAmountDue , and call the  clear method to reset
the cash register for the next sale.
12.  The  ArrayList<String> instance variable is
private, and the class users cannot acccess it.
420 Chapter 8 Designing Classes
13.  You need to supply an instance variable
that can hold the prices for all purchased
items. This could be an  ArrayList<Double> or
ArrayList<String> , or it could simply be a  String
to which you append lines. The instance vari-
able needs to be updated in the  recordPurchase
method. You also need a method that returns
the receipt.
14.  The tax ID of an employee does not change,
and no setter method should be supplied. The
salary of an employee can change, and both
getter and setter methods should be supplied.
15.  Section 8.2.3 suggests that a setter should
return  void , or perhaps a convenience value
that the user can also determine in some other
way. In this situation, the caller could check
whether  newName is blank, so the change is fine.
16.  It is an example of the “state pattern”
described in Section 8.3.5. The direction is a
state that changes when the bug turns, and it
affects how the bug moves.
17.  System.in and  System.out .
18.  Math.PI
19.  The method needs no data of any object. The
only required input is the  values argument.
20.  Yes, it works. Static methods can access static
variables of the same class. But it is a terrible
idea. As your programming tasks get more
complex, you will want to use objects and
classes to organize your programs.
21.  (a) No; (b) Yes; (c) Yes; (d) No
22.  No—you can use fully qualified names for
all other classes, such as  java.util.Random and
java.awt.Rectangle .
23.  /home/me/cs101/hw1/problem1 or, on Windows,
c:\Users\Me\cs101\hw1\problem1 .
24.  Here is one possible answer.
publicclassEarthquakeTest
{
@TestpublicvoidtestLevel4()
{
Earthquakequake=newEarthquake(4);
Assert.assertEquals(
"Feltbymanypeople,nodestruction",
quake.getDescription());
}
}
25.  It is a tolerance threshold for comparing
floating-point numbers. We want the equality
test to pass if there is a small roundoff error.
9
C h a p t e r
421
© Jason Hosking/Photodisc/Getty Images, Inc.
InherItanCe
to learn about inheritance
to implement subclasses that inherit and override superclass methods
to understand the concept of polymorphism
to be familiar with the common superclass  Object and its methods
C h a p t e r G o a l s
C h a p t e r C o n t e n t s
9.1 InherItance hIerarchIes 422
Programming Tip 9.1: Use a single Class for
Variation in Values, Inheritance for Variation
in Behavior 426
9.2 ImplementIng subclasses 426
Syntax 9.1: subclass Declaration 428
Common Error 9.1: replicating Instance Variables
from the superclass 430
Common Error 9.2: Confusing super- and
subclasses 430
9.3 OverrIdIng methOds 431
Syntax 9.2: Calling a superclass Method 431
Common Error 9.3: accidental overloading 435
Common Error 9.4: Forgetting to Use super
When Invoking a superclass Method 435
Special Topic 9.1: Calling the superclass
Constructor 436
Syntax 9.3: Constructor with
superclass Initializer 436
9.4 pOlymOrphIsm 437
Special Topic 9.2: Dynamic Method lookup and
the Implicit parameter 440
Special Topic 9.3: abstract Classes 441
Special Topic 9.4: Final Methods and Classes 442
Special Topic 9.5: protected access 442
How To 9.1: Developing an Inheritance
hierarchy 443
Worked Example 9.1: Implementing an employee
hierarchy for payroll processing
9.5 Object: the cOsmIc
superclass 448
Syntax 9.4: the instanceof operator 451
Common Error 9.5: Don’t Use type tests 452
Special Topic 9.6: Inheritance and the
tostring Method 453
Special Topic 9.7: Inheritance and the
equals Method 454
Computing & Society 9.1: Who Controls the
Internet? 454
422
Objects from related classes usually share common
behavior. For example, cars, bicycles, and buses all provide
transportation. In this chapter, you will learn how the
notion of inheritance expresses the relationship between
specialized and general classes. By using inheritance, you
will be able to share code between classes and provide
services that can be used by multiple classes.
9.1 Inheritance Hierarchies
In object-oriented design, inheritance is a relationship between a more general class
(called the super class) and a more specialized class (called the subclass). The subclass
inherits data and behavior from the superclass. For example, consider the relation-
ships between different kinds of vehicles depicted in Figure 1.
Every car is a vehicle. Cars share the common traits of all vehicles, such as the abil-
ity to transport peo ple from one place to another. We say that the class  Car inherits
from the class  Vehicle . In this relationship, the  Vehicle class is the superclass and the
Car class is the subclass. In Figure 2, the superclass and subclass are joined with an
arrow that points to the superclass.
When you use inheritance in your programs, you can reuse code instead of dupli-
cating it. This reuse comes in two forms. First, a subclass inherits the methods of the
superclass. For example, if the  Vehicle class has a  drive method, then a subclass  Car
automatically inherits the method. It need not be duplicated.
The second form of reuse is more subtle. You can reuse algorithms that manipulate
Vehicle objects. Because a car is a special kind of vehi cle, we can use a  Car object in
such an algorithm, and it will work correctly. The substitutionprinciple states that
A subclass inherits
data and behavior
from a superclass.
You can always use a
subclass object
in place of a
superclass object.
Figure 1  An Inheritance Hierarchy of Vehicle Classes
© Richard Stouffer/iStockphoto (vehicle); © Ed Hidden/iStockphoto (motorcycle); © YinYang/iStockphoto (car);
© Robert Pernell/iStockphoto (truck); Media Bakery (sedan); Cezary Wojtkowski/Age Fotostock America (SUV).
Vehicle
Motorcycle Car Truck
Sedan SUV
Jason Hosking/Photodisc/Getty Images, Inc.
9.1 Inheritance hierarchies 423
Figure 2 
an Inheritance Diagram
Vehicle
Car
that you can always use a subclass object when a superclass object is expected. For
example, con sider a method that takes an argument of type  Vehicle :
void processVehicle(Vehicle v)
Because  Car is a subclass of  Vehicle , you can call that method with a  Car object:
Car myCar = new Car(. . .);
processVehicle(myCar);
Why provide a method that processes  Vehicle objects instead of  Car objects? That
method is more useful because it can handle any kind of vehicle (including  Truck and
Motorcycle objects).
In this chapter, we will consider a simple hierarchy
of classes. Most likely, you have taken computer-
graded quizzes. A quiz consists of ques tions, and there
are different kinds of questions:
• Fill-in-the-blank
• Choice (single or multiple)
• Numeric (where an approximate answer is ok;
e.g., 1.33 when the actual answer is 4/3)
• Free response
Figure 3 shows an inheritance hierarchy for these question types.
© paul kline/iStockphoto.
We will develop a simple but
flexible quiz-taking program
to illustrate inheritance.
Figure 3 
Inheritance hierarchy
of Question types
Choice
Question
FillIn
Question
Numeric
Question
FreeResponse
Question
MultiChoice
Question
Question
424 Chapter 9 Inheritance
At the root of this hierarchy is the  Question type. A question can display its text,
and it can check whether a given response is a correct answer.
section_1/Question.java
1 /**
2 A question with a text and an answer.
3 */
4 public class Question
5 {
6 private String text;
7 private String answer;
8
9 /**
10 Constructs a question with empty question and answer.
11 */
12 public Question()
13 {
14 text = "";
15 answer = "";
16 }
17
18 /**
19 Sets the question text.
20 @param questionText  the text of this question
21 */
22 public void setText(String questionText)
23 {
24 text = questionText;
25 }
26
27 /**
28 Sets the answer for this question.
29 @param correctResponse  the answer
30 */
31 public void setAnswer(String correctResponse)
32 {
33 answer = correctResponse;
34 }
35
36 /**
37 Checks a given response for correctness.
38 @param response  the response to check
39 @return  true if the response was correct, false otherwise
40 */
41 public boolean checkAnswer(String response)
42 {
43 return response.equals(answer);
44 }
45
46 /**
47 Displays this question.
48 */
49 public void display()
50 {
51 System.out.println(text);
52 }
53 }
9.1 Inheritance hierarchies 425
This  Question class is very basic. It does not handle multiple-choice questions,
numeric questions, and so on. In the following sections, you will see how to form
subclasses of the  Question class.
Here is a simple test program for the  Question class:
section_1/Questiondemo1.java
1 import java.util.Scanner;
2
3 /**
4 This program shows a simple quiz with one question.
5 */
6 public class QuestionDemo1
7 {
8 public static void main(String[] args)
9 {
10 Scanner in = new Scanner(System.in);
11
12 Question q = new Question();
13 q.setText("Who was the inventor of Java?");
14 q.setAnswer("James Gosling");
15
16 q.display();
17 System.out.print("Your answer: ");
18 String response = in.nextLine();
19 System.out.println(q.checkAnswer(response));
20 }
21 }
program run
Who was the inventor of Java?
Your answer: James Gosling
true
1.  Consider classes  Manager and  Employee . Which should be the superclass and which
should be the sub class?
2.  What are the inheritance relationships between classes  BankAccount ,  Checking
Account , and  SavingsAccount ?
3.  What are all the superclasses of the  JFrame class? Consult the Java API documen-
tation or Appendix D.
4.  Consider the method  doSomething(Car c) . List all vehicle classes from Figure 1
whose objects cannot be passed to this method.
5.  Should a class  Quiz inherit from the class  Question ? Why or why not?
practice It  Now you can try these exercises at the end of the chapter: R9.1, R9.7, R9.9.
© Nicholas Homrich/iStockphoto.
s e l F  c h e c k
426 Chapter 9 Inheritance
use a single class for variation in values, Inheritance for
variation in behavior
The purpose of inheritance is to model objects with different behavior. When students first
learn about inheritance, they have a tendency to overuse it, by creating multiple classes even
though the variation could be expressed with a simple instance variable.
Consider a program that tracks the fuel efficiency of a fleet of cars by logging the distance
traveled and the refuel ing amounts. Some cars in the fleet are hybrids. Should you create a sub-
class  HybridCar ? Not in this application. Hybrids don’t behave any differently than other cars
when it comes to driving and refueling. They just have a better fuel efficiency. A single  Car class
with an instance variable
double milesPerGallon;
is entirely sufficient.
However, if you write a program that shows how to repair different kinds of vehicles, then
it makes sense to have a separate class  HybridCar . When it comes to repairs, hybrid cars behave
differently from other cars.
9.2 Implementing subclasses
In this section, you will see how to form a subclass and how a subclass automatically
inherits functional ity from its superclass.
Suppose you want to write a program that handles questions such as the following:
In which country was the inventor of Java born?
1. Australia
2. Canada
3. Denmark
4. United States
You could write a  ChoiceQuestion class from scratch, with methods to set up the ques-
tion, display it, and check the answer. But you don’t have to. Instead, use inheritance
and implement  ChoiceQuestion as a subclass of the  Question class (see Figure 4).
In Java, you form a subclass by specifying what makes the subclass different from
its superclass.
Subclass objects automatically have the instance variables that are declared in the
superclass. You only declare instance variables that are not part of the superclass
objects.
programming tip 9.1
© Eric Isselé/iStockphoto.
a subclass inherits all
methods that it does
not override.
Figure 4 
the  ChoiceQuestion Class is a
subclass of the  Question Class
Question
Choice
Question
9.2 Implementing subclasses 427
Like the manufacturer of a
stretch limo, who starts with a
regular car and modifies it, a
programmer makes a subclass
by modifying another class.
Media Bakery.
The subclass inherits all public methods from the superclass. You declare any
methods that are new to the subclass, and change the implementation of inherited
methods if the inherited behavior is not appro priate. When you supply a new imple-
mentation for an inherited method, you override the method.
A  ChoiceQuestion object differs from a  Question object in three ways:
• Its objects store the various choices for the answer.
• There is a method for adding answer choices.
• The  display method of the  ChoiceQuestion class shows these choices so that the
respondent can choose one of them.
When the  ChoiceQuestion class inherits from the  Question class, it needs to spell out
these three differences:
public class ChoiceQuestion extends Question
{
//  This instance variable is added to the subclass
private ArrayList<String> choices;
//  This method is added to the subclass
public void addChoice(String choice, boolean correct) { . . . }
//  This method overrides a method from the superclass
public void display() { . . . }
}
The reserved word  extends denotes inheritance. Figure 5 shows how the methods and
instance variables are captured in a UML diagram.
a subclass can
override a
superclass method
by providing a new
implementation.
the  extends reserved
word indicates that a
class inherits from a
superclass.
Figure 5 
the  ChoiceQuestion Class adds an Instance
Variable and a Method, and overrides a Method
ChoiceQuestion
choices
addChoice(String, boolean)
display()
Question
text
answer
setText(String)
setAnswer(String)
checkAnswer(String)
display()
428 Chapter 9 Inheritance
syntax 9.1  subclass Declaration
public class  SubclassName extends  SuperclassName
{
instance variables
methods
}
Syntax
public class ChoiceQuestion extends Question
{
private ArrayList<String> choices;
public void addChoice(String choice, boolean correct) { . . . }
public void display() { . . . }
}
Subclass Superclass
The reserved word  extends
denotes inheritance.
Declare methods that are
added to the subclass.
Declare instance variables
that are added to
the subclass.
Declare methods that
the subclass overrides.
Figure 6 shows the layout of a  ChoiceQuestion object. It has the  text and  answer
instance variables that are declared in the  Question superclass, and it adds an additional
instance variable,  choices .
The  addChoice method is specific to the  ChoiceQuestion class. You can only apply it to
ChoiceQuestion objects, not general  Question objects.
In contrast, the  display method is a method that already exists in the superclass.
The subclass overrides this method, so that the choices can be properly displayed.
All other methods of the  Question class are automatically inherited by the  Choice
Question class.
You can call the inherited methods on a subclass object:
choiceQuestion.setAnswer("2");
However, the private instance variables of the superclass are inaccessible. Because
these variables are private data of the superclass, only the superclass has access to
them. The subclass has no more access rights than any other class.
In particular, the  ChoiceQuestion methods cannot directly access the instance vari-
able  answer . These methods must use the public interface of the  Question class to access
its private data, just like every other method.
To illustrate this point, let’s implement the  addChoice method. The method has two
arguments: the choice to be added (which is appended to the list of choices), and a
Boolean value to indicate whether this choice is correct.
Figure 6 
Data layout of a
subclass object
text =
ChoiceQuestion
answer =
choices =
ChoiceQuestion
methods cannot access
these instance variables.
Question portion
9.2 Implementing subclasses 429
For example,
question.addChoice("Canada", true);
The first argument is added to the  choices variable. If the second argument is true, then
the  answer instance variable becomes the number of the current choice. For example, if
choices.size() is 2, then  answer is set to the string  "2" .
public void addChoice(String choice, boolean correct)
{
choices.add(choice);
if (correct)
{
//  Convert  choices.size() to string
String choiceString = "" + choices.size();
setAnswer(choiceString);
}
}
You can’t just access the  answer variable in the superclass. Fortunately, the  Ques
tion class has a  setAnswer method. You can call that method. On which object? The
question that you are currently modifying—that is, the implicit parameter of the
ChoiceQuestion.addChoice method. Remember, if you invoke a method on the implicit
parameter, you don’t have to specify the implicit parameter and can write just the
method name:
setAnswer(choiceString);
If you prefer, you can make it clear that the method is executed on the implicit
parameter:
this.setAnswer(choiceString);
6.  Suppose  q is an object of the class  Question and  cq an object of the class  Choice
Question . Which of the following calls are legal?
a.  q.setAnswer(response)
b.  cq.setAnswer(response)
c.  q.addChoice(choice, true)
d.  cq.addChoice(choice, true)
7.  Suppose the class  Employee is declared as follows:
public class Employee
{
private String name;
private double baseSalary;
public void setName(String newName) { . . . }
public void setBaseSalary(double newSalary) { . . . }
public String getName() { . . . }
public double getSalary() { . . . }
}
Declare a class  Manager that inherits from the class  Employee and adds an instance
variable  bonus for stor ing a salary bonus. Omit constructors and methods.
8.  Which instance variables does the  Manager class from Self Check 7 have?
9.  In the  Manager class, provide the method header (but not the implementation) for
a method that over rides the  getSalary method from the class  Employee .
Full cOde example
Go to  wiley.com/go/
javacode to download
a program that
shows a simple  Car
class extending a
Vehicle class.
© Nicholas Homrich/iStockphoto.
s e l F  c h e c k
430 Chapter 9 Inheritance
10.  Which methods does the  Manager class from Self Check 9 inherit?
practice It  Now you can try these exercises at the end of the chapter: R9.3, E9.6, E9.10.
replicating Instance variables from the superclass
A subclass has no access to the private instance variables of the superclass.
public ChoiceQuestion(String questionText)
{
text = questionText; //  Error—tries to access private superclass variable
}
When faced with a compiler error, beginners commonly “solve” this issue by adding another
instance variable with the same name to the subclass:
public class ChoiceQuestion extends Question
{
private ArrayList<String> choices;
private String text; //  Don’t!
. . .
}
Sure, now the constructor compiles, but it doesn’t set the correct text! Such a  ChoiceQuestion
object has two instance variables, both named  text . The constructor sets one of them, and the
display method displays the other. The correct solution is to access the instance variable of the
superclass through the public interface of the superclass. In our example, the  ChoiceQuestion
constructor should call the  setText method of the  Question class.
text =
ChoiceQuestion
answer =
choices =
text =
Question portion
.
confusing super- and subclasses
If you compare an object of type  ChoiceQuestion with an object of type  Question , you find that
• The reserved word  extends suggests that the  ChoiceQuestion object is an extended version of
a  Question .
• The  ChoiceQuestion object is larger; it has an added instance variable,  choices .
• The  ChoiceQuestion object is more capable; it has an  addChoice method.
It seems a superior object in every way. So why is  ChoiceQuestion called the subclass and
Question the superclass?
The super/sub terminology comes from set theory. Look at the set of all questions. Not all
of them are  ChoiceQues tion objects; some of them are other kinds of questions. Therefore, the
set of  ChoiceQuestion objects is a subset of the set of all  Question objects, and the set of  Question
objects is a superset of the set of  ChoiceQuestion objects. The more specialized objects in the
subset have a richer state and more capabilities.
Common error 9.1
© John Bell/iStockphoto.
Common error 9.2
© John Bell/iStockphoto.
9.3 Overriding Methods  431
9.3 Overriding Methods
The subclass inherits the methods from the superclass. If you are not satisfied with
the behavior of an inherited method, you override it by specifying a new implemen-
tation in the subclass.
Consider the  display method of the  ChoiceQuestion class. It overrides the superclass
display method in order to show the choices for the answer. This method extends the
functionality of the superclass version. This means that the subclass method carries
out the action of the superclass method (in our case, displaying the question text), and
it also does some additional work (in our case, displaying the choices). In other cases,
a subclass method replaces the functionality of a superclass method, implementing an
entirely different behavior.
Let us turn to the implementation of the  display method of the  ChoiceQuestion class.
The method needs to
•  Display the question text.
•  Display the answer choices.
The second part is easy because the answer choices are an instance variable of the
subclass.
public class ChoiceQuestion
{
. . .
public void display()
{
//  Display the question text
. . .
//  Display the answer choices
for (int i = 0; i < choices.size(); i++)
{
int choiceNumber = i + 1;
System.out.println(choiceNumber + ": " + choices.get(i));
}
}
}
But how do you get the question text? You can’t access the  text variable of the super-
class directly because it is private.
An overriding
method can extend
or replace the
functionality of the
superclass method.
Syntax 9.2  Calling a Superclass Method
super. methodName ( parameters ); Syntax
public void deposit(double amount)
{
transactionCount++;
super.deposit(amount);
}
Calls the method
of the superclass
instead of the method
of the current class.
If you omit  super , this method calls itself.
See page 435.
432 Chapter 9 Inheritance
Instead, you can call the  display method of the superclass, by using the reserved
word  super :
public void display()
{
//  Display the question text
super.display(); //  OK
//  Display the answer choices
. . .
}
If you omit the reserved word  super , then the method will not work as intended.
public void display()
{
//  Display the question text
display(); //  Error—invokes  this.display()
. . .
}
Because the implicit parameter  this is of type  ChoiceQuestion , and there is a method
named  display in the  ChoiceQuestion class, that method will be called—but that is just
the method you are currently writing! The method would call itself over and over.
Note that  super , unlike  this , is not a reference to an object. There is no separate
superclass object—the subclass object contains the instance variables of the super-
class. Instead,  super is simply a reserved word that forces execution of the superclass
method.
Here is the complete program that lets you take a quiz consisting of two  Choice
Question objects. We construct both objects and pass them to a method  present Question .
That method displays the question to the user and checks whether the user response
is correct.
section_3/Questiondemo2.java
1 import java.util.Scanner;
2
3 /**
4 This program shows a simple quiz with two choice questions.
5 */
6 public class QuestionDemo2
7 {
8 public static void main(String[] args)
9 {
10 ChoiceQuestion first = new ChoiceQuestion();
11 first.setText("What was the original name of the Java language?");
12 first.addChoice("*7", false);
13 first.addChoice("Duke", false);
14 first.addChoice("Oak", true);
15 first.addChoice("Gosling", false);
16
17 ChoiceQuestion second = new ChoiceQuestion();
18 second.setText("In which country was the inventor of Java born?");
19 second.addChoice("Australia", false);
20 second.addChoice("Canada", true);
21 second.addChoice("Denmark", false);
22 second.addChoice("United States", false);
23
24 presentQuestion(first);
25 presentQuestion(second);
Use the reserved
word  super to call a
superclass method.
Camera: © james steidl/iStockphoto.
Globe: © Alex Slobodkin/iStockphoto.
A N IM AT IO N
Inheritance
9.3 overriding Methods 433
26 }
27
28 /**
29 Presents a question to the user and checks the response.
30 @param q  the question
31 */
32 public static void presentQuestion(ChoiceQuestion q)
33 {
34 q.display();
35 System.out.print("Your answer: ");
36 Scanner in = new Scanner(System.in);
37 String response = in.nextLine();
38 System.out.println(q.checkAnswer(response));
39 }
40 }
section_3/choiceQuestion.java
1 import java.util.ArrayList;
2
3 /**
4 A question with multiple choices.
5 */
6 public class ChoiceQuestion extends Question
7 {
8 private ArrayList<String> choices;
9
10 /**
11 Constructs a choice question with no choices.
12 */
13 public ChoiceQuestion()
14 {
15 choices = new ArrayList<String>();
16 }
17
18 /**
19 Adds an answer choice to this question.
20 @param choice  the choice to add
21 @param correct  true if this is the correct choice, false otherwise
22 */
23 public void addChoice(String choice, boolean correct)
24 {
25 choices.add(choice);
26 if (correct)
27 {
28 //  Convert choices.size()  to string
29 String choiceString = "" + choices.size();
30 setAnswer(choiceString);
31 }
32 }
33
34 public void display()
35 {
36 //  Display the question text
37 super.display();
38 //  Display the answer choices
39 for (int i = 0; i < choices.size(); i++)
40 {
434 Chapter 9 Inheritance
41 int choiceNumber = i + 1;
42 System.out.println(choiceNumber + ": " + choices.get(i));
43 }
44 }
45 }
program run
What was the original name of the Java language?
1: *7
2: Duke
3: Oak
4: Gosling
Your answer: *7
false
In which country was the inventor of Java born?
1: Australia
2: Canada
3: Denmark
4: United States
Your answer:  2
true
11.  What is wrong with the following implementation of the  display method?
public class ChoiceQuestion
{
. . .
public void display()
{
System.out.println(text);
for (int i = 0; i < choices.size(); i++)
{
int choiceNumber = i + 1;
System.out.println(choiceNumber + ": " + choices.get(i));
}
}
}
12.  What is wrong with the following implementation of the  display method?
public class ChoiceQuestion
{
. . .
public void display()
{
this.display();
for (int i = 0; i < choices.size(); i++)
{
int choiceNumber = i + 1;
System.out.println(choiceNumber + ": " + choices.get(i));
}
}
}
13.  Look again at the implementation of the  addChoice method that calls the  setAnswer
method of the superclass. Why don’t you need to call  super. setAnswer ?
14.  In the  Manager class of Self Check 7, override the  getName method so that managers
have a  * before their name (such as  *Lin, Sally ).
© Nicholas Homrich/iStockphoto.
s e l F  c h e c k
9.3 overriding Methods 435
15.  In the  Manager class of Self Check 9, override the  getSalary method so that it re-
turns the sum of the sal ary and the bonus.
practice It  Now you can try these exercises at the end of the chapter: E9.1, E9.2, E9.11.
accidental Overloading
In Java, two methods can have the same name, provided they differ in their parameter types.
For example, the  PrintStream class has methods called  println with headers
void println(int x)
and
void println(String x)
These are different methods, each with its own implementation. The Java compiler considers
them to be completely unrelated. We say that the  println name is overloaded. This is different
from overriding, where a subclass method provides an implementation of a method whose
parameter variables have the same types.
If you mean to override a method but use a parameter variable with a different type, then
you accidentally introduce an over loaded method. For example,
public class ChoiceQuestion extends Question
{
. . .
public void display(PrintStream out)
//  Does not override  void display()
{
. . .
}
}
The compiler will not complain. It thinks that you want to provide a method just for  Print
Stream arguments, while inheriting another method  void display() .
When overriding a method, be sure to check that the types of the parameter variables match
exactly.
Forgetting to use  super  When Invoking a superclass method
A common error in extending the functionality of a superclass method is to forget the reserved
word  super . For example, to compute the salary of a manager, get the salary of the underlying
Employee object and add a bonus:
public class Manager
{
. . .
public double getSalary()
{
double baseSalary = getSalary();
//  Error: should be  super.getSalary()
return baseSalary + bonus;
}
}
Here  getSalary() refers to the  getSalary method applied to the implicit parameter of the
method. The implicit param eter is of type  Manager , and there is a  getSalary method in the
Common error 9.3
© John Bell/iStockphoto.
Common error 9.4
© John Bell/iStockphoto.
436 Chapter 9 Inheritance
Manager class. Calling that method is a recursive call, which will never stop. Instead, you must
tell the compiler to invoke the superclass method.
Whenever you call a superclass method from a subclass method with the same name, be
sure to use the reserved word  super .
calling the superclass constructor
Consider the process of constructing a subclass object. A subclass
constructor can only ini tialize the instance variables of the sub-
class. But the superclass instance variables also need to be initial-
ized. Unless you specify otherwise, the superclass instance variables
are initial ized with the constructor of the superclass that has no
arguments.
In order to specify another constructor, you use the  super reserved
word, together with the arguments of the superclass constructor, as
the first statement of the subclass construc tor.
For example, suppose the  Question superclass had a construc-
tor for setting the question text. Here is how a subclass constructor
could call that superclass constructor:
public ChoiceQuestion(String questionText)
{
super(questionText);
choices = new ArrayList<String>();
}
In our example program, we used the superclass constructor with no
arguments. However, if all superclass constructors have arguments,
you must use the  super syntax and provide the arguments for a super-
class constructor.
When the reserved word  super is followed by a parenthesis, it
indicates a call to the superclass constructor. When used in this way,
the constructor call must be the first state ment of the subclass
constructor. If  super is followed by a period and a method name, on the other hand, it indicates
a call to a superclass method, as you saw in the preceding sec tion. Such a call can be made any-
where in any subclass method.
special topic 9.1
© Eric Isselé/iStockphoto.
Unless specified
otherwise, the
subclass constructor
calls the superclass
constructor with
no arguments.
to call a superclass
constructor, use the
super reserved word
in the first statement
of the subclass
constructor.
the constructor of
a subclass can pass
arguments to a
super class con-
structor, using the
reserved word  super .
syntax 9.3  Constructor with superclass Initializer
public  ClassName ( parameterType parameterName , . . .)
{
super( arguments );
. . .
}
Syntax
public ChoiceQuestion(String questionText)
{
super(questionText);
choices = new ArrayList<String>;
}
The superclass
constructor
is called first.
If you omit the superclass
constructor call, the superclass
constructor with no arguments
is invoked.
The constructor
body can contain
additional statements.
9.4 Polymorphism  437
9.4 Polymorphism
In this section, you will learn how to use inheritance for processing objects of differ-
ent types in the same program.
Consider our first sample program. It presented two  Question objects to the user.
The second sample program presented two  ChoiceQuestion objects. Can we write a
program that shows a mixture of both question types?
With inheritance, this goal is very easy to realize. In order to present a question
to the user, we need not know the exact type of the question. We just display the
question and check whether the user supplied the correct answer. The  Question super-
class has methods for displaying and checking. Therefore, we can simply declare the
parameter variable of the  presentQues tion method to have the type  Question :
publicstaticvoidpresentQuestion(Questionq)
{
q.display();
System.out.print("Youranswer:");
Scannerin=newScanner(System.in);
Stringresponse=in.nextLine();
System.out.println(q.checkAnswer(response));
}
As discussed in Section 9.1, we can substitute a subclass object whenever a superclass
object is expected:
ChoiceQuestionsecond=newChoiceQuestion();
...
presentQuestion(second);// OK to pass a  ChoiceQuestion
When the  presentQuestion method executes, the object references stored in  second and
q refer to the same object of type  ChoiceQuestion (see Figure 7).
However, the variable  q knows less than the full story about the object to which it
refers (see Figure 8).
A subclass reference
can be used when a
superclass reference
is expected.
Figure 7  
Variables of Different 
Types Referring to the 
Same Object
text =
ChoiceQuestion
answer =
choices =
Variable of type
ChoiceQuestion
second =
q =
Variable of type
Question
Figure 8  
A  Question  Reference
Can Refer to an Object 
of Any Subclass 
of  Question
text =
?
answer =
q =
Variable of type
Question
438 Chapter 9 Inheritance
In the same way that vehicles can
differ in their method of locomo tion,
polymorphic objects carry out tasks
in different ways.
© Alpophoto/iStockphoto.
Because  q is a variable of type  Question , you can call the  display and  checkAnswer
methods. You cannot call the  addChoice method, though—it is not a method of the
Question superclass.
This is as it should be. After all, it happens that in this method call,  q refers to a
ChoiceQuestion . In another method call,  q might refer to a plain  Question or an entirely
different subclass of  Question .
Now let’s have a closer look inside the  presentQuestion method. It starts with the
call
q.display(); //  Does it call  Question.display or  ChoiceQuestion.display ?
Which  display method is called? If you look at the program output on page 439, you will
see that the method called depends on the contents of the parameter variable  q . In the
first case,  q refers to a  Question object, so the  Question.display method is called. But in
the second case,  q refers to a  ChoiceQuestion , so the  ChoiceQues tion.display method is
called, showing the list of choices.
In Java, method calls are always determined by the type of the actual object, not the
type of the variable containing the object reference. This is called dynamicmethod
lookup.
Dynamic method lookup allows us to treat objects of different classes in a uniform
way. This feature is called polymorphism. We ask multiple objects to carry out a task,
and each object does so in its own way.
Polymorphism makes programs easily extensible. Suppose we want to have a
new kind of question for calculations, where we are willing to accept an approxi-
mate answer. All we need to do is to declare a new class  NumericQuestion that extends
Question , with its own  check Answer method. Then we can call the  presentQuestion
method with a mixture of plain questions, choice questions, and numeric questions.
The  presentQuestion method need not be changed at all! Thanks to dynamic method
lookup, method calls to the  display and  checkAnswer methods automatically select the
correct method of the newly declared classes.
section_4/Questiondemo3.java
1 import java.util.Scanner;
2
3 /**
4 This program shows a simple quiz with two question types.
5 */
Camera: © james steidl/iStockphoto.
Globe: © Alex Slobodkin/iStockphoto.
A N IM AT IO N
Polymorphism
When the virtual
machine calls an
instance method,
it locates the
method of the
implicit parameter’s
class. this is called
dynamic method
lookup.
polymorphism
(“having multiple
forms”) allows
us to manipulate
objects that share
a set of tasks, even
though the tasks are
executed in
different ways.
9.4 polymorphism 439
6 public class QuestionDemo3
7 {
8 public static void main(String[] args)
9 {
10 Question first = new Question();
11 first.setText("Who was the inventor of Java?");
12 first.setAnswer("James Gosling");
13
14 ChoiceQuestion second = new ChoiceQuestion();
15 second.setText("In which country was the inventor of Java born?");
16 second.addChoice("Australia", false);
17 second.addChoice("Canada", true);
18 second.addChoice("Denmark", false);
19 second.addChoice("United States", false);
20
21 presentQuestion(first);
22 presentQuestion(second);
23 }
24
25 /**
26 Presents a question to the user and checks the response.
27 @param q  the question
28 */
29 public static void presentQuestion(Question q)
30 {
31 q.display();
32 System.out.print("Your answer: ");
33 Scanner in = new Scanner(System.in);
34 String response = in.nextLine();
35 System.out.println(q.checkAnswer(response));
36 }
37 }
program run
Who was the inventor of Java?
Your answer: Bjarne Stroustrup
false
In which country was the inventor of Java born?
1: Australia
2: Canada
3: Denmark
4: United States
Your answer:  2
true
16.  Assuming  SavingsAccount is a subclass of  BankAccount , which of the following code
fragments are valid in Java?
a.  BankAccount account = new SavingsAccount();
b.  SavingsAccount account2 = new BankAccount();
c.  BankAccount account = null;
d.  SavingsAccount account2 = account;
17.  If account  is a variable of type  BankAccount that holds a non- null reference, what
do you know about the object to which  account refers?
© Nicholas Homrich/iStockphoto.
s e l F  c h e c k
440 Chapter 9 Inheritance
18.  Declare an array  quiz that can hold a mixture of  Question and  ChoiceQuestion
objects.
19.  Consider the code fragment
ChoiceQuestion cq = . . .; //  A non-null value
cq.display();
Which actual method is being called?
20.  Is the method call  Math.sqrt(2) resolved through dynamic method lookup?
practice It  Now you can try these exercises at the end of the chapter: R9.6, E9.4, E9.14.
dynamic method lookup and the Implicit parameter
Suppose we add the  presentQuestion method to the  Question class itself:
void presentQuestion()
{
display();
System.out.print("Your answer: ");
Scanner in = new Scanner(System.in);
String response = in.nextLine();
System.out.println(checkAnswer(response));
}
Now consider the call
ChoiceQuestion cq = new ChoiceQuestion();
cq.setText("In which country was the inventor of Java born?");
. . .
cq.presentQuestion();
Which  display and  checkAnswer method will the  presentQuestion method call? If you look inside
the code of the  pre sentQuestion method, you can see that these methods are executed on the
implicit parameter:
public class Question
{
public void presentQuestion()
{
this.display();
System.out.print("Your answer: ");
Scanner in = new Scanner(System.in);
String response = in.nextLine();
System.out.println(this.checkAnswer(response));
}
}
The implicit parameter  this in our call is a reference to an object of type  ChoiceQuestion .
Because of dynamic method lookup, the  ChoiceQuestion versions of the  display and  check Answer
methods are called automatically. This happens even though the  presentQuestion method is
declared in the  Question class, which has no knowledge of the  ChoiceQues tion class.
As you can see, polymorphism is a very powerful mechanism. The  Question class supplies
a  presentQuestion method that specifies the common nature of presenting a question, namely
to display it and check the response. How the displaying and checking are carried out is left to
the subclasses.
special topic 9.2
© Eric Isselé/iStockphoto.
9.4 polymorphism 441
abstract classes
When you extend an existing class, you have the choice whether or not to override the meth-
ods of the superclass. Sometimes, it is desirable to force programmers to override a method.
That happens when there is no good default for the superclass, and only the subclass program-
mer can know how to implement the method properly.
Here is an example: Suppose the First National Bank of Java decides that every account
type must have some monthly fees. Therefore, a  deductFees method should be added to the
Account class:
public class Account
{
public void deductFees() { . . . }
. . .
}
But what should this method do? Of course, we could have the method do nothing. But then
a programmer imple menting a new subclass might simply forget to implement the  deductFees
method, and the new account would inherit the do-nothing method of the superclass. There is
a better way—declare the  deductFees method as an abstractmethod:
public abstract void deductFees();
An abstract method has no implementation. This forces the implementors of subclasses to
specify concrete implementations of this method. (Of course, some subclasses might decide
to implement a do-nothing method, but then that is their choice—not a silently inherited
default.)
You cannot construct objects of classes with abstract methods. For example, once the
Account class has an abstract method, the compiler will flag an attempt to create a  new Account()
as an error.
A class for which you cannot create objects is called an abstractclass. A class for which you
can create objects is sometimes called a concreteclass. In Java, you must declare all abstract
classes with the reserved word  abstract :
public abstract class Account
{
public abstract void deductFees();
. . .
}
public class SavingsAccount extends Account //  Not abstract
{
. . .
public void deductFees() //  Provides an implementation
{
. . .
}
}
If a class extends an abstract class without providing an implementation of all abstract meth-
ods, it too is abstract.
public abstract class BusinessAccount
{
//  No implementation of  deductFees
}
Note that you cannot construct an object of an abstract class, but you can still have an object
reference whose type is an abstract class. Of course, the actual object to which it refers must be
an instance of a concrete subclass:
special topic 9.3
© Eric Isselé/iStockphoto.
442 Chapter 9 Inheritance
Account anAccount; //  OK
anAccount = new Account(); //  Error— Account is abstract
anAccount = new SavingsAccount(); //  OK
anAccount = null; //  OK
When you declare a method as abstract, you force programmers to provide implementations
in subclasses. This is better than coming up with a default that might be inherited accidentally.
Final methods and classes
In Special Topic 9.3 you saw how you can force other programmers to create subclasses of
abstract classes and override abstract methods. Occasionally, you may want to do the opposite
and prevent other programmers from creating subclasses or from overriding certain methods.
In these situations, you use the  final reserved word. For example, the  String class in the stan-
dard Java library has been declared as
public final class String { . . . }
That means that nobody can extend the  String class. When you have a reference of type  String ,
it must contain a  String object, never an object of a subclass.
You can also declare individual methods as final:
public class SecureAccount extends BankAccount
{
. . .
public final boolean checkPassword(String password)
{
. . .
}
}
This way, nobody can override the  checkPassword method with another method that simply
returns  true .
protected access
We ran into a hurdle when trying to implement the  display method of the  ChoiceQuestion class.
That method wanted to access the instance variable  text of the superclass. Our remedy was to
use the appropriate method of the superclass to display the text.
Java offers another solution to this problem. The superclass can declare an instance variable
as protected:
public class Question
{
protected String text;
. . .
}
Protected data in an object can be accessed by the methods of the object’s class and all its
subclasses. For example,  ChoiceQuestion inherits from  Question , so its methods can access the
protected instance variables of the  Question superclass.
Some programmers like the  protected access feature because it seems to strike a balance
between absolute protection (making instance variables private) and no protection at all (mak-
ing instance variables public). However, experience has shown that protected instance vari-
ables are subject to the same kinds of problems as public instance variables. The designer of
the superclass has no control over the authors of subclasses. Any of the subclass methods can
corrupt the superclass data. Furthermore, classes with protected variables are hard to modify.
special topic 9.4
© Eric Isselé/iStockphoto.
special topic 9.5
© Eric Isselé/iStockphoto.
9.4 polymorphism 443
Even if the author of the superclass would like to change the data implementation, the pro-
tected variables cannot be changed, because someone somewhere out there might have writ ten
a subclass whose code depends on them.
In Java, protected variables have another drawback—they are accessible not just by sub-
classes, but also by other classes in the same package (see Special Topic 8.4).
It is best to leave all data private. If you want to grant access to the data to subclass methods
only, consider mak ing the accessor method protected.
step 1  List the classes that are part of the hierarchy.
In our case, the problem description yields two classes:  SavingsAccount and  Checking Account .
Of course, you could implement each of them separately. But that would not be a good idea
because the classes would have to repeat com mon functionality, such as updating an account
balance. We need another class that can be responsible for that com mon functionality. The
problem statement does not explicitly mention such a class. Therefore, we need to discover
it. Of course, in this case, the solution is simple. Savings accounts and checking accounts are
special cases of a bank account. Therefore, we will introduce a common superclass  BankAccount .
step 2  Organize the classes into an inheritance hierarchy.
Draw an inheritance diagram that shows super- and subclasses. Here is one for our example:
Savings
Account
Checking
Account
BankAccount
© Steve Simzer/iStockphoto.
hoW to 9.1  developing an Inheritance hierarchy
When you work with a set of classes, some of which are more general and others more spe-
cialized, you want to organize them into an inheritance hierarchy. This enables you to pro cess
objects of different classes in a uniform way.
As an example, we will consider a bank that offers customers the following account types:
• A savings account that earns interest. The interest compounds monthly and is computed on the min imum
monthly balance.
• A checking account that has no interest, gives you three free withdrawals per month, and charges a $1
transaction fee for each additional withdrawal.
problem statement  Design and implement a program that will manage a set of accounts
of both types. It should be structured so that other account types can be added without affect-
ing the main processing loop. Supply a menu
D)eposit W)ithdraw M)onth end Q)uit
For deposits and withdrawals, query the account number and amount. Print the balance of the
account after each transaction.
In the “Month end” command, accumulate interest or clear the transaction counter,
depending on the type of the bank account. Then print the balance of all accounts.
444 Chapter 9 Inheritance
step 3  Determine the common responsibilities.
In Step 2, you will have identified a class at the base of the hierarchy. That class needs to have
sufficient responsibili ties to carry out the tasks at hand. To find out what those tasks are, write
pseudocode for processing the objects.
For each user command
If it is a deposit or withdrawal
Deposit or withdraw the amount from the specified account.
Print the balance.
If it is month end processing
For each account
Call month end processing.
Print the balance.
From the pseudocode, we obtain the following list of common responsibilities that every bank
account must carry out:
Deposit money.
Withdraw money.
Get the balance.
Carry out month end processing.
step 4  Decide which methods are overridden in subclasses.
For each subclass and each of the common responsibilities, decide whether the behavior can be
inherited or whether it needs to be overridden. Be sure to declare any methods that are inher-
ited or overridden in the root of the hierar chy.
public class BankAccount
{
. . .
/**
Makes a deposit into this account.
@param amount the amount of the deposit
*/
public void deposit(double amount) { . . . }
/**
Makes a withdrawal from this account, or charges a penalty if
sufficient funds are not available.
@param amount  the amount of the withdrawal
*/
public void withdraw(double amount) { . . . }
/**
Carries out the end of month processing that is appropriate
for this account.
*/
public void monthEnd() { . . . }
/**
Gets the current balance of this bank account.
@return  the current balance
*/
public double getBalance() { . . . }
}
The  SavingsAccount and  CheckingAccount classes will both override the  monthEnd method. The
Savings Account class must also override the  withdraw method to track the minimum balance.
The  Checking Account class must update a transaction count in the  withdraw method.
9.4 polymorphism 445
step 5  Declare the public interface of each subclass.
Typically, subclasses have responsibilities other than those of the superclass. List those, as well
as the methods that need to be overridden. You also need to specify how the objects of the
subclasses should be constructed.
In this example, we need a way of setting the interest rate for the savings account. In addi-
tion, we need to specify constructors and overridden methods.
public class SavingsAccount extends BankAccount
{
. . .
/**
Constructs a savings account with a zero balance.
*/
public SavingsAccount() { . . . }
/**
Sets the interest rate for this account.
@param rate  the monthly interest rate in percent
*/
public void setInterestRate(double rate) { . . . }
//  These methods override superclass methods
public void withdraw(double amount) { . . . }
public void monthEnd() { . . . }
}
public class CheckingAccount extends BankAccount
{
. . .
/**
Constructs a checking account with a zero balance.
*/
public CheckingAccount() { . . . }
//  These methods override superclass methods
public void withdraw(double amount) { . . . }
public void monthEnd() { . . . }
}
step 6  Identify instance variables.
List the instance variables for each class. If you find an instance variable that is common to all
classes, be sure to place it in the base of the hierarchy.
All accounts have a balance. We store that value in the  BankAccount superclass:
public class BankAccount
{
private double balance;
. . .
}
The  SavingsAccount class needs to store the interest rate. It also needs to store the minimum
monthly balance, which must be updated by all withdrawals.
public class SavingsAccount extends BankAccount
{
private double interestRate;
private double minBalance;
. . .
}
446 Chapter 9 Inheritance
The  CheckingAccount class needs to count the withdrawals, so that the charge can be applied
after the free withdrawal limit is reached.
public class CheckingAccount extends BankAccount
{
private int withdrawals;
. . .
}
step 7  Implement constructors and methods.
The methods of the  BankAccount class update or return the balance.
public void deposit(double amount)
{
balance = balance + amount;
}
public void withdraw(double amount)
{
balance = balance  amount;
}
public double getBalance()
{
return balance;
}
At the level of the  BankAccount superclass, we can say nothing about end of month processing.
We choose to make that method do nothing:
public void monthEnd()
{
}
In the  withdraw method of the  SavingsAccount class, the minimum balance is updated. Note the
call to the superclass method:
public void withdraw(double amount)
{
super.withdraw(amount);
double balance = getBalance();
if (balance < minBalance)
{
minBalance = balance;
}
}
In the  monthEnd method of the  SavingsAccount class, the interest is deposited into the account.
We must call the  deposit method because we have no direct access to the  balance instance vari-
able. The minimum balance is reset for the next month.
public void monthEnd()
{
double interest = minBalance * interestRate / 100;
deposit(interest);
minBalance = getBalance();
}
The  withdraw method of the  CheckingAccount class needs to check the withdrawal count. If there
have been too many withdrawals, a charge is applied. Again, note how the method invokes the
superclass method:
public void withdraw(double amount)
{
9.4 polymorphism 447
final int FREE_WITHDRAWALS = 3;
final int WITHDRAWAL_FEE = 1;
super.withdraw(amount);
withdrawals++;
if (withdrawals > FREE_WITHDRAWALS)
{
super.withdraw(WITHDRAWAL_FEE);
}
}
End of month processing for a checking account simply resets the withdrawal count.
public void monthEnd()
{
withdrawals = 0;
}
step 8  Construct objects of different subclasses and process them.
In our sample program, we allocate five checking accounts and five savings accounts and store
their addresses in an array of bank accounts. Then we accept user commands and execute
deposits, withdrawals, and monthly processing.
BankAccount[] accounts = . . .;
. . .
Scanner in = new Scanner(System.in);
boolean done = false;
while (!done)
{
System.out.print("D)eposit W)ithdraw M)onth end Q)uit: ");
String input = in.next();
if (input.equals("D") || input.equals("W")) //  Deposit or withdrawal
{
System.out.print("Enter account number and amount: ");
int num = in.nextInt();
double amount = in.nextDouble();
if (input.equals("D")) { accounts[num].deposit(amount); }
else { accounts[num].withdraw(amount); }
System.out.println("Balance: " + accounts[num].getBalance());
}
else if (input.equals("M")) //  Month end processing
{
for (int n = 0; n < accounts.length; n++)
{
accounts[n].monthEnd();
System.out.println(n + " " + accounts[n].getBalance());
}
}
else if (input == "Q")
{
done = true;
}
}
Full cOde example
Go to  wiley.com/go/
javacode to download
the program with
BankAccount ,
SavingsAccount , and
CheckingAccount
classes.
448 Chapter 9 Inheritance
9.5  Object : the Cosmic superclass
In Java, every class that is declared without an explicit  extends clause automatically
extends the class  Object . That is, the class  Object is the direct or indirect superclass of
every class in Java (see Figure 9). The  Object class defines several very general meth-
ods, including
•  toString , which yields a string describing the object (Section 9.5.1).
•  equals , which compares objects with each other (Section 9.5.2).
•  hashCode , which yields a numerical code for storing the object in a set (see Special
Topic 15.1).
9.5.1 overriding the  toString Method
The  toString method returns a string representation for each object. It is often used
for debugging.
For example, consider the  Rectangle class in the standard Java library. Its  toString
method shows the state of a rectangle:
Rectangle box = new Rectangle(5, 10, 20, 30);
String s = box.toString();
//  Sets  s to  "java.awt.Rectangle[x=5,y=10,width=20,height=30]"
The  toString method is called automatically whenever you concatenate a string with
an object. Here is an example:
"box=" + box;
On one side of the  + concatenation operator is a string, but on the other side is an
object reference. The Java compiler automatically invokes the  toString method to
turn the object into a string. Then both strings are concatenated. In this case, the
result is the string
"box=java.awt.Rectangle[x=5,y=10,width=20,height=30]"
The compiler can invoke the  toString method, because it knows that every object has
a  toString method: Every class extends the  Object class, and that class declares  toString .
As you know, numbers are also converted to strings when they are concatenated
with other strings. For example,
int age = 18;
String s = "Harry's age is " + age;
//  Sets  s to  "Harry's age is 18"
WorkeD exaMple 9.1  Implementing an employee
hierarchy for payroll processing
Learn how to implement payroll processing that works
for different kinds of employees. Go to  www.wiley.com/go/
javaexamples and download Worked Example 9.1.
© Sean Locke/iStockphoto.
9.5 object: the Cosmic superclass 449
Figure 9  the  Object Class Is the superclass of every Java Class
Object
Question
ChoiceQuestion NumericQuestion
Rectangle String Scanner
In this case, the  toString method is not involved. Numbers are not objects, and there is
no  toString method for them. Fortunately, there is only a small set of primitive types,
and the compiler knows how to convert them to strings.
Let’s try the  toString method for the  BankAccount class:
BankAccount momsSavings = new BankAccount(5000);
String s = momsSavings.toString();
//  Sets  s to something like  "BankAccount@d24606bf"
That’s disappointing—all that’s printed is the name of the class, followed by the hash
code, a seemingly random code. The hash code can be used to tell objects apart—dif-
ferent objects are likely to have differ ent hash codes. (See Special Topic 15.1 for the
details.)
We don’t care about the hash code. We want to know what is inside the object.
But, of course, the  toString method of the  Object class does not know what is inside
the  BankAccount class. Therefore, we have to override the method and supply our own
version in the  BankAccount class. We’ll follow the same format that the  toString method
of the  Rectangle class uses: first print the name of the class, and then the values of the
instance variables inside brackets.
public class BankAccount
{
. . .
public String toString()
{
return "BankAccount[balance=" + balance + "]";
}
}
This works better:
BankAccount momsSavings = new BankAccount(5000);
String s = momsSavings.toString(); //  Sets  s to  "BankAccount[balance=5000]"
override the
toString method to
yield a string that
describes the
object’s state.
450 Chapter 9 Inheritance
9.5.2 the  equals Method
In addition to the  toString method, the  Object class
also provides an  equals method, whose purpose
is to check whether two objects have the same
contents:
if (stamp1.equals(stamp2)) . . .
//  Contents are the same—see Figure 10
This is different from the test with the  == operator,
which tests whether two references are identical,
referring to the same object:
if (stamp1 == stamp2) . . .
//  Objects are the same—see Figure 11
Let’s implement the  equals method for a  Stamp class. You need to override the  equals
method of the  Object class:
public class Stamp
{
private String color;
private int value;
. . .
public boolean equals(Object otherObject)
{
. . .
}
. . .
}
Now you have a slight problem. The  Object class knows nothing about stamps, so
it declares the  other Object parameter variable of the  equals method to have the type
Object . When overriding the method, you are not allowed to change the type of the
parameter variable. Cast the parameter variable to the class  Stamp :
Stamp other = (Stamp) otherObject;
© Ken Brown/iStockphoto.
The  equals method checks whether
two objects have the same contents.
the equals method
checks whether two
objects have the
same contents.
Figure 10  two references to equal objects
stamp1 =
color =
Stamp
value = 90
cyan
stamp2 =
color =
Stamp
value = 90
cyan
Figure 11  two references to the same object
stamp1 =
color =
Stamp
value = 90
cyan
stamp2 =
9.5 object: the Cosmic superclass 451
Then you can compare the two stamps:
public boolean equals(Object otherObject)
{
Stamp other = (Stamp) otherObject;
return color.equals(other.color)
&& value == other.value;
}
Note that this  equals method can access the instance variables of any  Stamp object: the
access  other.color is perfectly legal.
9.5.3 the  instanceof operator
As you have seen, it is legal to store a subclass reference in a superclass variable:
ChoiceQuestion cq = new ChoiceQuestion();
Question q = cq; //  OK
Object obj = cq; //  OK
Very occasionally, you need to carry out the opposite conversion, from a superclass
reference to a sub class reference.
For example, you may have a variable of type  Object , and you happen to know that
it actually holds a  Question reference. In that case, you can use a cast to convert the
type:
Question q = (Question) obj;
However, this cast is somewhat dangerous. If you are wrong, and  obj actually refers
to an object of an unrelated type, then a “class cast” exception is thrown.
To protect against bad casts, you can use the  instanceof operator. It tests whether
an object belongs to a particular type. For example,
obj instanceof Question
returns  true if the type of  obj is convertible to  Question . This happens if  obj refers to an
actual  Question or to a subclass such as  ChoiceQuestion .
If you know that an
object belongs to
a given class, use
a cast to convert
the type.
the  instanceof
operator tests
whether an object
belongs to a
particular type.
syntax 9.4  the  instanceof operator
if (anObject instanceof Question)
{
Question q = (Question) anObject;
. . .
}
If  anObject is  null ,
instanceof returns  false .
Returns  true if  anObject
can be cast to a  Question .
Two references
to the same object.
You can invoke  Question
methods on this variable.
The object may belong to a
subclass of  Question .
object instanceof  TypeName Syntax
452 Chapter 9 Inheritance
Using the  instanceof operator, a safe cast can be programmed as follows:
if (obj instanceof Question)
{
Question q = (Question) obj;
}
Note that  instanceof is not a method. It is an operator, just like  + or  < . However, it does
not operate on numbers. To the left is an object, and to the right a type name.
Do not use the  instanceof operator to bypass polymorphism:
if (q instanceof ChoiceQuestion) //  Don’t do this—see Common Error 9.5
{
//  Do the task the  ChoiceQuestion way
}
else if (q instanceof Question)
{
//  Do the task the  Question way
}
In this case, you should implement a method  doTheTask in the  Question class, override it
in  ChoiceQuestion , and call
q.doTheTask();
21.  Why does the call
System.out.println(System.out);
produce a result such as  java.io.PrintStream@7a84e4 ?
22.  Will the following code fragment compile? Will it run? If not, what error is
reported?
Object obj = "Hello";
System.out.println(obj.length());
23.  Will the following code fragment compile? Will it run? If not, what error is
reported?
Object obj = "Who was the inventor of Java?";
Question q = (Question) obj;
q.display();
24.  Why don’t we simply store all objects in variables of type Object ?
25.  Assuming that  x is an object reference, what is the value of  x instanceof Object ?
practice It  Now you can try these exercises at the end of the chapter: E9.7, E9.8, E9.12.
don’t use type tests
Some programmers use specific type tests in order to implement behavior that varies with each
class:
if (q instanceof ChoiceQuestion) //  Don’t do this
{
//  Do the task the  ChoiceQuestion way
}
else if (q instanceof Question)
{
Full cOde example
Go to  wiley.com/go/
javacode to download
a program that
demonstrates the
toString method
and the  instanceof
operator.
© Nicholas Homrich/iStockphoto.
s e l F  c h e c k
Common error 9.5
© John Bell/iStockphoto.
9.5 object: the Cosmic superclass 453
//  Do the task the  Question way
}
This is a poor strategy. If a new class such as  NumericQuestion is added, then you need to revise
all parts of your pro gram that make a type test, adding another case:
else if (q instanceof NumericQuestion)
{
//  Do the task the  NumericQuestion way
}
In contrast, consider the addition of a class  NumericQuestion to our quiz program. Nothing
needs to change in that program because it uses polymorphism, not type tests.
Whenever you find yourself trying to use type tests in a hierarchy of classes, reconsider
and use polymorphism instead. Declare a method  doTheTask in the superclass, override it in the
subclasses, and call
q.doTheTask();
Inheritance and the  toString  method
You just saw how to write a  toString method: Form a string consisting of the class name and
the names and values of the instance variables. However, if you want your  toString method to
be usable by subclasses of your class, you need to work a bit harder.
Instead of hardcoding the class name, call the  getClass method (which every class inherits
from the  Object class) to obtain an object that describes a class and its properties. Then invoke
the  getName method to get the name of the class:
public String toString()
{
return getClass().getName() + "[balance=" + balance + "]";
}
Then the  toString method prints the correct class name when you apply it to a subclass, say a
SavingsAccount .
SavingsAccount momsSavings = . . . ;
System.out.println(momsSavings);
//  Prints "SavingsAccount[balance=10000]"
Of course, in the subclass, you should override  toString and add the values of the subclass
instance variables. Note that you must call  super.toString to get the instance variables of the
superclass—the subclass can’t access them directly.
public class SavingsAccount extends BankAccount
{
. . .
public String toString()
{
return super.toString() + "[interestRate=" + interestRate + "]";
}
}
Now a savings account is converted to a string such as  SavingsAccount[balance=10000][interest
Rate=5] . The brackets show which variables belong to the superclass.
special topic 9.6
© Eric Isselé/iStockphoto.
454 Chapter 9 Inheritance
Inheritance and the  equals  method
You just saw how to write an  equals method: Cast the  otherObject parameter variable to the
type of your class, and then compare the instance variables of the implicit parameter and the
explicit parameter.
But what if someone called  stamp1.equals(x) where  x wasn’t a  Stamp object? Then the bad
cast would generate an exception. It is a good idea to test whether  otherObject really is an
instance of the  Stamp class. The easiest test would be with the  instanceof operator. However,
that test is not specific enough. It would be possible for  otherObject to belong to some subclass
of  Stamp . To rule out that possibility, you should test whether the two objects belong to the
same class. If not, return false.
if (getClass() != otherObject.getClass()) { return false; }
Moreover, the Java language specification demands that the  equals method return false when
otherObject is  null .
Here is an improved version of the  equals method that takes these two points into account:
public boolean equals(Object otherObject)
{
special topic 9.7
© Eric Isselé/iStockphoto.
In 1962, J.C.r. licklider
was head of the first
computer research program at Darpa,
the Defense advanced research proj-
ects agency. he wrote a series of
papers describing a “galactic network”
through which computer users could
access data and programs from other
sites. this was well before computer
networks were invented. By 1969, four
computers—three in California and one
in Utah—were connected to the arpa-
net, the precursor of the Internet. the
network grew quickly, linking comput-
ers at many universities and research
organizations. It was orig inally thought
that most network users wanted to run
programs on remote computers. Using
remote execution, a researcher at one
institution would be able to access an
underutilized computer at a different
site. It quickly became apparent that
remote execution was not what the
network was actually used for. Instead,
the “killer application” was electronic
mail: the transfer of messages between
computer users at different locations.
In 1972, Bob kahn proposed to
extend arpanet into the Internet: a
collection of interoperable networks.
all networks on the Internet share
common protocols for data transmis-
sion. kahn and Vinton Cerf developed
a proto col, now called tCp/Ip (trans-
mission Control protocol/Internet pro-
tocol). on January 1, 1983, all hosts on
the Internet simultaneously switched
to the tCp/Ip protocol (which is used
to this day).
over time, researchers, computer
scientists, and hobbyists published
increasing amounts of information
on the Internet. For example, project
Gutenberg makes available the text
of important classical books, whose
copyright has expired, in computer-
readable form ( www.gutenberg.org ). In
1989, tim Berners-lee, a computer sci-
entist at Cern (the european organiza-
tion for nuclear research) started work
on hyperlinked documents, allowing
users to browse by following links to
related documents. this infrastructure
is now known as the World Wide Web.
the first interfaces to retrieve this
information were, by today’s standards,
unbelievably clumsy and hard to use.
In March 1993, WWW traffic was 0.1
percent of all Internet traffic. all that
changed when Marc andreesen, then
a graduate student working for the
national Center for supercomputing
applications (nCsa), released Mosaic.
Mosaic dis played web pages in graphi-
cal form, using images, fonts, and col-
ors (see the figure). andreesen went on
to fame and fortune at netscape, and
Microsoft licensed the Mosaic code to
create Internet explorer. By 1996, WWW
traffic accounted for more than half of
the data transported on the Internet.
the Internet has a very democratic
structure. anyone can publish any-
thing, and anyone can read whatever
has been published. this does not
always sit well with governments and
corporations.
Many governments control the
Internet infrastructure in their country.
For example, an Internet user in China,
searching for the tiananmen square
Computing & Society 9.1  Who Controls the Internet?
© MediaBakery.
9.5 object: the Cosmic superclass 455
if (otherObject == null) { return false; }
if (getClass() != otherObject.getClass()) { return false; }
Stamp other = (Stamp) otherObject;
return color.equals(other.color) && value == other.value;
}
When you implement  equals in a subclass, you should first call  equals in the superclass to
check whether the superclass instance variables match. Here is an example:
public CollectibleStamp extends Stamp
{
private int year;
. . .
public boolean equals(Object otherObject)
{
if (!super.equals(otherObject)) { return false; }
CollectibleStamp other = (CollectibleStamp) otherObject;
return year == other.year;
}
}
massacre or air pollution in their
hometown, may find nothing. Viet-
nam blocks access to Facebook, per-
haps fearing that anti-government
protesters might use it to organize
themselves. the U.s. government has
required publicly funded libraries and
schools to install filters that block sex-
ually explicit and hate speech.
When the Internet is delivered by
phone or tV cable companies, those
companies sometimes interfere with
competing Internet offerings. Cell
phone companies refused to carry
Voice-over-Ip services, and cable com-
panies slowed down movie streaming.
the Internet has become a powerful
force for delivering information––both
good and bad. It is our responsibility as
citizens to demand of our government
that we can control which information
to access.
The NCSA Mosaic Browser
456 Chapter 9 Inheritance
explain the notions of inheritance, superclass, and subclass.
• A subclass inherits data and behavior from a
superclass.
• You can always use a subclass object in place
of a superclass object.
Implement subclasses in java.
• A subclass inherits all methods that it does not
override.
• A subclass can override a superclass method by
providing a new imple mentation.
• The  extends reserved word indicates that a class
inherits from a superclass.
Implement methods that override methods from a superclass.
• An overriding method can extend or replace the functionality of the superclass
method.
• Use the reserved word  super to call a superclass method.
• Unless specified otherwise, the subclass constructor calls the superclass con-
structor with no arguments.
• To call a superclass constructor, use the  super reserved word in the first statement
of the subclass constructor.
• The constructor of a subclass can pass arguments to a superclass constructor,
using the reserved word  super .
use polymorphism for processing objects of related types.
• A subclass reference can be used when a superclass reference is expected.
• When the virtual machine calls an instance method, it locates the method of
the implicit parameter’s class. This is called dynamic method lookup.
• Polymorphism (“having multiple forms”) allows us to manipulate objects
that share a set of tasks, even though the tasks are executed in different ways.
Work with the  Object  class and its methods.
• Override the  toString method to yield a string that describes the object’s state.
• The  equals method checks whether two objects have the same contents.
• If you know that an object belongs to a given class, use a cast to convert the type.
• The  instanceof operator tests whether an object belongs to a particular type.
C h a p t e r s U M M a rY
© Richard Stouffer/iStockphoto (vehicle); © Ed Hidden/iStockphoto
(motorcycle); © YinYang/iStockphoto (car); © Robert Pernell/iStockphoto
(truck).
Vehicle
Motorcycle Car Truck
Media Bakery.
© Alpophoto/iStockphoto.
review Questions 457
• r9.1  Identify the superclass and subclass in each of the following pairs of classes.
a.  Employee ,  Manager
b.  GraduateStudent ,  Student
c.  Person ,  Student
d.  Employee ,  Professor
e.  BankAccount ,  CheckingAccount
f.  Vehicle ,  Car
g.  Vehicle ,  Minivan
h.  Car ,  Minivan
i.  Truck ,  Vehicle
• r9.2  Consider a program for managing inventory in a small appliance store. Why isn’t it
useful to have a superclass  SmallAppliance and subclasses  Toaster ,  CarVacuum ,  Travel Iron ,
and so on?
• r9.3  Which methods does the  ChoiceQuestion class inherit from its superclass? Which
methods does it override? Which methods does it add?
• r9.4  Which methods does the  SavingsAccount class in How To 9.1 inherit from its super-
class? Which methods does it override? Which methods does it add?
• r9.5  List the instance variables of a  CheckingAccount object from How To 9.1.
•• r9.6  Suppose the class  Sub extends the class  Sandwich . Which of the following assignments
are legal?
Sandwich x = new Sandwich();
Sub y = new Sub();
a.  x = y;
b.  y = x;
c.  y = new Sandwich();
d.  x = new Sub();
• r9.7  Draw an inheritance diagram that shows the inheritance relationships between these
classes.
•  Person
•  Employee
•  Student
•  Instructor
•  Classroom
•  Object
• r9.8  In an object-oriented traffic simulation system, we have the classes listed below.
Draw an inheritance diagram that shows the relationships between these classes.
•  Vehicle
•  Car
•  Truck
•  Sedan
•  Coupe
•  PickupTruck
r e V I e W Q U e s t I o n s
•  SportUtilityVehicle
•  Minivan
•  Bicycle
•  Motorcycle
458 Chapter 9 Inheritance
• r9.9  What inheritance relationships would you establish among the following classes?  
•  Student
•  Professor
•  TeachingAssistant
•  Employee
•  Secretary
•  DepartmentChair
•  Janitor
•  SeminarSpeaker
•  Person
•  Course
•  Seminar
•  Lecture
•  ComputerLab
•• r9.10  How does a cast such as  (BankAccount) x  differ from a cast of number values such as
(int) x ?
••• r9.11  Which of these conditions returns  true? Check the Java documentation for the
inheritance patterns. Recall that  System.out is an object of the  PrintStream class.
a.  System.out instanceof PrintStream
b.  System.out instanceof OutputStream
c.  System.out instanceof LogStream
d.  System.out instanceof Object
e.  System.out instanceof String
f.  System.out instanceof Writer
•• e9.1  Add a class  NumericQuestion to the question hierarchy of Section 9.1. If the response
and the expected answer differ by no more than 0.01, then accept the response as
correct.
•• e9.2  Add a class  FillInQuestion to the question hierarchy of Section 9.1. Such a question is
constructed with a string that contains the answer, surrounded by  _ _ , for exam ple,
"The inventor of Java was _James Gosling_" . The question should be displayed as
The inventor of Java was _____
• e9.3  Modify the  checkAnswer method of the  Question class so that it does not take into
account different spaces or upper/lowercase characters. For example, the response
"JAMES gosling" should match an answer of  "James Gosling" .
•• e9.4  Add a class  AnyCorrectChoiceQuestion to the question hierarchy of Section 9.1 that
allows multiple correct choices. The respondent should provide any one of the cor-
rect choices. The answer string should contain all of the correct choices, separated by
spaces. Provide instructions in the question text.
•• e9.5  Add a class  MultiChoiceQuestion to the question hierarchy of Section 9.1 that allows
multiple correct choices. The respondent should provide all correct choices, sepa-
rated by spaces. Provide instructions in the question text.
•• e9.6  Add a method  addText to the  Question superclass and provide a different implementa-
tion of  ChoiceQuestion that calls  addText rather than storing an array list of choices.
• e9.7  Provide  toString methods for the  Question and  ChoiceQuestion classes.
•• e9.8  Implement a superclass  Person . Make two classes,  Student and  Instructor , that inherit
from  Person . A person has a name and a year of birth. A student has a major, and
an instructor has a salary. Write the class declarations, the constructors, and the
p r a C t I C e e x e r C I s e s
programming projects 459
methods  toString for all classes. Supply a test program that tests these classes and
methods.
•• e9.9  Make a class  Employee with a name and salary. Make a class  Manager inherit from
Employee . Add an instance variable, named  department , of type  String . Supply a method
toString that prints the manager’s name, department, and salary. Make a class
Executive inherit from  Manager . Supply appropriate  toString methods for all classes.
Supply a test program that tests these classes and methods.
•• e9.10  The  java.awt.Rectangle class of the standard Java library does not supply a method
to com pute the area or perimeter of a rectangle. Provide a subclass  BetterRectangle of
the  Rectangle class that has  getPerimeter and  getArea methods. Do not add any instance
variables. In the constructor, call the  setLocation and  setSize methods of the  Rectangle
class. Provide a program that tests the methods that you supplied.
••• e9.11  Repeat Exercise E9.10, but in the  BetterRectangle constructor, invoke the superclass
constructor.
•• e9.12  A labeled point has x- and y-coordinates and a string label. Provide a class  Labeled
Point with a constructor  LabeledPoint(int x, int y, String label) and a  toString
method that displays  x ,  y , and the label.
•• e9.13  Reimplement the  LabeledPoint class of Exercise E9.12 by storing the location in a
java.awt.Point object. Your  toString method should invoke the  toString method of
the  Point class.
•• business e9.14  Change the  CheckingAccount class in How To 9.1 so that a $1 fee is levied for depos-
its or withdrawals in excess of three free monthly transactions. Place the code for
com puting the fee into a separate method that you call from the  deposit and  withdraw
methods.
•• business p9.1  Implement a superclass  Appointment and subclasses
Onetime ,  Daily , and  Monthly . An appointment
has a description (for example, “see the dentist”)
and a date. Write a method  occursOn(int year,
int month, int day) that checks whether the
appointment occurs on that date. For example, for
a monthly appointment, you must check whether
the day of the month matches. Then fill an array of
Appointment objects with a mixture of appointments.
Have the user enter a date and print out all appointments that occur on that date.
•• business p9.2  Improve the appointment book program of Exercise P9.1. Give the user the option
to add new appointments. The user must specify the type of the appointment, the
description, and the date.
••• business p9.3  Improve the appointment book program of Exercise P9.1 and P9.2 by letting the
user save the appointment data to a file and reload the data from a file. The saving
part is straightforward: Make a method  save . Save the type, description, and date to
a file. The loading part is not so easy. First determine the type of the appointment to
be loaded, create an object of that type, and then call a  load method to load the data.
p r o G r a M M I n G p r o J e C t s
© Pali Rao/iStockphoto.
460 Chapter 9 Inheritance
•• science p9.4  Resonant circuits are used to select a signal (e.g., a radio station or TV channel)
from among other competing signals. Resonant circuits are characterized by the
frequency response shown in the figure below. The resonant frequency response
is completely described by three parameters: the resonant frequency,  ω o , the band-
width, B, and the gain at the resonant frequency, k.
Frequency (rad/s, log scale)
k
ω o
B 0.707k
Two simple resonant circuits are shown in the figure below. The circuit in (a) is
called a parallel resonant circuit. The circuit in (b) is called a series resonant circuit.
Both resonant circuits consist of a resistor having resistance R, a capacitor having
capacitance C, and an inductor having inductance L.
R L C
C
L
R
(a) Parallel resonant circuit (b) Series resonant circuit
These circuits are designed by determining values of R, C, and L that cause the
resonant frequency response to be described by specified values of  ω o , B, and k. The
design equations for the parallel resonant circuit are:
R k C
BR
L
C
= = = , , and
o
1 1
2
ω
Similarly, the design equations for the series resonant circuit are:
R
k
L
R
B
C
L
= = =
1 1
2
, , and
o
ω
Write a Java program that represents  ResonantCircuit as a superclass and represents
the  SeriesResonantCircuit and  ParallelResonantCircuit  as subclasses. Give the super-
class three private instance variables representing the parameters  ω o , B, and k of the
resonant frequency response. The superclass should provide public instance
methods to get and set each of these variables. The superclass should also provide a
display method that prints a description of the resonant frequency response.
Each subclass should provide a method that designs the corresponding resonant
circuit. The subclasses should also override the  display method of the superclass to
programming projects 461
print descriptions of both the frequency response (the values of  ω o , B, and k) and the
circuit (the values of R, C, and L).
All classes should provide appropriate constructors.
Supply a class that demonstrates that the subclasses all work properly.
••• science p9.5  In this problem, you will model a circuit consisting of an arbitrary configuration of
resistors. Provide a superclass  Circuit with a instance method  getResistance . Pro-
vide a subclass  Resistor representing a single resistor. Provide subclasses  Serial and
Parallel , each of which contains an  ArrayList<Circuit> . A  Serial circuit mod els a
series of circuits, each of which can be a single resistor or another circuit. Simi larly, a
Parallel circuit models a set of circuits in parallel. For example, the following circuit
is a  Parallel circuit containing a single resistor and one  Serial cir cuit:
A  Serial circuit
Use Ohm’s law to compute the combined resistance.
•• science p9.6  Part (a) of the figure below shows a symbolic representation of an electric circuit
called an amplifier. The input to the amplifier is the voltage v i and the output is the
voltage v o . The output of an amplifier is proportional to the input. The constant of
proportionality is called the “gain” of the amplifier.
–
+
–
+
–
+
R 2
R 1
R 1
v i
R 2
R 1 R 2
v o
v o v i
v i
v o
v o v i
(a) Amplifier (b) Inverting amplifier
(c) Noninverting amplifier (d) Voltage divider amplifier
Parts (b), (c), and (d) show schematics of three specific types of amplifier: the
inverting amplifier, noninverting amplifier, and voltage divider amplifier. Each of
these three amplifiers consists of two resistors and an op amp. The value of the gain
of each amplifier depends on the values of its resistances. In particular, the gain, g, of
the inverting amplifier is given by  g
R
R
= −
2
1
. Similarly the gains of the noninverting
amplifier and voltage divider amplifier are given by  g
R
R
= + 1
2
1
and  g
R
R R
=
+
2
1 2
,
respectively.
462 Chapter 9  Inheritance
Write a Java program that represents the amplifier as a superclass and represents
the inverting, noninverting, and voltage divider amplifiers as subclasses. Give the
subclass two methods,  getGain and a  getDescription method that returns a string
identifying the amplifier. Each subclass should have a constructor with two argu-
ments, the resistances of the amplifier.
The subclasses need to override the  getGain and  getDescription methods of the
superclass.
Supply a class that demonstrates that the subclasses all work properly for sample val-
ues of the resistances.
A n s w e r s  t o  s e l f - C h e C k  Q u e s t I o n s
1.  Because every manager is an employee but not
the other way around, the  Manager class is more
specialized. It is the subclass, and  Employee is
the superclass.
2.  CheckingAccount and  SavingsAccount both inherit
from the more general class  Bank Account .
3.  The classes  Frame ,  Window , and  Component in the
java.awt package, and the class  Object in the
java.lang package.
4.  Vehicle, truck, motorcycle
5.  It shouldn’t. A quiz isn’t a question; it has
questions.
6.  a, b, d
7.  publicclassManagerextendsEmployee
{
privatedoublebonus;
// Constructors and methods omitted
}
8.  name ,  baseSalary , and bonus
9.  publicclassManagerextendsEmployee
{
...
publicdoublegetSalary(){...}
}
10.  getName ,  setName ,  setBaseSalary
11.  The method is not allowed to access the
instance variable  text from the superclass.
12.  The type of the  this reference is  ChoiceQuestion .
Therefore, the  display method of  ChoiceQuestion
is selected, and the method calls itself.
13.  Because there is no ambiguity. The subclass
doesn’t have a  setAnswer method.
14.  publicStringgetName()
{
return"*"+super.getName();
}
15.  publicdoublegetSalary()
{
returnsuper.getSalary()+bonus;
}
16.  a only.
17.  It belongs to the class  BankAccount or one of its
subclasses.
18.  Question[]quiz=newQuestion[SIZE];
19.  You cannot tell from the fragment— cq may
be initialized with an object of a sub class of
ChoiceQuestion . The  display method of whatever
object  cq references is invoked.
20.  No. This is a static method of the  Math class.
There is no implicit parameter object that
could be used to dynamically look up a
method.
21.  Because the implementor of the  PrintStream
class did not supply a  toString method.
22.  The second line will not compile. The class
Object does not have a method  length .
23.  The code will compile, but the second line will
throw a class cast exception because  Question is
not a superclass of  String .
24.  There are only a few methods that can be
invoked on variables of type  Object .
25.  The value is  false if  x is  null and  true otherwise.
10
C h a p t e r
463
© supermimicry/iStockphoto.
InterfaCes
to be able to declare and use
interface types
to appreciate how interfaces can be
used to decouple classes
to learn how to implement helper classes
as inner classes
to implement event listeners in graphical applications
C h a p t e r G o a l s
C h a p t e r C o n t e n t s
10.1 Using interfaces for
algorithm reUse 464
Syntax 10.1: Declaring an Interface 465
Syntax 10.2: Implementing an Interface 467
Common Error 10.1: forgetting to Declare
Implementing Methods as public 470
Common Error 10.2: trying to Instantiate
an Interface 470
Special Topic 10.1: Constants in Interfaces 470
10.2 Working With interface
Variables 471
Worked Example 10.1: Investigating number
sequences
10.3 the comparable interface 473
Special Topic 10.2: the clone Method and the
Cloneable Interface 475
10.4 Using interfaces for
callbacks 477
10.5 inner classes 481
Special Topic 10.3: anonymous Classes 482
10.6 mock objects 483
10.7 eVent handling 484
Common Error 10.3: Modifying parameter types
in the Implementing Method 489
Common Error 10.4: trying to Call
listener Methods 490
10.8 bUilding applications
With bUttons 490
Common Error 10.5: forgetting to attach
a listener 493
Programming Tip 10.1: Don’t Use a Container as
a listener 493
10.9 processing timer eVents 494
Common Error 10.6: forgetting to repaint 496
10.10 moUse eVents 497
Special Topic 10.4: Keyboard events 500
Special Topic 10.5: event adapters 501
Computing & Society 10.1: open source and free
software 502
464
a mixer rotates any tools that will attach to its motor’s shaft.
In other words, a single motor can be used with multiple
tools. We want to be able to reuse software components in
the same way. In this chapter, you will learn an important
strategy for separating the reusable part of a computation
from the parts that vary in each reuse scenario. the reusable
part invokes methods of an interface, not caring how the
methods are implemented––just as the mixer doesn’t
care about the shape of the attachment. In a program, the
reusable code is combined with a class that implements the
interface methods. to produce a different application, you
plug in another class that implements the same interface.
10.1 Using Interfaces for algorithm reuse
It is often possible to make a service avail
able to a wide set of inputs by focusing on
the essential opera tions that the service
requires. Interface types are used to express
these common operations.
This restaurant is willing to serve anyone
who conforms to the  Customer interface
with  eat and  pay methods.
10.1.1 Defining an Interface type
Consider this  average method that provides a service, namely to compute the average
bank balance of an array of bank accounts:
public static double average(BankAccount[] objects)
{
double sum = 0;
for (BankAccount obj : objects)
{
sum = sum + obj.getBalance();
}
if (objects.length > 0) { return sum / objects.length; }
else { return 0; }
}
Now suppose we want to compute an average of other objects. We have to write that
method again. Here it is for  Country objects:
public static double average(Country[] objects)
{
double sum = 0;
for (Country obj : objects)
{
sum = sum + obj.getArea();
© Oxana Oleynichenko/iStockphoto.
supermimicry/iStockphoto.
10.1 Using Interfaces for algorithm reuse 465
}
if (objects.length > 0) { return sum / objects.length; }
else { return 0; }
}
Clearly, the algorithm for computing the average is the same in all cases, but the
details of measurement differ. We would like to provide a single method that provides
this service.
But there is a problem. Each class has a different name for the method that returns
the value that is being averaged. In the  BankAccount class, we call  getBalance . In the  Coun
try class, we call  getArea .
Suppose that the various classes agree on a method  getMeasure that obtains the mea
sure to be used in the data analysis. For bank accounts,  getMeasure returns the balance.
For countries,  getMeasure returns the area, and so on.
Then we can implement a single method that computes
sum = sum + obj.getMeasure();
But agreeing on the name of the method is only half the solution. In Java, we also
must declare the type of the variable  obj . Of course, you can’t write
BankAccount or Country or ... obj; //  No
We need to invent a new type that describes any class whose objects can be measured.
In Java, an interface type is used to specify required operations. We will declare an
interface type that we call  Measurable :
public interface Measurable
{
double getMeasure();
}
The interface declaration lists all methods that the interface type requires. The  Measur
able interface type requires a single method,  getMeasure . In general, an interface type
can require multiple methods.
An interface type is similar to a class, but there are several important differences:
• An interface type does not have instance variables.
• All methods in an interface type are abstract; that is, they have a name, param
eters, and a return type, but they don’t have an implementation.
• All methods in an interface type are automatically public.
• An interface type has no constructor. Interfaces are not classes, and you cannot
construct objects of an interface type.
a Java interface
type declares
methods but does
not provide their
implementations.
syntax 10.1  Declaring an Interface
public interface  InterfaceName
{
method headers
}
Syntax
public interface Measurable
{
double getMeasure();
}
The methods of an interface
are automatically public.
No implementation is provided.
466 Chapter 10 Interfaces
Now that we have a type that denotes measurability, we can implement a reusable
average method:
public static double average(Measurable[] objects)
{
double sum = 0;
for (Measurable obj : objects)
{
sum = sum + obj.getMeasure();
}
if (objects.length > 0) { return sum / objects.length; }
else { return 0; }
}
This method is useful for objects of any class that
conforms to the  Measurable type. In the next sec
tion, you will see what a class must do to make its
objects measurable.
Note that the  Measurable interface is not a type
in the standard library—it was created specifi
cally for this book, to provide a very simple
example for studying the interface concept.
This standmixer provides the “rotation”
service to any attachment that
conforms to a common interface.
Similarly, the  average method at the
end of this section works with any class
that implements a common interface.
10.1.2 Implementing an Interface type
The  average method of the preceding section can process objects of any class that
implements the  Measurable interface. A class implements an interface type if it
declares the interface in an  implements clause, like this:
public class BankAccount implements Measurable
The class should then imple ment the method or methods that the interface requires:
public class BankAccount implements Measurable
{
. . .
public double getMeasure()
{
return balance;
}
}
Note that the class must declare the method as  public , whereas the interface need
not—all methods in an interface are public.
Once the  BankAccount class implements the  Measurable interface type,  BankAccount
objects are instances of the  Measurable type:
Measurable obj = new BankAccount(); //  OK
© gregory horler/iStockphoto.
Use the  implements
reserved word to
indicate that a class
implements an
interface type.
10.1 Using Interfaces for Algorithm Reuse  467
Syntax 10.2  Implementing an Interface
public class  ClassName  implements  InterfaceName ,  InterfaceName , . . .
{
instance variables
methods
}
Syntax
public class BankAccount implements Measurable
{
. . .
public double getMeasure()
{
return balance;
}
. . .
}
List all interface types
that this class implements.
This method provides the implementation
for the method declared in the interface.
BankAccount
instance variables
Other
BankAccount methods
A variable of type  Measurable holds a reference to an object of some class that imple-
ments the  Measurable interface.
Similarly, it is an easy matter to modify the  Country class to implement the  Measur-
able interface:
public class Country implements Measurable
{
public double getMeasure()
{
return area;
}
. . .
}
The program at the end of this section uses a single  average method (placed in class
Data ) to compute the average of bank accounts and the average of countries.
This is a typical usage for interface types. By inventing the  Measurable interface
type, we have made the  average method reusable.
Figure 1 shows the relationships between the  Data class, the  Measurable interface,
and the classes that implement the interface. Note that the  Data class depends only on
the  Measurable interface. It is decou pled from the  BankAccount and  Country classes.
Use interface types to
make code 
more reusable.
Figure 1  
UML Diagram of the 
Data  Class and
the Classes that 
Implement the 
Measurable  Interface
BankAccount Country
‹‹interface››
Measurable
Data
The  BankAccount
and  Country classes
implement the  Measurable
interface type.
The  Data class uses
the  Measurable type
but not  BankAccount
or  Country .
468 Chapter 10 Interfaces
In the UML notation, interfaces are tagged with an indicator  «interface» . A dotted
arrow with a trian gular tip denotes the implements relationship between a class and
an interface. You have to look carefully at the arrow tips—a dotted line with an open
arrow tip ( ) denotes the uses relationship or dependency.
section_1/data.java
1 public class Data
2 {
3 /**
4 Computes the average of the measures of the given objects.
5 @param objects  an array of  Measurable objects
6 @return  the average of the measures
7 */
8 public static double average(Measurable[] objects)
9 {
10 double sum = 0;
11 for (Measurable obj : objects)
12 {
13 sum = sum + obj.getMeasure();
14 }
15 if (objects.length > 0) { return sum / objects.length;}
16 else { return 0; }
17 }
18 }
section_1/measurabletester.java
1 /**
2 This program demonstrates the measurable  BankAccount and  Country classes.
3 */
4 public class MeasurableTester
5 {
6 public static void main(String[] args)
7 {
8 Measurable[] accounts = new Measurable[3];
9 accounts[0] = new BankAccount(0);
10 accounts[1] = new BankAccount(10000);
11 accounts[2] = new BankAccount(2000);
12
13 double averageBalance = Data.average(accounts);
14 System.out.println("Average balance: " + averageBalance);
15 System.out.println("Expected: 4000");
16
17 Measurable[] countries = new Measurable[3];
18 countries[0] = new Country("Uruguay", 176220);
19 countries[1] = new Country("Thailand", 513120);
20 countries[2] = new Country("Belgium", 30510);
21
22 double averageArea = Data.average(countries);
23 System.out.println("Average area: " + averageArea);
24 System.out.println("Expected: 239950");
25 }
26 }
program run
Average balance: 4000
Expected: 4000
Average area: 239950
Expected: 239950
10.1 Using Interfaces for Algorithm Reuse  469
10.1.3 Comparing Interfaces and Inheritance
In Chapter 9, you saw how to use inheritance to model hierarchies of related classes,
such as different kinds of quiz questions or bank accounts. Multiple-choice questions
and fill-in questions are questions with specific characteristics.
Interfaces model a somewhat different relationship. Consider for example the
BankAccount and  Country classes in the preceding section. Both implement the  Measurable
interface type, but otherwise they have nothing in common. Being measurable is just
one aspect of what it means to be a bank account or country. It is useful to model this
common aspect, because it enables other programmers to write tools that exploit the
commonality, such as the method for computing averages.
A class can implement more than one interface, for example
public class Country implements Measurable, Named
Here,  Named is a different interface
public interface Named
{
String getName();
}
In contrast, a class can only extend (inherit from) a single superclass.
An interface merely specifies the behavior that an implementing class should sup-
ply. It provides no implementation. In contrast, a superclass provides some imple-
mentation that a subclass inherits.
Special Topic 9.3 introduced abstract classes, which defer the implementation of
some methods to subclasses. You can think of an interface as a class in which every
method is abstract. However, interfaces have an advantage over such abstract classes
––a class can implement more than one of them.
Generally, you will develop interfaces when you have code that processes objects
of different classes in a common way. For example, a drawing program might have
different objects that can be drawn, such as lines, images, text, and so on. In this situ-
ation, a  Drawable interface with a  draw method will be useful. Another example is a
traffic simulation that models the movement of people, cars, dogs, balls, and so on. In
this example, you might create a  Moveable interface with methods  move and  getPosition .
1.  Suppose you want to use the  average method to find the average salary of an ar-
ray of  Employee objects. What condition must the  Employee class fulfill?
2.  Why can’t the  average method have a parameter variable of type  Object[] ?
3.  Why can’t you use the  average method to find the average length of  String
objects?
4.  What is wrong with this code?
Measurable meas = new Measurable();
System.out.println(meas.getMeasure());
5.  What is wrong with this code?
Measurable meas = new Country("Uruguay", 176220);
System.out.println(meas.getName());
Practice It  Now you can try these exercises at the end of the chapter: E10.1, E10.2, E10.3.
© Nicholas Homrich/iStockphoto.
S e l f C h e C k
470 Chapter 10 Interfaces
forgetting to declare implementing methods as public
The methods in an interface are not declared as  public , because they are public by default.
However, the methods in a class are not public by default—their default access level is “pack
age” access, which we discussed in Chapter 8. It is a common error to forget the  public reserved
word when declaring a method from an interface:
public class BankAccount implements Measurable
{
. . .
double getMeasure() //  Oops—should be public
{
return balance;
}
}
Then the compiler complains that the method has a weaker access level, namely package access
instead of public access. The remedy is to declare the method as  public .
trying to instantiate an interface
You can declare variables whose type is an interface, for example:
Measurable meas;
However, you can never construct an object of an interface type:
Measurable meas = new Measurable(); //  Error
Interfaces aren’t classes. There are no objects whose types are interfaces. If an interface vari
able refers to an object, then the object must belong to some class—a class that implements the
interface:
Measurable meas = new BankAccount(); //  OK
constants in interfaces
Interfaces cannot have instance variables, but it is legal to specify constants. For example,
the  SwingConstants interface declares various constants, such as  SwingConstants.NORTH ,  Swing
Constants.EAST , and so on.
When declaring a constant in an interface, you can (and should) omit the reserved words
public static final , because all variables in an interface are automatically  public static final .
For example,
public interface SwingConstants
{
int NORTH = 1;
int NORTHEAST = 2;
int EAST = 3;
. . .
}
Common error 10.1
© John Bell/iStockphoto.
Common error 10.2
© John Bell/iStockphoto.
special topic 10.1
© Eric Isselé/iStockphoto.
10.2 Working with Interface Variables  471
10.2 Working with Interface Variables
In the following sections, you will learn how to work with variables whose types are
interfaces.
10.2.1 Converting from Classes to Interfaces
Have a close look at the call
double averageBalance = Data.average(accounts);
from the program of the preceding section. Here,  accounts is an array of  BankAccount
objects. However, the  average method expects an array of  Measurable objects:
public double average(Measurable[] objects)
It is legal to convert from the  BankAccount type to the  Measurable type. In general, you
can convert from a class type to the type of any interface that the class implements.
For example,
BankAccount account = new BankAccount(1000);
Measurable meas = account; //  OK
Alternatively, a  Measurable variable can refer to an object of the  Country class of the pre-
ceding section because that class also implements the  Measurable interface.
Country uruguay = new Country("Uruguay", 176220);
Measurable meas = uruguay; //  Also OK
However, the  Rectangle class from the standard library doesn’t implement the  Measur-
able interface. There fore, the following assignment is an error:
Measurable meas = new Rectangle(5, 10, 20, 30); //  Error
10.2.2 Invoking Methods on Interface Variables
Now suppose that the variable  meas has been initialized with a reference to an object
of some class that implements the  Measurable interface. You don’t know to which class
that object belongs. But you do know that the class implements the methods of the
interface type, and you can invoke them:
double result = meas.getMeasure();
You can convert
from a class type to
an interface type,
provided the class
implements the
interface.
Figure 2  Variables of Class and Interface Types
account =
balance =
BankAccount
1000
meas =
Variable has
type  BankAccount .
Variable has type  Measurable ;
can only invoke  getMeasure method.
472 Chapter 10  Interfaces
Now let’s think through the call to the  getMeasure method more carefully. Which  get-
Measure method is called? The  BankAccount and  Country classes provide two different
implementations of that method. How does the correct method get called if the caller
doesn’t even know the exact class to which  meas belongs?
This is again polymorphism in action. (See Section 9.4 for a discussion of polymor-
phism.) The Java virtual machine locates the correct method by first looking at the
class of the actual object, and then calling the method with the given name in that
class. That is, if  meas refers to a  BankAccount object, then the  BankAccount.getMeasure
method is called. If  meas refers to a  Country object, then the  Country.getMeasure method
is called.
10.2.3 Casting from Interfaces to Classes
Occasionally, it happens that you store an object in an interface reference and you
need to convert its type back. Consider this method that returns the object with the
larger measure:
public static Measurable larger(Measurable obj1, Measurable obj2)
{
if (obj1.getMeasure() > obj2.getMeasure())
{
return obj1;
}
else
{
return obj2;
}
}
The  larger method returns the object with the larger measure, as a  Measurable refer-
ence. It has no choice––it does not know the exact type of the object. Let’s use the
method:
Country uruguay = new Country("Uruguay", 176220);
Country thailand = new Country("Thailand", 513120);
Measurable max = larger(uruguay, thailand);
Now what can you do with the  max reference? You know it refers to a  Country object,
but the compiler doesn’t. For example, you cannot call the  getName method:
String countryName = max.getName(); //  Error
That call is an error, because the  Measurable type has no  getName method.
However, as long as you are absolutely sure that  max refers to a  Country object, you
can use the cast nota tion to convert its type back:
Country maxCountry = (Country) max;
String name = maxCountry.getName();
Method calls on an
interface reference
are polymorphic. 
The appropriate
method is deter-
mined at run time.
Figure 3  
An Interface Reference 
Can Refer to an Object 
of Any Class that 
Implements the Interface
?
meas =
Has a
getMeasure method
You need a cast to
convert from an
interface type to a
class type.
10.3 the Comparable Interface 473
If you are wrong, and the object doesn’t actually refer to a
country, a runtime exception will occur.
If a  Person object is actually a  Superhero , you need a cast
before you can apply any  Superhero methods.
6.  Can you use a cast  (BankAccount) meas to convert a  Measurable variable  meas to a
BankAccount reference?
7.  If both  BankAccount and  Country implement the  Measurable interface, can a  Country
reference be converted to a  BankAccount reference?
8.  Why is it impossible to construct a  Measurable object?
9.  Why can you nevertheless declare a variable whose type is  Measurable ?
10.  What does this code fragment print? Why is this an example of polymorphism?
Measurable[] data = { new BankAccount(10000), new Country("Belgium", 30510) };
System.out.println(average(data));
practice it  Now you can try these exercises at the end of the chapter: R10.1, R10.2, R10.3.
10.3 the  Comparable Interface
In the preceding sections, we defined the  Measurable interface and provided an  average
method that works with any classes implementing that interface. In this section, you
will learn about the  Comparable interface of the standard Java library.
The  Measurable interface is used for measuring a single object. The  Comparable inter
face is more complex because comparisons involve two objects. The interface declares
a  compareTo method. The call
a.compareTo(b)
must return a negative number if  a should come before  b , zero if  a and  b are the same,
and a positive num ber if  b should come before  a .
The  Comparable interface has a single method:
public interface Comparable
{
int compareTo(Object otherObject);
}
© Andrew Rich/iStockphoto.
fUll code example
Go to  wiley.com/go/
javacode to down-
load a demon stration
of conversions
between class and
interface types.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
WorKeD exaMple 10.1  investigating number sequences
Learn how to use a  Sequence interface to investigate properties of
arbitrary number sequences. Go to  wiley.com/go/javaexamples and
download Worked Example 10.1.
© Norebbo/iStockphoto
Implement the
Comparable interface
so that objects
of your class can
be compared, for
example, in a
sort method.
474 Chapter 10 Interfaces
For example, the  BankAccount class can implement  Comparable like this:
public class BankAccount implements Comparable
{
. . .
public int compareTo(Object otherObject)
{
BankAccount other = (BankAccount) otherObject;
if (balance < other.balance) { return 1; }
if (balance > other.balance) { return 1; }
return 0;
}
. . .
}
This  compareTo method compares bank accounts by their balance. Note that the
compareTo method has a parameter variable of type  Object . To turn it into a  BankAccount
reference, we use a cast:
BankAccount other = (BankAccount) otherObject;
Once the  BankAccount class implements the  Comparable interface, you can sort an array
of bank accounts with the  Arrays.sort method:
BankAccount[] accounts = new BankAccount[3];
accounts[0] = new BankAccount(10000);
accounts[1] = new BankAccount(0);
accounts[2] = new BankAccount(2000);
Arrays.sort(accounts);
The  accounts array is now sorted by increasing balance.
The  compareTo method checks whether
another object is larger or smaller.
11.  How can you sort an array of  Country objects by increasing area?
12.  Can you use the  Arrays.sort method to sort an array of  String objects? Check the
API documentation for the  String class.
13.  Can you use the  Arrays.sort method to sort an array of  Rectangle objects? Check
the API documentation for the  Rectangle class.
14.  Write a method  max that finds the larger of any two  Comparable objects.
15.  Write a call to the method of Self Check 14 that computes the larger of two bank
accounts, then prints its balance.
practice it  Now you can try these exercises at the end of the chapter: E10.6, E10.23.
fUll code example
Go to  wiley.com/go/
javacode to download
a program that
demonstrates the
Comparable interface
with bank accounts.
© Janis Dreosti/iStockphoto.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
10.3 The Comparable Interface  475
The  clone Method and the  Cloneable Interface
You know that copying an object reference simply gives you two references to the same object:
BankAccount account = new BankAccount(1000);
BankAccount account2 = account;
account2.deposit(500);
//  Now both  account and account2 refer to a bank account with a balance of 1500
What can you do if you actually want to make a copy of an object? That is the purpose of the
clone method. The  clone method must return a new object that has an identical state to the
existing object (see Figure 4).
Here is how to call it:
BankAccount clonedAccount = (BankAccount) account.clone();
The return type of the  clone method is the class  Object . When you call the method, you must
use a cast to inform the compiler that  account.clone() really returns a  BankAccount object.
The  Object.clone method is the starting point for the  clone methods in your own classes.
It creates a new object of the same type as the original object. It also automatically copies
the instance variables from the original object to the cloned object. Here is a first attempt to
implement the  clone method for the  BankAccount class:
public class BankAccount
{
. . .
public Object clone()
{
//  Not complete
Object clonedAccount = super.clone();
return clonedAccount;
}
}
However, this  Object.clone method must be used with care. It only shifts the problem of clon-
ing by one level; it does not completely solve it. Specifically, if an object contains a reference to
another object, then the  Object.clone method makes a copy of that object reference, not a clone
of that object. Figure 5 shows how the  Object.clone method works with a  Customer object that
has references to a  String object and a  BankAccount object. As you can see, the  Object.clone
method copies the references to the cloned  Customer object and does not clone the objects to
which they refer. Such a copy is called a shallow copy.
Special Topic 10.2
© Eric Isselé/iStockphoto.
© Alex Gumerov/iStockphoto.
The  clone method makes an
identical copy of an object.
Figure 4  Cloning Objects
account =
balance =
BankAccount
10000
clonedAccount =
balance =
BankAccount
10000
476 Chapter 10 Interfaces
figure 5 
the  Object.clone
Method Makes a
shallow Copy
String
balance =
BankAccount
10000
name =
Customer
account =
name =
Customer
account =
There is a reason why the  Object.clone method does not systematically clone all sub
objects. In some situations, it is unnecessary. For example, if an object contains a reference to a
string, there is no harm in copying the string refer ence, because Java string objects can never
change their contents. The  Object.clone method does the right thing if an object contains only
numbers, Boolean values, and strings. But it must be used with caution when an object con
tains references to mutable objects.
For that reason, there are two safeguards built into the  Object.clone method to ensure that
it is not used acciden tally. First, the method is declared  protected (see Special Topic 9.5). This
prevents you from accidentally calling  x.clone() if the class to which  x belongs hasn’t declared
clone to be public.
As a second precaution,  Object.clone checks that the object being cloned implements the
Cloneable interface. If not, it throws an exception. The  Object.clone method looks like this:
public class Object
{
protected Object clone()
throws CloneNotSupportedException
{
if (this instanceof Cloneable)
{
//  Copy the instance variables
. . .
}
else
{
throw new CloneNotSupportedException();
}
}
}
Unfortunately, all that safeguarding means that the legitimate callers of  Object.clone() pay a
price—they must catch that exception (see Chapter 11) even if their class implements  Cloneable .
public class BankAccount implements Cloneable
{
. . .
public Object clone()
{
try
{
return super.clone();
}
10.4 Using Interfaces for Callbacks 477
catch (CloneNotSupportedException e)
{
//  Can’t happen because we implement Cloneable  but we still must catch it.
return null;
}
}
}
If an object contains a reference to another mutable object, then you must call  clone for that
reference. For example, suppose the  Customer class has an instance variable of class  BankAccount .
You can implement  Customer.clone as follows:
public class Customer implements Cloneable
{
private String name;
private BankAccount account;
. . .
public Object clone()
{
try
{
Customer cloned = (Customer) super.clone();
cloned.account = (BankAccount) account.clone();
return cloned;
}
catch(CloneNotSupportedException e)
{ //  Can’t happen because we implement Cloneable
return null;
}
}
}
In general, implementing the  clone method requires these steps:
• Make the class implement the  Cloneable interface type.
• In the  clone method, call  super.clone() . Catch the  CloneNotSupportedException if the
superclass is  Object .
• Clone any mutable instance variables.
10.4 Using Interfaces for Callbacks
In this section, we introduce the notion of a callback, show how it leads to a more
flexible  average method, and study how a callback can be implemented in Java by
using interface types.
To understand why a further improvement to the  average method is desirable, con
sider these limitations of the  Measurable interface:
• You can add the  Measurable interface only to classes under your control. If you
want to process a set of  Rectangle objects, you cannot make the  Rectangle class
implement another interface—it is a library class, which you cannot change.
• You can measure an object in only one way. If you want to analyze a set of cars
both by speed and price, you are stuck.
Therefore, let’s rethink the  average method. The method measures objects, requiring
them to be of type  Measurable . The responsibility of measuring lies with the added
objects themselves. That is the cause for the limitations.
478 Chapter 10 Interfaces
A callback object waits to be called.
The algorithm that has the callback object
only calls it when it needs to have the
information that the callback can provide.
© Dan Herrick/iStockphoto.
It would be better if we could give the average method the data to be averaged, and
separately a method for measuring the objects. When collecting rect angles, we might
give it a method for computing the area of a rectangle. When collecting cars, we might
give it a method for getting the car’s price.
Such a method is called a callback. A callback is a mechanism for bundling up a
block of code so that it can be invoked at a later time.
In some programming languages, it is possible to specify callbacks directly, as
blocks of code or names of methods. But Java is an objectoriented programming
language. Therefore, you turn callbacks into objects. This process starts by declaring
an interface for the callback:
public interface Measurer
{
double measure(Object anObject);
}
The  measure method measures an object and returns its measurement. Here we use the
fact that all objects can be converted to the type  Object .
The code that makes the call to the callback receives an object of a class that imple
ments this interface. In our case, the improved  average method receives a  Measurer
object.
public static double average(Object[] objects, Measurer meas)
{
double sum = 0;
for (Object obj : objects)
{
sum = sum + meas.measure(obj);
}
if (objects.length > 0) { return sum / objects.length; }
else { return 0; }
}
The  average method simply makes a callback to the  measure method whenever it needs
to measure any object.
Finally, a specific callback is obtained by implementing the  Measurer interface. For
example, here is how you can measure rectangles by area. Provide a class
public class AreaMeasurer implements Measurer
{
public double measure(Object anObject)
{
Rectangle aRectangle = (Rectangle) anObject;
double area = aRectangle.getWidth() * aRectangle.getHeight();
return area;
}
}
a callback is a
mechanism for
specifying code that
is executed at a
later time.
10.4 Using Interfaces for Callbacks 479
figure 6  UMl Diagram of the  Data Class and the  Measurer Interface
AreaMeasurer Rectangle
‹‹interface››
Measurer
Data
Note that the  measure method has a parameter variable of type  Object , even though this
particular mea surer just wants to measure rectangles. The method parameter types
must match those of the  measure method in the  Measurer interface. Therefore, the
anObject parameter variable is cast to the  Rectangle type:
Rectangle aRectangle = (Rectangle) anObject;
What can you do with an  AreaMeasurer ? You need it to compute the average area of
rectangles. Construct an object of the  AreaMeasurer class and pass it to the  average
method:
Measurer areaMeas = new AreaMeasurer();
Rectangle[] rects
= { new Rectangle(5, 10, 20, 30), new Rectangle(10, 20, 30, 40) };
double averageArea = average(rects, areaMeas);
The  average method will ask the  AreaMeasurer object to measure the rectangles.
Figure 6 shows the UML diagram of the classes and interfaces of this solution.
As in Figure 1, the  Data class (which holds the  average method) is decoupled from the
class whose objects it processes ( Rectangle ). However, unlike in Figure 1, the  Rectangle
class is no longer coupled with another class. Instead, to process rectangles, you pro
vide a small “helper” class  AreaMeasurer . This helper class has only one purpose: to tell
the  average method how to measure its objects.
Here is the complete program:
section_4/measurer.java
1 /**
2 Describes any class whose objects can measure other objects.
3 */
4 public interface Measurer
5 {
6 /**
7 Computes the measure of an object.
8 @param anObject  the object to be measured
9 @return  the measure
10 */
11 double measure(Object anObject);
12 }
480 Chapter 10 Interfaces
section_4/areameasurer.java
1 import java.awt.Rectangle;
2
3 /**
4 Objects of this class measure rectangles by area.
5 */
6 public class AreaMeasurer implements Measurer
7 {
8 public double measure(Object anObject)
9 {
10 Rectangle aRectangle = (Rectangle) anObject;
11 double area = aRectangle.getWidth() * aRectangle.getHeight();
12 return area;
13 }
14 }
section_4/data.java
1 public class Data
2 {
3 /**
4 Computes the average of the measures of the given objects.
5 @param objects  an array of objects
6 @param meas  the measurer for the objects
7 @return  the average of the measures
8 */
9 public static double average(Object[] objects, Measurer meas)
10 {
11 double sum = 0;
12 for (Object obj : objects)
13 {
14 sum = sum + meas.measure(obj);
15 }
16 if (objects.length > 0) { return sum / objects.length; }
17 else { return 0; }
18 }
19 }
section_4/measurertester.java
1 import java.awt.Rectangle;
2
3 /**
4 This program demonstrates the use of a  Measurer .
5 */
6 public class MeasurerTester
7 {
8 public static void main(String[] args)
9 {
10 Measurer areaMeas = new AreaMeasurer();
11
12 Rectangle[] rects = new Rectangle[]
13 {
14 new Rectangle(5, 10, 20, 30),
15 new Rectangle(10, 20, 30, 40),
16 new Rectangle(20, 30, 5, 15)
17 };
18
10.5 Inner Classes 481
19 double averageArea = Data.average(rects, areaMeas);
20 System.out.println("Average area: " + averageArea);
21 System.out.println("Expected: 625");
22 }
23 }
program run
Average area: 625
Expected: 625
16.  Suppose you want to use the  average method of Section 10.1 to find the average
length of  String objects. Why can’t this work?
17.  How can you use the  average class of this section to find the average length of
String objects?
18.  Why does the  measure method of the  Measurer interface have one more argument
than the  getMeasure method of the  Measurable interface?
19.  Write a method  max with three arguments that finds the larger of any two objects,
using a  Measurer to compare them.
20.  Write a call to the method of Self Check 19 that computes the larger of two rect
angles, then prints its width and height.
practice it  Now you can try these exercises at the end of the chapter: R10.7, E10.4, E10.5.
10.5 Inner Classes
The  AreaMeasurer class of the preceding section is a very trivial class. We need this class
only because the  average method needs an object of some class that implements the
Measurer interface. When you have a class that serves a very lim ited purpose, such as
this one, you can declare the class inside the method that needs it:
public class MeasurerTester
{
public static void main(String[] args)
{
class AreaMeasurer implements Measurer
{
. . .
}
. . .
Measurer areaMeas = new AreaMeasurer();
double averageArea = Data.average(rects, areaMeas);
. . .
}
}
A class that is declared inside another class, such as the AreaMeasurer class in this exam
ple, is called an inner class. This arrangement signals to the reader of your program
that the  AreaMeasurer class is not interesting beyond the scope of this method. Since
an inner class inside a method is not a publicly accessi ble feature, you don’t need to
document it as thoroughly.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
© maureenpr/iStockphoto.
An inner class is a
class that is declared
inside another class.
an inner class is
declared inside
another class.
482 Chapter 10 Interfaces
You can also declare an inner class inside an enclosing class, but outside of its meth
ods. Then the inner class is available to all methods of the enclosing class.
public class MeasurerTester
{
class AreaMeasurer implements Measurer
{
. . .
}
public static void main(String[] args)
{
Measurer areaMeas = new AreaMeasurer();
double averageArea = Data.average(rects, areaMeas);
. . .
}
}
When you compile the source files for a program that uses inner classes, have a look
at the class files in your program directory—you will find that the inner classes are
stored in files with curious names, such as  MeasurerTester$1AreaMeasurer.class . The
exact names aren’t important. The point is that the com piler turns an inner class into
a regular class file.
21.  Why would you use an inner class instead of a regular class?
22.  When would you place an inner class inside a class but outside any methods?
23.  How many class files are produced when you compile the  MeasurerTester pro
gram from this section?
practice it  Now you can try these exercises at the end of the chapter: E10.7, E10.9.
anonymous classes
An entity is anonymous if it does not have a name. In a program, something that is only used
once doesn’t usually need a name. For example, you can replace
Country belgium = new Country("Belgium", 30510);
countries.add(belgium);
with
countries.add(new Country("Belgium", 30510));
if the country is not used elsewhere in the same method. The object  new Country("Belgium",
30510) is an anonymous object. Pro grammers like anonymous objects, because they don’t
have to go through the trouble of coming up with a name. If you have struggled with the deci
sion whether to call a coin  c ,  dime , or  aCoin , you’ll understand this sentiment.
Inner classes often give rise to a similar situation. After a single object of the  AreaMeasurer
has been con structed, the class is never used again. In Java, it is possible to declare anonymous
classes if all you ever need is a sin gle object of the class.
public static void main(String[] args)
{
//  Construct an object of an anonymous class
Measurer m = new Measurer()
// Class declaration starts here
{
Inner classes are
commonly used for
utility classes that
should not be
visible elsewhere
in a program.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
special topic 10.3
© Eric Isselé/iStockphoto.
testing track 10.6 Mock objects 483
public double measure(Object anObject)
{
Rectangle aRectangle = (Rectangle) anObject;
return aRectangle.getWidth() * aRectangle.getHeight();
}
};
double result = Data.average(rectangles, m);
. . .
}
This means: Construct an object of a class that implements the  Measurer interface by declaring
the  measure method as specified. Many programmers like this style, but we will not use it in this
book.
10.6 Mock objects
When you work on a program that consists of multiple classes, you often want to test
some of the classes before the entire program has been completed. A very effective
technique for this purpose is the use of mock objects. A mock object provides the
same services as another object, but in a simplified manner.
Consider a grade book application that manages quiz scores for students. This
calls for a class  GradeBook with methods such as
public void addScore(int studentId, double score)
public double getAverageScore(int studentId)
public void save(String filename)
Now consider the class  GradingProgram that manipulates a  GradeBook object. That class
calls the methods of the  GradeBook class. We would like to test the  GradingProgram class
without having a fully functional  Grade Book class.
To make this work, declare an interface type with the same methods that the  Grade
Book class provides. A common convention is to use the letter  I as the prefix for such
an interface:
public interface IGradeBook
{
void addScore(int studentId, double score);
double getAverageScore(int studentId);
void save(String filename);
. . .
}
If you just want to practice arranging the Christmas
decorations, you don’t need a real tree. Similarly,
when you develop a computer program, you can
use mock objects to test parts of your program.
a mock object
provides the same
services as another
object, but in a
simplified manner.
© Don Nichols/iStockphoto.
484 Chapter 10 Interfaces  Graphics track
The  GradingProgram class should only use this interface, never the  GradeBook class. Of
course, the  GradeBook class will implement this interface, but as already mentioned, it
may not be ready for some time.
In the meantime, provide a mock implementation that makes some simplifying
assumptions. Saving is not actually necessary for testing the user interface. We can
temporarily restrict to the case of a single stu dent.
public class MockGradeBook implements IGradeBook
{
private ArrayList<Double> scores;
public MockGradeBook() { scores = new ArrayList<Double>(); }
public void addScore(int studentId, double score)
{
//  Ignore  studentId
scores.add(score);
}
public double getAverageScore(int studentId)
{
double total = 0;
for (double x : scores) { total = total + x; }
return total / scores.size();
}
public void save(String filename)
{
//  Do nothing
}
. . .
}
Now construct an instance of  MockGradeBook and use it in the  GradingProgram class. You
can immediately test the  GradingProgram class. When you are ready to test the actual
class, simply use a  GradeBook instance instead. Don’t erase the mock class—it will still
come in handy for regression testing.
24.  Why is it necessary that the real class and the mock class implement the same
interface type?
25.  Why is the technique of mock objects particularly effective when the  GradeBook
and  GradingProgram class are developed by two programmers?
practice it  Now you can try these exercises at the end of the chapter: P10.12, P10.13.
10.7 event handling
This and the following sections continue the book’s graphics track. You will learn
how interfaces are used when programming graphical user interfaces.
In the applications that you have written so far, user input was under control of
the program. The pro gram asked the user for input in a specific order. For example,
a program might ask the user to supply first a name, then a dollar amount. But the
programs that you use every day on your computer don’t work like that. In a pro
gram with a graphical user interface, the user is in control. The user can use both the
Both the mock class
and the actual class
implement the
same interface.
fUll code example
Go to  wiley.com/go/
javacode to download
a program that
demonstrates the
use of mock objects
for testing.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
Graphics track 10.7 event handling 485
mouse and the keyboard and can manipulate many parts of the user interface in any
desired order. For example, the user can enter information into text fields, pull down
menus, click buttons, and drag scroll bars in any order. The program must react to
the user commands in whatever order they arrive. Having to deal with many possible
inputs in random order is quite a bit harder than simply forcing the user to supply
input in a fixed order.
In the following sections, you will learn how to write Java programs that can react
to userinterface events, such as menu selections and mouse clicks. The Java window
ing toolkit has a very sophisticated mechanism that allows a program to specify the
events in which it is interested and which objects to notify when one of these events
occurs.
10.7.1 listening to events
Whenever the user of a graphical program types
characters or uses the mouse anywhere inside one
of the windows of the program, the Java window
ing toolkit sends a notification to the program that
an event has occurred. The windowing toolkit gen
erates huge numbers of events. For example, when
ever the mouse moves a tiny interval over a win
dow, a “mouse move” event is generated. Whenever
the mouse button is clicked, “mouse pressed” and
“mouse released” events are generated. In addition,
higherlevel events are generated when a user selects
a menu item or button.
Most programs don’t want to be flooded by irrel
evant events. For example, consider what happens
when selecting a menu item with the mouse. The
mouse moves over the menu item, then the mouse
button is pressed, and finally the mouse button is released. Rather than receiving
all these mouse events, a program can indicate that it only cares about menu selec
tions, not about the underlying mouse events. However, if the mouse input is used
for drawing shapes on a virtual canvas, it is necessary to closely track mouse events.
Every program must indicate which events it needs to receive. It does that by
installing event listener objects. An event listener object belongs to a class that you
provide. The methods of your event listener classes contain the instructions that you
want to have executed when the events occur.
To install a listener, you need to know the event source. The event source is the
userinterface compo nent that generates a particular event. You add an event listener
object to the appropriate event sources. Whenever the event occurs, the event source
calls the appropriate methods of all attached event listeners.
This sounds somewhat abstract, so let’s run through an extremely simple program
that prints a mes sage whenever a button is clicked (see Figure 7). Button listeners
must belong to a class that implements the  ActionListener interface:
public interface ActionListener
{
void actionPerformed(ActionEvent event);
}
User-interface events
include key presses,
mouse moves,
button clicks, menu
selections, and so on.
© Seriy Tryapitsyn/iStockphoto.
In an event-driven user interface,
the program receives an event
whenever the user manipulates
an input component.
an event listener
belongs to a class
that is provided
by the application
programmer. Its
methods describe
the actions to be
taken when an
event occurs.
event sources report
on events. When an
event occurs, the
event source notifies
all event listeners.
486 Chapter 10 Interfaces  Graphics track
figure 7  Implementing an action listener
This particular interface has a single method,  actionPerformed . It is your job to supply
a class whose  action Performed method contains the instructions that you want exe
cuted whenever the button is clicked. Here is a very simple example of such a listener
class:
section_7_1/clicklistener.java
1 import java.awt.event.ActionEvent;
2 import java.awt.event.ActionListener;
3
4 /**
5 An action listener that prints a message.
6 */
7 public class ClickListener implements ActionListener
8 {
9 public void actionPerformed(ActionEvent event)
10 {
11 System.out.println("I was clicked.");
12 }
13 }
We ignore the values of the  event parameter variable of the  actionPerformed method—it
contains additional details about the event, such as the time at which it occurred.
Once the listener class has been declared, we need to construct an object of the
class and add it to the button:
ActionListener listener = new ClickListener();
button.addActionListener(listener);
Whenever the button is clicked, it calls
listener.actionPerformed(event);
As a result, the message is printed.
You can think of the  actionPerformed method as another example of a callback,
similar to the  measure method of the  Measurer class. The windowing toolkit calls the
actionPerformed method whenever the button is pressed, whereas the  Data class calls
the  measure method whenever it needs to measure an object.
The  ButtonViewer class, shown below, constructs a frame with a button and adds
a  ClickListener to the button. You can test this program out by opening a console
win dow, starting the  ButtonViewer program from that console window, clicking the
button, and watching the messages in the console window.
Use  JButton
components for
buttons. attach an
ActionListener to
each button.
Graphics track 10.7 event handling 487
section_7_1/buttonViewer.java
1 import java.awt.event.ActionListener;
2 import javax.swing.JButton;
3 import javax.swing.JFrame;
4
5 /**
6 This program demonstrates how to install an action listener.
7 */
8 public class ButtonViewer
9 {
10 private static final int FRAME_WIDTH = 100;
11 private static final int FRAME_HEIGHT = 60;
12
13 public static void main(String[] args)
14 {
15 JFrame frame = new JFrame();
16 JButton button = new JButton("Click me!");
17 frame.add(button);
18
19 ActionListener listener = new ClickListener();
20 button.addActionListener(listener);
21
22 frame.setSize(FRAME_WIDTH, FRAME_HEIGHT);
23 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
24 frame.setVisible(true);
25 }
26 }
10.7.2 Using Inner Classes for listeners
In the preceding section, you saw how the code that is executed when a button is
clicked is placed into a listener class. It is common to implement listener classes as
inner classes like this:
JButton button = new JButton(". . .");
//  This inner class is declared in the same method as the button variable
class MyListener implements ActionListener
{
. . .
};
ActionListener listener = new MyListener();
button.addActionListener(listener);
There are two advantages to making a listener class into an inner class. First, listener
classes tend to be very short. You can put the inner class close to where it is needed,
without cluttering up the remainder of the project. Moreover, inner classes have a
very attractive feature: Their methods can access instance variables and methods of
the surrounding class.
This feature is particularly useful when implementing event handlers. It allows
the inner class to access variables with out having to receive them as constructor or
method arguments.
Let’s look at an example. Suppose we want to add interest to a bank account when
ever a button is clicked.
Methods of an inner
class can access
variables from the
surrounding class.
488 Chapter 10 Interfaces  Graphics track
JButton button = new JButton("Add Interest");
final BankAccount account = new BankAccount(INITIAL_BALANCE);
//  This inner class is declared in the same method as the  account and  button variables.
class AddInterestListener implements ActionListener
{
public void actionPerformed(ActionEvent event)
{
//  The listener method accesses the  account variable
//  from the surrounding block
double interest = account.getBalance() * INTEREST_RATE / 100;
account.deposit(interest);
}
};
ActionListener listener = new AddInterestListener();
button.addActionListener(listener);
There is a technical wrinkle. An inner class can access surrounding local variables
only if they are declared as  final . That sounds like a restriction, but it is usually not
an issue in practice. Keep in mind that an object variable is  final when the variable
always refers to the same object. The state of the object can change, but the variable
can’t refer to a different object. For example, in our program, we never intended to
have the  account variable refer to multiple bank accounts, so there was no harm in
declaring it as  final .
An inner class can also access instance variables of the surrounding class, again
with a restriction. The instance variable must belong to the object that constructed
the inner class object. If the inner class object was created inside a static method, it can
only access static variables.
Here is the source code for the program:
section_7_2/investmentViewer1.java
1 import java.awt.event.ActionEvent;
2 import java.awt.event.ActionListener;
3 import javax.swing.JButton;
4 import javax.swing.JFrame;
5
6 /**
7 This program demonstrates how an action listener can access
8 a variable from a surrounding block.
9 */
10 public class InvestmentViewer1
11 {
12 private static final int FRAME_WIDTH = 120;
13 private static final int FRAME_HEIGHT = 60;
14
15 private static final double INTEREST_RATE = 10;
16 private static final double INITIAL_BALANCE = 1000;
17
18 public static void main(String[] args)
19 {
20 JFrame frame = new JFrame();
21
22 //  The button to trigger the calculation
23 JButton button = new JButton("Add Interest");
24 frame.add(button);
25
local variables that
are accessed by an
inner class method
must be declared
as  final .
Graphics track 10.7 event handling 489
26 //  The application adds interest to this bank account
27 final BankAccount account = new BankAccount(INITIAL_BALANCE);
28
29 class AddInterestListener implements ActionListener
30 {
31 public void actionPerformed(ActionEvent event)
32 {
33 //  The listener method accesses the  account variable
34 //  from the surrounding block
35 double interest = account.getBalance() * INTEREST_RATE / 100;
36 account.deposit(interest);
37 System.out.println("balance: " + account.getBalance());
38 }
39 }
40
41 ActionListener listener = new AddInterestListener();
42 button.addActionListener(listener);
43
44 frame.setSize(FRAME_WIDTH, FRAME_HEIGHT);
45 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
46 frame.setVisible(true);
47 }
48 }
program run
balance: 1100.0
balance: 1210.0
balance: 1331.0
balance: 1464.1
26.  Which objects are the event source and the event listener in the  ButtonViewer
program?
27.  Why is it legal to assign a  ClickListener object to a variable of type  ActionListener ?
28.  When do you call the  actionPerformed method?
29.  Why would an inner class method want to access a variable from a surrounding
scope?
30.  If an inner class accesses a local variable from a surrounding scope, what special
rule applies?
practice it  Now you can try these exercises at the end of the chapter: R10.14, R10.20, E10.13.
modifying parameter types in the implementing method
When you implement an interface, you must declare each method exactly as it is specified in
the interface. Acciden tally making small changes to the parameter types is a common error.
Here is the classic example:
class MyListener implements ActionListener
{
public void actionPerformed()
//  Oops . . .  forgot ActionEvent  parameter variable
{
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
Common error 10.3
© John Bell/iStockphoto.
490 Chapter 10 Interfaces  Graphics track
. . .
}
}
As far as the compiler is concerned, this class fails to provide the method
public void actionPerformed(ActionEvent event)
You have to read the error message carefully and pay attention to the parameter and return
types to find your error.
trying to call listener methods
Some students try to call the listener methods themselves:
ActionEvent event = new ActionEvent(. ..); //  Don’t do this
listener.actionPerformed(event);
You should not call the listener. The Java user interface calls it when the program user has
clicked a button.
10.8 Building applications with Buttons
In this section, you will learn how to structure a graphical application that contains
buttons. We will put a button to work in our simple investment viewer program.
Whenever the button is clicked, interest is added to a bank account, and the new bal
ance is displayed (see Figure 8).
First, we construct an object of the  JButton class, passing the button label to the
constructor, like this:
JButton button = new JButton("Add Interest");
We also need a userinterface component that displays a message, namely the current
bank balance. Such a component is called a label. You pass the initial message string
to the  JLabel constructor, like this:
JLabel label = new JLabel("balance: " + account.getBalance());
The frame of our application contains both the button and the label. However, we
cannot simply add both components directly to the frame—they would be placed on
top of each other. The solution is to put them into a panel, a container for other user
interface components, and then add the panel to the frame:
JPanel panel = new JPanel();
panel.add(button);
panel.add(label);
frame.add(panel);
Common error 10.4
© John Bell/iStockphoto.
Use a  JPanel
container to
group multiple
user-interface
components
together.
figure 8  an application with a Button
Graphics track 10.8 Building applications with Buttons 491
Whenever a button is pressed, the  actionPerformed
method is called on all listeners.
© Eduard Andras/iStockphoto.
Now we are ready for the hard part—the event listener that handles button clicks. As
in the preceding section, it is necessary to provide a class that implements the  Action
Listener interface, and to place the but ton action into the  actionPerformed method. Our
listener class adds interest to the account and displays the new balance:
class AddInterestListener implements ActionListener
{
public void actionPerformed(ActionEvent event)
{
double interest = account.getBalance() * INTEREST_RATE / 100;
account.deposit(interest);
label.setText("balance: " + account.getBalance());
}
}
There is just a minor technicality. The  actionPerformed method manipulates the  account
and  label variables. These are local variables of the  main method of the investment
viewer program, not instance variables of the  AddInterestListener class. We there
fore need to declare the  account and  label variables as  final so that the  actionPerformed
method can access them.
Let’s put the pieces together:
public static void main(String[] args)
{
. . .
JButton button = new JButton("Add Interest");
final BankAccount account = new BankAccount(INITIAL_BALANCE);
final JLabel label = new JLabel("balance: " + account.getBalance());
class AddInterestListener implements ActionListener
{
public void actionPerformed(ActionEvent event)
{
double interest = account.getBalance() * INTEREST_RATE / 100;
account.deposit(interest);
label.setText("balance: " + account.getBalance());
}
}
ActionListener listener = new AddInterestListener();
button.addActionListener(listener);
. . .
}
With a bit of practice, you will learn to glance at this code and translate it into plain
English: “When the button is clicked, add interest and set the label text.”
specify button
click actions
through classes
that implement
the  ActionListener
interface.
492 Chapter 10 Interfaces  Graphics track
Here is the complete program. It demonstrates how to add multiple components
to a frame, by using a panel, and how to implement listeners as inner classes.
section_8/investmentViewer2.java
1 import java.awt.event.ActionEvent;
2 import java.awt.event.ActionListener;
3 import javax.swing.JButton;
4 import javax.swing.JFrame;
5 import javax.swing.JLabel;
6 import javax.swing.JPanel;
7
8 /**
9 This program displays the growth of an investment.
10 */
11 public class InvestmentViewer2
12 {
13 private static final int FRAME_WIDTH = 400;
14 private static final int FRAME_HEIGHT = 100;
15
16 private static final double INTEREST_RATE = 10;
17 private static final double INITIAL_BALANCE = 1000;
18
19 public static void main(String[] args)
20 {
21 JFrame frame = new JFrame();
22
23 //  The button to trigger the calculation
24 JButton button = new JButton("Add Interest");
25
26 //  The application adds interest to this bank account
27 final BankAccount account = new BankAccount(INITIAL_BALANCE);
28
29 //  The label for displaying the results
30 final JLabel label = new JLabel("balance: " + account.getBalance());
31
32 //  The panel that holds the userinterface components
33 JPanel panel = new JPanel();
34 panel.add(button);
35 panel.add(label);
36 frame.add(panel);
37
38 class AddInterestListener implements ActionListener
39 {
40 public void actionPerformed(ActionEvent event)
41 {
42 double interest = account.getBalance() * INTEREST_RATE / 100;
43 account.deposit(interest);
44 label.setText("balance: " + account.getBalance());
45 }
46 }
47
48 ActionListener listener = new AddInterestListener();
49 button.addActionListener(listener);
50
51 frame.setSize(FRAME_WIDTH, FRAME_HEIGHT);
52 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
53 frame.setVisible(true);
54 }
55 }
Graphics track 10.8 Building applications with Buttons 493
31.  How do you place the  "balance: . . ." message to the left of the  "Add Interest"
button?
32.  Why was it not necessary to declare the  button variable as  final ?
practice it  Now you can try these exercises at the end of the chapter: E10.14, E10.15, E10.16.
forgetting to attach a listener
If you run your program and find that your buttons seem to be dead, doublecheck that you
attached the button lis tener. The same holds for other userinterface components. It is a sur
prisingly common error to program the listener class and the event handler action without
actually attaching the listener to the event source.
don’t Use a container as a listener
In this book, we use inner classes for event listeners. That approach works for many different
event types. Once you master the technique, you don’t have to think about it anymore. Many
development environments automatically generate code with inner classes, so it is a good idea
to be familiar with them.
However, some programmers bypass the event listener classes and instead turn a container
(such as a panel or frame) into a listener. Here is a typical example. The  actionPerformed method
is added to the viewer class. That is, the viewer implements the  ActionListener interface.
public class InvestmentViewer
implements ActionListener //  This approach is not recommended
{
public InvestmentViewer()
{
JButton button = new JButton("Add Interest");
button.addActionListener(this);
. . .
}
public void actionPerformed(ActionEvent event)
{
. . .
}
. . .
}
Now the  actionPerformed method is a part of the  InvestmentViewer class rather than part of a
separate listener class. The listener is installed as  this .
This technique has two major flaws. First, it separates the button declaration from the
button action. Also, it doesn’t scale well. If the viewer class contains two buttons that each
generate action events, then the  actionPerformed method must investigate the event source,
which leads to code that is tedious and errorprone.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
Common error 10.5
© John Bell/iStockphoto.
programming tip 10.1
© Eric Isselé/iStockphoto.
494 Chapter 10 Interfaces  Graphics track
10.9 processing timer events
In this section we will study timer events and show how you can use them to imple
ment simple animations.
The  Timer class in the  javax.swing package generates
a sequence of action events, spaced at even time inter
vals. (You can think of a timer as an invisible button
that is automatically clicked.) This is useful whenever
you want to have an object updated at regular inter
vals. For example, in an animation, you may want to
update a scene ten times per second and redisplay the
image to give the illusion of movement.
When you use a timer, you specify the frequency
of the events and an object of a class that implements
the  ActionListener interface. Place whatever action you
want to occur inside the  action Performed method. Finally, start the timer.
class MyListener implements ActionListener
{
public void actionPerformed(ActionEvent event)
{
Action that is executed at each timer event
}
}
MyListener listener = new MyListener();
Timer t = new Timer(interval, listener);
t.start();
Then the timer calls the  actionPerformed method of the  listener object every  interval
milliseconds.
Our sample program will display a moving rectangle. We first supply a  Rectangle
Component class with a  moveRectangleBy method that moves the rectangle by a given
amount.
section_9/rectanglecomponent.java
1 import java.awt.Graphics;
2 import java.awt.Graphics2D;
3 import java.awt.Rectangle;
4 import javx.swing.JComponent;
5
6 /**
7 This component displays a rectangle that can be moved.
8 */
9 public class RectangleComponent extends JComponent
10 {
11 private static final int BOX_X = 100;
12 private static final int BOX_Y = 100;
13 private static final int BOX_WIDTH = 20;
14 private static final int BOX_HEIGHT = 30;
15
16 private Rectangle box;
17
18 public RectangleComponent()
19 {
© jeff giniewicz/iStockphoto.
A Swing timer notifies a listener
with each “tick”.
a timer generates
timer events at
fixed intervals.
Graphics track 10.9 processing timer events 495
20 //  The rectangle that the  paintComponent method draws
21 box = new Rectangle(BOX_X, BOX_Y, BOX_WIDTH, BOX_HEIGHT);
22 }
23
24 public void paintComponent(Graphics g)
25 {
26 Graphics2D g2 = (Graphics2D) g;
27 g2.draw(box);
28 }
29
30 /**
31 Moves the rectangle by a given amount.
32 @param dx  the amount to move in the xdirection
33 @param dy  the amount to move in the ydirection
34 */
35 public void moveRectangleBy(int dx, int dy)
36 {
37 box.translate(dx, dy);
38 repaint();
39 }
40 }
Note the call to  repaint in the  moveRectangleBy method. This call is necessary to ensure
that the component is repainted after the state of the rectangle object has been
changed. Keep in mind that the component object does not contain the pixels that
show the drawing. The component merely contains a  Rectangle object, which itself
contains four coordinate values. Calling  translate updates the rectangle coordinate
values. The call to  repaint forces a call to the  paintComponent method. The  paintCompo
nent method redraws the component, causing the rectangle to appear at the updated
location.
The  actionPerformed method of the timer listener simply calls  component.moveBy(1, 1) .
This moves the rectangle one pixel down and to the right. Because the  actionPerformed
method is called many times per sec ond, the rectangle appears to move smoothly
across the frame.
section_9/rectangleframe.java
1 import java.awt.event.ActionEvent;
2 import java.awt.event.ActionListener;
3 import javax.swing.JFrame;
4 import javax.swing.Timer;
5
6 /**
7 This frame contains a moving rectangle.
8 */
9 public class RectangleFrame extends JFrame
10 {
11 private static final int FRAME_WIDTH = 300;
12 private static final int FRAME_HEIGHT = 400;
13
14 private RectangleComponent scene;
15
16 class TimerListener implements ActionListener
17 {
18 public void actionPerformed(ActionEvent event)
19 {
20 scene.moveRectangleBy(1, 1);
the  repaint method
causes a component
to repaint itself. Call
repaint whenever
you modify the
shapes that the
paintComponent
method draws.
496 Chapter 10 Interfaces  Graphics track
21 }
22 }
23
24 public RectangleFrame()
25 {
26 scene = new RectangleComponent();
27 add(scene);
28
29 setSize(FRAME_WIDTH, FRAME_HEIGHT);
30
31 ActionListener listener = new TimerListener();
32
33 final int DELAY = 100; //  Milliseconds between timer ticks
34 Timer t = new Timer(DELAY, listener);
35 t.start();
36 }
37 }
section_9/rectangleViewer.java
1 import javax.swing.JFrame;
2
3 /**
4 This program moves the rectangle.
5 */
6 public class RectangleViewer
7 {
8 public static void main(String[] args)
9 {
10 JFrame frame = new RectangleFrame();
11 frame.setTitle("An animated rectangle");
12 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
13 frame.setVisible(true);
14 }
15 }
33.  Why does a timer require a listener object?
34.  What would happen if you omitted the call to  repaint in the  moveBy method?
practice it  Now you can try these exercises at the end of the chapter: E10.20, E10.21.
forgetting to repaint
You have to be careful when your event handlers change the data in a painted component.
When you make a change to the data, the component is not automatically painted with the
new data. You must call the  repaint method of the component, either in the event handler or
in the component’s mutator methods. Your component’s  paintComponent method will then be
invoked with an appropriate  Graphics object. Note that you should not call the  paintComponent
method directly.
This is a concern only for your own painted components. When you make a change to a
standard Swing compo nent such as a  JLabel , the component is automatically repainted.
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
Common error 10.6
© John Bell/iStockphoto.
Graphics track 10.10 Mouse events 497
10.10 Mouse events
If you write programs that show drawings, and you want users to manipulate the
drawings with a mouse, then you need to process mouse events. Mouse events are
more complex than button clicks or timer ticks.
A mouse listener must implement the  Mouse Listener interface, which contains the
following five meth ods:
public interface MouseListener
{
void mousePressed(MouseEvent event);
//  Called when a mouse button has been pressed on a component
void mouseReleased(MouseEvent event);
//  Called when a mouse button has been released on a component
void mouseClicked(MouseEvent event);
//  Called when the mouse has been clicked on a component
void mouseEntered(MouseEvent event);
//  Called when the mouse enters a component
void mouseExited(MouseEvent event);
//  Called when the mouse exits a component
}
The  mousePressed and  mouseReleased methods are
called whenever a mouse button is pressed or
released. If a button is pressed and released in
quick succession, and the mouse has not moved,
then the  mouseClicked method is called as well.
The  mouseEntered and  mouseExited methods can
be used to paint a userinterface component in
a special way whenever the mouse is pointing
inside it.
The most commonly used method is  mouse
Pressed . Users generally expect that their actions
are pro cessed as soon as the mouse button is
pressed.
You add a mouse listener to a component by calling the  addMouseListener method:
public class MyMouseListener implements MouseListener
{
//  Implements five methods
}
MouseListener listener = new MyMouseListener();
component.addMouseListener(listener);
In our sample program, a user clicks on a component containing a rectangle. When
ever the mouse button is pressed, the rectangle is moved to the mouse location. We
first enhance the  RectangleComponent class and add a  moveRectangleTo method to move
the rectangle to a new position.
section_10/rectanglecomponent2.java
1 import java.awt.Graphics;
2 import java.awt.Graphics2D;
3 import java.awt.Rectangle;
4 import javax.swing.JComponent;
Use a mouse listener
to capture
mouse events.
© james Brey/iStockphoto.
In Swing, a mouse event isn’t a gather-
ing of rodents; it’s notification of a
mouse click by the program user.
498 Chapter 10 Interfaces  Graphics track
5
6 /**
7 This component displays a rectangle that can be moved.
8 */
9 public class RectangleComponent2 extends JComponent
10 {
11 private static final int BOX_X = 100;
12 private static final int BOX_Y = 100;
13 private static final int BOX_WIDTH = 20;
14 private static final int BOX_HEIGHT = 30;
15
16 private Rectangle box;
17
18 public RectangleComponent2()
19 {
20 //  The rectangle that the  paintComponent method draws
21 box = new Rectangle(BOX_X, BOX_Y, BOX_WIDTH, BOX_HEIGHT);
22 }
23
24 public void paintComponent(Graphics g)
25 {
26 Graphics2D g2 = (Graphics2D) g;
27 g2.draw(box);
28 }
29
30 /**
31 Moves the rectangle to the given location.
32 @param x  the xposition of the new location
33 @param y  the yposition of the new location
34 */
35 public void moveRectangleTo(int x, int y)
36 {
37 box.setLocation(x, y);
38 repaint();
39 }
40 }
Note the call to  repaint in the  moveRectangleTo method. As explained in the preced
ing section, this call causes the component to repaint itself and show the rectangle in
the new position.
Now, add a mouse listener to the component. Whenever the mouse is pressed, the
listener moves the rectangle to the mouse location.
class MousePressListener implements MouseListener
{
public void mousePressed(MouseEvent event)
{
int x = event.getX();
int y = event.getY();
component.moveRectangleTo(x, y);
}
//  Donothing methods
public void mouseReleased(MouseEvent event) {}
public void mouseClicked(MouseEvent event) {}
public void mouseEntered(MouseEvent event) {}
public void mouseExited(MouseEvent event) {}
}
Graphics track 10.10 Mouse events 499
figure 9 
Clicking the Mouse Moves
the rectangle
It often happens that a particular listener specifies actions only for one or two of the
listener methods. Nevertheless, all five methods of the interface must be imple
mented. The unused methods are simply implemented as donothing methods.
Go ahead and run the  RectangleViewer2 program. Whenever you click the mouse
inside the frame, the topleft corner of the rectangle moves to the mouse pointer (see
Figure 9).
section_10/rectangleframe2.java
1 import java.awt.event.MouseListener;
2 import java.awt.event.MouseEvent;
3 import javax.swing.JFrame;
4
5 /** 
6 This frame contains a moving rectangle.
7 */
8 public class RectangleFrame2 extends JFrame
9 {
10 private static final int FRAME_WIDTH = 300;
11 private static final int FRAME_HEIGHT = 400;
12
13 private RectangleComponent2 scene;
14
15 class MousePressListener implements MouseListener
16 {
17 public void mousePressed(MouseEvent event)
18 {
19 int x = event.getX();
20 int y = event.getY();
21 scene.moveRectangleTo(x, y);
22 }
23
24 //  Donothing methods
25 public void mouseReleased(MouseEvent event) {}
26 public void mouseClicked(MouseEvent event) {}
27 public void mouseEntered(MouseEvent event) {}
28 public void mouseExited(MouseEvent event) {}
500 Chapter 10 Interfaces  Graphics track
29 }
30
31 public RectangleFrame2()
32 {
33 scene = new RectangleComponent2();
34 add(scene);
35
36 MouseListener listener = new MousePressListener();
37 scene.addMouseListener(listener);
38
39 setSize(FRAME_WIDTH, FRAME_HEIGHT);
40 }
41 }
section_10/rectangleViewer2.java
1 import javax.swing.JFrame;
2
3 /**
4 This program displays a rectangle that can be moved with the mouse.
5 */
6 public class RectangleViewer2
7 {
8 public static void main(String[] args)
9 {
10 JFrame frame = new RectangleFrame2();
11 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
12 frame.setVisible(true);
13 }
14 }
35.  Why was the  moveRectangleBy method in the  RectangleComponent replaced with a
moveRectangleTo method?
36.  Why must the  MousePressListener class supply five methods?
practice it  Now you can try these exercises at the end of the chapter: R10.17, E10.22.
keyboard events
If you program a game, you may want to process keystrokes, such as the arrow keys. Add a
key listener to the component on which you draw the game scene. The  KeyListener interface
has three methods. As with a mouse listener, you are most interested in key press events, and
you can leave the other two methods empty. Your key listener class should look like this:
class MyKeyListener implements KeyListener
{
public void keyPressed(KeyEvent event)
{
String key = KeyStroke.getKeyStrokeForEvent(event).toString();
key = key.replace("pressed ", "");
Process key.
}
//  Donothing methods
public void keyReleased(KeyEvent event) {}
© Nicholas Homrich/iStockphoto.
s e l f  c h e c k
special topic 10.4
© Eric Isselé/iStockphoto.
Graphics track 10.10 Mouse events 501
public void keyTyped(KeyEvent event) {}
}
The call  KeyStroke.getKeyStrokeForEvent(event).toString()
turns the event object into a text description of the key,
such as  "pressed LEFT" . In the next line, we eliminate the
"pressed "  prefix. The remainder is a string such as  "LEFT"
or  "A" that describes the key that was pressed. You can
find a list of all key names in the API documentation of
the  KeyStroke class.
As always, remember to attach the listener to the event
source:
KeyListener listener = new MyKeyListener();
scene.addKeyListener(listener);
In order to receive key events, your component must call
scene.setFocusable(true);
event adapters
In the preceding section you saw how to install a mouse listener into a mouse event source and
how the listener methods are called when an event occurs. Usually, a program is not interested
in all listener notifications. For exam ple, a program may only be interested in mouse clicks and
may not care that these mouse clicks are composed of “mouse pressed” and “mouse released”
events. Of course, the program could supply a listener that implements all those methods in
which it has no interest as “donothing” methods, for example:
class MouseClickListener implements MouseListener
{
public void mouseClicked(MouseEvent event)
{
Mouse click action
}
//  Four donothing methods
public void mouseEntered(MouseEvent event) {}
public void mouseExited(MouseEvent event) {}
public void mousePressed(MouseEvent event) {}
public void mouseReleased(MouseEvent event) {}
}
To avoid this labor, some friendly soul has created a  MouseAdapter class that implements the
MouseListener interface such that all methods do nothing. You can extend that class, inheriting
the donothing methods and over riding the methods that you care about, like this:
class MouseClickListener extends MouseAdapter
{
public void mouseClicked(MouseEvent event)
{
Mouse click action
}
}
There is also a  KeyAdapter class that implements the  KeyListener interface with three donothing
methods.
© Dmitry Shironosov/iStockphoto.
Whenever the program user presses
a key, a key event is generated.
fUll code example
Go to  wiley.com/go/
javacode to download
a program that uses
the arrow keys to
move a rectangle.
special topic 10.5
© Eric Isselé/iStockphoto.
502 Chapter 10 Interfaces
Use interfaces for making a service available to multiple classes.
• A Java interface type declares methods but does not
provide their implementations.
• Use the  implements reserved word to indicate that a
class implements an interface type.
• Use interface types to make code more reusable.
Most companies that
produce  software
regard the source code as a trade
secret. after all, if customers or com-
petitors had access to the source
code, they could study it and create
similar programs without paying the
original vendor. for the same reason,
customers dislike secret source code.
If a company goes out of business or
decides to discontinue support for a
computer program, its users are left
stranded. they are unable to fix bugs
or adapt the program to a new operat-
ing system. nowadays, some software
packages are distributed with “open
source” or “free software” licenses.
here, the term “free” doesn’t refer to
price, but to the freedom to inspect
and modify the source code. richard
stallman, a famous computer scientist
and winner of a Macarthur “genius”
grant, pioneered the concept of free
software. he is the inventor of the
emacs text editor and the originator of
the GnU project that aims to create an
entirely free version of a UnIx compat-
ible operating system. all programs of
the GnU project are licensed under the
General public license (Gpl). the Gpl
allows you to make as many copies as
you wish, make any modifications to
the source, and redistribute the origi-
nal and modified programs, charging
nothing at all or whatever the market
will bear. In return, you must agree that
your modifications also fall under the
Gpl. You must give out the source code
to any changes that you distrib ute,
and anyone else can distribute them
under the same conditions. the Gpl,
and similar open source licenses, form
a social contract. Users of the software
enjoy the freedom to use and modify
the software, and in return they are
obligated to share any improvements
that they make. Many programs, such
as the linux operating system and
the GnU C++ compiler, are distributed
under the Gpl.
some commercial software ven dors
have attacked the Gpl as “viral” and
“undermining the commercial software
sector”. other companies have a more
nuanced strategy, pro ducing propri-
etary software while also contributing
to open source projects.
frankly, open source is not a pana-
cea and there is plenty of room for
the commercial software sector. open
source software often lacks the polish
of commercial software because many
of the programmers are volunteers
who are interested in solving their
own problems, not in making a prod-
uct that is easy to use by others. some
product categories are not available at
all as open source software because
the development work is unattractive
when there is little promise of com-
mercial gain. open source software has
been most successful in areas that are
of interest to programmers, such as
the linux operating system, Web serv-
ers, and programming tools.
on the positive side, the open soft-
ware community can be very competi-
tive and creative. It is quite common
to see several competing projects that
take ideas from each other, all rap-
idly becoming more capable. having
many programmers involved, all read-
ing the source code, often means that
bugs tend to get squashed quickly.
eric ray mond describes open source
develop ment in his famous article “the
Cathedral and the Bazaar” ( http://
catb.org/~esr/writings/cathedral
bazaar/cathedralbazaar/index.html ).
he writes “Given enough eyeballs, all
bugs are shallow”.
Courtesy of Richard Stallman.
Richard Stallman, a pioneer of the
free source movement
Computing & Society 10.1  open source and free software
© MediaBakery.
C h a p t e r s U M M a rY
© Oxana Oleynichenko/iStockphoto.
Chapter summary 503
describe how to convert between class and interface types.
• You can convert from a class type to an interface type, provided the class imple
ments the interface.
• Method calls on an interface reference are polymorphic. The appropriate method
is determined at run time.
• You need a cast to convert from an interface type to a class type.
Use the  Comparable  interface from the java library.
• Implement the  Comparable interface so that objects of your class can be compared,
for example, in a sort method.
describe how to use interface types for providing callbacks.
• A callback is a mechanism for specifying code that is executed at a later time.
Use inner classes to limit the scope of a utility class.
• An inner class is declared inside another class.
• Inner classes are commonly used for utility classes that
should not be visible elsewhere in a program.
Use mock objects for supplying test versions of classes.
• A mock object provides the same services as another object, but in a simplified
manner.
• Both the mock class and the actual class implement the same interface.
implement event listeners to react to events in user-interface programming.
• Userinterface events include key presses, mouse moves, button clicks, menu
selections, and so on.
• An event listener belongs to a class that is provided by the application program
mer. Its methods describe the actions to be taken when an event occurs.
• Event sources report on events. When an event occurs, the event source notifies
all event listeners.
• Use  JButton components for buttons. Attach an  ActionListener to each button.
• Methods of an inner class can access local and instance variables from the
surrounding scope.
• Local variables that are accessed by an inner class method must be declared
as  final .
build graphical applications that use buttons.
• Use a  JPanel container to group multiple userinterface components together.
• Specify button click actions through classes that implement the  ActionListener
interface.
© Andrew Rich/iStockphoto.
© maureenpr/iStockphoto.
© Eduard Andras/iStockphoto.
504 Chapter 10 Interfaces
Use a timer for drawing animations.
• A timer generates timer events at fixed intervals.
• The  repaint method causes a component to repaint itself. Call  repaint whenever
you modify the shapes that the  paintComponent method draws.
Write programs that process mouse events.
• Use a mouse listener to capture mouse events.
• r10.1  Suppose  C is a class that implements the interfaces  I and  J . Which of the following
assignments require a cast?
C c = . . .;
I i = . . .;
J j = . . .;
a.  c = i;
b.  j = c;
c.  i = j;
• r10.2  Suppose  C is a class that implements the interfaces  I and  J , and suppose  i is declared
as:  I i = new C();
Which of the following statements will throw an exception?
a.  C c = (C) i;
b.  J j = (J) i;
c.  i = (I) null;
• r10.3  Suppose the class  Sandwich implements the  Edible interface, and you are given the
variable declarations
Sandwich sub = new Sandwich();
Rectangle cerealBox = new Rectangle(5, 10, 20, 30);
Edible e = null;
© jeff giniewicz/iStockphoto.
© james Brey/iStockphoto.
java.awt.Component
addKeyListener
addMouseListener
repaint
setFocusable
java.awt.Container
add
java.awt.Dimension
java.awt.Rectangle
setLocation
java.awt.event.ActionListener
actionPerformed
java.awt.event.KeyEvent
java.awt.event.KeyListener
keyPressed
keyReleased
keyTyped
java.awt.event.MouseEvent
getX
getY
java.awt.event.MouseListener
mouseClicked
mouseEntered
mouseExited
mousePressed
mouseReleased
javax.swing.AbstractButton
addActionListener
javax.swing.JButton
javax.swing.JLabel
javax.swing.JPanel
javax.swing.KeyStroke
getKeyStrokeForEvent
javax.swing.Timer
start
stop
s ta n D a r D l I B r a rY I t e M s I n t r o D U C e D I n t h I s C h ap t e r
r e V I e W Q U e s t I o n s
review Questions 505
Which of the following assignment statements are legal?
a.  e = sub;
b.  sub = e;
c.  sub = (Sandwich) e;
d.  sub = (Sandwich) cerealBox;
e.  e = cerealBox;
f.  e = (Edible) cerealBox;
g.  e = (Rectangle) cerealBox;
h.  e = (Rectangle) null;
•• r10.4  The classes  Rectangle2D.Double ,  Ellipse2D.Double , and  Line2D.Double implement the  Shape
interface. The  Graphics2D class depends on the  Shape interface but not on the rectangle,
ellipse, and line classes. Draw a UML diagram denoting these facts.
•• r10.5  Suppose  r contains a reference to a  new Rectangle(5, 10, 20, 30) . Which of the fol
lowing assignments is legal? (Look inside the API documentation to check which
interfaces the  Rectangle class implements.)
a.  Rectangle a = r;
b.  Shape b = r;
c.  String c = r;
d.  ActionListener d = r;
e.  Measurable e = r;
f.  Serializable f = r;
g.  Object g = r;
•• r10.6  Classes such as  Rectangle2D.Double ,  Ellipse2D.Double , and  Line2D.Double implement the
Shape interface. The  Shape interface has a method
Rectangle getBounds()
that returns a rectangle completely enclosing the shape. Consider the method call:
Shape s = . . .;
Rectangle r = s.getBounds();
Explain why this is an example of polymorphism.
•• r10.7  Suppose you need to process an array of employees to find the average salary.
Discuss what you need to do to use the  Data.average method in Section 10.1 (which
processes  Measurable objects). What do you need to do to use the second implementa
tion (in Section 10.4)? Which is easier?
• r10.8  What happens if you try to use an array of  String objects with the  Data.average
method in Section 10.1?
•• r10.9  How can you use the  Data.average method in Section 10.4 if you want to compute the
average length of the strings?
•• r10.10  What happens if you pass an array of strings and an AreaMeasurer to the  Data.average
method of Section 10.4?
•• r10.11  Consider this toplevel and inner class. Which variables can the  f method access?
public class T
{
private int t;
public void m(final int x, int y)
{
int a;
final int b;
class C implements I
{
506 Chapter 10 Interfaces
public void f()
{
. . .
}
}
final int c;
. . .
}
}
•• r10.12  What happens when an inner class tries to access a non final local variable? Try it
out and explain your findings.
••• graphics r10.13  How would you reorganize the  InvestmentViewer1 program if you needed to make
AddInterestListener into a toplevel class (that is, not an inner class)?
• graphics r10.14  What is an event object? An event source? An event listener?
• graphics r10.15  From a programmer’s perspective, what is the most important difference between
the user interfaces of a console application and a graphical application?
• graphics r10.16  What is the difference between an  ActionEvent and a  MouseEvent ?
•• graphics r10.17  Why does the  ActionListener interface have only one method, whereas the
Mouse Lis tener has five methods?
•• graphics r10.18  Can a class be an event source for multiple event types? If so, give an example.
•• graphics r10.19  What information does an action event object carry? What additional information
does a mouse event object carry?
••• graphics r10.20  Why are we using inner classes for event listeners? If Java did not have inner classes,
could we still implement event listeners? How?
•• graphics r10.21  What is the difference between the  paintComponent and  repaint methods?
• graphics r10.22  What is the difference between a frame and a panel?
•• e10.1  Add a method
public static Measurable max(Measurable[] objects)
to the  Data class that returns the object with the largest measure.
• e10.2  Implement a class  Quiz that implements the  Measurable interface. A quiz has a score
and a letter grade (such as  B+ ). Use the  Data class of Exercise E10.1 to process an array
of quizzes. Display the average score and the quiz with the highest score (both letter
grade and score).
• e10.3  A person has a name and a height in centimeters. Use the  Data class of Exercise E10.1
to process an array of  Person objects. Display the aver age height and the name of the
tallest person.
•• e10.4  Add a method to the  Data class that returns the object with the largest measure, as
measured by the supplied measurer:
public static Object max(Object[] objects, Measurer m)
p r a C t I C e e x e r C I s e s
practice exercises 507
• e10.5  Using a different  Measurer object, process a set of  Rectangle objects to find the rectan
gle with the largest perimeter.
• e10.6  Modify the  Coin class from Chapter 8 to have it implement the  Comparable interface.
• e10.7  Repeat Exercise E10.5, making the  Measurer into an inner class inside the  main method.
• e10.8  Repeat Exercise E10.5, making the  Measurer into an inner class outside the  main
method.
•• e10.9  Implement a class  Bag that stores items represented as strings. Items can be repeated.
Supply methods for adding an item, and for counting how many times an item has
been added:
public void add(String itemName)
public int count(String itemName)
Your  Bag class should store the data in an  ArrayList<Item> , where  Item is an inner class
with two instance variables: the name of the item and the quantity.
•• e10.10  Implement a class  Grid that stores measurements in a rectangular grid. The grid has
a given number of rows and columns, and a description string can be added for any
grid location. Supply the following constructor and methods:
public Grid(int numRows, int numColumns)
public void add(int row, int column, String description)
public String getDescription(int row, int column)
public ArrayList<Location> getDescribedLocations()
Here,  Location is an inner class that encapsulates the row and the column of a grid
location.
••• e10.11  Reimplement Exercise E10.10 where the grid is unbounded. The constructor has no
arguments, and the row and column parameter variables of the  add and  getDescription
methods can be arbitrary integers.
••• graphics e10.12  Write a method  randomShape that randomly generates objects implementing the  Shape
interface in the Java library API: some mixture of rectangles, ellipses, and lines, with
random positions. Call it ten times and draw all of them.
• graphics e10.13  Enhance the  ButtonViewer program so that it prints a message “I was clicked n times!”
whenever the button is clicked. The value n should be incremented with each click.
•• graphics e10.14  Enhance the  ButtonViewer program so that it has two buttons, each of which prints a
message “I was clicked n times!” whenever the button is clicked. Each button should
have a separate click count.
•• graphics e10.15  Enhance the  ButtonViewer program so that it has two buttons labeled A and B, each of
which prints a message “Button x was clicked!”, where x is A or B.
•• graphics e10.16  Implement a  ButtonViewer program as in Exercise E10.15, using only a single listener
class.
• graphics e10.17  Enhance the  ButtonViewer program so that it prints the time at which the button was
clicked.
••• graphics e10.18  Implement the  AddInterestListener in the  InvestmentViewer1 program as a regular class
(that is, not an inner class). Hint: Store a reference to the bank account. Add a con
structor to the listener class that sets the reference.
508 Chapter 10 Interfaces
••• graphics e10.19  Implement the  AddInterestListener in the  InvestmentViewer2 program as a regular class
(that is, not an inner class). Hint: Store references to the bank account and the label
in the listener. Add a constructor to the listener class that sets the references.
•• graphics e10.20  Write a program that uses a timer to print the current time once a second. Hint: The
following code prints the current time:
Date now = new Date();
System.out.println(now);
The  Date class is in the  java.util package.
••• graphics e10.21  Change the  RectangleComponent for the animation program in Section 10.9 so that
the rectangle bounces off the edges of the component rather than simply moving
out side.
• graphics e10.22  Change the  RectangleComponent for the mouse listener program in Section 10.10 so that
a new rectangle is added to the component whenever the mouse is clicked.
Hint: Keep an  ArrayList<Rectangle> and draw all rectangles in the  paintComponent
method.
• e10.23  Supply a class  Person that implements the  Comparable interface. Compare persons
by their names. Ask the user to input ten names and generate ten  Person objects.
Using the  compareTo method, determine the first and last person among them and
print them.
•• p10.1  Modify the  display method of the  LastDigitDistribution class of Worked Example
10.1 so that it produces a histogram, like this:
0: *************
1: ******************
2: *************
Scale the bars so that widest one has length 40.
•• p10.2  Write a class  PrimeSequence that implements the  Sequence interface of Worked Example
10.1, and produces the sequence of prime numbers.
• p10.3  Add a method  hasNext to the  Sequence interface of Worked Example 10.1 that returns
false if the sequence has no more values. Implement a class  MySequence producing a
sequence of real data of your choice, such as populations of cities or countries, tem
peratures, or stock prices. Obtain the data from the Internet and reformat the values
so that they are placed into an array. Return one value at a time in the  next method,
until you reach the end of the data. Your  SequenceDemo class should display the distri
bution of the last digits of all sequence values.
• p10.4  Provide a class  FirstDigitDistribution that works just like the  LastDigitDistribution
class of Worked Example 10.1, except that it counts the distribution of the first digit
of each value. (It is a wellknown fact that the first digits of random values are not
uniformly distributed. This fact has been used to detect accounting fraud, when
sequences of transaction amounts had an unnatural distribution of their first digits.)
p r o G r a M M I n G p r o J e C t s
programming projects 509
•• p10.5  Declare an interface  Filter as follows:
public interface Filter
{
boolean accept(Object x);
}
Modify the implementation of the  Data class in Section 10.4 to use both a  Measurer and
a  Filter object. Only objects that the filter accepts should be processed. Demonstrate
your modification by processing a collection of bank accounts, filtering out all
accounts with balances less than $1,000.
•• p10.6  The  System.out.printf method has predefined formats for printing integers, float ing
point numbers, and other data types. But it is also extensible. If you use the  S format,
you can print any class that implements the  Formattable interface. That inter face has a
single method:
void formatTo(Formatter formatter, int flags, int width, int precision)
In this exercise, you should make the  BankAccount class implement the  Formattable
interface. Ignore the flags and precision and simply format the bank balance, using
the given width. In order to achieve this task, you need to get an  Appendable reference
like this:
Appendable a = formatter.out();
Appendable is another interface with a method
void append(CharSequence sequence)
CharSequence is yet another interface that is implemented by (among others) the  String
class. Construct a string by first converting the bank balance into a string and then
padding it with spaces so that it has the desired width. Pass that string to the  append
method.
••• p10.7  Enhance the  formatTo method of Exercise P10.6 by taking into account the precision.
••• graphics p10.8  Write a program that displays a scrolling message in a panel. Use a timer for the
scrolling effect. In the timer’s action listener, move the starting position of the mes
sage and repaint. When the message has left the window, reset the starting position
to the other corner. Provide a user interface to customize the message text, font,
foreground and background colors, and the scrolling speed and direction.
••• graphics p10.9  Write a program that allows the user to specify a triangle with three mouse presses.
After the first mouse press, draw a small dot. After the second mouse press, draw a
line joining the first two points. After the third mouse press, draw the entire triangle.
The fourth mouse press erases the old triangle and starts a new one.
••• graphics p10.10  Implement a program that allows two players to play tictactoe.
Draw the game grid and an indication of whose turn it is (X or O).
Upon the next click, check that the mouse click falls into an empty
location, fill the location with the mark of the current player, and
give the other player a turn. If the game is won, indicate the win
ner. Also supply a button for starting over.
••• graphics p10.11  Write a program that lets users design bar charts with a mouse. When the user clicks
inside a bar, the next mouse click extends the length of the bar to the xcoordinate of
the mouse click. (If it is at or near 0, the bar is removed.) When the user clicks below
the last bar, a new bar is added whose length is the xcoordinate of the mouse click.
© KathyMuller/iStockphoto.
510 Chapter 10 Interfaces
• testing p10.12  Consider the task of writing a program that plays tictactoe against a human oppo
nent. A user interface  TicTacToeUI reads the user’s moves and displays the computer’s
moves and the board. A class  TicTacToeStrategy determines the next move that the
computer makes. A class  TicTacToeBoard represents the current state of the board.
Complete all classes except for the strategy class. Instead, use a mock class that sim
ply picks the first available empty square.
•• testing p10.13  Consider the task of translating a plain text book from Project Gutenberg ( http://
gutenberg.org ) to HTML. For example, here is the start of the first chapter of
Tol stoy’s Anna Karenina:
Chapter 1
Happy families are all alike; every unhappy family is unhappy in its own way.
Everything was in confusion in the Oblonskys' house. The wife had discovered
that the husband was carrying on an intrigue with a French girl, who had been a
governess in their family, and she had announced to her husband that she could
not go on living in the same house with him ...
The equivalent HTML is:
<h1>Chapter 1</h1>
<p>Happy families are all alike; every unhappy
family is unhappy in its own way.</p>
<p>Everything was in confusion in the
Oblonskys&rsquo; house. The wife had discovered
that the husband was carrying on an intrigue
with a French girl, who had been a governess in
their family, and she had announced to her
husband that she could not go on living in the
same house with him ...</p>
The HTML conversion can be carried out in two
steps. First, the plain text is assembled into
segments, blocks of text of the same kind (heading,
paragraph, and so on). Then each segment is
converted, by sur rounding it with the HTML
tags and converting special characters.
Fetching the text from the Internet and breaking it into segments is a challenging
task. Provide an inter face and a mock implementation. Combine it with a class that
uses the mock implementation to finish the formatting task.
•• graphics p10.14  Write a program that demonstrates the growth of a roach population. Start with two
roaches and double the number of roaches with each
button click.
•• graphics p10.15  Write a program that animates a car so that it moves across a frame.
••• graphics p10.16  Write a program that animates two cars moving across a frame in opposite direc tions
(but at different heights so that they don’t collide.)
•• graphics p10.17  Write a program that prompts the user to enter the x and ypositions of the cen
ter and a radius, using  JOptionPane dialogs.When the user clicks a “Draw” button,
prompt for the inputs and draw a circle with that center and radius in a component.
•• graphics p10.18  Write a program that allows the user to specify a circle by clicking on the center and
then typing the radius in a  JOptionPane . Note that you don’t need a “Draw” button.
plain text htMl
“ ” &ldquo; (left) or
&rdquo; (right)
‘ ’ &lsquo; (left) or
&rsquo; (right)
— &emdash;
< &lt;
> &gt;
& &amp;
answers to self-Check Questions 511
••• graphics p10.19  Write a program that allows the user to specify a circle with two mouse presses,
the first one on the center and the second on a point on the periphery. Hint: In the
mouse press handler, you must keep track of whether you already received the cen
ter point in a previous mouse press.
• graphics p10.20  Design an interface  MoveableShape that can be used as a generic mechanism for ani
mating a shape. A moveable shape must have two methods:  move and  draw . Write
a generic  AnimationPanel that paints and moves any  MoveableShape (or array list of
Move able Shape objects). Supply moveable rectangle and car shapes.
• p10.21  Your task is to design a general program for managing board games with two play
ers. Your program should be flexible enough to handle games such as tictactoe,
chess, or the Game of Nim of Project 6.2.
Design an interface  Game that describes a board game. Think about what your pro
gram needs to do. It asks the first player to input a move—a string in a gamespecific
format, such as  Be3 in chess. Your program knows nothing about specific games, so
the  Game interface must have a method such as
boolean isValidMove(String move)
Once the move is found to be valid, it needs to be executed—the interface needs
another method  execute Move . Next, your program needs to check whether the game is
over. If not, the other player’s move is pro cessed. You should also provide some
mechanism for displaying the current state of the board.
Design the  Game interface and provide two implementations of your choice—such as
Nim and  Chess (or  Tic TacToe if you are less ambitious). Your  GamePlayer class should
manage a  Game reference without knowing which game is played, and process the
moves from both players. Supply two programs that differ only in the initialization
of the  Game reference.
a n s W e rs t o s e l f - C h e C K Q U e s t I o n s
1.  It must implement the  Measurable interface, and
its  getMeasure method must return the salary.
2.  The  Object class doesn’t have a  getMeasure
method.
3.  You cannot modify the  String class to imple
ment  Measurable — String is a library class.
See Section 10.4 for a solution.
4.  Measurable is not a class. You cannot construct
objects of type  Measurable .
5.  The variable  meas is of type  Measurable , and that
type has no  getName method.
6.  Only if  meas  actually refers to a  BankAccount
object.
7.  No––a  Country reference can be converted to a
Measurable reference, but if you attempt to cast
that reference to a  BankAccount , an exception
occurs.
8.  Measurable is an interface. Interfaces have
no instance variables and no method
implementations.
9.  That variable never refers to a  Measurable
object. It refers to an object of some class—a
class that implements the  Measurable interface.
10.  The code fragment prints 20255. The average
method calls  getMeasure on each object in the
array. In the first call, the object is a  BankAccount .
In the second call, the object is a  Country . A dif
ferent  getMeasure method is called in each case.
The first call returns the account balance, the
second one the area, which are then averaged.
11.  Have the  Country class implement the  Comparable
interface, as shown below, and call  Arrays.sort .
public class Country implements Comparable
{
. . .
512 Chapter 10 Interfaces
public int compareTo(Object otherObject)
{
Country other = (Country) otherObject;
if (area < other.area) { return 1; }
if (area > other.area) { return 1; }
return 0;
}
}
12.  Yes, you can, because  String implements the
Comparable interface type.
13.  No. The  Rectangle class does not implement the
Comparable interface.
14.  public static Comparable max(
Comparable a, Comparable b)
{
if (a.compareTo(b) > 0) { return a; }
else { return b; }
}
15.  BankAccount larger
= (BankAccount) max(first, second);
System.out.println(larger.getBalance());
Note that the result must be cast from
Comparable to  BankAccount so that you can invoke
the  getBalance method.
16.  The  String class doesn’t implement the
Measurable interface.
17.  Implement a class  StringMeasurer that imple
ments the  Measurer interface.
18.  A  Measurer measures an object, whereas  get
Measure measures “itself”, that is, the implicit
parameter.
19.  public static Object max(
Object a, Object b, Measurer m)
{
if (m.getMeasure(a) > m.getMeasure(b))
{
return a;
}
else { return b; }
}
20.  Rectangle larger = (Rectangle) max(
first, second, areaMeas);
System.out.println(larger.getWidth() + " by "
+ larger.getHeight());
Note that the result of  max must be cast from
Object to  Rectangle so that you can invoke the
getWidth and  getHeight methods.
21.  Inner classes are convenient for insignificant
classes. Also, their methods can access local
and instance variables from the surrounding
scope.
22.  When the inner class is needed by more than
one method of the classes.
23.  Four: one for the outer class, one for the inner
class, and two for the  Data and  Mea surer classes.
24.  You want to implement the  GradingProgram class
in terms of that interface so that it doesn’t have
to change when you switch between the mock
class and the actual class.
25.  Because the developer of  GradingProgram
doesn’t have to wait for the  GradeBook class to be
complete.
26.  The  button object is the event source. The  lis
tener object is the event listener.
27.  The  ClickListener class implements the  Action
Listener interface.
28.  You don’t. It is called whenever the button is
clicked.
29.  Direct access is simpler than the alternative—
passing the variable as an argument to a con
structor or method.
30.  The local variable must be declared as  final .
31.  First add  label to the  panel , then add  button .
32.  The  actionPerformed method does not access
that variable.
33.  The timer needs to call some method whenever
the time interval expires. It calls the  action
Performed method of the listener object.
34.  The moved rectangles won’t be painted, and
the rectangle will appear to be stationary until
the frame is repainted for an external reason.
35.  Because you know the current mouse posi
tion, not the amount by which the mouse has
moved.
36.  It implements the  MouseListener interface,
which has five methods.
11
C h a p t e r
513
James King-Holmes/Bletchley ParkTrust/Photo Researchers, Inc.
to read and write text files
to process command line arguments
to throw and catch exceptions
to implement programs that propagate checked exceptions
C h a p t e r G o a l s
C h a p t e r C o n t e n t s
11.1 Reading and WRiting
text Files 514
Common Error 11.1: Backslashes in
File names 517
Common Error 11.2: Constructing a scanner
with a string 517
Special Topic 11.1: reading Web pages 517
Special Topic 11.2: File Dialog Boxes 517
Special Topic 11.3: Character encodings 518
11.2 text input and Output 519
Special Topic 11.4: regular expressions 526
11.3 COmmand line aRguments 527
How To 11.1: processing text Files 530
Worked Example 11.1: analyzing Baby names
Computing & Society 11.1: encryption
algorithms 533
11.4 exCeptiOn Handling 534
Syntax 11.1: throwing an exception 534
Syntax 11.2: Catching exceptions 536
Syntax 11.3: the throws Clause 539
Syntax 11.4: the finally Clause 540
Programming Tip 11.1: throw early,
Catch late 542
Programming Tip 11.2: Do not squelch
exceptions 542
Programming Tip 11.3: Do not Use catch and
finally in the same try statement 542
Programming Tip 11.4: Do throw specific
exceptions 543
Special Topic 11.5: assertions 543
Special Topic 11.6: automatic resource
Management in Java 7 544
Computing & Society 11.2: the ariane rocket
Incident 544
11.5 appliCatiOn: Handling
input eRRORs 545
InpUt/oUtpUt
anD exCeptIon
hanDlInG
514
In this chapter, you will learn how to read and write
files—a very useful skill for processing real world data. as
an application, you will learn how to encrypt data. (the
enigma machine shown at left is an encryption device used
by Germany in World War II. pioneering British computer
scientists broke the code and were able to intercept
encoded messages, which was a significant help in winning
the war.) the remainder of this chapter tells you how your
programs can report and recover from problems, such as
missing files or malformed content, using the exception-
handling mechanism of the Java language.
11.1 reading and Writing text Files
We begin this chapter by discussing the common task of reading and writing files that
contain text. Exam ples of text files include not only files that are created with a simple
text editor, such as Windows Note pad, but also Java source code and HTML files.
In Java, the most convenient mechanism for reading text is to use the  Scanner class.
You already know how to use a  Scanner for reading console input. To read input from
a disk file, the  Scanner class relies on another class,  File , which describes disk files and
directories. (The  File class has many methods that we do not discuss in this book; for
example, methods that delete or rename a file.)
To begin, construct a  File object with the name of the input file:
File inputFile = new File("input.txt");
Then use the  File object to construct a  Scanner object:
Scanner in = new Scanner(inputFile);
This  Scanner object reads text from the file  input.txt . You can use the  Scanner methods
(such as  nextInt ,  next Double , and  next ) to read data from the input file.
For example, you can use the following loop to process numbers in the input file:
while (in.hasNextDouble())
{
double value = in.nextDouble();
Process value.
}
To write output to a file, you construct a  PrintWriter object with the desired file name,
for example
PrintWriter out = new PrintWriter("output.txt");
If the output file already exists, it is emptied before the new data are written into it. If
the file doesn’t exist, an empty file is created.
The  PrintWriter class is an enhancement of the  PrintStream class that you already
know— System.out is a  PrintStream object. You can use the familiar  print ,  println , and
printf methods with any  PrintWriter object:
out.println("Hello, World!");
out.printf("Total: %8.2f\n", total);
Use the  Scanner class
for reading text files.
When writing
text files, use the
PrintWriter class and
the  print/println /
printf methods.
Bletchley ParkTrust/Photo Researchers, Inc.
11.1 reading and Writing text Files 515
When you are done processing a file, be sure to close the  Scanner or  PrintWriter :
in.close();
out.close();
If your program exits without closing the  PrintWriter , some of the output may not be
written to the disk file.
The following program puts these concepts to work. It reads a file containing
numbers and writes the numbers, lined up in a column and followed by their total, to
another file.
For example, if the input file has the contents
32 54 67.5 29 35 80
115 44.5 100 65
then the output file is
32.00
54.00
67.50
29.00
35.00
80.00
115.00
44.50
100.00
65.00
Total: 622.00
There is one additional issue that we need to tackle. If the input or output file for a
Scanner doesn’t exist, a  FileNotFoundException occurs when the  Scanner object is con-
structed. The compiler insists that we specify what the program should do when that
happens. Similarly, the  PrintWriter constructor generates this exception if it cannot
open the file for writing. (This can happen if the name is illegal or the user does not
have the authority to create a file in the given location.) In our sample program, we
want to terminate the  main method if the exception occurs. To achieve this, we label
the  main method with a  throws declara tion:
public static void main(String[] args) throws FileNotFoundException
You will see in Section 11.4 how to deal with exceptions in a more professional way.
The  File ,  PrintWriter , and  FileNotFoundException classes are contained in the  java.io
package.
section_1/total.java
1 import java.io.File;
2 import java.io.FileNotFoundException;
3 import java.io.PrintWriter;
4 import java.util.Scanner;
5
6 /**
7 This program reads a file with numbers, and writes the numbers to another
8 file, lined up in a column and followed by their total.
9 */
10 public class Total
11 {
12 public static void main(String[] args) throws FileNotFoundException
13 {
Close all files
when you are done
processing them.
516 Chapter 11 Input/output and exception handling
14 //  Prompt for the input and output file names
15
16 Scanner console = new Scanner(System.in);
17 System.out.print("Input file: ");
18 String inputFileName = console.next();
19 System.out.print("Output file: ");
20 String outputFileName = console.next();
21
22 //  Construct the  Scanner and  PrintWriter objects for reading and writing
23
24 File inputFile = new File(inputFileName);
25 Scanner in = new Scanner(inputFile);
26 PrintWriter out = new PrintWriter(outputFileName);
27
28 //  Read the input and write the output
29
30 double total = 0;
31
32 while (in.hasNextDouble())
33 {
34 double value = in.nextDouble();
35 out.printf("%15.2f\n", value);
36 total = total + value;
37 }
38
39 out.printf("Total: %8.2f\n", total);
40
41 in.close();
42 out.close();
43 }
44 }
1.  What happens when you supply the same name for the input and output files to
the  Total pro gram? Try it out if you are not sure.
2.  What happens when you supply the name of a nonexistent input file to the  Total
program? Try it out if you are not sure.
3.  Suppose you wanted to add the total to an existing file instead of writing a new
file. Self Check 1 indicates that you cannot simply do this by specifying the same
file for input and output. How can you achieve this task? Provide the pseudo-
code for the solution.
4.  How do you modify the program so that it shows the average, not the total, of
the inputs?
5.  How can you modify the  Total program so that it writes the values in two
columns, like this:
32.00 54.00
67.50 29.00
35.00 80.00
115.00 44.50
100.00 65.00
Total: 622.00
practice it  Now you can try these exercises at the end of the chapter: R11.1, R11.2, E11.1.
© Nicholas Homrich/iStockphoto.
s e l F  C H e C k
11.1 reading and Writing text Files 517
Backslashes in File names
When you specify a file name as a string literal, and the name contains backslash characters (as
in a Windows file name), you must supply each backslash twice:
File inputFile = new File("c:\\homework\\input.dat");
A single backslash inside a quoted string is an escape character that is combined with the fol-
lowing character to form a special meaning, such as  \n for a newline character. The  \\  combi-
nation denotes a single backslash.
When a user supplies a file name to a program, however, the user should not type the back-
slash twice.
Constructing a  Scanner  with a  String
When you construct a  PrintWriter with a string, it writes to a file:
PrintWriter out = new PrintWriter("output.txt");
However, this does not work for a  Scanner . The statement
Scanner in = new Scanner("input.txt"); //  Error?
does not open a file. Instead, it simply reads through the string:  in.next() returns the string
"input.txt" . (This is occasionally useful—see Section 11.2.5.)
You must simply remember to use  File objects in the  Scanner constructor:
Scanner in = new Scanner(new File("input.txt")); //  OK
Reading Web pages
You can read the contents of a web page with this sequence of commands:
String address = "http://horstmann.com/index.html";
URL pageLocation = new URL(address);
Scanner in = new Scanner(pageLocation.openStream());
Now simply read the contents of the web page with the
Scanner in the usual way. The  URL constructor and the  open
Stream method can throw an  IOException , so you need to tag
the  main method with  throws IOException . (See Section 11.4.3
for more information on the  throws clause.)
The  URL class is contained in the  java.net package.
File dialog Boxes
In a program with a graphical user interface, you will want to use a file dialog box (such as the
one shown in the fig ure below) whenever the users of your program need to pick a file. The
JFileChooser class implements a file dialog box for the Swing user-interface toolkit.
The  JFileChooser class has many options to fine-tune the display of the dialog box, but in its
most basic form it is quite simple: Construct a file chooser object; then call the  showOpenDialog
or  showSaveDialog method. Both methods show the same dialog box, but the button for select-
ing a file is labeled “Open” or “Save”, depending on which method you call.
Common error 11.1
© John Bell/iStockphoto.
Common error 11.2
© John Bell/iStockphoto.
special topic 11.1
© Eric Isselé/iStockphoto.
Full COde example
Go to  wiley.com/go/
javacode to download
a program that
reads data from a
web page.
special topic 11.2
© Eric Isselé/iStockphoto.
518 Chapter 11 Input/output and exception handling
For better placement of the dialog box on the screen, you can specify the user-interface
component over which to pop up the dialog box. If you don’t care where the dialog box pops
up, you can simply pass  null . The  showOpenDialog and  showSaveDialog methods return either
JFileChooser.APPROVE_OPTION , if the user has chosen a file, or  JFi leChooser.CANCEL_OPTION , if the
user canceled the selection. If a file was chosen, then you call the  getSelectedFile method to
obtain a  File object that describes the file. Here is a complete example:
JFileChooser chooser = new JFileChooser();
Scanner in = null;
if (chooser.showOpenDialog(null) == JFileChooser.APPROVE_OPTION)
{
File selectedFile = chooser.getSelectedFile();
in = new Scanner(selectedFile);
. . .
}
Character encodings
A character (such as the letter A, the digit 0, the accented character é, the Greek letter π, the
symbol ∫, or the Chinese character 中) is encoded as a sequence of bytes. Each byte is a value
between 0 and 255.
Unfortunately, the encoding is not uniform. In 1963, ASCII (the American Standard Code
for Information Interchange) defined an encoding for 128 characters, which you can find in
Appendix A. ASCII encodes all upper- and lowercase Latin letters and digits, as well as com-
mon symbols such as + * %, as values between 0 and 127. For example, the code for the letter
A is 65.
As different populations felt the need to encode their own alphabets, they designed their
own codes. Many of them built upon ASCII, using the values in the range from 128 to 255 for
their own language. For example, in Spain, the letter é was encoded as 233. But in Greece, the
code 233 denoted the letter ι (a lowercase iota). As you can imagine, if a Spanish tourist named
José sent an e-mail to a Greek hotel, this created a problem.
Full COde example
Go to  wiley.com/go/
javacode to download
a program that
demonstrates how to
use a file chooser.
A  JFileChooser Dialog Box
Call with
showOpenDialog
method
Button is “Save” when
showSaveDialog method
is called
special topic 11.3
© Eric Isselé/iStockphoto.
11.2 text Input and output 519
To resolve this issue, the design of Unicode was begun in 1987. As described in Computing
& Society 4.2, each character in the world is given a unique integer value. However, there
are still multiple encodings of those integers in binary. The most popular encoding is called
UTF-8. It encodes each character as a sequence of one to four bytes. For example, an A is still
65, as in ASCII, but an é is 195 169. The details of the encoding don’t matter, as long as you
specify that you want UTF-8 when you read and write a file.
As this book goes to print, the Windows and Macintosh operating systems have not yet
made the switch to UTF-8. Java picks up the character encoding from the operating system.
Unless you specifally request otherwise, the  Scanner and  PrintWriter classes will read and write
files in that encoding. That’s fine if your files contain only ASCII characters, or if the cre-
ator and the recipient use the same encoding. But if you need to process files with accented
characters, Chinese characters, or special symbols, you should specifically request the UTF-8
encoding. Construct a scanner with
Scanner in = new Scanner(file, "UTF8");
and a print writer with
PrintWriter out = new PrintWriter(file, "UTF8");
You may wonder why Java can’t just figure out the character encoding. However, consider the
string José. In UTF-8, that’s 74 111 115 195 169. The first three bytes, for Jos, are in the ASCII
range and pose no problem. But the next two bytes, 195 169, could be é in UTF-8 or Ã¡ in
the traditional Spanish encoding. The  Scanner object doesn’t understand Spanish, and it can’t
decide which encoding to choose.
Therefore, you should always specify the UTF-8 encoding when you exchange files with
users from other parts of the world.
11.2 text Input and output
In the following sections, you will learn how to process text with complex contents,
and you will learn how to cope with challenges that often occur with real data.
11.2.1 reading Words
The  next method of the  Scanner class reads the next string. Consider the loop
while (in.hasNext())
{
String input = in.next();
System.out.println(input);
}
If the user provides the input:
Mary had a little lamb
this loop prints each word on a separate line:
Mary
had
a
little
lamb
However, the words can contain punctuation marks and other symbols. The  next
method returns any sequence of characters that is not white space. White space
the  next method
reads a string that
is delimited by
white space.
520 Chapter 11 Input/output and exception handling
includes spaces, tab characters, and the new line characters that separate lines. For
example, the following strings are considered “words” by the  next method:
snow.
1729
C++
(Note the period after  snow —it is considered a part of the word because it is not white
space.)
Here is precisely what happens when the  next method is executed. Input characters
that are white space are consumed—that is, removed from the input. However, they
do not become part of the word. The first character that is not white space becomes
the first character of the word. More characters are added until either another white
space character occurs, or the end of the input file has been reached. However, if the
end of the input file is reached before any character was added to the word, a “no such
element exception” occurs.
Sometimes, you want to read just the words and discard anything that isn’t a letter.
You achieve this task by calling the  useDelimiter method on your  Scanner object:
Scanner in = new Scanner(. . .);
in.useDelimiter("[^AZaz]+");
Here, we set the character pattern that separates words to “any sequence of charac-
ters other than letters”. (See Special Topic 11.4.) With this setting, punctuation and
numbers are not included in the words returned by the  next method.
11.2.2 reading Characters
Sometimes, you want to read a file one character at a time. You will see an example in
Section 11.3 where we encrypt the characters of a file. You achieve this task by calling
the  useDelimiter method on your  Scan ner object with an empty string:
Scanner in = new Scanner(. . .);
in.useDelimiter("");
Now each call to  next returns a string consisting of a single character. Here is how you
can process the characters:
while (in.hasNext())
{
char ch = in.next().charAt(0);
Process ch.
}
11.2.3 Classifying Characters
When you read a character, or when you analyze the characters in a word or line,
you often want to know what kind of character it is. The  Character class declares sev-
eral useful methods for this purpose. Each of them has an argument of type  char and
returns a  boolean value (see Table 1).
For example, the call
Character.isDigit(ch)
returns  true if  ch is a digit ( '0' . . .  '9' or a digit in another writing system—see
Computing & Society 4.2),  false otherwise.
the  Character
class has methods
for classifying
characters.
11.2 text Input and output 521
table 1 Character testing Methods
Method
examples of
accepted Characters
isDigit 0, 1, 2
isLetter A, B, C, a, b, c
isUpperCase A, B, C
isLowerCase a, b, c
isWhiteSpace space, newline, tab
11.2.4 reading lines
When each line of a file is a data record, it is often best to read entire lines with the
nextLine method:
String line = in.nextLine();
The next input line (without the newline character) is placed into the string  line . You
can then take the line apart for further processing.
The  hasNextLine method returns  true if there is at least one more line in the input,
false when all lines have been read. To ensure that there is another line to process, call
the  hasNextLine method before calling  nextLine .
Here is a typical example of processing lines in a file. A file with population data
from the CIA Fact Book site ( https://www.cia.gov/library/publications/theworld
factbook/index.html ) contains lines such as the follow ing:
China 1330044605
India 1147995898
United States 303824646
. . .
Because some country names have more than one word, it would be tedious to read
this file using the  next method. For example, after reading  United , how would your pro-
gram know that it needs to read another word before reading the population count?
Instead, read each input line into a string:
while (in.hasNextLine())
{
String line = nextLine();
Process line.
}
Use the  isDigit and  isWhiteSpace methods in Table 1 to find out where the name ends
and the number starts.
Locate the first digit:
int i = 0;
while (!Character.isDigit(line.charAt(i))) { i++; }
Then extract the country name and population:
String countryName = line.substring(0, i);
String population = line.substring(i);
the  nextLine method
reads an entire line.
522 Chapter 11 Input/output and exception handling
However, the country name contains one or more spaces at the end. Use the  trim
method to remove them:
countryName = countryName.trim();
s e t a t S d e t i n U 3 0 3 8 2 4 6 4 6
i starts here i ends here
Use  trim to
remove this space.
countryName
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
population
The  trim method returns the string with all white space at the beginning and end
removed.
There is one additional problem. The population is stored in a string, not a num-
ber. In Section 11.2.6, you will see how to convert the string to a number.
11.2.5 scanning a string
In the preceding section, you saw how to break a string into parts by looking at
individual characters. Another approach is occasionally easier. You can use a  Scanner
object to read the characters from a string:
Scanner lineScanner = new Scanner(line);
Then you can use  lineScanner like any other  Scanner object, reading words and
numbers:
String countryName = lineScanner.next(); //  Read first word
//  Add more words to  countryName until number encountered
while (!lineScanner.hasNextInt())
{
countryName = countryName + " " + lineScanner.next();
}
int populationValue = lineScanner.nextInt();
11.2.6 Converting strings to numbers
Sometimes you have a string that contains a number, such as the  population string
in Section 11.2.4. For example, suppose that the string is the character sequence
"303824646" . To get the integer value  303824646 , you use the  Integer.parseInt method:
int populationValue = Integer.parseInt(population);
// populationValue is the integer 303824646
To convert a string containing floating-point digits to its floating-point value, use the
Double.parseDouble method. For example, suppose  input is the string  "3.95" .
double price = Double.parseDouble(input);
// price  is the floating-point number 3.95
You need to be careful when calling the  Integer.parseInt and  Double.parseDouble  meth-
ods. The argument must be a string contain ing the digits of an integer, without any
additional characters. Not even spaces are allowed! In our situa tion, we happen to
If a string contains
the digits of a
number, you use the
Integer.parseInt or
Double.parseDouble
method to obtain the
number value.
11.2 text Input and output 523
know that there won’t be any spaces at the beginning of the string, but there might be
some at the end. Therefore, we use the  trim method:
int populationValue = Integer.parseInt(population.trim());
How To 11.1 on page 530 continues this example.
11.2.7 avoiding errors When reading numbers
You have used the  nextInt and  nextDouble methods of the  Scanner class many times, but
here we will have a look at what happens in “abnormal” situations. Suppose you call
int value = in.nextInt();
The  nextInt method recognizes numbers such as  3 or  21 . However, if the input is not
a properly formatted number, an “input mismatch exception” occurs. For example,
consider an input containing the characters
2 1 s t c e n t u r y
White space is consumed and the word  21st is read. However, this word is not a prop-
erly formatted num ber, causing an input mismatch exception in the  nextInt method.
If there is no input at all when you call  nextInt or  nextDouble , a “no such element
exception” occurs. To avoid exceptions, use the  hasNextInt method to screen the input
when reading an integer. For example,
if (in.hasNextInt())
{
int value = in.nextInt();
. . .
}
Similarly, you should call the  hasNextDouble method before calling  nextDouble .
11.2.8 Mixing number, Word, and line Input
The  nextInt ,  nextDouble , and  next methods do not consume the white space that follows
the number or word. This can be a problem if you alternate between calling  next Int /
nextDouble / next and  nextLine . Suppose a file con tains country names and population
values in this format:
China
1330044605
India
1147995898
United States
303824646
Now suppose you read the file with these instructions:
while (in.hasNextLine())
{
String countryName = in.nextLine();
int population = in.nextInt();
Process the country name and population.
}
524 Chapter 11 Input/output and exception handling
Initially, the input contains
4 0 0 3 3 1 \n a n i h C 4 6 0 \n 5 I n d i a \n
After the first call to the  nextLine method, the input contains
4 0 0 3 3 1 4 6 0 \n 5 I n d i a \n
After the call to  nextInt , the input contains
\n I n d i a \n
Note that the  nextInt call did not consume the newline character. Therefore, the sec-
ond call to  nextLine reads an empty string!
The remedy is to add a call to  nextLine after reading the population value:
String countryName = in.nextLine();
int population = in.nextInt();
in.nextLine(); //  Consume the newline
The call to  nextLine consumes any remaining white space and the newline character.
11.2.9 Formatting output
When you write numbers or strings, you often want to control how they appear. For
exam ple, dollar amounts are usually formatted with two significant digits, such as
Cookies: 3.20
You know from Section 4.3.2 how to achieve this output with the  printf method. In
this section, we discuss additional options of the  printf method.
Suppose you need to print a table of items and prices, each stored in an array, such as
Cookies: 3.20
Linguine: 2.95
Clams: 17.29
Note that the item strings line up to the left, whereas the numbers line up to the right.
By default, the  printf method lines up values to the right.
table 2 Format Flags
Flag Meaning example
 Left alignment 1.23 followed by spaces
0 Show leading zeroes 001.23
+ Show a plus sign for positive numbers +1.23
( Enclose negative numbers in parentheses (1.23)
, Show decimal separators 12,300
^ Convert letters to uppercase 1.23E+1
11.2 text Input and output 525
To specify left alignment, you add a hyphen (-) before the field width:
System.out.printf(“%10s%10.2f”, items[i] + “:”, prices[i]);
Here, we have two format specifiers.
•  %10s formats a left-justified string. The string  items[i] + ":" is padded with spaces
so it becomes ten charac ters wide. The   indicates that the string is placed on the
left, followed by sufficient spaces to reach a width of 10.
•  %10.2f formats a floating-point number, also in a field that is ten characters wide.
However, the spaces appear to the left and the value to the right.
C l a m s : 1 7 . 2 9
Two digits after
the decimal point
A left-justified
string
width 10 width 10
A construct such as  %10s or  %10.2f is called a format specifier: it describes how a value
should be formatted.
A format specifier has the following structure:
• The first character is a  % .
• Next, there are optional “flags” that modify the format, such as   to indicate left
alignment. See Table 2 for the most common format flags.
• Next is the field width, the total number of characters in the field (including the
spaces used for padding), followed by an optional precision for floating-point
numbers.
• The format specifier ends with the format type, such as  f for floating-point values
or  s for strings. There are quite a few format types—Table 3 shows the most
important ones.
table 3 Format types
Code type example
d Decimal integer 123
f Fixed floating-point  12.30
e Exponential floating-point 1.23e+1
g General floating-point
(exponential notation is used for
very large or very small values)
12.3
s String Tax:
Full COde example
Go to  wiley.com/go/
javacode to download
a program that
processes a file
containing a mixture
of text and numbers.
526 Chapter 11 Input/output and exception handling
6.  Suppose the input contains the characters  Hello, World! . What are the values of
word and  input after this code fragment?
String word = in.next();
String input = in.nextLine();
7.  Suppose the input contains the characters  995.0 Fred . What are the values of
number and  input after this code fragment?
int number = 0;
if (in.hasNextInt()) { number = in.nextInt(); }
String input = in.next();
8.  Suppose the input contains the characters  6E6 6,995.00 . What are the values of  x1
and  x2 after this code fragment?
double x1 = in.nextDouble();
double x2 = in.nextDouble();
9.  Your input file contains a sequence of numbers, but sometimes a value is not
available and is marked as  N/A . How can you read the numbers and skip over the
markers?
10.  How can you remove spaces from the country name in Section 11.2.4 without
using the  trim method?
practice it  Now you can try these exercises at the end of the chapter: E11.2, E11.4, E11.5.
Regular expressions
Regular expressions describe character patterns. For example, numbers have a simple form.
They contain one or more digits. The regular expression describing numbers is  [09]+ . The set
[09] denotes any digit between 0 and 9, and the  + means “one or more”.
The search commands of professional programming editors understand regular expres-
sions. Moreover, several utility programs use regular expressions to locate matching text. A
commonly used program that uses regular expres sions is grep (which stands for “global regu-
lar expression print”). You can run grep from a command line or from inside some compila-
tion environments. Grep is part of the UNIX operating system, and versions are available for
Windows. It needs a regular expression and one or more files to search. When grep runs, it
displays a set of lines that match the regular expression.
Suppose you want to find all magic numbers (see Programming Tip 4.1) in a file.
grep [09]+ Homework.java
lists all lines in the file  Homework.java that contain sequences of digits. That isn’t terribly useful;
lines with variable names  x1 will be listed. OK, you want sequences of digits that do not imme-
diately follow letters:
grep [^AZaz][09]+ Homework.java
The set  [^AZaz] denotes any characters that are not in the ranges  A to  Z and  a to  z . This works
much better, and it shows only lines that contain actual numbers.
The  useDelimiter method of the  Scanner class accepts a regular expression to describe delim-
iters—the blocks of text that separate words. As already mentioned, if you set the delimiter
pattern to [^AZaz]+ , a delimiter is a sequence of one or more characters that are not letters.
For more information on regular expressions, consult one of the many tutorials on the
Internet by pointing your search engine to “regular expression tutorial”.
© Nicholas Homrich/iStockphoto.
s e l F  C H e C k
special topic 11.4
© Eric Isselé/iStockphoto.
11.3 Command line arguments 527
11.3 Command line arguments
Depending on the operating system and Java development environment used, there
are different methods of starting a program—for example, by selecting “Run” in the
compilation environment, by clicking on an icon, or by typing the name of the pro-
gram at the prompt in a command shell window. The latter method is called “invok-
ing the program from the command line”. When you use this method, you must of
course type the name of the program, but you can also type in additional information
that the program can use. These additional strings are called command line argu-
ments. For example, if you start a pro gram with the command line
java ProgramClass v input.dat
then the program receives two command line arguments: the strings  "v" and  "input.
dat" . It is entirely up to the program what to do with these strings. It is customary to
interpret strings starting with a hyphen (  ) as program options.
Should you support command line arguments for your programs, or should you
prompt users, per haps with a graphical user interface? For a casual and infrequent
user, an interactive user interface is much better. The user interface guides the user
along and makes it possible to navigate the application without much knowledge. But
for a frequent user, a command line interface has a major advantage: it is easy to auto-
mate. If you need to process hundreds of files every day, you could spend all your
time typing file names into file chooser dialog boxes. However, by using batch files or
shell scripts (a feature of your computer’s operating system), you can automatically
call a program many times with different command line arguments.
Your program receives its command line arguments in the  args parameter of the
main method:
public static void main(String[] args)
In our example,  args is an array of length 2, containing the strings
args[0]: "v"
args[1]: "input.dat"
Let us write a program that encrypts a file—that is, scram-
bles it so that it is unreadable except to those who know the
decryption method. Ignoring 2,000 years of progress in the
field of encryption, we will use a method familiar to Julius
Caesar, replacing A with a D, B with an E, and so on (see
Figure 1).
The emperor Julius Caesar used a
simple scheme to encrypt messages.
programs that start
from the command
line receive the
command line
arguments in the
main method.
© xyno/iStockphoto.
Figure 1 
Caesar Cipher
M e e t m e a t t h e
P h h w p h d w w k h
Plain text
Encrypted text
528 Chapter 11 Input/output and exception handling
The program takes the following command line arguments:
• An optional  d flag to indicate decryption instead of encryption
• The input file name
• The output file name
For example,
java CaesarCipher input.txt encrypt.txt
encrypts the file  input.txt and places the result into  encrypt.txt .
java CaesarCipher d encrypt.txt output.txt
decrypts the file  encrypt.txt and places the result into  output.txt .
section_3/CaesarCipher.java
1 import java.io.File;
2 import java.io.FileNotFoundException;
3 import java.io.PrintWriter;
4 import java.util.Scanner;
5
6 /**
7 This program encrypts a file using the Caesar cipher.
8 */
9 public class CaesarCipher
10 {
11 public static void main(String[] args) throws FileNotFoundException
12 {
13 final int DEFAULT_KEY = 3;
14 int key = DEFAULT_KEY;
15 String inFile = "";
16 String outFile = "";
17 int files = 0; //  Number of command line arguments that are files
18
19 for (int i = 0; i < args.length; i++)
20 {
21 String arg = args[i];
22 if (arg.charAt(0) == '')
23 {
24 //  It is a command line option
25
26 char option = arg.charAt ( 1);
27 if (option == 'd') { key = key; }
28 else { usage(); return; }
29 }
30 else
31 {
32 //  It is a file name
33
34 files++;
35 if (files == 1) { inFile = arg; }
36 else if (files == 2) { outFile = arg; }
37 }
38 }
39 if (files != 2) { usage(); return; }
40
11.3 Command line arguments 529
41 Scanner in = new Scanner(new File(inFile));
42 in.useDelimiter(""); //  Process individual characters
43 PrintWriter out = new PrintWriter(outFile);
44
45 while (in.hasNext())
46 {
47 char from = in.next().charAt(0);
48 char to = encrypt(from, key);
49 out.print(to);
50 }
51 in.close();
52 out.close();
53 }
54
55 /**
56 Encrypts upper- and lowercase characters by shifting them
57 according to a key.
58 @param ch  the letter to be encrypted
59 @param key  the encryption key
60 @return  the encrypted letter
61 */
62 public static char encrypt(char ch, int key)
63 {
64 int base = 0;
65 if ('A' <= ch && ch <= 'Z') { base = 'A'; }
66 else if ('a' <= ch && ch <= 'z') { base = 'a'; }
67 else { return ch; } //  Not a letter
68 int offset = ch  base + key;
69 final int LETTERS = 26; //  Number of letters in the Roman alphabet
70 if (offset > LETTERS) { offset = offset  LETTERS; }
71 else if (offset < 0) { offset = offset + LETTERS; }
72 return (char) (base + offset);
73 }
74
75 /**
76 Prints a message describing proper usage.
77 */
78 public static void usage()
79 {
80 System.out.println("Usage: java CaesarCipher [d] infile outfile");
81 }
82 }
11.  If the program is invoked with  java CaesarCipher d file1.txt , what are the
elements of  args ?
12.  Trace the program when it is invoked as in Self Check 11.
13.  Will the program run correctly if the program is invoked with  java CaesarCipher
file1.txt file2.txt d ? If so, why? If not, why not?
14.  Encrypt CAESAR using the Caesar cipher.
15.  How can you modify the program so that the user can specify an encryption key
other than 3 with a  k option, for example
java CaesarCipher k15 input.txt output.txt
practice it  Now you can try these exercises at the end of the chapter: R11.4, E11.8, E11.9.
© Nicholas Homrich/iStockphoto.
s e l F  C H e C k
530 Chapter 11  Input/Output and Exception Handling
Step 1  Understand the processing task.
As always, you need to have a clear understand ing of the task before designing a solution. Can
you carry out the task by hand (perhaps with smaller input files)? If not, get more information
about the problem.
One important consideration is whether you can process the data as it becomes available,
or whether you need to store it first. For example, if you are asked to write out sorted data,
you first need to collect all input, perhaps by placing it in an array list. However, it is often
possible to process the data “on the go”, without storing it.
In our example, we can read each file one line at a time and compute the density for each
line because our input files store the population and area data in the same order.
The following pseudocode describes our processing task.
While there are more lines to be read
Read a line from each file.
Extract the country name.
population = number following the country name in the line from the first file
area = number following the country name in the line from the second file
If area != 0
density = population / area
Print country name and density.
Step 2  Determine which files you need to read and write.
This should be clear from the problem. In our example, there are two input files, the popula-
tion data and the area data, and one output file.
Step 3  Choose a mechanism for obtaining the file names.
There are three options:
• Hard-coding the file names (such as  "worldpop.txt" ).
• Asking the user:
Scanner in = new Scanner(System.in);
System.out.print("Enter filename: ");
String inFile = in.nextLine();
• Using command-line arguments for the file names.
In our example, we use hard-coded file names for simplicity.
© Steve Simzer/iStockphoto.
HOw TO 11.1  Processing Text Files
Processing text files that contain real data can be
surprisingly challenging. This How To gives you step-
by-step guid ance using world population data.
Problem Statement  Read two country data files,
worldpop.txt and  worldarea.txt (supplied with your
source code). Both files contain the same countries in
the same order. Write a file  world_pop_density.txt that
contains country names and population densi ties (peo-
ple per square km), with the country names aligned left
and the numbers aligned right:
Afghanistan 50.56
Akrotiri 127.64
Albania 125.91
Algeria 14.18
American Samoa 288.92
. . .
© Oksana Perkins/iStockphoto.
Singapore is one of the most densely
populated countries in the world.
11.3 Command line arguments 531
step 4  Choose between line, word, and character-based input.
As a rule of thumb, read lines if the input data is grouped by lines. That is the case with tabular
data, such as in our example, or when you need to report line numbers.
When gathering data that can be distributed over several lines, then it makes more sense to
read words. Keep in mind that you lose all white space when you read words.
Reading characters is mostly useful for tasks that require access to individual characters.
Examples include ana lyzing character frequencies, changing tabs to spaces, or encryption.
step 5  With line-oriented input, extract the required data.
It is simple to read a line of input with the  nextLine method. Then you need to get the data out
of that line. You can extract substrings, as described in Section 11.2.4.
Typically, you will use methods such as  Character.isWhitespace and  Character.isDigit to
find the boundaries of substrings.
If you need any of the substrings as numbers, you must convert them, using  Integer.parseInt
or  Double.parseDou ble .
step 6  Use classes and methods to factor out common tasks.
Processing input files usually has repetitive tasks, such as skipping over white space or extract-
ing numbers from strings. It really pays off to isolate these tedious operations from the
remainder of the code.
In our example, we have a task that occurs twice: splitting an input line into the country
name and the value that follows. We implement a simple  CountryValue class for this purpose,
using the technique described in Section 11.2.4.
Here is the complete source code:
how_to_1/CountryValue.java
1 /**
2 Describes a value that is associated with a country.
3 */
4 public class CountryValue
5 {
6 private String country;
7 private double value;
8
9 /**
10 Constructs a  CountryValue from an input line.
11 @param line  a line containing a country name, followed by a value
12 */
13 public CountryValue(String line)
14 {
15 int i = 0; //  Locate the start of the first digit
16 while (!Character.isDigit(line.charAt(i))) { i++; }
17 int j = i  1; //  Locate the end of the preceding word
18 while (Character.isWhitespace(line.charAt(j))) { j; }
19 country = line.substring(0, j + 1); //  Extract the country name
20 value = Double.parseDouble(line.substring(i).trim()); //  Extract the value
21 }
22
23 /**
24 Gets the country name.
25 @return  the country name
26 */
27 public String getCountry() { return country; }
28
532 Chapter 11 Input/output and exception handling
29 /**
30 Gets the associated value.
31 @return  the value associated with the country
32 */
33 public double getValue() { return value; }
34 }
how_to_1/populationdensity.java
1 import java.io.File;
2 import java.io.FileNotFoundException;
3 import java.io.PrintWriter;
4 import java.util.Scanner;
5
6 public class PopulationDensity
7 {
8 public static void main(String[] args) throws FileNotFoundException
9 {
10 //  Open input files
11 Scanner in1 = new Scanner(new File("worldpop.txt"));
12 Scanner in2 = new Scanner(new File("worldarea.txt"));
13
14 //  Open output file
15 PrintWriter out = new PrintWriter("world_pop_density.txt");
16
17 //  Read lines from each file
18 while (in1.hasNextLine() && in2.hasNextLine())
19 {
20 CountryValue population = new CountryValue(in1.nextLine());
21 CountryValue area = new CountryValue(in2.nextLine());
22
23 //  Compute and print the population density
24 double density = 0;
25 if (area.getValue() != 0) //  Protect against division by zero
26 {
27 density = population.getValue() / area.getValue();
28 }
29 out.printf("%40s%15.2f\n", population.getCountry(), density);
30 }
31
32 in1.close();
33 in2.close();
34 out.close();
35 }
36 }
WorkeD exaMple 11.1  analyzing Baby names
Learn how to use data from the Social Security
Administration to analyze the most popular baby
names. Go to  wiley.com/go/javaexamples and down-
load Worked Example 11.1.
11.3 Command line arguments 533
this chapter’s exercise
section gives a few
algorithms for encrypting text. Don’t
actually use any of those meth ods to
send secret messages to your lover.
any skilled cryptographer can break
these schemes in a very short time—
that is, reconstruct the original text
without knowing the secret key word.
In 1978 ron rivest, adi shamir,
and leonard adleman introduced an
encryption method that is much more
powerful. the method is called RSA
encryption, after the last names of its
inventors. the exact scheme is too
complicated to present here, but it is
not actually difficult to follow. You can
find the details in http://the ory.lcs.
mit.edu/~rivest/rsapaper.pdf .
rsa is a remarkable encryption
method. there are two keys: a pub-
lic key and a private key (see the fig-
ure). You can print the public key on
your business card (or in your e-mail
signature block) and give it to any-
one. then anyone can send you mes-
sages that only you can decrypt. even
though everyone else knows the pub lic
key, and even if they intercept all the
messages coming to you, they cannot
break the scheme and actually read
the messages. In 1994, hundreds of
researchers, collaborating over the
Internet, cracked an rsa message
encrypted with a 129-digit key. Mes-
sages encrypted with a key of 230 dig-
its or more are expected to be secure.
the inventors of the algorithm
obtained a patent for it. a patent is
a deal that society makes with an
inven tor. For a period of 20 years, the
inventor has an exclusive right to its
commercialization, may collect royal-
ties from others wishing to manufac-
ture the invention, and may even stop
competitors from using it altogether.
In return, the inventor must publish
the invention, so that others may learn
from it, and must relinquish all claim
to it after the monopoly period ends.
the presumption is that in the absence
of patent law, inventors would be
reluctant to go through the trouble of
inventing, or they would try to cloak
their techniques to prevent others from
copying their devices.
there has been some controversy
about the rsa patent. had there not
been patent protection, would the
inventors have published the method
anyway, thereby giving the benefit to
society without the cost of the 20-year
monopoly? In this case, the answer is
probably yes. the inventors were aca-
demic researchers who live on sala-
ries rather than sales receipts and are
usually rewarded for their discover-
ies by a boost in their reputation and
careers. Would their followers have
been as active in discovering (and pat-
enting) improvements? there is no way
of knowing, of course. Is an algo rithm
even patentable, or is it a math ematical
fact that belongs to nobody? the patent
office did take the latter attitude for a
long time. the rsa inventors and many
others described their inventions in
terms of imaginary electronic devices,
rather than algo rithms, to circumvent
that restriction. nowadays, the pat-
ent office will award software patents.
there is another interesting aspect
to the rsa story. a programmer, phil
Zimmermann, developed a program
called pGp (for Pretty Good Privacy)
that is based on rsa. anyone can use
the program to encrypt messages, and
decryption is not feasible even with
the most powerful computers. You can
get a copy of a free pGp implementa-
tion from the GnU project ( http://www.
gnupg.org ). the existence of strong
encryption  methods  bothers  the
United states government to no end.
Criminals and foreign agents can send
communications that the police and
intelligence agencies cannot deci pher.
the government considered charging
Zimmermann with breaching a law
that forbids the unauthorized export
of munitions, arguing that he should
have known that his program would
appear on the Internet. there have
been serious proposals to make it ille-
gal for private citizens to use these
encryption methods, or to keep the
keys secret from law enforcement.
Public-Key Encryption
(mobile phone) © Anna Khomulo/iStockphoto.
Meet
me at
the
toga
party
Meet
me at
the
toga
party
Xwya
Txu%
*(Wt
&93ya
=9
Alice
The message
is encrypted with
Bob’s public key
Bob
Decrypted
text
Plain
text
Encrypted
text
The message is
decypted with Bob’s
matching private key
Computing & Society 11.1  encryption algorithms
© MediaBakery.
534 Chapter 11 Input/output and exception handling
11.4 exception handling
There are two aspects to dealing with program errors: detection and handling. For
example, the  Scanner constructor can detect an attempt to read from a non-existent
file. However, it cannot handle that error. A satisfactory way of handling the error
might be to terminate the program, or to ask the user for another file name. The  Scan
ner class cannot choose between these alternatives. It needs to report the error to
another part of the program.
In Java, exception handling provides a flexible mechanism for passing control from
the point of error detection to a handler that can deal with the error. In the following
sections, we will look into the details of this mechanism.
11.4.1 throwing exceptions
When you detect an error condition, your job is really easy. You just throw an appro-
priate exception object, and you are done. For exam ple, suppose someone tries to
withdraw too much money from a bank account.
if (amount > balance)
{
//  Now what?
}
First look for an appropriate exception class. The Java library pro vides many classes
to signal all sorts of exceptional conditions. Figure 2 shows the most useful ones.
(The classes are arranged as a tree-shaped inheritance hierarchy, with more special-
ized classes at the bottom of the tree.)
Look around for an exception type that might describe your situa tion. How about
the  ArithmeticException ? Is it an arithmetic error to have a negative balance? No—Java
can deal with negative numbers. Is the amount to be withdrawn illegal? Indeed it is. It
is just too large. Therefore, let’s throw an  IllegalArgument Exception .
if (amount > balance)
{
throw new IllegalArgumentException("Amount exceeds balance");
}
to signal an
exceptional
condition, use the
throw statement
to throw an
exception object.
syntax 11.1  throwing an exception
A new
exception object
is constructed,
then thrown.
if (amount > balance)
{
throw new IllegalArgumentException("Amount exceeds balance");
}
balance = balance - amount;
Most exception objects
can be constructed with
an error message.
This line is not executed when
the exception is thrown.
throw  exceptionObject ; Syntax
11.4 exception handling 535
When you throw an exception, execution does not con-
tinue with the next statement but with an excep tion
handler. That is the topic of the next section.
When you throw an exception, the normal control flow
is terminated. This is similar to a circuit breaker that
cuts off the flow of electricity in a dangerous situation.
When you throw
an exception,
processing
continues in an
exception handler.
© Lisa F. Young/iStockphoto.
Figure 2 
a part of the hierarchy
of exception Classes
ClassNot
Found
Exception
IndexOut
OfBounds
Exception
Illegal
Argument
Exception
ClassCast
Exception
Arithmetic
Exception
Runtime
Exception
Exception
IOException
FileNotFound
Exception
MalformedURL
Exception
UnknownHost
Exception
NumberFormat
Exception
NullPointer
Exception
NoSuch
Element
Exception
Throwable
Error
InputMismatch
Exception
Import from
java.io
When constructing
a scanner or writer
with a non-existent file
Import from
java.util
When calling
Integer.parseInt or
Double.parseDouble with
an illegal argument
When calling
next ,  nextInt , or  nextDouble
on a scanner and no input
is available
When calling
nextInt or  nextDouble on a
scanner and the input is not
of the expected form
536 Chapter 11 Input/output and exception handling
11.4.2 Catching exceptions
Every exception should be handled somewhere in your program. If an exception has
no handler, an error message is printed, and your program terminates. Of course,
such an unhandled exception is confusing to program users.
You handle exceptions with the  try / catch statement. Place the statement into a
location of your pro gram that knows how to handle a particular exception. The  try
block contains one or more statements that may cause an exception of the kind that
you are willing to handle. Each  catch clause contains the han dler for an exception
type. Here is an example:
try
{
String filename = . . .;
Scanner in = new Scanner(new File(filename));
String input = in.next();
int value = Integer.parseInt(input);
. . .
}
catch (IOException exception)
{
exception.printStackTrace();
}
catch (NumberFormatException exception)
{
System.out.println(exception.getMessage());
}
place the statements
that can cause an
exception inside a
try block, and the
handler inside a
catch clause.
syntax 11.2  Catching exceptions
try
{
statement
statement
. . .
}
catch ( ExceptionClass exceptionObject )
{
statement
statement
. . .
}
Syntax
This constructor can throw a
FileNotFoundException .
try
{
Scanner in = new Scanner(new File("input.txt"));
String input = in.next();
process(input);
}
catch (IOException exception)
{
System.out.println("Could not open input file");
}
catch (Exception except)
{
System.out.println(except.getMessage());
}
This is the exception that was thrown.
A  FileNotFoundException
is a special case of an  IOException .
When an  IOException is thrown,
execution resumes here.
Additional catch clauses
can appear here. Place
more specific exceptions
before more general ones.
11.4 exception handling 537
Three exceptions may be thrown in this  try block:
• The  Scanner constructor can throw a
FileNotFoundException .
•  Scanner.next can throw a  NoSuchElementException .
•  Integer.parseInt can throw a  NumberFormat Exception .
If any of these exceptions is actually thrown, then the
rest of the instructions in the  try block are skipped.
Here is what happens for the various exception types:
• If a  FileNotFoundException is thrown, then the  catch clause for the  IOException is
executed. (If you look at Figure 2, you will note that  FileNotFoundException is a
descendant of  IOException .) If you want to show the user a different message for a
FileNotFoundException , you must place the  catch clause before the clause for an
IOException .
• If a  NumberFormatException occurs, then the second  catch clause is executed.
• A  NoSuchElementException is not caught by any of the  catch clauses. The exception
remains thrown until it is caught by another  try block.
Each  catch clause contains a handler. When the  catch (IOException exception) block is
executed, then some method in the  try block has failed with an  IOException (or one of
its descendants).
In this handler, we pro duce a printout of the chain of method calls that led to the
exception, by calling
exception.printStackTrace()
In the second exception handler, we call  exception.getMessage() to retrieve the mes-
sage associated with the exception. When the  parseInt method throws a  NumberFormat
Exception , the message contains the string that it was unable to format. When you
throw an exception, you can provide your own message string. For example, when
you call
throw new IllegalArgumentException("Amount exceeds balance");
the message of the exception is the string provided in the constructor.
In these sample  catch clauses, we merely inform the user of the source of the prob-
lem. Often, it is better to give the user another chance to provide a correct input—see
Section 11.5 for a solution.
11.4.3 Checked exceptions
In Java, the exceptions that you can throw and catch fall into three categories.
• Internal errors are reported by descendants of the type  Error . One example is the
OutOfMemoryError , which is thrown when all available computer memory has been
used up. These are fatal errors that happen rarely, and we will not consider them
in this book.
• Descendants of  RuntimeException , such as as  IndexOutOfBoundsException or  Illegal
ArgumentException indicate errors in your code. They are called unchecked
exceptions.
© Andraz Cerar/iStockphoto.
You should only catch those
exceptions that you can handle.
Camera: © james steidl/iStockphoto.
Globe: © Alex Slobodkin/iStockphoto.
A N IM AT IO N
Exception Handling
538 Chapter 11 Input/output and exception handling
• All other exceptions are checked exceptions. These exceptions indicate that
something has gone wrong for some external reason beyond your control. In
Figure 2, the checked exceptions are shaded in a darker color.
Why have two kinds of exceptions? A checked exception describes a problem that
can occur, no matter how careful you are. For example, an  IOException can be caused
by forces beyond your control, such as a disk error or a broken network connection.
The compiler takes checked exceptions very seriously and ensures that they are han-
dled. Your program will not compile if you don’t indicate how to deal with a checked
exception.
The unchecked exceptions, on the other hand, are your fault. The compiler does
not check whether you handle an unchecked exception, such as an  IndexOutOfBounds
Exception . After all, you should check your index values rather than install a handler
for that exception.
If you have a handler for a checked exception in the same method that may throw
it, then the compiler is satisfied. For example,
try
{
File inFile = new File(filename);
Scanner in = new Scanner(inFile); //  Throws  FileNotFoundException
. . .
}
catch (FileNotFoundException exception) //  Exception caught here
{
. . .
}
However, it commonly happens that the current method cannot handle the excep-
tion. In that case, you need to tell the compiler that you are aware of this exception
and that you want your method to be termi nated when it occurs. You supply the
method with a  throws clause:
public void readData(String filename) throws FileNotFoundException
{
File inFile = new File(filename);
Scanner in = new Scanner(inFile);
. . .
}
The  throws clause signals to the caller of your method that it may encounter
a  FileNotFoundException . Then the caller needs to make the same decision—
han dle the exception, or declare that the exception may be thrown.
It sounds somehow irresponsible not to handle an exception when you
know that it happened. Actually, the opposite is true. Java provides an
excep tion handling facility so that an exception can be sent to the appropri-
ate han dler. Some methods detect errors, some methods handle them, and
some methods just pass them along. The  throws clause simply ensures that no
exceptions get lost along the way.
Just as trucks with large or hazardous loads carry warning signs,
the  throws clause warns the caller that an exception may occur.
Checked exceptions
are due to external
circumstances that
the programmer
cannot prevent.
the compiler
checks that your
program handles
these exceptions.
add a  throws clause
to a method that
can throw a
checked exception.
© tillsonburg/iStockphoto.
11.4 exception handling 539
11.4.4
syntax 11.3  the  throws Clause
You may also list unchecked exceptions. You must specify all checked exceptions
that this method may throw.
public void readData(String filename)
throws FileNotFoundException, NumberFormatException
modifiers returnType methodName ( parameterType parameterName , . . .)
throws  ExceptionClass ,  ExceptionClass , . . .
Syntax
the  finally Clause
Occasionally, you need to take some action whether or not an exception is thrown.
The  finally clause is used to handle this situation.
Here is a typical example: It is important to close a  PrintWriter to ensure that all
output is written to the file. In the following code segment, we open a stream, call one
or more methods, and then close the stream:
PrintWriter out = new PrintWriter(filename);
writeData(out);
out.close(); //  May never get here
Now suppose that one of the methods before the last line throws an exception. Then
the call to  close is never executed! You solve this problem by placing the call to  close
inside a  finally clause:
PrintWriter out = new PrintWriter(filename);
try
{
writeData(out);
}
finally
{
out.close();
}
In a normal case, there will be no problem. When the
try block is completed, the  finally clause is executed,
and the writer is closed. However, if an exception
occurs, the  finally clause is also executed before the
exception is passed to its handler.
Use the  finally clause whenever you need to do
some clean up, such as closing a file, to ensure that the
clean up happens no matter how the method exits.
All visitors to a foreign country have to go through
passport control, no matter what happened on their
trip. Similarly, the code in a  finally clause is always
executed, even when an exception has occurred.
once a  try block
is entered, the
statements in a
finally clause are
guaranteed to be
executed, whether
or not an exception
is thrown.
© archives/iStockphoto.
Full COde example
Go to  wiley.com/go/
javacode to download
a program that
demonstrates
throwing and
catching exceptions.
540 Chapter 11 Input/output and exception handling
11.4.5
syntax 11.4  the  finally Clause
try
{
statement
statement
. . .
}
finally
{
statement
statement
. . .
}
Syntax
PrintWriter out = new PrintWriter(filename);
try
{
writeData(out);
}
finally
{
out.close();
}
This variable must be declared outside the  try block
so that the  finally clause can access it.
This code is
always executed,
even if an exception occurs.
This code may
throw exceptions.
Designing Your own exception types
Sometimes none of the standard exception types describe your particular error con-
dition well enough. In that case, you can design your own exception class. Consider
a bank account. Let’s report an  Insufficient FundsException when an attempt is made to
withdraw an amount from a bank account that exceeds the cur rent balance.
if (amount > balance)
{
throw new InsufficientFundsException(
"withdrawal of " + amount + " exceeds balance of " + balance);
}
Now you need to provide the  InsufficientFundsException class. Should it be a checked
or an unchecked exception? Is it the fault of some external event, or is it the fault of
the programmer? We take the position that the programmer could have prevented the
exceptional condition—after all, it would have been an easy matter to check whether
amount <= account.getBalance() before calling the  withdraw method. Therefore, the
exception should be an unchecked exception and extend the  Runtime Exception class or
one of its sub classes.
It is a good idea to extend an appropriate class in the exception hierarchy. For
example, we can con sider an  InsufficientFundsException a special case of an  Illegal
ArgumentException . This enables other pro grammers to catch the exception as an
Illegal ArgumentException if they are not interested in the exact nature of the problem.
to describe an
error condition,
provide a subclass
of an existing
exception class.
11.4 exception handling 541
It is customary to provide two constructors for an exception class: a constructor
with no arguments and a constructor that accepts a message string describing the rea-
son for the exception. Here is the decla ration of the exception class:
public class InsufficientFundsException extends IllegalArgumentException
{
public InsufficientFundsException() {}
public InsufficientFundsException(String message)
{
super(message);
}
}
When the exception is caught, its message string can be retrieved using the  get Message
method of the  Throw able class.
16.  Suppose  balance is 100 and  amount is 200. What is the value of  bal ance after these
statements?
if (amount > balance)
{
throw new IllegalArgumentException("Amount exceeds balance");
}
balance = balance  amount;
17.  When depositing an amount into a bank account, we don’t have to worry about
overdrafts—except when the amount is negative. Write a statement that throws
an appropriate exception in that case.
18.  Consider the method
public static void main(String[] args)
{
try
{
Scanner in = new Scanner(new File("input.txt"));
int value = in.nextInt();
System.out.println(value);
}
catch (IOException exception)
{
System.out.println("Error opening file.");
}
}
Suppose the file with the given file name exists and has no contents. Trace the
flow of execution.
19.  Why is an  ArrayIndexOutOfBoundsException not a checked exception?
20.  Is there a difference between catching checked and unchecked exceptions?
21.  What is wrong with the following code, and how can you fix it?
public static void writeAll(String[] lines, String filename)
{
PrintWriter out = new PrintWriter(filename);
for (String line : lines)
{
out.println(line.toUpperCase());
}
out.close();
© Nicholas Homrich/iStockphoto.
s e l F  C H e C k
542 Chapter 11 Input/output and exception handling
}
22.  What is the purpose of the call  super(message) in the second  Insufficient Funds
Exception constructor?
23.  Suppose you read bank account data from a file. Contrary to your expectation,
the next input value is not of type  double . You decide to implement a  BadData
Exception . Which exception class should you extend?
practice it  Now you can try these exercises at the end of the chapter: R11.7, R11.8, R11.9.
throw early, Catch late
When a method detects a problem that it cannot solve, it is better
to throw an exception rather than try to come up with an imperfect
fix. For example, suppose a method expects to read a number from a
file, and the file doesn’t contain a number. Simply using a zero value
would be a poor choice because it hides the actual problem and per-
haps causes a different problem elsewhere.
Conversely, a method should only catch an exception if it can
really remedy the situa tion. Otherwise, the best remedy is simply to have the exception propa-
gate to its caller, allowing it to be caught by a competent handler.
These principles can be summarized with the slogan “throw early, catch late”.
do not squelch exceptions
When you call a method that throws a checked exception and you haven’t specified a handler,
the compiler com plains. In your eagerness to continue your work, it is an understandable
impulse to shut the compiler up by squelch ing the exception:
try
{
Scanner in = new Scanner(new File(filename));
//  Compiler complained about FileNotFoundException
. . .
}
catch (FileNotFoundException e) {} //  So there!
The do-nothing exception handler fools the compiler into thinking that the exception has
been handled. In the long run, this is clearly a bad idea. Exceptions were designed to transmit
problem reports to a competent handler. Install ing an incompetent handler simply hides an
error condition that could be serious.
do not use  catch  and  finally  in the same  try  statement
It is possible to have a  finally clause following one or more  catch clauses. Then the code in the
finally clause is exe cuted whenever the  try block is exited in any of three ways:
1.  After completing the last statement of the  try block
2.  After completing the last statement of a  catch clause, if this  try block caught an
exception
3.  When an exception was thrown in the  try block and not caught
programming tip 11.1
© Eric Isselé/iStockphoto.
throw an exception
as soon as a
problem is detected.
Catch it only
when the problem
can be handled.
programming tip 11.2
© Eric Isselé/iStockphoto.
programming tip 11.3
© Eric Isselé/iStockphoto.
11.4 exception handling 543
It is tempting to combine  catch and  finally clauses, but the resulting code can be hard to
understand, and it is often incorrect. Instead, use two statements:
• a  try / finally statement to close resources
• a separate  try / catch statement to handle errors
For example,
try
{
PrintWriter out = new PrintWriter(filename);
try
{
Write output.
}
finally
{
out.close();
}
}
catch (IOException exception)
{
Handle exception.
}
Note that the nested statements work correctly even if the  PrintWriter constructor throws an
exception.
do throw specific exceptions
When throwing an exception, you should choose an exception class that describes the situ-
ation as closely as possible. For example, it would be a bad idea to simply throw a  Runtime 
Exception object when a bank account has insufficient funds. This would make it far too diffi-
cult to catch the exception. After all, if you caught all exceptions of type  Runtime Exception , your
catch clause would also be activated by exceptions of the type  NullPointer Exception ,  Array
In dexOutOfBoundsException , and so on. You would then need to carefully examine the exception
object and attempt to deduce whether the exception was caused by insufficient funds.
If the standard library does not have an exception class that describes your particular error
situation, simply pro vide a new exception class.
assertions
An assertion is a condition that you believe to be true at all times in a particular program loca-
tion. An assertion check tests whether an assertion is true. Here is a typical assertion check:
public double deposit (double amount)
{
assert amount >= 0;
balance = balance + amount;
}
In this method, the programmer expects that the quantity amount can never be negative.
When the assertion is correct, no harm is done, and the program works in the normal way. If,
for some reason, the assertion fails, and assertion checking is enabled, then the  assert state-
ment throws an exception of type  AssertionError , causing the program to terminate.
However, if assertion checking is disabled, then the assertion is never checked, and the pro-
gram runs at full speed. By default, assertion checking is disabled when you execute a program.
programming tip 11.4
© Eric Isselé/iStockphoto.
special topic 11.5
© Eric Isselé/iStockphoto.
544 Chapter 11  Input/Output and Exception Handling
To execute a program with assertion checking turned on, use this command:
java -enableassertions MainClass
You can also use the shortcut  -ea instead of  -enableassertions . You should turn assertion
checking on during program development and testing.
Automatic Resource Management in Java 7
In Java 7, you can use a new form of the  try block that automatically closes a  PrintWriter or
Scanner object. Here is the syntax:
try (PrintWriter out = new PrintWriter(filename))
{
Write output to  out.
}
The  close method is automatically invoked on the  out object when the  try block ends, whether
or not an exception has occurred. A  finally statement is not required.
Special Topic 11.6
© Eric Isselé/iStockphoto.
The European Space
Agency (ESA), Europe’s
counterpart to NASA, had developed a
rocket model called Ariane that it had
successfully used several times to
launch satellites and scientific experi-
ments into space. However, when a new
version, the Ariane 5, was launched on
June 4, 1996, from ESA’s launch site
in Kourou, French Guiana, the rocket
veered off course about 40 seconds
after liftoff. Flying at an angle of more
than 20 degrees, rather than straight
up, exerted such an aerodynamic force
that the boosters separated, which trig-
gered the automatic self-destruction
mechanism. The rocket blew itself up.
The ultimate cause of this accident
was an unhandled exception! The
rocket contained two identical devices
(called inertial reference systems) that
processed flight data from  measuring
devices and turned the data into infor-
mation about the rocket position.
The onboard computer used the posi-
tion information for controlling the
boosters. The same inertial reference
systems and computer software had
worked fine on the Ariane 4.
However, due to design changes
to the rocket, one of the sensors mea-
sured a larger acceleration force than
had been encountered in the Ariane 4.
That value, expressed as a floating-
point value, was stored in a 16-bit
integer (like a  short  variable in Java).
Unlike Java, the Ada language, used
for the device software, generates an
exception if a float ing-point number is
too large to be converted to an integer.
Unfortunately, the programmers of the
device had decided that this situation
would never happen and didn’t provide
an exception handler.
When the overflow did happen, the
exception was triggered and, because
there was no handler, the device shut
itself off. The onboard computer sensed
the failure and switched over to the
backup device. However, that device
had shut itself off for exactly the same
reason, something that the designers
of the rocket had not expected. They
fig ured that the devices might fail for
mechanical reasons, but the chance
of them having the same mechanical
failure was remote. At that point, the
rocket was without reliable position
information and went off course.
Perhaps it would have been better if
the software hadn’t been so thorough?
If it had ignored the overflow, the
device wouldn’t have been shut off. It
would have computed bad data. But
then the device would have reported
wrong position data, which could have
been just as fatal. Instead, a correct
implementation should have caught
over flow exceptions and come up with
some strategy to recompute the flight
data. Clearly, giving up was not a
reason able option in this context.
The advantage of
the exception-handling
mechanism is that it
makes  these  issues
explicit  to  program-
mers—some thing  to
think about when you
curse the Java compiler
for complaining about
uncaught exceptions.
© AP/Wide World Photos.
The Explosion of the Ariane Rocket
Computing & Society 11.2  The Ariane Rocket Incident
© MediaBakery.
11.5 application: handling Input errors 545
11.5 application: handling Input errors
This section walks through a complete example of a program with exception han-
dling. The program asks a user for the name of a file. The file is expected to con-
tain data values. The first line of the file contains the total number of values, and the
remaining lines contain the data. A typical input file looks like this:
3
1.45
2.1
0.05
What can go wrong? There are two principal risks.
• The file might not exist.
• The file might have data in the wrong format.
Who can detect these faults? The  Scanner constructor will throw an exception when
the file does not exist. The methods that process the input values need to throw an
exception when they find an error in the data format.
What exceptions can be thrown? The  Scanner constructor throws a  FileNot
FoundException when the file does not exist, which is appropriate in our situation.
When the file data is in the wrong format, we will throw a  BadDataException , a custom
checked exception class. We use a checked exception because cor ruption of a data file
is beyond the control of the programmer.
Who can remedy the faults that the exceptions report? Only the  main method of the
DataAnalyzer pro gram interacts with the user. It catches the exceptions, prints appro-
priate error messages, and gives the user another chance to enter a correct file.
section_5/dataanalyzer.java
1 import java.io.FileNotFoundException;
2 import java.io.IOException;
3 import java.util.Scanner;
4
5 /**
6 This program reads a file containing numbers and analyzes its contents.
7 If the file doesn’t exist or contains strings that are not numbers, an
8 error message is displayed.
9 */
10 public class DataAnalyzer
11 {
12 public static void main(String[] args)
13 {
14 Scanner in = new Scanner(System.in);
15 DataSetReader reader = new DataSetReader();
16
17 boolean done = false;
18 while (!done)
19 {
20 try
21 {
22 System.out.println("Please enter the file name: ");
23 String filename = in.next();
24
25 double[] data = reader.readFile(filename);
26 double sum = 0;
When designing
a program, ask
yourself what
kinds of exceptions
can occur.
For each exception,
you need to decide
which part of
your program can
competently
handle it.
546 Chapter 11 Input/output and exception handling
27 for (double d : data) { sum = sum + d; }
28 System.out.println("The sum is " + sum);
29 done = true;
30 }
31 catch (FileNotFoundException exception)
32 {
33 System.out.println("File not found.");
34 }
35 catch (BadDataException exception)
36 {
37 System.out.println("Bad data: " + exception.getMessage());
38 }
39 catch (IOException exception)
40 {
41 exception.printStackTrace();
42 }
43 }
44 }
45 }
The  catch clauses in the  main method give a human-readable error report if the file was
not found or bad data was encountered.
The following  readFile method of the  DataSetReader class constructs the  Scanner
object and calls the  read Data method. It is completely unconcerned with any excep-
tions. If there is a problem with the input file, it simply passes the exception to its
caller.
public double[] readFile(String filename) throws IOException
{
File inFile = new File(filename);
Scanner in = new Scanner(inFile);
try
{
readData(in);
return data;
}
finally
{
in.close();
}
}
The method throws an  IOException , the common superclass of  FileNotFoundException
(thrown by the  Scanner constructor) and  BadDataException (thrown by the  readData
method).
Next, here is the  readData method of the  DataSetReader class. It reads the number of
values, constructs an array, and calls  readValue for each data value.
private void readData(Scanner in) throws BadDataException
{
if (!in.hasNextInt())
{
throw new BadDataException("Length expected");
}
int numberOfValues = in.nextInt();
data = new double[numberOfValues];
for (int i = 0; i < numberOfValues; i++)
{
11.5 application: handling Input errors 547
readValue(in, i);
}
if (in.hasNext())
{
throw new BadDataException("End of file expected");
}
}
This method checks for two potential errors. The file might not start with an integer,
or it might have additional data after reading all values.
However, this method makes no attempt to catch any exceptions. Plus, if the  read
Value method throws an exception—which it will if there aren’t enough values in the
file—the exception is simply passed on to the caller.
Here is the  readValue method:
private void readValue(Scanner in, int i) throws BadDataException
{
if (!in.hasNextDouble())
{
throw new BadDataException("Data value expected");
}
data[i] = in.nextDouble();
}
To see the exception handling at work, look at a specific error scenario:
1.  DataAnalyzer.main calls  DataSetReader.readFile.
2.  readFile calls  readData .
3.  readData calls  readValue .
4.  readValue doesn’t find the expected value and throws a  BadDataException .
5.  readValue has no handler for the exception and terminates immediately.
6.  readData has no handler for the exception and terminates immediately.
7.  readFile has no handler for the exception and terminates immediately after
executing the  finally clause and closing the  Scanner object.
8.  DataAnalyzer.main has a handler for a  BadDataException . That handler prints a
message to the user. Afterward, the user is given another chance to enter a file
name. Note that the statements comput ing the sum of the values have been
skipped.
This example shows the separation between error detection (in the  DataSetReader.
readValue method) and error handling (in the  DataAnalyzer.main method). In between
the two are the  readData and  readFile meth ods, which just pass exceptions along.
section_5/datasetReader.java
1 import java.io.File;
2 import java.io.IOException;
3 import java.util.Scanner;
4
5 /**
6 Reads a data set from a file. The file must have the format
7 numberOfValues
8 value1
9 value2
10 . . .
548 Chapter 11 Input/output and exception handling
11 */
12 public class DataSetReader
13 {
14 private double[] data;
15
16 /**
17 Reads a data set.
18 @param filename  the name of the file holding the data
19 @return  the data in the file
20 */
21 public double[] readFile(String filename) throws IOException
22 {
23 File inFile = new File(filename);
24 Scanner in = new Scanner(inFile);
25 try
26 {
27 readData(in);
28 return data;
29 }
30 finally
31 {
32 in.close();
33 }
34 }
35
36 /**
37 Reads all data.
38 @param in  the scanner that scans the data
39 */
40 private void readData(Scanner in) throws BadDataException
41 {
42 if (!in.hasNextInt())
43 {
44 throw new BadDataException("Length expected");
45 }
46 int numberOfValues = in.nextInt();
47 data = new double[numberOfValues];
48
49 for (int i = 0; i < numberOfValues; i++)
50 {
51 readValue(in, i);
52 }
53
54 if (in.hasNext())
55 {
56 throw new BadDataException("End of file expected");
57 }
58 }
59
60 /**
61 Reads one data value.
62 @param in  the scanner that scans the data
63 @param i  the position of the value to read
64 */
65 private void readValue(Scanner in, int i) throws BadDataException
66 {
67 if (!in.hasNextDouble())
68 {
69 throw new BadDataException("Data value expected");
70 }
Chapter summary 549
71 data[i] = in.nextDouble();
72 }
73 }
section_5/Baddataexception.java
1 import java.io.IOException;
2
3 /**
4 This class reports bad input data.
5 */
6 public class BadDataException extends IOException
7 {
8 public BadDataException() {}
9 public BadDataException(String message)
10 {
11 super(message);
12 }
13 }
24.  Why doesn’t the  DataSetReader.readFile method catch any exceptions?
25.  Suppose the user specifies a file that exists and is empty. Trace the flow of
execution.
26.  If the  readValue method had to throw a  NoSuchElementException instead of a  Bad
DataException when the next input isn’t a floating-point number, how would the
implementation change?
27.  Consider the  try/finally statement in the  readFile method. Why was the  in vari-
able declared outside the  try block?
28.  How can the program be simplified when you use the “automatic resource man-
agement” feature described in Special Topic 11.6?
practice it  Now you can try these exercises at the end of the chapter: R11.15, R11.16, E11.11.
develop programs that read and write files.
• Use the  Scanner class for reading text files.
• When writing text files, use the  PrintWriter class and the  print/println/printf
methods.
• Close all files when you are done processing them.
Be able to process text in files.
• The  next method reads a string that is delimited by white space.
• The  Character class has methods for classifying characters.
• The  nextLine method reads an entire line.
• If a string contains the digits of a number, you use the  Integer.parseInt or
Double.parseDouble method to obtain the number value.
© Nicholas Homrich/iStockphoto.
s e l F  C H e C k
C h a p t e r s U M M a rY
550 Chapter 11 Input/output and exception handling
process the command line arguments of a program.
• Programs that start from the command line receive the command line arguments
in the  main method.
use exception handling to transfer control from an error location to an error handler.
• To signal an exceptional condition, use the  throw statement to throw an exception
object.
• When you throw an exception, processing continues in an exception handler.
• Place the statements that can cause an exception inside a  try
block, and the handler inside a  catch clause.
• Checked exceptions are due to external circumstances that the
programmer cannot prevent. The compiler checks that your
program handles these exceptions.
• Add a  throws clause to a method that can throw a checked exception.
• Once a  try block is entered, the statements in a  finally clause are guaranteed to be
executed, whether or not an exception is thrown.
• To describe an error condition, provide a subclass of an existing
exception class.
• Throw an exception as soon as a problem is detected. Catch it
only when the problem can be handled.
use exception handling in a program that processes input.
• When designing a program, ask yourself what kinds of exceptions can occur.
• For each exception, you need to decide which part of your program can compe-
tently handle it.
© xyno/iStockphoto.
© Lisa F. Young/iStockphoto.
© Andraz Cerar/iStockphoto.
© tillsonburg/iStockphoto.
© archives/iStockphoto.
java.io.File
java.io.FileNotFoundException
java.io.IOException
java.io.PrintWriter
close
java.lang.Character
isDigit
isLetter
isLowerCase
isUpperCase
isWhiteSpace
java.lang.Double
parseDouble
java.lang.Error
java.lang.Integer
parseInt
java.lang.IllegalArgumentException
java.lang.NullPointerException
java.lang.NumberFormatException
java.lang.RuntimeException
java.lang.Throwable
getMessage
printStackTrace
java.net.URL
openStream
java.util.InputMismatchException
java.util.NoSuchElementException
java.util.Scanner
close
hasNextLine
nextLine
useDelimiter
javax.swing.JFileChooser
getSelectedFile
showOpenDialog
showSaveDialog
s ta n D a r D l I B r a rY I t e M s I n t r o D U C e D I n t h I s C h ap t e r
review Questions 551
•• R11.1  What happens if you try to open a file for reading that doesn’t exist? What happens if
you try to open a file for writing that doesn’t exist?
•• R11.2  What happens if you try to open a file for writing, but the file or device is write-
protected (sometimes called read-only)? Try it out with a short test program.
• R11.3  How do you open a file whose name contains a backslash, like  c:temp\output.dat ?
• R11.4  If a program Woozle is started with the command
java Woozle Dname=piglet I\eeyore v heff.txt a.txt lump.txt
what are the values of  args[0] ,  args[1] , and so on?
• R11.5  What is the difference between throwing an exception and catching an exception?
• R11.6  What is a checked exception? What is an unchecked exception? Give an example for
each. Which exceptions do you need to declare with the  throws reserved word?
•• R11.7  Why don’t you need to declare that your method might throw an  IndexOutOfBounds 
Exception ?
•• R11.8  When your program executes a  throw statement, which statement is executed next?
•• R11.9  What happens if an exception does not have a matching  catch clause?
•• R11.10  What can your program do with the exception object that a  catch clause receives?
•• R11.11  Is the type of the exception object always the same as the type declared in the  catch
clause that catches it? If not, why not?
• R11.12  What is the purpose of the  finally clause? Give an example of how it can be used.
•• R11.13  What happens when an exception is thrown, the code of a  finally clause executes,
and that code throws an exception of a different kind than the original one? Which
one is caught by a surrounding  catch clause? Write a sample program to try it out.
•• R11.14  Which exceptions can the  next and  nextInt methods of the  Scanner class throw? Are
they checked exceptions or unchecked exceptions?
•• R11.15  Suppose the program in Section 11.5 reads a file containing the following values:
1
2
3
4
What is the outcome? How could the program be improved to give a more accurate
error report?
•• R11.16  Can the  readFile method in Section 11.5 throw a  NullPointer Exception ? If so, how?
••• R11.17  Suppose the code in Programming Tip 11.3 had been condensed to a single  try/catch/
finally statement:
PrintWriter out = new PrintWriter(filename);
try
{
Write output.
r e v I e W Q U e s t I o n s
552 Chapter 11 Input/output and exception handling
}
catch (IOException exception)
{
Handle exception.
}
finally
{
out.close();
}
What is the disadvantage of this version? (Hint: What happens when the  PrintWriter
constructor throws an exception?) Why can’t you solve the problem by moving the
declaration of the  out variable inside the  try block?
• e11.1  Write a program that carries out the following tasks:
Open a file with the name hello.txt.
Store the message "Hello, World!" in the file.
Close the file.
Open the same file again.
Read the message into a string variable and print it.
• e11.2  Write a program that reads a file containing text. Read each line and send it to the
output file, preceded by line numbers. If the input file is
Mary had a little lamb
Whose fleece was white as snow.
And everywhere that Mary went,
The lamb was sure to go!
then the program produces the output file
/* 1 */ Mary had a little lamb
/* 2 */ Whose fleece was white as snow.
/* 3 */ And everywhere that Mary went,
/* 4 */ The lamb was sure to go!
The line numbers are enclosed in  /* */ delimiters so that the program can be used for
numbering Java source files.
Prompt the user for the input and output file names.
• e11.3  Repeat Exercise E11.2, but allow the user to specify the file name on the command-
line. If the user doesn’t specify any file name, then prompt the user for the name.
• e11.4  Write a program that reads a file containing two columns of floating-point numbers.
Prompt the user for the file name. Print the average of each column.
•• e11.5  Write a program that asks the user for a file name and prints the number of charac-
ters, words, and lines in that file.
•• e11.6  Write a program  Find that searches all files specified on the command line and prints
out all lines containing a specified word. For example, if you call
java Find ring report.txt address.txt Homework.java
p r a C t I C e e x e r C I s e s
© Chris Price/iStockphoto.
practice exercises 553
then the program might print
report.txt: has broken up an international ring of DVD bootleggers that
address.txt: Kris Kringle, North Pole
address.txt: Homer Simpson, Springfield
Homework.java: String filename;
The specified word is always the first command line argument.
•• e11.7  Write a program that checks the spelling of all words in a file. It should read each
word of a file and check whether it is contained in a word list. A word list is avail-
able on most Linux systems in the file  /usr/share/dict/words . (If you don’t have access
to a Linux system, your instructor should be able to get you a copy.) The program
should print out all words that it cannot find in the word list.
•• e11.8  Write a program that replaces each line of a file with its reverse. For example, if you
run
java Reverse HelloPrinter.java
then the contents of  HelloPrinter.java are changed to
retnirPolleH ssalc cilbup
{
)sgra ][gnirtS(niam diov citats cilbup
{
wodniw elosnoc eht ni gniteerg a yalpsiD //
;)"!dlroW ,olleH"(nltnirp.tuo.metsyS
}
}
Of course, if you run  Reverse twice on the same file, you get back the original file.
•• e11.9  Write a program that reads each line in a file, reverses its lines, and writes them to
another file. For example, if the file  input.txt contains the lines
Mary had a little lamb
Its fleece was white as snow
And everywhere that Mary went
The lamb was sure to go.
and you run
reverse input.txt output.txt
then  output.txt contains
The lamb was sure to go.
And everywhere that Mary went
Its fleece was white as snow
Mary had a little lamb
•• e11.10  Get the data for names in prior decades from the Social Security Administration.
Paste the table data in files named  babynames80s.txt , etc. Modify the  worked_example_1/
BabyNames.java program so that it prompts the user for a file name. The numbers in
the files have comma separators, so modify the program to handle them. Can you
spot a trend in the frequencies?
•• e11.11  Write a program that asks the user to input a set of floating-point values. When the
user enters a value that is not a number, give the user a second chance to enter the
value. After two chances, quit reading input. Add all correctly specified values and
print the sum when the user is done entering data. Use exception handling to detect
improper inputs.
554 Chapter 11 Input/output and exception handling
• e11.12  Modify the  BankAccount class to throw an  IllegalArgumentException when the account is
constructed with a negative balance, when a negative amount is deposited, or when
an amount that is not between 0 and the current balance is withdrawn. Write a test
program that causes all three exceptions to occur and that catches them all.
•• e11.13  Repeat Exercise E11.12, but throw exceptions of three exception types that
you pro vide.
•• e11.14  Modify the  DataSetReader class so that you do not call  hasNextInt or  hasNextDouble .
Simply have  nextInt and  nextDouble throw a  NoSuchElementException and catch it in the
main method.
•• p11.1  Write a program that reads in  worked_example_1/babynames.txt and produces two files,
boynames.txt and  girlnames.txt , separating the data for the boys and girls.
••• p11.2  Write a program that reads a file in the same format as  worked_example_1/babynames.txt
and prints all names that are both boy and girl names (such as Alexis or Morgan).
•• p11.3  Using the mechanism described in Special Topic 11.1, write a program that reads
all data from a web page and writes them to a file. Prompt the user for the web page
URL and the file.
•• p11.4  Using the mechanism described in Special Topic 11.1, write a program that reads all
data from a web page and prints all hyperlinks of the form
<a href=" link "> link text </a>
Extra credit if your program can follow the links that it finds and find links in those
web pages as well. (This is the method that search engines such as Google use to find
web sites.)
•• p11.5  Write a program that reads in a set of coin descriptions from a file. The input file has
the format
coinName1 coinValue1
coinName2 coinValue2
. . .
Add a method
void read(Scanner in) throws FileNotFoundException
to the  Coin class of Section 8.2. Throw an exception if the current line is not properly
formatted. Then implement a method
static ArrayList<Coin> readFile(String filename) throws FileNotFoundException
In the  main method, call  readFile . If an exception is thrown, give the user a chance to
select another file. If you read all coins successfully, print the total value.
••• p11.6  Design a class  Bank that contains a number of bank accounts. Each account has an
account number and a current balance. Add an  accountNumber field to the  BankAccount
class. Store the bank accounts in an array list. Write a  readFile method of the  Bank
class for reading a file with the format
accountNumber1 balance1
accountNumber2 balance2
. . .
p r o G r a M M I n G p r o J e C t s
programming projects 555
Implement  read methods for the  Bank and  BankAccount classes. Write a sample program
to read in a file with bank accounts, then print the account with the highest balance.
If the file is not properly formatted, give the user a chance to select another file.
•• Business p11.7  A hotel salesperson enters sales in a text file. Each line contains the following,
separated by semicolons: The name of the client, the service sold (such as Dinner,
Conference, Lodging, and so on), the amount of the sale, and the date of that event.
Write a program that reads such a file and displays the total amount for each service
category. Display an error if the file does not exist or the format is incorrect.
•• Business p11.8  Write a program that reads a text file as described in Exercise P11.7, and that writes a
separate file for each service category, containing the entries for that category. Name
the output files  Dinner.txt ,  Conference.txt , and so on.
•• Business p11.9  A store owner keeps a record of daily cash transactions in a text file. Each line
contains three items: The invoice number, the cash amount, and the letter P if the
amount was paid or R if it was received. Items are separated by spaces. Write a pro-
gram that prompts the store owner for the amount of cash at the beginning and end
of the day, and the name of the file. Your program should check whether the actual
amount of cash at the end of the day equals the expected value.
••• science p11.10  After the switch in the figure below closes, the voltage (in volts) across the capacitor
is represented by the equation
v t B e
t RC
( ) = −
( )
−
1
( )
+
–
v(t)
+
–
C
t = 0
R
B
Suppose the parameters of the electric circuit are B = 12 volts, R = 500 Ω, and
C = 0.25 μF. Consequently
v t e
t
( ) = −
−
( )
12 1
0 008 .
where t has units of μs. Read a file  params.txt containing the values for B, R, C, and
the starting and ending values for t. Write a file  rc.txt of values for the time t and the
corresponding capacitor voltage v(t), where t goes from the given starting value to
the given ending value in 100 steps. In our example, if t goes from 0 to 1,000 μs, the
twelfth entry in the output file would be:
110 7.02261
••• science p11.11  The figure at right shows a plot of the capacitor voltage from
the circuit shown in Exercise P11.10. The capacitor voltage
increases from 0 volts to B volts. The “rise time” is defined as
the time required for the capacitor voltage to change from v 1 =
0.05 × B to v 2 = 0.95 × B.
0
t (µs)
0
B
556 Chapter 11 Input/output and exception handling
The file  rc.txt contains a list of values of time t and the corresponding capacitor
voltage v(t). A time in μs and the corresponding voltage in volts are printed on the
same line. For example, the line
110 7.02261
indicates that the capacitor voltage is 7.02261 volts when the time is 110 μs. The time
is increasing in the data file.
Write a program that reads the file  rc.txt  and uses the data to calculate the rise time.
Approximate B by the voltage in the last line of the file, and find the data points that
are closest to 0.05 × B and 0.95 × B.
•• science p11.12  Suppose a file contains bond energies and bond lengths for covalent bonds in the
following format:
single, double,
or triple bond
Bond energy
(kJ/mol)
Bond length
(nm)
C|C 370 0.154
C||C 680 0.13
C|||C 890 0.12
C|H 435 0.11
C|N 305 0.15
C|O 360 0.14
C|F 450 0.14
C|Cl 340 0.18
O|H 500 0.10
O|O 220 0.15
O|Si 375 0.16
N|H 430 0.10
N|O 250 0.12
F|F 160 0.14
H|H 435 0.074
Write a program that accepts data from one column and returns the corresponding
data from the other columns in the stored file. If input data matches different rows,
then return all matching row data. For example, a bond length input of 0.12 should
return triple bond C|||C and bond energy 890 kJ̸mol and single bond N|O and bond
energy 250 kJ̸mol.
© Chris Dascher/iStockphoto.
answers to self-Check Questions 557
a n s W e rs t o s e l F - C h e C k Q U e s t I o n s
1.  When the  PrintWriter object is created, the out-
put file is emptied. Sadly, that is the same file as
the input file. The input file is now empty and
the  while loop exits immediately.
2.  The program throws a FileNotFoundException
and terminates.
3.  Open a scanner for the file.
For each number in the scanner
Add the number to an array.
Close the scanner.
Set total to 0.
Open a print writer for the file.
For each number in the array
Write the number to the print writer.
Add the number to total.
Write total to the print writer.
Close the print writer.
4.  Add a variable  count that is incremented when-
ever a number is read. At the end, print the
average, not the total, as
out.printf("Average: %8.2f\n", total / count);
Because the string  "Average" is three characters
longer than  "Total" , change the other output to
out.printf("%18.2f\n", value) .
5.  Add a variable  count that is incremented when-
ever a number is read. Only write a new line
when it is even.
count++;
out.printf("%8.2f", value);
if (count % 2 == 0) { out.println(); }
At the end of the loop, write a new line if count
is odd, then write the total:
if (count % 2 == 1) { out.println(); }
out.printf("Total: %10.2f\n", total);
6.  word is  "Hello," and  input is  "World!"
7.  Because  995.0  is not an integer, the call
in.hasNextInt() returns false, and the call
in.nextInt() is skipped. The value of  number
stays 0, and  input is set to the string  "995.0" .
8.  x1 is set to 6000000. Because a comma is not
considered a part of a floating-point number
in Java, the second call to  nextDouble causes an
input mismatch exception and  x2 is not set.
9.  Read them as strings, and convert those strings
to numbers that are not equal to  N/A :
String input = in.next();
if (!input.equals("N/A"))
{
double value = Double.parseDouble(input);
Process value.
}
10.  Locate the last character of the country name:
int j = i  1;
while (!Character.isWhiteSpace(line.charAt(j)))
{
j;
}
Then extract the country name:
String countryName = line.substring(0, j + 1);
11.  args[0] is  "d" and  args[1] is  "file1.txt"
12.
Then the program prints a message
Usage: java CaesarCi pher [d] infile out file
13.  The program will run correctly. The loop that
parses the options does not depend on the
positions in which the options appear.
14.  FDHVDU
15.  Add the lines
else if (option == 'k')
{
key = Integer.parseInt(
args[i].substring(2));
}
after line 27 and update the usage information.
16.  It is still 100. The last statement was not
executed because the exception was thrown.
17.  if (amount < 0)
{
throw new IllegalArgumentException(
"Negative amount");
}
18.  The  Scanner constructor succeeds because
the file exists. The  nextInt method throws
a  NoSuchElementException . This is not an
key  inFile  outFile  i  arg
3  null  null  0  -d
-3  file1.txt  1  file1.txt
2
558 Chapter 11 Input/output and exception handling
IO Exception . Therefore, the error is not caught.
Because there is no other handler, an error
message is printed and the program terminates.
19.  Because programmers should simply check
that their array index values are valid instead
of trying to handle an  ArrayIndexOutOfBounds
Exception .
20.  No. You can catch both exception types in the
same way, as you can see in the code example
on page 536.
21.  There are two mistakes. The  PrintWriter con-
structor can throw a  FileNotFoundExcep tion . You
should supply a  throws clause. And if one of the
array elements is  null , a  NullPointerException is
thrown. In that case, the  out.close() statement
is never exe cuted. You should use a  try / finally
statement.
22.  To pass the exception message string to the
IllegalArgumentException superclass.
23.  Because file corruption is beyond the control
of the programmer, this should be a checked
exception, so it would be wrong to extend
RuntimeException or  Illegal ArgumentException .
Because the error is related to input,  IOExcep
tion would be a good choice.
24.  It would not be able to do much with them.
The  DataSetReader class is a reusable class
that may be used for systems with differ-
ent languages and different user inter faces.
Thus, it cannot engage in a dialog with the
program user.
25.  DataAnalyzer.main calls  DataSetReader.readFile ,
which calls  readData . The call  in.hasNextInt()
returns  false , and  readData throws a  BadData
Exception . The  read File method doesn’t catch it,
so it propagates back to  main , where it is caught.
26.  It could simply be
private void readValue(Scanner in, int i)
{
data[i] = in.nextDouble();
}
The  nextDouble method throws a  NoSuchElement
Exception or a  InputMismatchException (which is
a subclass of  NoSuchElementException ) when the
next input isn’t a floating-point number. That
exception isn’t a checked exception, so it need
not be declared.
27.  If it had been declared inside the  try block, its
scope would only have extended until the end
of the  try block, and it would not have been
accessible in the  finally clause.
28.  The  try/finally statement in the  readFile
method can be rewritten as
try (Scanner in = new Scanner(inFile))
{
readData(in);
return data;
}
12
C h a p t e r
559
ObjeCt-
Oriented
design
to learn how to discover new classes
and methods
to use CrC cards for class discovery
to identify inheritance, aggregation, and
dependency relationships between classes
to describe class relationships using UML class diagrams
to apply object-oriented design techniques to building
complex programs
C h a p t e r g O a L s
C h a p t e r C O n t e n t s
12.1 Classes and Their
responsibiliTies 560
12.2 relaTionships beTween
Classes 563
How To 12.1: Using CrC Cards and UML diagrams
in program design 566
Special Topic 12.1: attributes and Methods in
UML diagrams 567
Special Topic 12.2: Multiplicities 568
Special Topic 12.3: aggregation, association,
and Composition 568
12.3 appliCaTion: prinTing
an invoiCe 569
Computing & Society 12.1: databases
and privacy 580
Worked Example 12.1: simulating an automatic
teller Machine
© Petrea Alexandru/iStockphoto.
560
.
successfully implementing a software system—as simple as
your next homework project or as complex as the next air
traffic monitoring system—requires a great deal of planning
and design. in fact, for larger projects, the amount of time
spent on planning and design is much greater than the
amount of time spent on programming and testing.
do you find that most of your homework time is spent in
front of the computer, keying in code and fixing bugs? if
so, you can probably save time by focusing on a better
design before you start coding. this chapter tells you how
to approach the design of an object-oriented program in a
systematic manner.
12.1 Classes and their responsibilities
When you design a program, you work from a requirements specification, a descrip-
tion of what your program should do. The designer’s task is to discover structures
that make it possible to implement the requirements in a computer program. In the
following sections, we will examine the steps of the design process.
12.1.1 discovering Classes
When you solve a problem using objects and classes, you need to determine the
classes required for the implementation. You may be able to reuse existing classes, or
you may need to implement new ones.
One simple approach for discovering classes and methods is to look for the nouns
and verbs in the requirements specification. Often, nouns correspond to classes, and
verbs correspond to methods.
For example, suppose your job is to print an invoice such as the one in Figure 1.
to discover classes,
look for nouns in the
problem description.
Figure 1 
an invoice
I N V O I C E
Sam’s Small Appliances
100 Main Street
Anytown, CA 98765
Item Qty Price Total
Toaster 3 $29.95 $89.85
Hair Dryer 1 $24.95 $24.95
Car Vacuum 2 $19.99 $39.98
AMOUNT DUE: $154.78
12.1 Classes and their responsibilities 561
Obvious classes that come to mind are  Invoice ,  LineItem , and  Customer . It is a good
idea to keep a list of can didate classes on a whiteboard or a sheet of paper. As you
brainstorm, simply put all ideas for classes onto the list. You can always cross out the
ones that weren’t useful after all.
In general, concepts from the problem domain, be it science, business, or a game,
often make good classes. Exam ples are
•  Cannonball
•  CashRegister
•  Monster
The name for such a class should be a noun that describes the concept.
Not all classes can be discovered from the program requirements. Most complex
programs need classes for tactical purposes, such as file or database access, user inter-
faces, control mechanisms, and so on.
Some of the classes that you need may already exist, either in the standard library
or in a program that you developed previously. You also may be able to use inheri-
tance to extend existing classes into classes that match your needs.
A common error is to overdo the class discovery process. For example, should an
address be an object of an  Address class, or should it simply be a string? There is no
perfect answer—it depends on the task that you want to solve. If your software needs
to analyze addresses (for example, to determine shipping costs), then an  Address class
is an appropriate design. However, if your software will never need such a capability,
you should not waste time on an overly complex design. It is your job to find a bal-
anced design; one that is neither too limiting nor excessively general.
12.1.2 the CrC Card Method
Once you have identified a set of classes, you define the behavior for each class. Find
out what methods you need to provide for each class in order to solve the program-
ming problem. A simple rule for finding these methods is to look for verbs in the task
description, then match the verbs to the appropriate objects. For example, in the
invoice program, a class needs to compute the amount due. Now you need to figure
out which class is responsible for this method. Do customers compute what they
owe? Do invoices total up the amount due? Do the items total themselves up? The
best choice is to make “compute amount due” the responsibility of the  Invoice class.
Concepts from the
problem domain
are good candidates
for classes.
In a class scheduling system, potential
classes from the problem domain include
Class, LectureHall, Instructor, and Student.
© Oleg Prikhodko/iStockphoto.
562 Chapter 12 Object-Oriented design
An excellent way to carry out this task is the “CRC card method.” CRC stands
for “classes”, “respon sibilities”, “collaborators”, and in its simplest form, the method
works as follows: Use an index card for each class (see Figure 2). As you think about
verbs in the task description that indicate methods, you pick the card of the class that
you think should be responsible, and write that responsibility on the card.
For each responsibility, you record which other classes are needed to fulfill it.
Those classes are the col laborators.
For example, suppose you decide that an invoice should compute the amount due.
Then you write “compute amount due” on the left-hand side of an index card with
the title  Invoice .
If a class can carry out that responsibility by itself, do nothing further. But if the
class needs the help of other classes, write the names of these collaborators on the
right-hand side of the card.
To compute the total, the invoice needs to ask each line item about its total price.
Therefore, the  LineItem class is a collaborator.
This is a good time to look up the index card for the  LineItem class. Does it have a
“get total price” method? If not, add one.
How do you know that you are on the right track? For each responsibility, ask
yourself how it can actually be done, using the responsibilities written on the various
cards. Many people find it helpful to group the cards on a table so that the collabora-
tors are close to each other, and to simulate tasks by mov ing a token (such as a coin)
from one card to the next to indicate which object is currently active.
Keep in mind that the responsibilities that you list on the CRC card are on a high
level. Sometimes a single responsibility may need two or more Java methods for car-
rying it out. Some researchers say that a CRC card should have no more than three
distinct responsibilities.
The CRC card method is informal on purpose, so that you can be creative and dis-
cover classes and their properties. Once you find that you have settled on a good set
of classes, you will want to know how they are related to each other. Can you find
classes with common properties, so that some responsibilities can be taken care of by
a common superclass? Can you organize classes into clusters that are independent of
each other? Finding class relationships and documenting them with diagrams is the
topic of Section 12.2.
a CrC card
describes a class,
its responsibilities,
and its collaborating
classes.
Figure 2  a CrC Card
compute amount due LineItem
Invoice
Class
Responsibilities
Collaborators
12.2  Relationships Between Classes  563
1.  What is the rule of thumb for finding classes?
2.  Your job is to write a program that plays chess. Might  ChessBoard be an appropri-
ate class? How about  MovePiece ?
3.  Suppose the invoice is to be saved to a file. Name a likely collaborator.
4.  Looking at the invoice in Figure 1, what is a likely responsibility of the
Customer class?
5.  What do you do if a CRC card has ten responsibilities?
Practice It  Now you can try these exercises at the end of the chapter: R12.4, R12.8.
12.2  Relationships Between Classes
When designing a program, it is useful to document the relationships between classes.
This helps you in a number of ways. For example, if you find classes with common
behavior, you can save effort by placing the common behavior into a superclass. If
you know that some classes are not related to each other, you can assign different
programmers to implement each of them, without worrying that one of them has to
wait for the other.
In the following sections, we will describe the most common types of relationships.
12.2.1 Dependency
Many classes need other classes in order to do their jobs.
For example, in Section 8.2.2, we described a design of a
CashRegister class that depends on the  Coin class to deter-
mine the value of the payment.
The dependency relationship is sometimes nick-
named the “knows about” relationship. The cash
register in Section 8.2.2 knows that there are coin
objects. In contrast, the  Coin class does not depend on
the  Cash Register class. Coins have no idea that they are
being collected in cash registers, and they can carry out
their work without ever calling any method in the
CashRegister class.
As you saw in Section 8.2, dependency is denoted
by a dashed line with a -shaped open arrow tip. The
arrow tip points to the class on which the other class
depends. Figure 3 shows a class diagram indicating that
the  CashRegister class depends on the  Coin class.
If many classes of a program depend on each other, then we say that the coupling
between classes is high. Conversely, if there are few dependencies between classes,
then we say that the coupling is low (see Figure 4).
Figure 3  
Dependency Relationship Between the  CashRegister  and  Coin  Classes
© Nicholas Homrich/iStockphoto.
S e l F C h e C k
© visual7/iStockphoto.
Too many dependencies make
a system difficult to manage.
A class depends on
another class if it
uses objects of 
that class.
CashRegister
Coin
564 Chapter 12 Object-Oriented design
Figure 4  high and Low Coupling between Classes
Low coupling High coupling
Why does coupling matter? If the  Coin class changes in the next release of the pro-
gram, all the classes that depend on it may be affected. If the change is drastic, the
coupled classes must all be updated. Fur thermore, if we would like to use a class in
another program, we have to take with it all the classes on which it depends. Thus, we
want to remove unnecessary coupling between classes.
12.2.2 aggregation
Another fundamental relationship between classes is the “aggregation” relationship
(which is informally known as the “has-a” relationship).
The aggregation relationship states that objects of one class contain objects of
another class. Consider a quiz that is made up of questions. Because each quiz has one
or more questions, we say that the class  Quiz aggregates the class  Question . In the UML
notation, aggregation is denoted by a line with a diamond-shaped symbol attached to
the aggregating class (see Figure 5).
Finding out about aggregation is very helpful for deciding how to implement
classes. For example, when you implement the  Quiz class, you will want to store the
questions of a quiz as an instance variable.
Because a quiz can have any number of questions, an array or array list is a good
choice for collecting them:
publicclassQuiz
{
privateArrayList<Question>questions;
...
}
Aggregation is a stronger form of dependency. If a class has objects of another class, it
certainly knows about the other class. However, the converse is not true. For exam-
ple, a class may use the  Scanner class without ever declaring an instance variable of
it is a good practice
to minimize the
coupling (i.e.,
dependency)
between classes.
a class aggregates
another if its objects
contain objects of the
other class.
Full Code example
go to  wiley.com/go/
javacode to download
the complete  Quiz
and  Question classes.
Figure 5 
Class diagram
showing aggregation
Quiz Question
12.2 relationships between Classes 565
class
A car has a motor and tires.
In object-oriented design,
this “has-a” relationship
is called aggregation.
© bojan fatur/iStockphoto.
Scanner . The class may simply construct a local variable of type  Scanner , or its
methods may receive  Scanner objects as arguments. This use is not aggregation because
the objects of the class don’t contain  Scanner objects—they just create or receive them
for the duration of a single method.
Generally, you need aggregation when an object needs to remember another object
between method calls.
12.2.3 inheritance
Inheritance is a relationship between a more general class (the superclass) and a more
specialized class (the subclass). This relationship is often described as the “is-a” rela-
tionship. Every truck is a vehicle. Every savings account is a bank account.
Inheritance is sometimes abused. For example, consider a  Tire class that describes a
car tire. Should the class  Tire be a subclass of a class  Circle ? It sounds convenient.
There are quite a few useful methods in the  Circle class—for example, the  Tire class
may inherit methods that compute the radius, perimeter, and center point, which
should come in handy when drawing tire shapes. Though it may be convenient for
the programmer, this arrangement makes no sense conceptually. It isn’t true that
every tire is a circle. Tires are car parts, whereas circles are geometric objects. There is
a relationship between tires and circles, though. A tire has a circle as its boundary.
Use aggregation:
publicclassTire
{
privateStringrating;
privateCircleboundary;
...
}
Here is another example: Every car is a vehicle. Every car has a
tire (in fact, it typically has four or, if you count the spare, five).
Thus, you would use inheritance from  Vehicle and use aggregation
of  Tire objects (see Figure 6 for the UML diagram):
publicclassCarextendsVehicle
{
privateTire[]tires;
...
}
Figure 6 
UML notation for inheritance and aggregation
inheritance (the
is-a relationship)
is some times
inappropriately
used when the has-a
relationship would be
more appropriate.
Vehicle
Car
Tire
aggregation (the
has-a relationship)
denotes that objects
of one class contain
references to objects
of another class.
566 Chapter 12 Object-Oriented design
The arrows in the UML notation can get confusing. Table 1 shows a summary of
the four UML rela tionship symbols that we use in this book.
table 1 UML relationship symbols
relationship symbol Line style arrow tip
Inheritance Solid Triangle
Interface Implementation Dotted Triangle
Aggregation Solid Diamond
Dependency Dotted Open
6.  Consider the  CashRegisterTester class of Section 8.2. On which classes does it
depend?
7.  Consider the  Question and  ChoiceQuestion objects of Chapter 9. How are
they related?
8.  Consider the  Quiz class described in Section 12.2.2. Suppose a quiz contains a
mixture of  Question and  ChoiceQuestion objects. Which classes does the  Quiz class
depend on?
9.  Why should coupling be minimized between classes?
10.  In an e-mail system, messages are stored in a mailbox. Draw a UML diagram
that shows the appro priate aggregation relationship.
11.  You are implementing a system to manage a library, keeping track of which
books are checked out by whom. Should the  Book class aggregate  Patron or the
other way around?
12.  In a library management system, what would be the relationship between classes
Patron and  Author ?
practice it  Now you can try these exercises at the end of the chapter: R12.5, R12.6, R12.10.
step 1  Discover classes.
Highlight the nouns in the problem description. Make a list of the nouns. Cross out those that
don’t seem to be rea sonable candidates for classes.
You need to be able
to distinguish the
UML notation for
inheritance, interface
implementation,
aggregation, and
dependency.
© Nicholas Homrich/iStockphoto.
s e l F  C h e C k
© Steve Simzer/iStockphoto.
hOw tO 12.1  using CrC Cards and uml diagrams in program design
Before writing code for a complex problem, you need to design a solution. The methodology
introduced in this chapter suggests that you follow a design process that is composed of the
following tasks:
• Discover classes.
• Determine the responsibilities of each class.
• Describe the relationships between the classes.
CRC cards and UML diagrams help you discover and record this information.
12.2 relationships between Classes 567
step 2  Discover responsibilities.
Make a list of the major tasks that your system needs to fulfill. From those tasks, pick one
that is not trivial and that is intuitive to you. Find a class that is responsible for carrying out
that task. Make an index card and write the name and the task on it. Now ask yourself how
an object of the class can carry out the task. It probably needs help from other objects. Then
make CRC cards for the classes to which those objects belong and write the responsibilities
on them.
Don’t be afraid to cross out, move, split, or merge responsibilities. Rip up cards if they
become too messy. This is an informal process.
You are done when you have walked through all major tasks and are satisfied that they can
all be solved with the classes and responsibilities that you discovered.
step 3  Describe relationships.
Make a class diagram that shows the relationships between all the classes that you discovered.
Start with inheritance—the is-a relationship between classes. Is any class a specialization of
another? If so, draw inheritance arrows. Keep in mind that many designs, especially for simple
programs, don’t use inheritance exten sively.
The “collaborators” column of the CRC card tells you which classes are used by that class.
Draw dependency arrows for the col laborators on the CRC cards.
Some dependency relationships give rise to aggregations. For each of the dependency rela-
tionships, ask yourself: How does the object locate its collaborator? Does it navigate to it
directly because it stores a reference? In that case, draw an aggregation arrow. Or is the collab-
orator a method parameter variable or return value? Then simply draw a depen dency arrow.
attributes and methods in uml diagrams
Sometimes it is useful to indicate class attributes and methods in a class diagram. An attribute
is an externally observable property that objects of a class have. For example,  name and  price
would be attributes of the  Product class. Usually, attributes correspond to instance variables.
But they don’t have to—a class may have a different way of organizing its data. For example,
a  GregorianCalendar object from the Java library has attributes  day ,  month , and  year , and it would
be appropriate to draw a UML diagram that shows these attributes. However, the class doesn’t
actually have instance variables that store these quantities. Instead, it internally represents all
dates by counting the millisec onds from January 1, 1970—an implementation detail that a
class user certainly doesn’t need to know about.
You can indicate attributes and methods in a class diagram by dividing a class rectangle into
three compartments, with the class name in the top, attributes in the middle, and methods in
the bottom (see the figure below). You need not list all attributes and methods in a particular
diagram. Just list the ones that are helpful for understanding whatever point you are making
with a particular diagram.
Also, don’t list as an attribute what you also draw as an aggregation. If you denote by
aggregation the fact that a  Car has  Tire objects, don’t add an attribute  tires .
Attributes and Methods
in a Class Diagram
Attributes
balance
deposit()
withdraw()
BankAccount
Methods
special topic 12.1
© Eric Isselé/iStockphoto.
568 Chapter 12 Object-Oriented design
multiplicities
Some designers like to write multiplicities at the end(s) of an aggregation relationship to denote
how many objects are aggregated. The notations for the most common multiplicities are:
• any number (zero or more): *
• one or more: 1..*
• zero or one: 0..1
• exactly one: 1
The figure below shows that a customer has one or more bank accounts.
An Aggregation Relationship with Multiplicities
Customer BankAccount
1..*
aggregation, association, and Composition
Some designers find the aggregation or has-a terminology unsatisfactory. For example, con-
sider customers of a bank. Does the bank “have” customers? Do the customers “have” bank
accounts, or does the bank “have” them? Which of these “has” relationships should be mod-
eled by aggregation? This line of thinking can lead us to premature imple mentation decisions.
Early in the design phase, it makes sense to use a more general relationship between classes
called association. A class is associated with another if you can navigate from objects of one
class to objects of the other class. For exam ple, given a  Bank object, you can navigate to  Customer
objects, perhaps by accessing an instance variable, or by making a database lookup.
The UML notation for an association relationship is a solid line, with optional arrows that
show in which direc tions you can navigate the relationship. You can also add words to the line
ends to further explain the nature of the relationship. The figure below shows that you can
navigate from  Bank objects to  Customer objects, but you cannot navigate the other way around.
That is, in this particular design, the  Customer class has no mechanism to determine in which
banks it keeps its money.
An Association Relationship
Bank Customer
serves
The UML standard also recognizes a stronger form of the aggregation relationship called com-
position, where the aggregated objects do not have an existence independent of the containing
object. For example, composition models the relationship between a bank and its accounts.
If a bank closes, the account objects cease to exist as well. In the UML notation, composition
looks like aggregation with a filled-in diamond.
special topic 12.2
© Eric Isselé/iStockphoto.
special topic 12.3
© Eric Isselé/iStockphoto.
12.3 application: printing an invoice 569
A Composition Relationship
Bank BankAccount
Frankly, the differences between aggregation, association, and composition can be confusing,
even to experienced designers. If you find the distinction helpful, by all means use the rela-
tionship that you find most appropriate. But don’t spend time pondering subtle differences
between these concepts. From the practical point of view of a Java programmer, it is useful to
know when objects of one class have references to objects of another class. The aggregation or
has-a relationship accurately describes this phenomenon.
12.3 application: printing an invoice
In this book, we discuss a five-part program development process that is particularly
well suited for beginning programmers:
1.  Gather requirements.
2.  Use CRC cards to find classes, responsibilities, and collaborators.
3.  Use UML diagrams to record class relationships.
4.  Use  javadoc to document method behavior.
5.  Implement your program.
There isn’t a lot of notation to learn. The class diagrams are simple to draw. The deliv-
erables of the design phase are obviously useful for the implementation phase—you
simply take the source files and start add ing the method code. Of course, as your
projects get more complex, you will want to learn more about formal design meth-
ods. There are many techniques to describe object scenarios, call sequencing, the
large-scale structure of programs, and so on, that are very beneficial even for rel-
atively simple projects. The Unified Modeling Language User Guide gives a good
overview of these techniques.
In this section, we will walk through the object-oriented design technique with
a very simple example. In this case, the methodology may feel overblown, but it is a
good introduction to the mechanics of each step. You will then be better prepared for
the more complex programs that you will encounter in the future.
12.3.1 requirements
Before you begin designing a solution, you should gather all requirements for your
program in plain English. Write down what your program should do. It is helpful to
include typical scenarios in addition to a general description.
The task of our sample program is to print out an invoice. An invoice describes
the charges for a set of products in certain quantities. (We omit complexities such as
dates, taxes, and invoice and customer num bers.) The program simply prints the bill-
ing address, all line items, and the amount due. Each line item contains the descrip-
tion and unit price of a product, the quantity ordered, and the total price.
start the
development
process by gathering
and documenting
program
requirements.
570 Chapter 12 Object-Oriented design
INVOICE
Sam'sSmallAppliances
100MainStreet
Anytown,CA98765
DescriptionPriceQtyTotal
Toaster29.95389.85
Hairdryer24.95124.95
Carvacuum19.99239.98
AMOUNTDUE:$154.78
Also, in the interest of simplicity, we do not provide a user interface. We just supply a
test program that adds line items to the invoice and then prints it.
12.3.2 CrC Cards
When designing an object-oriented program, you need to discover classes. Classes
correspond to nouns in the requirements specification. In this problem, it is pretty
obvious what the nouns are:
InvoiceAddressLineItemProduct
DescriptionPriceQuantityTotalAmountdue
(Of course,  Toaster doesn’t count—it is the description of a  LineItem object and there-
fore a data value, not the name of a class.)
Description and price are attributes of the  Product class. What about the quantity?
The quantity is not an attribute of a  Product . Just as in the printed invoice, let’s have a
class  LineItem that records the product and the quantity (such as “3 toasters”).
The total and amount due are computed—not stored anywhere. Thus, they don’t
lead to classes.
After this process of elimination, we are left with four candidates for classes:
InvoiceAddressLineItemProduct
Each of them represents a useful concept, so let’s make them all into classes.
The purpose of the program is to print an invoice. However, the  Invoice class won’t
necessarily know whether to display the output in  System.out , in a text area, or in a file.
Therefore, let’s relax the task slightly and make the invoice responsible for formatting
the invoice. The result is a string (containing multiple lines) that can be printed out or
displayed. Record that responsibility on a CRC card:
format the invoice
Invoice
© Scott Cramer/iStockphoto. An invoice lists the
charges for each item
and the amount due.
Use CrC cards
to find classes,
responsibilities,
and collaborators.
12.3 application: printing an invoice 571
How does an invoice format itself? It must format the billing address, format all
line items, and then add the amount due. How can the invoice format an address? It
can’t—that really is the responsibility of the  Address class. This leads to a second CRC
card:
format the address
Address
Similarly, formatting of a line item is the responsibility of the  LineItem class.
The  format method of the  Invoice class calls the  format methods of the  Address and
LineItem classes. Whenever a method uses another class, you list that other class as a
collaborator. In other words,  Address and  LineItem are collaborators of  Invoice :
format the invoice Address
LineItem
Invoice
When formatting the invoice, the invoice also needs to compute the total amount due.
To obtain that amount, it must ask each line item about the total price of the item.
How does a line item obtain that total? It must ask the product for the unit price,
and then multiply it by the quantity. That is, the  Product class must reveal the unit
price, and it is a collaborator of the  LineItem class.
get description
get unit price
Product
572 Chapter 12 Object-Oriented design
format the item Product
get total price
LineItem
Finally, the invoice must be populated with products and quantities, so that it makes
sense to format the result. That too is a responsibility of the  Invoice class.
format the invoice Address
LineItem add a product and quantity
Product
Invoice
We now have a set of CRC cards that completes the CRC card process.
12.3.3 UML diagrams
After you have discovered classes and their relationships with CRC cards, you should
record your find ings in a UML diagram. The dependency relationships come from
the collaboration column on the CRC cards. Each class depends on the classes with
which it collaborates. In our example, the  Invoice class col laborates with the  Address ,
LineItem , and  Product classes. The  LineItem class collaborates with the  Product class.
Now ask yourself which of these dependencies are actually aggregations. How
does an invoice know about the address, line item, and product objects with which it
collaborates? An invoice object must hold references to the address and the line items
when it formats the invoice. But an invoice object need not hold a reference to a prod-
uct object when adding a product. The product is turned into a line item, and then it is
the item’s responsibility to hold a reference to it.
Therefore, the  Invoice class aggregates the  Address and  LineItem classes. The  LineItem
class aggregates the  Product class. However, there is no has-a relationship between an
invoice and a product. An invoice doesn’t store products directly—they are stored in
the  LineItem objects.
There is no inheritance in this example.
Figure 7 shows the class relationships that we discovered.
Use UML diagrams
to record class
relationships.
12.3 application: printing an invoice 573
12.3.4
Figure 7  the relationships between the invoice Classes
Invoice Address
Product LineItem
Method documentation
The final step of the design phase is to write the documentation of the discovered
classes and methods. Simply write a Java source file for each class, write the method
comments for those methods that you have discovered, and leave the bodies of the
methods blank.
/**
 Describes an invoice for a set of purchased products. 
*/
publicclassInvoice
{
/**
 Adds a charge for a product to this invoice. 
@paramaProduct the product that the customer ordered 
@paramquantity the quantity of the product 
*/
publicvoidadd(ProductaProduct,intquantity)
{
}
/**
 Formats the invoice. 
@return the formatted invoice 
*/
publicStringformat()
{
}
}
/**
 Describes a quantity of an article to purchase. 
*/
publicclassLineItem
{
/**
 Computes the total cost of this line item. 
@return the total price 
*/
publicdoublegetTotalPrice()
{
Use  javadoc
comments (with the
method bodies left
blank) to record the
behavior of classes.
574 Chapter 12 Object-Oriented design
}

/**
 Formats this item. 
@return a formatted string of this item 
*/
publicStringformat()
{
}
}
/**
 Describes a product with a description and a price. 
*/
publicclassProduct
{
/**
 Gets the product description. 
@return the description 
*/
publicStringgetDescription()
{
}
/**
 Gets the product price. 
@return the unit price 
*/
publicdoublegetPrice()
{
}
}
/**
 Describes a mailing address. 
*/
publicclassAddress
{
/**
 Formats the address. 
@return the address as a string with three lines 
*/
publicStringformat()
{
}
}
Then run the  javadoc program to obtain a neatly formatted version of your documen-
tation in HTML format (see Figure 8).
This approach for documenting your classes has a number of advantages. You can
share the HTML documentation with others if you work in a team. You use a format
that is immediately useful—Java source files that you can carry into the implementa-
tion phase. And, most importantly, you supply the comments for the key methods—
a task that less prepared programmers leave for later, and often neglect for lack of time.
12.3 application: printing an invoice 575
12.3.5
Figure 8 
Class documentation
in htML Format
implementation
After you have completed the object-oriented design, you are ready to implement the
classes.
You already have the method parameter variables and comments from the previ-
ous step. Now look at the UML diagram to add instance variables. Aggregated classes
yield instance variables. Start with the  Invoice class. An invoice aggregates  Address and
Line Item . Every invoice has one billing address, but it can have many line items. To
store multiple  LineItem objects, you can use an array list. Now you have the instance
variables of the  Invoice class:
publicclassInvoice
{
privateAddressbillingAddress;
privateArrayList<LineItem>items;
...
}
A line item needs to store a  Product object and the product quantity. That leads to the
following instance variables:
publicclassLineItem
{
privateintquantity;
privateProducttheProduct;
...
}
after completing the
design, implement
your classes.
576 Chapter 12 Object-Oriented design
The methods themselves are now easy to implement. Here is a typical example. You
already know what the  getTotalPrice method of the  LineItem class needs to do—get the
unit price of the product and multiply it with the quantity.
/**
 Computes the total cost of this line item. 
@return the total price 
*/
publicdoublegetTotalPrice()
{
returntheProduct.getPrice()*quantity;
}
We will not discuss the other methods in detail—they are equally straightforward.
Finally, you need to supply constructors, another routine task.
The entire program is shown below. It is a good practice to go through it in detail
and match up the classes and methods to the CRC cards and UML diagram.
Worked Example 12.1 (at  wiley.com/go/javaexamples ) demonstrates the design pro-
cess with a more challenging problem, a simulated automatic teller machine. You
should download and study that example as well.
In this chapter, you learned a systematic approach for building a relatively com-
plex program. However, object-oriented design is definitely not a spectator sport.
To really learn how to design and implement programs, you have to gain experience
by repeating this process with your own projects. It is quite possi ble that you don’t
immediately home in on a good solution and that you need to go back and reorganize
your classes and responsibilities. That is normal and only to be expected. The purpose
of the object-ori ented design process is to spot these problems in the design phase,
when they are still easy to rectify, instead of in the implementation phase, when mas-
sive reorganization is more difficult and time consum ing.
section_3/invoiceprinter.java
1  /**
2   This program demonstrates the invoice classes by
3   printing a sample invoice. 
4  */
5  publicclassInvoicePrinter
6  {
7  publicstaticvoidmain(String[]args)
8  {
9  AddresssamsAddress
10  =newAddress("Sam’sSmallAppliances",
11  "100MainStreet","Anytown","CA","98765");
12 
13  InvoicesamsInvoice=newInvoice(samsAddress);
14  samsInvoice.add(newProduct("Toaster",29.95),3);
15  samsInvoice.add(newProduct("Hairdryer",24.95),1);
16  samsInvoice.add(newProduct("Carvacuum",19.99),2);
17 
18  System.out.println(samsInvoice.format());
19  }
20  }
section_3/invoice.java
1  importjava.util.ArrayList;
2 
12.3 application: printing an invoice 577
3  /**
4   Describes an invoice for a set of purchased products. 
5  */
6  publicclassInvoice
7  {
8  privateAddressbillingAddress;
9  privateArrayList<LineItem>items;
10 
11  /**
12   Constructs an invoice. 
13  @paramanAddress the billing address 
14  */
15  publicInvoice(AddressanAddress)
16  {
17  items=newArrayList<LineItem>();
18  billingAddress=anAddress;
19  }
20 
21  /**
22   Adds a charge for a product to this invoice. 
23  @paramaProduct the product that the customer ordered 
24  @paramquantity the quantity of the product 
25  */
26  publicvoidadd(ProductaProduct,intquantity)
27  {
28  LineItemanItem=newLineItem(aProduct,quantity);
29  items.add(anItem);
30  }
31 
32  /**
33   Formats the invoice. 
34  @return the formatted invoice 
35  */
36  publicStringformat()
37  {
38  Stringr="INVOICE\n\n"
39  +billingAddress.format()
40  +String.format("\n\n%-30s%8s%5s%8s\n",
41  "Description","Price","Qty","Total");
42 
43  for(LineItemitem:items)
44  {
45  r=r+item.format()+"\n";
46  }
47 
48  r=r+String.format("\nAMOUNTDUE:$%8.2f",getAmountDue());
49 
50  returnr;
51  }
52 
53  /**
54   Computes the total amount due. 
55  @return the amount due 
56  */
57  privatedoublegetAmountDue()
58  {
59  doubleamountDue=0;
60  for(LineItemitem:items)
61  {
62  amountDue=amountDue+item.getTotalPrice();
578 Chapter 12 Object-Oriented design
63  }
64  returnamountDue;
65  }
66  }
section_3/lineitem.java
1  /**
2   Describes a quantity of an article to purchase. 
3  */
4  publicclassLineItem
5  {
6  privateintquantity;
7  privateProducttheProduct;
8 
9  /**
10   Constructs an item from the product and quantity. 
11  @paramaProduct the product 
12  @paramaQuantity the item quantity 
13  */
14  publicLineItem(ProductaProduct,intaQuantity)
15  {
16  theProduct=aProduct;
17  quantity=aQuantity;
18  }
19  
20  /**
21   Computes the total cost of this line item. 
22  @return the total price 
23  */
24  publicdoublegetTotalPrice()
25  {
26  returntheProduct.getPrice()*quantity;
27  }
28  
29  /**
30   Formats this item. 
31  @return a formatted string of this line item 
32  */
33  publicStringformat()
34  {
35  returnString.format("%-30s%8.2f%5d%8.2f",
36  theProduct.getDescription(),theProduct.getPrice(),
37  quantity,getTotalPrice());
38  }
39  }
section_3/product.java
1  /**
2   Describes a product with a description and a price. 
3  */
4  publicclassProduct
5  {
6  privateStringdescription;
7  privatedoubleprice;
8 
9  /**
10   Constructs a product from a description and a price. 
11  @paramaDescription the product description 
12.3 application: printing an invoice 579
12  @paramaPrice the product price 
13  */
14  publicProduct(StringaDescription,doubleaPrice)
15  {
16  description=aDescription;
17  price=aPrice;
18  }
19  
20  /**
21   Gets the product description. 
22  @return the description 
23  */
24  publicStringgetDescription()
25  {
26  returndescription;
27  }
28 
29  /**
30   Gets the product price. 
31  @return the unit price 
32  */
33  publicdoublegetPrice()
34  {
35  returnprice;
36  }
37  }
section_3/address.java
1  /**
2   Describes a mailing address. 
3  */
4  publicclassAddress
5  {
6  privateStringname;
7  privateStringstreet;
8  privateStringcity;
9  privateStringstate;
10  privateStringzip;
11 
12  /**
13   Constructs a mailing address. 
14  @paramaName the recipient name 
15  @paramaStreet the street 
16  @paramaCity the city 
17  @paramaState the two-letter state code 
18  @paramaZip the ZIP postal code 
19  */
20  publicAddress(StringaName,StringaStreet,
21  StringaCity,StringaState,StringaZip)
22  {
23  name=aName;
24  street=aStreet;
25  city=aCity;
26  state=aState;
27  zip=aZip;
28  }
29 
30  /**
31   Formats the address. 
580 Chapter 12 Object-Oriented design
32  @return the address as a string with three lines 
33  */
34  publicStringformat()
35  {
36  returnname+"\n"+street+"\n"
37  +city+","+state+""+zip;
38  }
39  }
13.  Which class is responsible for computing the amount due? What are its collabo-
rators for this task?
14.  Why do the  format methods return  String objects instead of directly printing to
System.out ?
practice it  Now you can try these exercises at the end of the chapter: R12.15, E12.4, E12.5.
© Nicholas Homrich/iStockphoto.
s e l F  C h e C k
Most companies use
computers  to  keep
huge databases of customer records
and other business information. data-
bases not only lower the cost of doing
business, they improve the quality
of service that companies can offer.
nowa days it is almost unimaginable
how time-consuming it used to be to
withdraw money from a bank branch
or to make travel reservations.
as these databases became ubiqui-
tous, they started creating problems
for citizens. Consider the “no fly list”
maintained by the U.s. government,
which lists names used by suspected
terrorists. On March 1, 2007, professor
walter Murphy, a constitutional scholar
of princeton University and a decorated
former Marine, was denied a boarding
pass. the airline employee asked him,
“have you been in any peace marches?
we ban a lot of people from flying
because of that.” as Murphy tells it, “i
explained that i had not so marched
but had, in september 2006, given a
lecture at princeton, televised and put
on the web, highly critical of george
bush for his many violations of the con-
stitution. ‘that’ll do it,’ the man said.”
we do not actually know if professor
Murphy’s name was on the list because
he was critical of the bush adminis-
tration or because some other poten-
tially dangerous person had traveled
under the same name. travelers with
similar misfortunes had serious diffi-
culties trying to get themselves off the
list.
problems  such  as  these  have
become  commonplace.  Companies
and the government routinely merge
multiple databases, derive information
about us that may be quite inaccurate,
and then use that information to make
deci sions. an insurance company may
deny coverage, or charge a higher pre-
mium, if it finds that you have too many
rel atives with a certain disease. You
may be denied a job because of a credit
or medical report. You do not usually
know what information about you is
stored or how it is used. in cases where
the information can be checked—such
as credit reports—it is often difficult to
correct errors.
another issue of concern is privacy.
Most people do something, at one time
or another in their lives, that they do
not want everyone to know about. as
judge Louis brandeis wrote in 1928,
“privacy is the right to be alone––the
most comprehensive of rights, and the
right most valued by civilized man.”
when employers can see your old
Face book posts, divorce lawyers have
access to tollroad records, and google
mines your e-mails and searches to
present you “targeted” advertising, you
have little privacy left.
the 1948 “universal declaration of
human rights” by the United nations
states, “no one shall be subjected to
arbitrary interference with his privacy,
family,  home  or  correspondence,
nor to attacks upon his honour and
reputa tion. everyone has the right to
the protection of the law against such
interference or attacks.” the United
states has surprisingly few legal pro-
tections against privacy invasion,
apart from federal laws protecting stu-
dent records and video rentals (the lat-
ter was passed after a supreme Court
nominee’s video rental records were
published). Other industrialized coun-
tries have gone much further and rec-
ognize every citizen’s right to control
what information about them should
be communicated to others and under
what circumstances.
© Greg Nicholas/iStockphoto.
If you pay road or bridge tolls with an
electronic pass, your records may not
be private.
Computing & Society 12.1  databases and privacy
© MediaBakery.
review Questions 581
recognize how to discover classes and their responsibilities.
• To discover classes, look for nouns in the problem description.
• Concepts from the problem domain are good candidates for classes.
• A CRC card describes a class, its responsibilities, and its collaborating classes.
Categorize class relationships and produce uml diagrams that describe them.
• A class depends on another class if it uses objects of that class.
• It is a good practice to minimize the coupling (i.e., dependency) between classes.
• A class aggregates another if its objects contain objects of the other class.
• Inheritance (the is-a relationship) is sometimes inappropriately used when the
has-a relationship would be more appropriate.
• Aggregation (the has-a relationship) denotes that objects of one class contain
references to objects of another class.
• You need to be able to distinguish the UML notation for inheritance, interface
implementation, aggregation, and dependency.
apply an object-oriented development process to designing a program.
• Start the development process by gathering and documenting program
requirements.
• Use CRC cards to find classes, responsibilities, and collaborators.
• Use UML diagrams to record class relationships.
• Use  javadoc comments (with the method bodies left blank) to record the
behavior of classes.
• After completing the design, implement your classes.
•• r12.1  List the steps in the process of object-oriented design that this chapter recommends
for student use.
• r12.2  Give a rule of thumb for how to find classes when designing a program.
• r12.3  Give a rule of thumb for how to find methods when designing a program.
•• r12.4  After discovering a method, why is it important to identify the object that is respon-
sible for carrying out the action?
wOrked exaMpLe 12.1  simulating an automatic Teller machine
Learn to apply the object-oriented design methodology to the
simulation of an automatic teller machine that works with both a
console-based and graphical user interface. Go to  wiley.com/go/
javaexamples and download Worked Example 12.1.
© Mark Evans/iStockphoto.
C h a p t e r s U M M a rY
© Oleg Prikhodko/iStockphoto.
© bojan fatur/iStockphoto.
r e v i e w Q U e s t i O n s
582 Chapter 12 Object-Oriented design
•• r12.5  What relationship is appropriate between the following classes: aggregation, inher-
itance, or neither?
a.  University—Student
b.  Student—TeachingAssistant
c.  Student—Freshman
d.  Student—Professor
e.  Car—Door
f.  Truck—Vehicle
g.  Traffic—TrafficSign
h.  TrafficSign—Color
•• r12.6  Every BMW is a vehicle. Should a class  BMW inherit from the class  Vehicle ? BMW is a
vehicle manufacturer. Does that mean that the class  BMW should inherit from the class
VehicleManufacturer ?
•• r12.7  Some books on object-oriented programming recommend using inheritance so that
the class  Circle extends the class  java.awt.Point . Then the  Circle class inherits the
setLocation method from the  Point superclass. Explain why the  setLocation method
need not be overridden in the subclass. Why is it nevertheless not a good idea to have
Circle inherit from  Point ? Conversely, would inheriting  Point from  Circle fulfill the
is-a rule? Would it be a good idea?
• r12.8  Write CRC cards for the  Coin and  CashRegister classes described in Section 8.2.
• r12.9  Write CRC cards for the  Quiz and  Question classes in Section 12.2.2.
•• r12.10  Draw a UML diagram for the  Quiz ,  Question , and  ChoiceQuestion classes. The  Quiz class
is described in Section 12.2.2.
••• r12.11  A file contains a set of records describing countries. Each record consists of the name
of the country, its population, and its area. Suppose your task is to write a program
that reads in such a file and prints
• The country with the largest area
• The country with the largest population
• The country with the largest population density (people per square kilometer)
Think through the problems that you need to solve. What classes and methods
will you need? Produce a set of CRC cards, a UML diagram, and a set of  javadoc
comments.
••• r12.12  Discover classes and methods for generating a student report card that lists all
classes, grades, and the grade point average for a semester. Produce a set of CRC
cards, a UML diagram, and a set of  javadoc comments.
•• r12.13  Consider the following problem description:
Users place coins in a vending machine and select a product by pushing a button. If the inserted coins
are sufficient to cover the purchase price of the product, the product is dispensed and change is given.
Otherwise, the inserted coins are returned to the user.
What classes should you use to implement a solution?
•• r12.14  Consider the following problem description:
Employees receive their biweekly paychecks. They are paid their hourly rates for each hour worked;
however, if they worked more than 40 hours per week, they are paid overtime at 150 percent of
their regular wage.
What classes should you use to implement a solution?
practice exercises 583
•• r12.15  Consider the following problem description:
Customers order products from a store. Invoices are generated to list the items and quantities ordered,
payments received, and amounts still due. Products are shipped to the shipping address of the cus-
tomer, and invoices are sent to the billing address.
Draw a UML diagram showing the aggregation relationships between the classes
Invoice ,  Address ,  Customer , and  Product .
• e12.1  Enhance the invoice-printing program by providing for two kinds of line items: One
kind describes products that are purchased in certain numerical quantities (such as
“3 toasters”), another describes a fixed charge (such as “shipping: $5.00”). Hint: Use
inheritance. Produce a UML diagram of your modified implementation.
•• e12.2  The invoice-printing program is somewhat unrealistic because the formatting of the
LineItem objects won’t lead to good visual results when the prices and quantities have
varying numbers of digits. Enhance the  format method in two ways: Accept an  int[]
array of column widths as an argument. Use the  NumberFormat class to format the cur-
rency values.
•• e12.3  The invoice-printing program has an unfortunate flaw—it mixes “application logic”
(the computation of total charges) and “presentation” (the visual appearance of the
invoice). To appreciate this flaw, imagine the changes that would be necessary to
draw the invoice in HTML for presentation on the Web. Reimplement the pro gram,
using a separate  InvoiceFormatter class to format the invoice. That is, the  Invoice and
LineItem methods are no longer responsible for formatting. However, they will
acquire other responsibilities, because the  InvoiceFormatter class needs to query them
for the values that it requires.
••• e12.4  Write a program that teaches arithmetic to a young child. The program tests addition
and subtraction. In level 1, it tests only addition of numbers less than 10 whose sum
is less than 10. In level 2, it tests addition of arbitrary one-digit numbers. In level 3, it
tests subtraction of one-digit numbers with a nonnegative difference.
Generate random problems and get the player’s input. The player gets up to two
tries per problem. Advance from one level to the next when the player has achieved a
score of five points.
••• e12.5  Implement a simple e-mail messaging system. A message has a recipient, a sender,
and a message text. A mailbox can store messages. Supply a number of mailboxes for
different users and a user interface for users to log in, send messages to other users,
read their own messages, and log out. Follow the design process that was described
in this chapter.
•• e12.6  Modify the implementation of the classes in the ATM simulation in Worked Exam-
ple 12.1 so that the bank manages a collection of bank accounts and a separate collec-
tion of customers. Allow joint accounts in which some accounts can have more than
one customer.
p r a C t i C e e x e r C i s e s
584 Chapter 12 Object-Oriented design
•• p12.1  Write a program that simulates a vending machine. Products can be purchased by
inserting coins with a value at least equal to the cost of the product. A user selects a
product from a list of available products, adds coins, and either gets the product or
gets the coins returned. The coins are returned if insufficient money was supplied
or if the product is sold out. The machine does not give change if too much money
was added. Products can be restocked and money removed by an operator. Follow
the design process that was described in this chapter. Your solution should include a
class  VendingMachine that is not coupled with the  Scanner or  PrintStream classes.
••• p12.2  Write a program to design an appointment calendar. An appointment includes the
date, starting time, ending time, and a description; for example,
Dentist2012/10/117:3018:30
CS1class2012/10/208:3010:00
Supply a user interface to add appointments, remove canceled appointments, and
print out a list of appointments for a particular day. Follow the design process that
was described in this chapter. Your solution should include a class  Appointment-
Calendar that is not coupled with the  Scanner or  PrintStream classes.
••• p12.3  Write a program that administers and grades quizzes. A quiz consists of questions.
There are four types of questions: text questions, number questions, choice ques-
tions with a single answer, and choice questions with multiple answers. When grad-
ing a text question, ignore leading or trailing spaces and letter case. When grading a
numeric question, accept a response that is approximately the same as the answer.
A quiz is specified in a text file. Each question starts with a letter indicating the
question type (T, N, S, M), followed by a line containing the question text. The next
line of a non-choice question contains the answer. Choice questions have a list of
choices that is terminated by a blank line. Each choice starts with + (correct) or
- (incorrect). Here is a sample file:
T
WhichJavareservedwordisusedtodeclareasubclass?
extends
S
WhatistheoriginalnameoftheJavalanguage?
-*7
-C--
+Oak
-Gosling
M
WhichofthefollowingtypesaresupertypesofRectangle?
-PrintStream
+Shape
+RectangularShape
+Object
-String
N
Whatisthesquarerootof2?
1.41421356
Your program should read in a quiz file, prompt the user for responses to all ques-
tions, and grade the responses. Follow the design process described in this chapter.
p r O g r a M M i n g p r O j e C t s
programming projects 585
•• p12.4  Produce a requirements document for a program that allows a company to send out
personalized mailings, either by e-mail or through the postal service. Template files
contain the message text, together with variable fields (such as Dear [Title] [Last
Name] . . .). A database (stored as a text file) contains the field values for each recip-
ient. Use HTML as the output file format. Then design and implement the pro gram.
••• p12.5  Write a tic-tac-toe game that allows a human player to play against the computer.
Your program will play many turns against a human opponent, and it will learn.
When it is the computer’s turn, the computer randomly selects an empty field,
except that it won’t ever choose a losing combination. For that purpose, your pro-
gram must keep an array of losing combinations. Whenever the human wins, the
immediately preceding combination is stored as losing. For example, suppose that
X = computer and O = human.
Suppose the current combination is
Now it is the human’s turn, who will
of course choose
The computer should then remember
the preceding combination
as a losing combination. As a result, the computer will never again choose
that combination from
X
O
O
or
Discover classes and supply a UML diagram before you begin to program.
••• business p12.6  Airline seating. Write a program that assigns seats on an airplane. Assume the
airplane has 20 seats in first class (5 rows of 4 seats each, separated by an aisle) and
90 seats in economy class (15 rows of 6 seats each, separated by an aisle). Your pro-
gram should take three commands: add passengers, show seating, and quit. When
passengers are added, ask for the class (first or economy), the number of passengers
traveling together (1 or 2 in first class; 1 to 3 in economy), and the seating prefer-
ence (aisle or window in first class; aisle, center, or window in economy). Then try
to find a match and assign the seats. If no match exists, print a message. Your solu-
tion should include a class  Airplane that is not coupled with the  Scanner or  PrintStream
classes. Follow the design process that was described in this chapter.
••• business p12.7  In an airplane, each passenger has a touch screen for ordering a drink and a snack.
Some items are free and some are not. The system prepares two reports for speeding
up service:
1.  A list of all seats, ordered by row, showing the charges that must be collected.
2.  A list of how many drinks and snacks of each type must be prepared for the
front and the rear of the plane.
X
O
X O
X
O
X O
O
X
O
X O
O
X O
586 Chapter 12 Object-Oriented design
1.  Look for nouns in the problem description.
2.  Yes ( ChessBoard ) and no ( MovePiece ).
3.  PrintStream .
4.  To produce the shipping address of the
customer.
5.  Reword the responsibilities so that they are at
a higher level, or come up with more classes to
handle the responsibilities.
6.  The  CashRegisterTester class depends on the
CashRegister ,  Coin ,and  System classes.
7.  The  ChoiceQuestion class inherits from the
Question class.
8.  The  Quiz class depends on the  Question class
but probably not  ChoiceQuestion , if we assume
that the methods of the  Quiz class manipu-
late generic  Question objects, as they did in
Chapter 9.
9.  If a class doesn’t depend on another, it is not
affected by interface changes in the other class.
10.
11.  Typically, a library system wants to track
which books a patron has checked out, so it
makes more sense to have  Patron aggregate  Book .
However, there is not always one true answer
in design. If you feel strongly that it is impor-
tant to identify the patron who checked out a
particular book (perhaps to notify the patron
to return it because it was requested by some-
one else), then you can argue that the aggrega-
tion should go the other way around.
12.  There would be no relationship.
13.  The  Invoice class is responsible for comput-
ing the amount due. It collaborates with the
LineItem class.
14.  This design decision reduces coupling. It
enables us to reuse the classes when we want to
show the invoice in a dialog box or on a
web page.
Mailbox Message
Follow the design process that was described in this chapter to identify classes, and
implement a program that simulates the system.
••• graphics p12.8  Implement a program to teach a young child to read the clock. In the game, present
an analog clock, such as the one shown at left. Generate random times and display
the clock. Accept guesses from the player. Reward the player for correct guesses.
After two incorrect guesses, display the correct answer and make a new random
time. Implement several levels of play. In level 1, only show full hours. In level 2,
show quarter hours. In level 3, show five-minute multiples, and in level 4, show any
number of minutes. After a player has achieved five correct guesses at one level,
advance to the next level.
••• graphics p12.9  Write a program that can be used to design a suburban scene, with houses, streets,
and cars. Users can add houses and cars of various colors to a street. Write more spe-
cific requirements that include a detailed description of the user interface. Then, dis-
cover classes and methods, provide UML diagrams, and implement your pro gram.
••• graphics p12.10  Write a simple graphics editor that allows users to add a mixture of shapes (ellipses,
rectangles, and lines in different colors) to a panel. Supply commands to load and
save the picture. Discover classes, supply a UML diagram, and implement your
program.
An Analog Clock
a n s w e r s t O s e L F - C h e C k Q U e s t i O n s
A
A P P E N D I X
A-1
ThE BAsIc LATIN AND LATIN-1
suBsETs of uNIcoDE
This appendix lists the Unicode characters that are most commonly used for process-
ing Western European languages. A complete listing of Unicode characters can be
found at  http://unicode.org .
Table 1 selected control characters
character code Decimal Escape sequence
Tab '\u0009' 9 '\t'
Newline '\u000A' 10 '\n'
Return '\u000D' 13 '\r'
Space '\u0020' 32
A-2 Appendix A The Basic Latin and Latin-1 subsets of unicode
Table 2 The Basic Latin (AscII) subset of unicode
char. code Dec. char. code Dec. char. code Dec.
@ '\u0040' 64 ` '\u0060' 96
! '\u0021' 33 A '\u0041' 65 a '\u0061' 97
" '\u0022' 34 B '\u0042' 66 b '\u0062' 98
# '\u0023' 35 C '\u0043' 67 c '\u0063' 99
$ '\u0024' 36 D '\u0044' 68 d '\u0064' 100
% '\u0025' 37 E '\u0045' 69 e '\u0065' 101
& '\u0026' 38 F '\u0046' 70 f '\u0066' 102
' '\u0027' 39 G '\u0047' 71 g '\u0067' 103
( '\u0028' 40 H '\u0048' 72 h '\u0068' 104
) '\u0029' 41 I '\u0049' 73 i '\u0069' 105
* '\u002A' 42 J '\u004A' 74 j '\u006A' 106
+ '\u002B' 43 K '\u004B' 75 k '\u006B' 107
, '\u002C' 44 L '\u004C' 76 l '\u006C' 108
- '\u002D' 45 M '\u004D' 77 m '\u006D' 109
. '\u002E' 46 N '\u004E' 78 n '\u006E' 110
/ '\u002F' 47 O '\u004F' 79 o '\u006F' 111
0 '\u0030' 48 P '\u0050' 80 p '\u0070' 112
1 '\u0031' 49 Q '\u0051' 81 q '\u0071' 113
2 '\u0032' 50 R '\u0052' 82 r '\u0072' 114
3 '\u0033' 51 S '\u0053' 83 s '\u0073' 115
4 '\u0034' 52 T '\u0054' 84 t '\u0074' 116
5 '\u0035' 53 U '\u0055' 85 u '\u0075' 117
6 '\u0036' 54 V '\u0056' 86 v '\u0076' 118
7 '\u0037' 55 W '\u0057' 87 w '\u0077' 119
8 '\u0038' 56 X '\u0058' 88 x '\u0078' 120
9 '\u0039' 57 Y '\u0059' 89 y '\u0079' 121
: '\u003A' 58 Z '\u005A' 90 z '\u007A' 122
; '\u003B' 59 [ '\u005B' 91 { '\u007B' 123
< '\u003C' 60 \' '\u005C' 92 | '\u007C' 124
= '\u003D' 61 ] '\u005D' 93 } '\u007D' 125
> '\u003E' 62 ˆ '\u005E' 94 ~ '\u007E' 126
? '\u003F' 63 _ '\u005F' 95
Appendix A The Basic Latin and Latin-1 subsets of unicode A-3
Table 3 The Latin-1 subset of unicode
char. code Dec. char. code Dec. char. code Dec.
À '\u00C0' 192 à '\u00E0' 224
¡ '\u00A1' 161 Á '\u00C1' 193 á '\u00E1' 225
¢ '\u00A2' 162 Â '\u00C2' 194 â '\u00E2' 226
£ '\u00A3' 163 Ã '\u00C3' 195 ã '\u00E3' 227
¤ '\u00A4' 164 Ä '\u00C4' 196 ä '\u00E4' 228
¥ '\u00A5' 165 Å '\u00C5' 197 å '\u00E5' 229
¦ '\u00A6' 166 Æ '\u00C6' 198 æ '\u00E6' 230
§ '\u00A7' 167 Ç '\u00C7' 199 ç '\u00E7' 231
¨ '\u00A8' 168 È '\u00C8' 200 è '\u00E8' 232
© '\u00A9' 169 É '\u00C9' 201 é '\u00E9' 233
ª '\u00AA' 170 Ê '\u00CA' 202 ê '\u00EA' 234
« '\u00AB' 171 Ë '\u00CB' 203 ë '\u00EB' 235
¬ '\u00AC' 172 Ì '\u00CC' 204 ì '\u00EC' 236
- '\u00AD' 173 Í '\u00CD' 205 í '\u00ED' 237
® '\u00AE' 174 Î '\u00CE' 206 î '\u00EE' 238
¯ '\u00AF' 175 Ï '\u00CF' 207 ï '\u00EF' 239
° '\u00B0' 176 Ð '\u00D0' 208 ð '\u00F0' 240
± '\u00B1' 177 Ñ '\u00D1' 209 ñ '\u00F1' 241
2
'\u00B2' 178 Ò '\u00D2' 210 ò '\u00F2' 242
3
'\u00B3' 179 Ó '\u00D3' 211 ó '\u00F3' 243
´ '\u00B4' 180 Ô '\u00D4' 212 ô '\u00F4' 244
µ '\u00B5' 181 Õ '\u00D5' 213 õ '\u00F5' 245
¶ '\u00B6' 182 Ö '\u00D6' 214 ö '\u00F6' 246
· '\u00B7' 183 × '\u00D7' 215 ÷ '\u00F7' 247
¸ '\u00B8' 184 Ø '\u00D8' 216 ø '\u00F8' 248
1
'\u00B9' 185 Ù '\u00D9' 217 ù '\u00F9' 249
º '\u00BA' 186 Ú '\u00DA' 218 ú '\u00FA' 250
» '\u00BB' 187 Û '\u00DB' 219 û '\u00FB' 251
¼ '\u00BC' 188 Ü '\u00DC' 220 ü '\u00FC' 252
½ '\u00BD' 189 Ý '\u00DD' 221 ý '\u00FD' 253
¾ '\u00BE' 190 Þ '\u00DE' 222 þ '\u00FE' 254
¿ '\u00BF' 191 ß '\u00DF' 223 ÿ '\u00FF' 255
B
A P P E N D I X
A-5
JAvA OPErAtOr
SummAry
The Java operators are listed in groups of decreasing precedence in the table below.
The horizontal lines in the table indicate a change in operator precedence. Opera-
tors with higher precedence bind more strongly than those with lower precedence.
For example,  x + y * z means  x + (y * z) because the  * operator has higher precedence
than the  + operator. Looking at the table below, you can tell that  x && y || z means
(x && y) || z because the  || operator has lower precedence.
The associativity of an operator indicates whether it groups left to right, or right
to left. For example, the  - operator binds left to right. Therefore,  x - y - z means
(x - y) - z . But the  = operator binds right to left, and  x = y = z means  x = (y = z) .
Operator Description Associativity
.  Access class feature
Left to right [ ]  Array subscript
()  Function call
++  Increment
Right to left
--  Decrement
!  Boolean not
~  Bitwise not
+  (unary) (Has no effect)
-  (unary) Negative
( TypeName ) Cast
new  Object allocation
* Multiplication
Left to right / Division or integer division
% Integer remainder
+ Addition, string concatenation
Left to right
- Subtraction
<< Shift left
Left to right >> Right shift with sign extension
>>> Right shift with zero extension
A-6 Appendix B Java Operator Summary
Operator Description Associativity
< Less than
Left to right
<= Less than or equal
> Greater than
>= Greater than or equal
instanceof Tests whether an object’s type is a
given type or a subtype thereof
== Equal
Left to right
!= Not equal
& Bitwise and Left to right
^ Bitwise exclusive or Left to right
| Bitwise or Left to right
&& Boolean “short circuit” and Left to right
|| Boolean “short circuit” or  Left to right
? : Conditional Right to left
= Assignment
Right to left
op = Assignment with binary operator (op is
one of  + ,  - ,  * ,  / ,  & ,  | ,  ^ ,  << ,  >> ,  >>> )
C
A P P E N D I X
A-7
JAvA REsERvED
WoRD summARy
Reserved Word Description
abstract An abstract class or method
assert An assertion that a condition is fulfilled
boolean The Boolean type
break Breaks out of the current loop or labeled statement
byte The 8-bit signed integer type
case A label in a  switch statement
catch The handler for an exception in a  try block
char The 16-bit Unicode character type
class Defines a class
const Not used
continue Skips the remainder of a loop body
default The default label in a  switch statement
do A loop whose body is executed at least once
double The 64-bit double-precision floating-point type
else The alternative clause in an  if statement
enum An enumeration type
extends Indicates that a class is a subclass of another class
final A value that cannot be changed after it has been initialized, a method that
cannot be overridden, or a class that cannot be extended
finally A clause of a  try block that is always executed
float The 32-bit single-precision floating-point type
for A loop with initialization, condition, and update expressions
goto Not used
if A conditional branch statement
implements Indicates that a class realizes an interface
A-8 Appendix C Java Reserved Word summary
Reserved Word Description
import Allows the use of class names without the package name
instanceof Tests whether an object’s type is a given type or a subtype thereof
int The 32-bit integer type
interface An abstract type with only abstract methods and constants
long The 64-bit integer type
native A method implemented in non-Java code
new Allocates an object
package A collection of related classes
private A feature that is accessible only by methods of the same class
protected A feature that is accessible only by methods of the same class, a subclass,
or another class in the same package
public A feature that is accessible by all methods
return Returns from a method
short The 16-bit integer type
static A feature that is defined for a class, not for individual instances
strictfp Uses strict rules for floating-point computations
super Invokes the superclass constructor or a superclass method
switch A selection statement
synchronized A block of code that is accessible to only one thread at a time
this The implicit parameter of a method; or invocation of another constructor
of the same class
throw Throws an exception
throws Indicates the exceptions that a method may throw
transient Instance variables that should not be serialized
try A block of code with exception handlers or a  finally handler
void Tags a method that doesn’t return a value
volatile A variable that may be accessed by multiple threads without
synchronization
while A loop statement
D
A P P E N D I X
A-9
ThE JAvA LIbrAry
This appendix lists all classes and methods from the standard Java library that are
used in this book.
In the following inheritance hierarchy, superclasses that are not used in this book are
shown in gray type. Some classes implement interfaces not covered in this book; they
are omitted. Classes are sorted first by package, then alphabetically within a package.
java.awt.Shape
java.lang.Cloneable
java.lang.Object
java.awt.BorderLayout
java.awt.Color
java.awt.Component
java.awt.Container
javax.swing.JComponent
javax.swing.AbstractButton
javax.swing.JButton
javax.swing.JMenuItem
javax.swing.JMenu
javax.swing.JToggleButton
javax.swing.JCheckBox
javax.swing.JRadioButton
javax.swing.JComboBox
javax.swing.JFileChooser
javax.swing.JLabel
javax.swing.JMenuBar
javax.swing.JPanel
javax.swing.JOptionPane
javax.swing.JScrollPane
javax.swing.JSlider
javax.swing.text.JTextComponent
javax.swing.JTextArea
javax.swing.JTextField
java.awt.Window
java.awt.Frame
javax.swing.JFrame
java.awt.Dimension2D
java.awt.Dimension implements Cloneable
java.awt.FlowLayout
java.awt.Font
java.awt.Graphics
java.awt.Graphics2D;
java.awt.GridLayout
java.awt.event.MouseAdapter implements MouseListener
java.awt.geom.Line2D implements Cloneable, Shape
java.awt.geom.Line2D.Double
java.awt.geom.Point2D implements Cloneable
java.awt.geom.Point2D.Double
java.awt.geom.RectangularShape implements Cloneable, Shape
A-10 Appendix D The Java Library
java.awt.geom.Rectangle2D
java.awt.Rectangle
java.awt.geom.Ellipse2D
java.awt.geom.Ellipse2D.Double
java.io.File implements Comparable<File>
java.io.InputStream
java.io.FileInputStream
java.io.OutputStream
java.io.FileOutputStream
java.io.FilterOutputStream
java.io.PrintStream
java.io.Writer
java.io.PrintWriter
java.lang.Boolean implements Comparable<Boolean>
java.lang.Character implements Comparable<Character>
java.lang.Class
java.lang.Math
java.lang.Number
java.math.BigDecimal implements Comparable<BigDecimal>
java.math.BigInteger implements Comparable<BigInteger>
java.lang.Double implements Comparable<Double>
java.lang.Integer implements Comparable<Integer>
java.lang.String implements Comparable<String>
java.lang.System
java.lang.Throwable
java.lang.Error
java.lang.Exception
java.lang.CloneNotSupportedException
java.lang.InterruptedException
java.io.IOException
java.io.EOFException
java.io.FileNotFoundException
java.lang.RuntimeException
java.lang.IllegalArgumentException
java.lang.NumberFormatException
java.lang.IllegalStateException
java.util.NoSuchElementException
java.util.InputMismatchException
java.lang.NullPointerException
java.net.URL
java.net.URLConnection
java.text.Format
java.text.DateFormat
java.util.AbstractCollection<E>
java.util.AbstractList<E>
java.util.AbstractSequentialList<E>
java.util.LinkedList<E> implements List<E>, Queue<E>
java.util.ArrayList<E> implements List<E>
java.util.AbstractQueue<E>
java.util.PriorityQueue<E>
java.util.AbstractSet<E>
java.util.HashSet<E> implements Set<E>
java.util.TreeSet<E> implements SortedSet<E>
java.util.AbstractMap<K, V>
java.util.HashMap<K, V> implements Map<K, V>
java.util.LinkedHashMap<K, V>
java.util.TreeMap<K, V> implements Map<K, V>
java.util.Arrays
java.util.Collections
Appendix D The Java Library A-11
java.util.Calendar
java.util.GregorianCalendar
java.util.Date
java.util.EventObject
java.awt.AWTEvent
java.awt.event.ActionEvent
java.awt.event.ComponentEvent
java.awt.event.InputEvent
java.awt.event.KeyEvent
java.awt.event.MouseEvent
javax.swing.event.ChangeEvent
java.util.Random
java.util.Scanner
java.util.logging.Level
java.util.logging.Logger
javax.swing.ButtonGroup
javax.swing.ImageIcon
javax.swing.Keystroke
javax.swing.Timer
javax.swing.border.AbstractBorder
javax.swing.border.EtchedBorder
javax.swing.border.TitledBorder
java.lang.Comparable<T>
java.util.Collection<E>
java.util.List<E>
java.util.Set<E>
java.util.SortedSet<E>
java.util.Comparator<T>
java.util.EventListener
java.awt.event.ActionListener
java.awt.event.KeyListener
java.awt.event.MouseListener
javax.swing.event.ChangeListener
java.util.Iterator<E>
java.util.ListIterator<E>
java.util.Map<K, V>
java.util.Queue<E> extends Collection<E>
In the following descriptions, the phrase “this object” (“this component”, “this con-
tainer”, and so forth) means the object (component, container, and so forth) on which
the method is invoked (the implicit parameter,  this ).
Package java.awt
Class java.awt.BorderLayout
•  BorderLayout()
This constructs a border layout. A border layout has five regions for adding com-
ponents, called  "NORTH" ,  "EAST" ,  "SOUTH" ,  "WEST" , and  "CENTER" .
•  static final int CENTER
This value identifies the center position of a border layout.
•  static final int EAST
This value identifies the east position of a border layout.
Package java.awt
A-12 Appendix D The Java Library
•  static final int NORTH
This value identifies the north position of a border layout.
•  static final int SOUTH
This value identifies the south position of a border layout.
•  static final int WEST
This value identifies the west position of a border layout.
Class  java.awt.Color
•  Color(int red, int green, int blue)
This creates a color with the specified red, green, and blue values between 0
and 255.
Parameters: red  The red component
green  The green component
blue  The blue component
Class  java.awt.Component
•  void addKeyListener(KeyListener listener)
This method adds a key listener to the component.
Parameters: listener  The key listener to be added
•  void addMouseListener(MouseListener listener)
This method adds a mouse listener to the component.
Parameters: listener  The mouse listener to be added
•  int getHeight()
This method gets the height of this component.
Returns:  The height in pixels
•  int getWidth()
This method gets the width of this component.
Returns:  The width in pixels
•  void repaint()
This method repaints this component by scheduling a call to the  paint method.
•  void setFocusable(boolean focusable)
This method controls whether or not the component can receive input focus.
Parameters: focusable true to have focus, or  false to lose focus
•  void setPreferredSize(Dimension preferredSize)
This method sets the preferred size of this component.
•  void setSize(int width, int height)
This method sets the size of this component.
Parameters: width  the component width
height the component height
•  void setVisible(boolean visible)
This method shows or hides the component.
Parameters: visible true to show the component, or  false to hide it
Package java.awt
Appendix D The Java Library A-13
Class java.awt.Container
•  void add(Component c)
•  void add(Component c, Object position)
These methods add a component to the end of this container. If a position is given,
the layout manager is called to position the component.
Parameters: c  The component to be added
position  An object expressing position information for the
layout manager
•  void setLayout(LayoutManager manager)
This method sets the layout manager for this container.
Parameters: manager  A layout manager
Class java.awt.Dimension
•  Dimension(int width, int height)
This constructs a  Dimension object with the given width and height.
Parameters: width  The width
height  The height
Class java.awt.FlowLayout
•  FlowLayout()
This constructs a new flow layout. A flow layout places as many components as
possible in a row, without chang ing their size, and starts new rows as needed.
Class java.awt.Font
•  Font(String name, int style, int size)
This constructs a font object from the specified name, style, and point size.
Parameters: name  The font name, either a font face name or a logical font
name, which must be one of  "Dialog" ,  "DialogInput" ,  "Monospaced" ,
"Serif" , or  "SansSerif"
style  One of  Font.PLAIN ,  Font.ITALIC ,  Font.BOLD , or
Font.ITALIC+Font.BOLD
size  The point size of the font
Class java.awt.Frame
•  void setTitle(String title)
This method sets the frame title.
Parameters: title  The title to be displayed in the border of the frame
Class java.awt.Graphics
•  void drawLine(int x1, int y1, int x2, int y2)
Draws a line between two points.
Parameters: x1 ,  y1  The starting point
x2 ,  y2  The endpoint
Package java.awt
A-14 Appendix D The Java Library
•  void setColor(Color c)
This method sets the current color. After the method call, all graphics operations
use this color.
Parameters: c  The new drawing color
Class  java.awt.Graphics2D
•  void draw(Shape s)
This method draws the outline of the given shape. Many classes—among them
Rectangle and  Line2D.Double —implement the  Shape interface.
Parameters: s  The shape to be drawn
•  void drawString(String s, int x, int y)
•  void drawString(String s, float x, float y)
These methods draw a string in the current font.
Parameters: s  The string to draw
x,y  The basepoint of the first character in the string
•  void fill(Shape s)
This method draws the given shape and fills it with the current color.
Parameters: s  The shape to be filled
Class java.awt.GridLayout
•  GridLayout(int rows, int cols)
This constructor creates a grid layout with the specified number of rows and col-
umns. The components in a grid layout are arranged in a grid with equal widths
and heights. One, but not both, of  rows and  cols can be zero, in which case any
number of objects can be placed in a row or in a column, respectively.
Parameters: rows  The number of rows in the grid
cols  The number of columns in the grid
Class java.awt.Rectangle
•  Rectangle()
This constructs a rectangle with a top-left corner at (0, 0) and width and height
set to 0.
•  Rectangle(int x, int y, int width, int height)
This constructs a rectangle with given top-left corner and size.
Parameters: x ,  y  The top-left corner
width  The width
height  The height
•  double getHeight()
•  double getWidth()
These methods get the height and width of the rectangle.
•  double getX()
•  double getY()
These methods get the x- and y-coordinates of the top-left corner of the rectangle.
Package java.awt
Appendix D The Java Library A-15
•  void grow(int dw, int dh)
This method adjusts the width and height of this rectangle.
Parameters: dw  The amount to add to the width (can be negative)
dh  The amount to add to the height (can be negative)
•  Rectangle intersection(Rectangle other)
This method computes the intersection of this rectangle with the specified
rectangle.
Parameters: other  A rectangle
Returns:  The largest rectangle contained in both  this  and  other
•  void setLocation(int x, int y)
This method moves this rectangle to a new location.
Parameters: x ,  y  The new top-left corner
•  void setSize(int width, int height)
This method sets the width and height of this rectangle to new values.
Parameters: width  The new width
height  The new height
•  void translate(int dx, int dy)
This method moves this rectangle.
Parameters: dx  The distance to move along the x-axis
dy  The distance to move along the y-axis
•  Rectangle union(Rectangle other)
This method computes the union of this rectangle with the specified rectangle.
This is not the set-theoretic union but the smallest rectangle that contains both
this and  other .
Parameters: other  A rectangle
Returns:  The smallest rectangle containing both  this and  other
Interface  java.awt.Shape
This interface describes shapes that can be drawn and filled by a  Graphics2D object.
Package  java.awt.event
Interface  java.awt.event.ActionListener
•  void actionPerformed(ActionEvent e)
The event source calls this method when an action occurs.
Class  java.awt.event.KeyEvent
This event is passed to the  KeyListener methods. Use the  KeyStroke class to obtain
the key information from the key event.
Interface  java.awt.event.KeyListener
•  void keyPressed(KeyEvent e)
•  void keyReleased(KeyEvent e)
These methods are called when a key has been pressed or released.
Package java.awt
Package java.awt.event
A-16 Appendix D The Java Library
•  void keyTyped(KeyEvent e)
This method is called when a keystroke has been composed by pressing and
releasing one or more keys.
Class java.awt.event.MouseEvent
•  int getX()
This method returns the horizontal position of the mouse when the event
occurred.
Returns:  The x-position of the mouse
•  int getY()
This method returns the vertical position of the mouse when the event occurred.
Returns:  The y-position of the mouse
Interface  java.awt.event.MouseListener
•  void mouseClicked(MouseEvent e)
This method is called when the mouse has been clicked (that is, pressed and
released in quick succession).
•  void mouseEntered(MouseEvent e)
This method is called when the mouse has entered the component to which this
listener was added.
•  void mouseExited(MouseEvent e)
This method is called when the mouse has exited the component to which this
listener was added.
•  void mousePressed(MouseEvent e)
This method is called when a mouse button has been pressed.
•  void mouseReleased(MouseEvent e)
This method is called when a mouse button has been released.
Package  java.awt.geom
Class  java.awt.geom.Ellipse2D.Double
•  Ellipse2D.Double(double x, double y, double w, double h)
This constructs an ellipse from the specified coordinates.
Parameters: x, y  The top-left corner of the bounding rectangle
w  The width of the bounding rectangle
h  The height of the bounding rectangle
Class  java.awt.geom.Line2D
•  double getX1()
•  double getX2()
•  double getY1()
•  double getY2()
These methods get the requested coordinate of an endpoint of this line.
Returns:  The x- or y-coordinate of the first or second endpoint
Package java.awt.event Package java.awt.geom
Appendix D The Java Library A-17
•  void setLine(double x1, double y1, double x2, double y2)
This methods sets the endpoints of this line.
Parameters: x1, y1  A new endpoint of this line
x2, y2  The other new endpoint
Class  java.awt.geom.Line2D.Double
•  Line2D.Double(double x1, double y1, double x2, double y2)
This constructs a line from the specified coordinates.
Parameters: x1, y1  One endpoint of the line
x2, y2  The other endpoint
•  Line2D.Double(Point2D p1, Point2D p2)
This constructs a line from the two endpoints.
Parameters: p1, p2  The endpoints of the line
Class  java.awt.geom.Point2D
•  double getX()
•  double getY()
These methods get the requested coordinates of this point.
Returns:  The x- or y-coordinate of this point
•  void setLocation(double x, double y)
This method sets the x- and y-coordinates of this point.
Parameters: x, y  The new location of this point
Class  java.awt.geom.Point2D.Double
•  Point2D.Double(double x, double y)
This constructs a point with the specified coordinates.
Parameters: x, y  The coordinates of the point
Class  java.awt.geom.RectangularShape
•  int getHeight()
•  int getWidth()
These methods get the height or width of the bounding rectangle of this
rectangular shape.
Returns:  The height or width, respectively
•  double getCenterX()
•  double getCenterY()
•  double getMaxX()
•  double getMaxY()
•  double getMinX()
•  double getMinY()
These methods get the requested coordinate value of the corners or center of the
bounding rectangle of this shape.
Returns:  The center, maximum, or minimum x- and y-coordinates
Package java.awt.geom
A-18 Appendix D The Java Library
Package java.io
Class java.io.EOFException
•  EOFException(String message)
This constructs an “end of file” exception object.
Parameters: message  The detail message
Class java.io.File
•  File(String name)
This constructs a  File object that describes a file (which may or may not exist)
with the given name.
Parameters: name  The name of the file
•  boolean exists()
This method checks whether there is a file in the local file system that matches this
File object.
Returns:  true  if there is a matching file,  false otherwise
•  static final String pathSeparator
The sytem-dependent separator between path names. A colon ( : ) in Linux or Mac
OS X; a semicolon ( ; ) in Win dows.
Class java.io.FileInputStream
•  FileInputStream(File f)
This constructs a file input stream and opens the chosen file. If the file cannot be
opened for reading, a  FileNot FoundException is thrown.
Parameters: f  The file to be opened for reading
•  FileInputStream(String name)
This constructs a file input stream and opens the named file. If the file cannot be
opened for reading, a  FileNot FoundException is thrown.
Parameters: name  The name of the file to be opened for reading
Class  java.io.FileNotFoundException
This exception is thrown when a file could not be opened.
Class  java.io.FileOutputStream
•  FileOutputStream(File f)
This constructs a file output stream and opens the chosen file. If the file cannot be
opened for writing, a  FileNot FoundException  is thrown.
Parameters: f  The file to be opened for writing
•  FileOutputStream(String name)
This constructs a file output stream and opens the named file. If the file cannot be
opened for writing, a  FileNot FoundException is thrown.
Parameters: name  The name of the file to be opened for writing
Package java.io
Appendix D The Java Library A-19
Class  java.io.InputStream
•  void close()
This method closes this input stream (such as a  FileInputStream ) and releases any
system resources associated with the stream.
•  int read()
This method reads the next byte of data from this input stream.
Returns:  The next byte of data, or –1 if the end of the stream is reached
Class  java.io.InputStreamReader
•  InputStreamReader(InputStream in)
This constructs a reader from a specified input stream.
Parameters: in  The stream to read from
Class  java.io.IOException
This type of exception is thrown when an input /output error is encountered.
Class  java.io.OutputStream
•  void close()
This method closes this output stream (such as a  FileOutputStream ) and releases any
system resources associated with this stream. A closed stream cannot perform
output operations and cannot be reopened.
•  void write(int b)
This method writes the lowest byte of  b to this output stream.
Parameters: b  The integer whose lowest byte is written
Class  java.io.PrintStream / Class  java.io.PrintWriter
•  PrintStream(String name)
•  PrintWriter(String name)
This constructs a  PrintStream or  PrintWriter and opens the named file. If the file
cannot be opened for writing, a  FileNotFoundException is thrown.
Parameters: name  The name of the file to be opened for writing
•  void close()
This method closes this stream or writer and releases any associated system
resources.
•  void print(int x)
•  void print(double x)
•  void print(Object x)
•  void print(String x)
•  void println()
•  void println(int x)
•  void println(double x)
Package java.io
A-20 Appendix D The Java Library
•  void println(Object x)
•  void println(String x)
These methods print a value to this  PrintStream or  PrintWriter . The  println methods
print a newline after the value. Objects are printed by converting them to strings
with their  toString methods.
Parameters: x  The value to be printed
•  PrintStream printf(String format, Object... values)
•  Printwriter printf(String format, Object... values)
These methods print the format string to this  PrintStream or  PrintWriter , substitut-
ing the given values for place holders that start with  % .
Parameters: format  The format string
values  The values to be printed. You can supply any number
of values
Returns:  The implicit parameter
Package java.lang
Class java.lang.Boolean
•  Boolean(boolean value)
This constructs a wrapper object for a  boolean value.
Parameters: value  The value to store in this object
•  boolean booleanValue()
This method returns the value stored in this  boolean object.
Returns:  The Boolean value of this object
Class  java.lang.Character
•  static boolean isDigit(ch)
This method tests whether a given character is a Unicode digit.
Parameters: ch  The character to test
Returns:  true if the character is a digit
•  static boolean isLetter(ch)
This method tests whether a given character is a Unicode letter.
Parameters: ch  The character to test
Returns:  true if the character is a letter
•  static boolean isLowerCase(ch)
This method tests whether a given character is a lowercase Unicode letter.
Parameters: ch  The character to test
Returns:  true if the character is a lowercase letter
•  static boolean isUpperCase(ch)
This method tests whether a given character is an uppercase Unicode letter.
Parameters: ch  The character to test
Returns:  true if the character is an uppercase letter
Package java.lang Package java.io
Appendix D The Java Library A-21
Class java.lang.Class
•  static Class forName(String className)
This method loads a class with a given name. Loading a class initializes its static
variables.
Parameters: className  The name of the class to load
Returns:  The type descriptor of the class
Interface  java.lang.Cloneable
A class implements this interface to indicate that the  Object.clone method is
allowed to make a shallow copy of its instance variables.
Class  java.lang.CloneNotSupportedException
This exception is thrown when a program tries to use  Object.clone to make a shal-
low copy of an object of a class that does not implement the  Cloneable interface.
Interface  java.lang.Comparable<T>
•  int compareTo(T other)
This method compares this object with the  other object.
Parameters: other  The object to be compared
Returns:  A negative integer if this object is less than the other, zero if they
are equal, or a positive integer other wise
Class  java.lang.Double
•  Double(double value)
This constructs a wrapper object for a double-precision floating-point number.
Parameters: value  The value to store in this object
•  double doubleValue()
This method returns the floating-point value stored in this  Double wrapper object.
Returns:  The value stored in the object
•  static double parseDouble(String s)
This method returns the floating-point number that the string represents. If the
string cannot be interpreted as a number, a  NumberFormatException is thrown.
Parameters: s  The string to be parsed
Returns:  The value represented by the string argument
Class  java.lang.Error
This is the superclass for all unchecked system errors.
Class  java.lang.IllegalArgumentException
•  IllegalArgumentException()
This constructs an  IllegalArgumentException with no detail message.
Package java.lang
A-22 Appendix D The Java Library
Class  java.lang.IllegalStateException
This exception is thrown if the state of an object indicates that a method cannot
currently be applied.
Class  java.lang.Integer
•  Integer(int value)
This constructs a wrapper object for an integer.
Parameters: value  The value to store in this object
•  int intValue()
This method returns the integer value stored in this wrapper object.
Returns:  The value stored in the object
•  static int parseInt(String s)
This method returns the integer that the string represents. If the string cannot be
interpreted as an integer, a  Num berFormatException is thrown.
Parameters: s  The string to be parsed
Returns:  The value represented by the string argument
•  static Integer parseInt(String s, int base)
This method returns the integer value that the string represents in a given number
system. If the string cannot be interpreted as an integer, a  NumberFormatException is
thrown.
Parameters: s  The string to be parsed
base  The base of the number system (such as 2 or 16)
Returns:  The value represented by the string argument
•  static String toString(int i)
•  static String toString(int i, int base)
This method creates a string representation of an integer in a given number sys-
tem. If no base is given, a decimal representation is created.
Parameters: i  An integer number
base  The base of the number system (such as 2 or 16)
Returns:  A string representation of the argument in the number system
•  static final int MAX_VALUE
This constant is the largest value of type  int .
•  static final int MIN_VALUE
This constant is the smallest (negative) value of type  int .
Class  java.lang.InterruptedException
This exception is thrown to interrupt a thread, usually with the intention of
terminating it.
Class  java.lang.Math
•  static double abs(double x)
This method returns the absolute value | x |.
Parameters: x  A floating-point value
Returns:  The absolute value of the argument
Package java.lang
Appendix D The Java Library A-23
•  static double acos(double x)
This method returns the angle with the given cosine, cos –1 x ∈ [0,  π ].
Parameters: x  A floating-point value between -1 and 1
Returns:  The arc cosine of the argument, in radians
•  static double asin(double x)
This method returns the angle with the given sine, sin –1 x ∈ [- π /2,  π /2].
Parameters: x  A floating-point value between -1 and 1
Returns:  The arc sine of the argument, in radians
•  static double atan(double x)
This method returns the angle with the given tangent, tan –1 x (- π /2,  π /2).
Parameters: x  A floating-point value
Returns:  The arc tangent of the argument, in radians
•  static double atan2(double y, double x)
This method returns the arc tangent, tan –1 (y/x) ∈ (- π ,  π ). If x can equal zero, or if
it is necessary to distinguish “northwest” from “southeast” and “northeast” from
“southwest”, use this method instead of  atan(y/x) .
Parameters: y ,  x  Two floating-point values
Returns:  The angle, in radians, between the points (0,0) and (x,y)
•  static double ceil(double x)
This method returns the smallest integer ≥ x (as a  double) .
Parameters: x  A floating-point value
Returns:  The smallest integer greater than or equal to the argument
•  static double cos(double radians)
This method returns the cosine of an angle given in radians.
Parameters: radians  An angle, in radians
Returns:  The cosine of the argument
•  static double exp(double x)
This method returns the value e x , where e is the base of the natural logarithms.
Parameters: x  A floating-point value
Returns:  e x
•  static double floor(double x)
This method returns the largest integer ≤ x (as a  double ).
Parameters: x  A floating-point value
Returns:  The largest integer less than or equal to the argument
•  static double log(double x)
•  static double log10(double x)
This method returns the natural (base e) or decimal (base 10) logarithm of x, ln x.
Parameters: x  A number greater than 0.0
Returns:  The natural logarithm of the argument
•  static int max(int x, int y)
•  static double max(double x, double y)
These methods return the larger of the given arguments.
Parameters: x ,  y  Two integers or floating-point values
Returns:  The maximum of the arguments
Package java.lang
A-24 Appendix D The Java Library
•  static int min(int x, int y)
•  static double min(double x, double y)
These methods return the smaller of the given arguments.
Parameters: x ,  y  Two integers or floating-point values
Returns:  The minimum of the arguments
•  static double pow(double x, double y)
This method returns the value x y (x > 0, or x = 0 and y > 0, or x < 0 and y is an
integer).
Parameters: x ,  y  Two floating-point values
Returns:  The value of the first argument raised to the power of the second
argument
•  static long round(double x)
This method returns the closest  long integer to the argument.
Parameters: x  A floating-point value
Returns:  The argument rounded to the nearest  long  value
•  static double sin(double radians)
This method returns the sine of an angle given in radians.
Parameters: radians  An angle, in radians
Returns:  The sine of the argument
•  static double sqrt(double x)
This method returns the square root of x,  x .
Parameters: x  A nonnegative floating-point value
Returns:  The square root of the argument
•  static double tan(double radians)
This method returns the tangent of an angle given in radians.
Parameters: radians  An angle, in radians
Returns:  The tangent of the argument
•  static double toDegrees(double radians)
This method converts radians to degrees.
Parameters: radians  An angle, in radians
Returns:  The angle in degrees
•  static double toRadians(double degrees)
This methods converts degrees to radians.
Parameters: degrees  An angle, in degrees
Returns:  The angle in radians
•  static final double E
This constant is the value of e, the base of the natural logarithms.
•  static final double PI
This constant is the value of  π .
Class  java.lang.NullPointerException
This exception is thrown when a program tries to use an object through a  null
reference.
Package java.lang
Appendix D The Java Library A-25
Class  java.lang.NumberFormatException
This exception is thrown when a program tries to parse the numerical value of a
string that is not a number.
Class  java.lang.Object
•  protected Object clone()
This constructs and returns a shallow copy of this object whose instance vari-
ables are copies of the instance vari ables of this object. If an instance variable
of the object is an object reference itself, only the reference is copied, not the
object itself. However, if the class does not implement the  Cloneable interface, a
CloneNotSupportedExcep tion is thrown. Subclasses should redefine this method to
make a deep copy.
Returns:  A copy of this object
•  boolean equals(Object other)
This method tests whether  this and the other object are equal. This method tests
only whether the object refer ences are to the same object. Subclasses should rede-
fine this method to compare the instance variables.
Parameters: other  The object with which to compare
Returns:  true if the objects are equal,  false otherwise
•  String toString()
This method returns a string representation of this object. This method produces
only the class name and loca tions of the objects. Subclasses should redefine this
method to print the instance variables.
Returns:  A string describing this object
Class  java.lang.RuntimeException
This is the superclass for all unchecked exceptions.
Class  java.lang.String
•  int compareTo(String other)
This method compares this string and the other string lexicographically.
Parameters: other  The other string to be compared
Returns:  A value less than 0 if this string is lexicographically less than
the other, 0 if the strings are equal, and a value greater than 0
otherwise
•  boolean equals(String other)
•  boolean equalsIgnoreCase(String other)
These methods test whether two strings are equal, or whether they are equal when
letter case is ignored.
Parameters: other  The other string to be compared
Returns:  true if the strings are equal
Package java.lang
A-26 Appendix D The Java Library
•  static String format(String format, Object... values)
This method formats the given string by substituting placeholders beginning with
% with the given values.
Parameters: format  The string with the placeholders
values  The values to be substituted for the placeholders
Returns:  The formatted string, with the placeholders replaced by the given
values
•  int length()
This method returns the length of this string.
Returns:  The count of characters in this string
•  String replace(String match, String replacement)
This method replaces matching substrings with a given replacement.
Parameters: match  The string whose matches are to be replaced
replacement  The string with which matching substrings are
replaced
Returns:  A string that is identical to this string, with all matching sub-
strings replaced by the given replacement
•  String substring(int begin)
•  String substring(int begin, int pastEnd)
These methods return a new string that is a substring of this string, made up of
all characters starting at position  begin and up to either position  pastEnd - 1 , if it is
given, or the end of the string.
Parameters: begin  The beginning index, inclusive
pastEnd  The ending index, exclusive
Returns:  The specified substring
•  String toLowerCase()
This method returns a new string that consists of all characters in this string con-
verted to lowercase.
Returns:  A string with all characters in this string converted to lowercase
•  String toUpperCase()
This method returns a new string that consists of all characters in this string con-
verted to uppercase.
Returns:  A string with all characters in this string converted to uppercase
Class  java.lang.System
•  static long currentTimeMillis()
This method returns the difference, measured in milliseconds, between the cur-
rent time and midnight, Universal Time, January 1, 1970.
Returns:  The current time in milliseconds since January 1, 1970.
•  static void exit(int status)
This method terminates the program.
Parameters: status  Exit status. A nonzero status code indicates abnormal
termination
•  static final InputStream in
This object is the “standard input” stream. Reading from this stream typically
reads keyboard input.
Package java.lang
Appendix D The Java Library A-27
•  static final PrintStream out
This object is the “standard output” stream. Printing to this stream typically
sends output to the console window.
Class  java.lang.Throwable
This is the superclass of exceptions and errors.
•  Throwable()
This constructs a  Throwable with no detail message.
•  String getMessage()
This method gets the message that describes the exception or error.
Returns:  The message
•  void printStackTrace()
This method prints a stack trace to the “standard error” stream. The stack trace
contains lists this object and all calls that were pending when it was created.
Package  java.math
Class  java.math.BigDecimal
•  BigDecimal(String value)
This constructs an arbitrary-precision floating-point number from the digits in
the given string.
Parameters: value  A string representing the floating-point number
•  BigDecimal add(BigDecimal other)
•  BigDecimal multiply(BigDecimal other)
•  BigDecimal subtract(BigDecimal other)
These methods return a  BigDecimal whose value is the sum, difference, product, or
quotient of this number and the other.
Parameters: other  The other number
Returns:  The result of the arithmetic operation
Class  java.math.BigInteger
•  BigInteger(String value)
This constructs an arbitrary-precision integer from the digits in the given string.
Parameters: value  A string representing an arbitrary-precision integer
•  BigInteger add(BigInteger other)
•  BigInteger divide(BigInteger other)
•  BigInteger mod(BigInteger other)
•  BigInteger multiply(BigInteger other)
•  BigInteger subtract(BigInteger other)
These methods return a  BigInteger whose value is the sum, quotient, remainder,
product, or difference of this number and the other.
Parameters: other  The other number
Returns:  The result of the arithmetic operation
Package java.lang Package java.math
A-28 Appendix D The Java Library
Package  java.net
Class  java.net.URL
•  URL(String s)
This constructs a  URL object from a string containing the URL.
Parameters: s  The URL string, such as  "http://horstmann.com/index.html"
•  InputStream openStream()
This method gets the input stream through which the client can read the informa-
tion that the server sends.
Returns:  The input stream associated with this URL
Package  java.util
Class  java.util.ArrayList<E>
•  ArrayList()
This constructs an empty array list.
•  boolean add(E element)
This method appends an element to the end of this array list.
Parameters: element  The element to add
Returns:  true (This method returns a value because it overrides a method
in the  List interface.)
•  void add(int index, E element)
This method inserts an element into this array list at the given position.
Parameters: index  Insert position
element  The element to insert
•  E get(int index)
This method gets the element at the specified position in this array list.
Parameters: index  Position of the element to return
Returns:  The requested element
•  E remove(int index)
This method removes the element at the specified position in this array list and
returns it.
Parameters: index  Position of the element to remove
Returns:  The removed element
•  E set(int index, E element)
This method replaces the element at a specified position in this array list.
Parameters: index  Position of element to replace
element  Element to be stored at the specified position
Returns:  The element previously at the specified position
•  int size()
This method returns the number of elements in this array list.
Returns:  The number of elements in this array list
Package java.net Package java.util
Appendix D The Java Library A-29
Class  java.util.Arrays
•  static int binarySearch(Object[] a, Object key)
This method searches the specified array for the specified object using the binary
search algorithm. The array ele ments must implement the  Comparable interface.
The array must be sorted in ascending order.
Parameters: a  The array to be searched
key  The value to be searched for
Returns:  The position of the search key, if it is contained in the array;
otherwise, -index - 1, where index is the position where the
element may be inserted
•  static T[] copyOf(T[] a, int newLength)
This method copies the elements of the array  a , or the first  newLength elements if
a.length > newLength , into an array of length  newLength and returns that array. T can
be a primitive type, class, or interface type.
Parameters: a  The array to be copied
key  The value to be searched for
Returns:  The position of the search key, if it is contained in the array;
otherwise, -index - 1, where index is the position where the
element may be inserted
•  static void sort(Object[] a)
This method sorts the specified array of objects into ascending order. Its elements
must implement the  Comparable  interface.
Parameters: a  The array to be sorted
•  static String toString(T[] a)
This method creates and returns a string containing the array elements. T can be a
primitive type, class, or inter face type.
Parameters: a  An array
Returns:  A string containing a comma-separated list of string representa-
tions of the array elements, surrounded by brackets.
Class  java.util.Calendar
•  int get(int field)
This method returns the value of the given field.
Parameters: field One of  Calendar.YEAR ,  Calendar.MONTH ,
Calendar.DAY_OF_MONTH ,  Calendar.HOUR ,  Calendar.MINUTE ,
Calendar.SECOND , or  Calendar.MILLISECOND 
Interface  java.util.Collection<E>
•  boolean add(E element)
This method adds an element to this collection.
Parameters: element  The element to add
Returns:  true if adding the element changes the collection
•  boolean contains(E element)
This method tests whether an element is present in this collection.
Parameters: element  The element to find
Returns:  true if the element is contained in the collection
Package java.util
A-30 Appendix D The Java Library
•  Iterator iterator()
This method returns an iterator that can be used to traverse the elements of this
collection.
Returns:  An object of a class implementing the  Iterator interface
•  boolean remove(E element)
This method removes an element from this collection.
Parameters: element  The element to remove
Returns:  true if removing the element changes the collection
•  int size()
This method returns the number of elements in this collection.
Returns:  The number of elements in this collection
Class  java.util.Collections
•  static <T> int binarySearch(List<T> a, T key)
This method searches the specified list for the specified object using the binary
search algorithm. The list elements must implement the  Comparable interface. The
list must be sorted in ascending order.
Parameters: a  The list to be searched
key  The value to be searched for
Returns:  The position of the search key, if it is contained in the list;
otherwise, -index - 1, where index is the position where the
element may be inserted
•  static <T> void sort(List<T> a)
This method sorts the specified list of objects into ascending order. Its elements
must implement the  Comparable  interface.
Parameters: a  The list to be sorted
Interface  java.util.Comparator<T>
•  int compare(T first, T second)
This method compares the given objects.
Parameters: first , s econd  The objects to be compared
Returns:  A negative integer if the first object is less than the second, zero if
they are equal, or a positive integer otherwise
Class  java.util.Date
•  Date()
This constructs an object that represents the current date and time.
Class  java.util.EventObject
•  Object getSource()
This method returns a reference to the object on which this event initially
occurred.
Returns:  The source of this event
Package java.util
Appendix D The Java Library A-31
Class  java.util.GregorianCalendar
•  GregorianCalendar()
This constructs a calendar object that represents the current date and time.
•  GregorianCalendar(int year, int month, int day)
This constructs a calendar object that represents the start of the given date.
Parameters: year ,  month ,  day  The given date
Class  java.util.HashMap<K, V>
•  HashMap<K, V>()
This constructs an empty hash map.
Class  java.util.HashSet<E>
•  HashSet<E>()
This constructs an empty hash set.
Class  java.util.InputMismatchException
This exception is thrown if the next available input item does not match the type
of the requested item.
Interface  java.util.Iterator<E>
•  boolean hasNext()
This method checks whether the iterator is past the end of the list.
Returns:  true if the iterator is not yet past the end of the list
•  E next()
This method moves the iterator over the next element in the linked list. This
method throws an exception if the iterator is past the end of the list.
Returns:  The object that was just skipped over
•  void remove()
This method removes the element that was returned by the last call to  next or
previous . This method throws an exception if there was an  add or  remove operation
after the last call to  next or  previous .
Class  java.util.LinkedList<E>
•  void addFirst(E element)
•  void addLast(E element)
These methods add an element before the first or after the last element in this list.
Parameters: element  The element to be added
•  E getFirst()
•  E getLast()
These methods return a reference to the specified element from this list.
Returns:  The first or last element
Package java.util
A-32 Appendix D The Java Library
•  E removeFirst()
•  E removeLast()
These methods remove the specified element from this list.
Returns:  A reference to the removed element
Interface  java.util.List<E>
•  ListIterator<E> listIterator()
This method gets an iterator to visit the elements in this list.
Returns:  An iterator that points before the first element in this list
Interface  java.util.ListIterator<E>
Objects implementing this interface are created by the  listIterator methods of list
classes.
•  void add(E element)
This method adds an element after the iterator position and moves the iterator
after the new element.
Parameters: element  The element to be added
•  boolean hasPrevious()
This method checks whether the iterator is before the first element of the list.
Returns:  true if the iterator is not before the first element of the list
•  E previous()
This method moves the iterator over the previous element in the linked list. This
method throws an exception if the iterator is before the first element of the list.
Returns:  The object that was just skipped over
•  void set(E element)
This method replaces the element that was returned by the last call to  next or
previous . This method throws an exception if there was an  add or  remove operation
after the last call to  next or  previous .
Parameters: element  The element that replaces the old list element
Interface  java.util.Map<K, V>
•  V get(K key)
Gets the value associated with a key in this map.
Parameters: key  The key for which to find the associated value
Returns:  The value associated with the key, or  null if the key is not present
in the map
•  Set<K> keySet()
This method returns all keys this map.
Returns:  A set of all keys in this map
•  V put(K key, V value)
This method associates a value with a key in this map.
Parameters: key  The lookup key
value  The value to associate with the key
Returns:  The value previously associated with the key, or  null if the key
was not present in the map
Package java.util
Appendix D The Java Library A-33
•  V remove(K key)
This method removes a key and its associated value from this map.
Parameters: key  The lookup key
Returns:  The value previously associated with the key, or  null if the key
was not present in the map
Class  java.util.NoSuchElementException
This exception is thrown if an attempt is made to retrieve a value that does not
exist.
Class  java.util.PriorityQueue<E>
•  PriorityQueue<E>()
This constructs an empty priority queue. The element type  E must implement the
Comparable interface.
•  E remove()
This method removes the smallest element in the priority queue.
Returns:  The removed value
Interface  java.util.Queue<E>
•  E peek()
Gets the element at the head of the queue without removing it.
Returns:  The head element or  null if the queue is empty
Class  java.util.Random
•  Random()
This constructs a new random number generator.
•  double nextDouble()
This method returns the next pseudorandom, uniformly distributed floating-
point number between 0.0 (inclusive) and 1.0 (exclusive) from this random num-
ber generator’s sequence.
Returns:  The next pseudorandom floating-point number
•  int nextInt(int n)
This method returns the next pseudorandom, uniformly distributed integer
between 0 (inclusive) and the speci fied value (exclusive) drawn from this random
number generator’s sequence.
Parameters: n  Number of values to draw from
Returns:  The next pseudorandom integer
Class  java.util.Scanner
•  Scanner(File in)
•  Scanner(InputStream in)
•  Scanner(Reader in)
These construct a scanner that reads from the given file, input stream, or reader.
Parameters: in  The file, input stream, or reader from which to read
Package java.util
A-34 Appendix D The Java Library
•  void close()
This method closes this scanner and releases any associated system resources.
•  boolean hasNext()
•  boolean hasNextDouble()
•  boolean hasNextInt()
•  boolean hasNextLine()
These methods test whether it is possible to read any non-empty string, a
floating-point value, an integer, or a line, as the next item.
Returns:  true if it is possible to read an item of the requested type,  false
otherwise (either because the end of the file has been reached,
or because a number type was tested and the next item is not a
number)
•  String next()
•  double nextDouble()
•  int nextInt()
•  String nextLine()
These methods read the next whitespace-delimited string, floating-point value,
integer, or line.
Returns:  The value that was read
•  Scanner useDelimiter(String pattern)
Sets the pattern for the delimiters between input tokens.
Parameters: pattern  A regular expression for the delimiter pattern
Returns:  This scanner
Interface  java.util.Set<E>
This interface describes a collection that contains no duplicate elements.
Class  java.util.TreeMap<K, V>
•  TreeMap<K, V>()
This constructs an empty tree map. The iterator of a  TreeMap visits the entries in
sorted order.
Class  java.util.TreeSet<E>
•  TreeSet<E>()
This constructs an empty tree set.
Package  java.util.logging
Class  java.util.logging.Level
•  static final int INFO
This value indicates informational logging.
•  static final int OFF
This value indicates logging of no messages.
Package java.util java.util.logging
Appendix D The Java Library A-35
Class  java.util.logging.Logger
•  static Logger getGlobal()
This method gets the global logger. For Java 5 and 6, use  getLogger(“global”)
instead.
Returns:  The global logger that, by default, displays messages with level
INFO or a higher severity on the con sole.
•  void info(String message)
This method logs an informational message.
Parameters: message  The message to log
•  void setLevel(Level aLevel)
This method sets the logging level. Logging messages with a lesser severity than
the current level are ignored.
Parameters: aLevel  The minimum level for logging messages
Package  javax.swing
Class  javax.swing.AbstractButton
•  void addActionListener(ActionListener listener)
This method adds an action listener to the button.
Parameters: listener  The action listener to be added
•  boolean isSelected()
This method returns the selection state of the button.
Returns:  true if the button is selected
•  void setSelected(boolean state)
This method sets the selection state of the button. This method updates the but-
ton but does not trigger an action event.
Parameters: state true to select,  false to deselect
Class  javax.swing.ButtonGroup
•  void add(AbstractButton button)
This method adds the button to the group.
Parameters: button  The button to add
Class  javax.swing.ImageIcon
•  ImageIcon(String filename)
This constructs an image icon from the specified graphics file.
Parameters: filename  A string specifying a file name
Class  javax.swing.JButton
•  JButton(String label)
This constructs a button with the given label.
Parameters: label  The button label
Package java.util.logging Package javax.swing
A-36 Appendix D The Java Library
Class  javax.swing.JCheckBox
•  JCheckBox(String text)
This constructs a check box with the given text, which is initially deselected.
(Use the  set Selected  method to make the box selected; see the  javax.swing.
Abstract Button  class.)
Parameters: text  The text displayed next to the check box
Class  javax.swing.JComboBox
•  JComboBox()
This constructs a combo box with no items.
•  void addItem(Object item)
This method adds an item to the item list of this combo box.
Parameters: item  The item to add
•  Object getSelectedItem()
This method gets the currently selected item of this combo box.
Returns:  The currently selected item
•  boolean isEditable()
This method checks whether the combo box is editable. An editable combo box
allows the user to type into the text field of the combo box.
Returns:  true if the combo box is editable
•  void setEditable(boolean state)
This method is used to make the combo box editable or not.
Parameters: state true to make editable,  false to disable editing
•  void setSelectedItem(Object item)
This method sets the item that is shown in the display area of the combo box as
selected.
Parameters: item The item to be displayed as selected
Class  javax.swing.JComponent
•  protected void paintComponent(Graphics g)
Override this method to paint the surface of a component. Your method needs to
call  super.paintComponent(g) .
Parameters: g  The graphics context used for drawing
•  void setBorder(Border b)
This method sets the border of this component.
Parameters: b  The border to surround this component
•  void setFont(Font f)
Sets the font used for the text in this component.
Parameters: f  A font
Class  javax.swing.JFileChooser
•  JFileChooser()
This constructs a file chooser.
Package javax.swing
Appendix D The Java Library A-37
•  File getSelectedFile()
This method gets the selected file from this file chooser.
Returns:  The selected file
•  int showOpenDialog(Component parent)
This method displays an “Open File” file chooser dialog box.
Parameters: parent  The parent component or  null
Returns:  The return state of this file chooser after it has been closed by
the user: either  APPROVE_OPTION or  CANCEL_OPTION . If  APPROVE_OPTION is
returned, call  get Selected File() on this file chooser to get the file
•  int showSaveDialog(Component parent)
This method displays a “Save File” file chooser dialog box.
Parameters: parent  The parent component or  null
Returns:  The return state of the file chooser after it has been closed by the
user:  either APPROVE_OPTION  or  CANCEL_OPTION
Class  javax.swing.JFrame
•  void setDefaultCloseOperation(int operation)
This method sets the default action for closing the frame.
Parameters: operation  The desired close operation. Choose among
DO_NOTHING_ON_CLOSE ,  HIDE_ON_CLOSE (the default),  DISPOSE_ON_CLOSE ,
or  EXIT_ON_CLOSE
•  void setJMenuBar(JMenuBar mb)
This method sets the menu bar for this frame.
Parameters: mb  The menu bar. If  mb is  null , then the current menu bar is
removed
•  static final int EXIT_ON_CLOSE
This value indicates that when the user closes this frame, the application is to exit.
Class  javax.swing.JLabel
•  JLabel(String text)
•  JLabel(String text, int alignment)
These containers create a  JLabel instance with the specified text and horizontal
alignment.
Parameters: text  The label text to be displayed by the label
alignment  One of  SwingConstants.LEFT ,  SwingConstants.CENTER , or
SwingConstants.RIGHT
Class  javax.swing.JMenu
•  JMenu()
This constructs a menu with no items.
•  JMenuItem add(JMenuItem menuItem)
This method appends a menu item to the end of this menu.
Parameters: menuItem  The menu item to be added
Returns:  The menu item that was added
Package javax.swing
A-38 Appendix D  The Java Library
Class  javax.swing.JMenuBar
•  JMenuBar()
This constructs a menu bar with no menus.
•  JMenu add(JMenu menu)
This method appends a menu to the end of this menu bar.
Parameters:  menu  The menu to be added
Returns:  The menu that was added
Class  javax.swing.JMenuItem
•  JMenuItem(String text)
This constructs a menu item.
Parameters:  text  The text to appear in the menu item
Class  javax.swing.JOptionPane
•  static String showInputDialog(Object prompt)
This method brings up a modal input dialog box, which displays a prompt and
waits for the user to enter an input in a text field, preventing the user from doing
anything else in this program.
Parameters:  prompt  The prompt to display
Returns:  The string that the user typed
•  static void showMessageDialog(Component parent, Object message)
This method brings up a confirmation dialog box that displays a message and
waits for the user to confirm it.
Parameters:  parent  The parent component or  null
message  The message to display
Class  javax.swing.JPanel
This class is a component without decorations. It can be used as an invisible con-
tainer for other components.
Class  javax.swing.JRadioButton
•  JRadioButton(String text)
This constructs a radio button having the given text that is initially deselected.
(Use the  setSelected method to select it; see the  javax.swing.AbstractButton class.)
Parameters:  text  The string displayed next to the radio button
Class  javax.swing.JScrollPane
•  JScrollPane(Component c)
This constructs a scroll pane around the given component.
Parameters:  c  The component that is decorated with scroll bars
Package javax.swing
Appendix D The Java Library A-39
Class  javax.swing.JSlider
•  JSlider(int min, int max, int value)
This constructor creates a horizontal slider using the specified minimum, maxi-
mum, and value.
Parameters: min  The smallest possible slider value
max  The largest possible slider value
value  The initial value of the slider
•  void addChangeListener(ChangeListener listener)
This method adds a change listener to the slider.
Parameters: listener  The change listener to add
•  int getValue()
This method returns the slider’s value.
Returns:  The current value of the slider
Class  javax.swing.JTextArea
•  JTextArea()
This constructs an empty text area.
•  JTextArea(int rows, int columns)
This constructs an empty text area with the specified number of rows and
columns.
Parameters: rows  The number of rows
columns  The number of columns
•  void append(String text)
This method appends text to this text area.
Parameters: text  The text to append
Class  javax.swing.JTextField
•  JTextField()
This constructs an empty text field.
•  JTextField(int columns)
This constructs an empty text field with the specified number of columns.
Parameters: columns  The number of columns
Class  javax.swing.KeyStroke
•  static KeyStroke getKeyStrokeForEvent(KeyEvent event)
Gets a  KeyStroke object describing the key stroke that caused the event.
Parameters: event The key event to be analyzed
Returns:  A  KeyStroke object. Call  toString on this object to get a string
representation such as  "pressed LEFT"
Package javax.swing
A-40 Appendix D The Java Library
Class  javax.swing.Timer
•  Timer(int millis, ActionListener listener)
This constructs a timer that notifies an action listener whenever a time interval has
elapsed.
Parameters: millis  The number of milliseconds between timer notifications
listener  The object to be notified when the time interval has
elapsed
•  void start()
This method starts the timer. Once the timer has started, it begins notifying its
listener.
•  void stop()
This method stops the timer. Once the timer has stopped, it no longer notifies its
listener.
Package  javax.swing.event
Class  javax.swing.event.ChangeEvent
Components such as sliders emit change events when they are manipulated by
the user.
Interface  javax.swing.event.ChangeListener
•  void stateChanged(ChangeEvent e)
This event is called when the event source has changed its state.
Parameters: e  A change event
Package  javax.swing.text
Class  javax.swing.text.JTextComponent
•  String getText()
This method returns the text contained in this text component.
Returns:  The text
•  boolean isEditable()
This method checks whether this text component is editable.
Returns:  true if the component is editable
•  void setEditable(boolean state)
This method is used to make this text component editable or not.
Parameters: state true to make editable,  false to disable editing
•  void setText(String text)
This method sets the text of this text component to the specified text. If the
argument is the empty string, the old text is deleted.
Parameters: text  The new text to be set
Package javax.swing Package javax.swing.event Package javax.swing.text
G-1
Glossary
Abstract class  A class that cannot be instantiated.
Abstract method  A method with a name, parameter variable types, and return type but
without an implementation.
Access specifier  A reserved word that indicates the accessibility of a feature, such as
private  or  public .
Accessor method  A method that accesses an object but does not change it.
Aggregation  The has-a relationship between classes.
Algorithm  An unambiguous, executable, and terminating specification of a way to solve a
problem.
Anonymous class  A class that does not have a name.
Anonymous object  An object that is not stored in a named variable.
API (Application Programming Interface)  A code library for building programs.
API Documentation  Information about each class in the Java library.
Applet  A graphical Java program that executes inside a web browser or applet viewer.
Argument  A value supplied in a method call, or one of the values combined by an operator.
Array  A collection of values of the same type stored in contiguous memory locations, each
of which can be accessed by an integer index.
Array list  A Java class that implements a dynamically-growable array of objects.
Assert  A claim that a certain condition holds in a particular program location.
Assignment  Placing a new value into a variable.
Association  A relationship between classes in which one can navigate from objects of one
class to objects of the other class, usually by following object references.
Asymmetric bounds  Bounds that include the starting index but not the ending index.
Attribute  A named property that an object is responsible for maintaining.
Auto-boxing  Automatically converting a primitive type value into a wrapper type object.
Big-Oh notation  The notation g(n) = O(f(n)), which denotes that the function g grows
at a rate that is bounded by the growth rate of the function f with respect to n. For example,
10n 2  + 100n - 1000 = O(n 2 ).
Binary operator  An operator that takes two arguments, for example + in x + y.
Binary search  A fast algorithm for finding a value in a sorted array. It narrows the search
down to half of the array in every step.
Bit  Binary digit; the smallest unit of information, having two possible values: 0 and 1. A data
element consisting of n bits has 2 n  possible values.
Black-box testing  Testing a method without knowing its implementation.
Block  A group of statements bracketed by  {} .
Body  All statements of a method or block.
Boolean operator  An operator that can be applied to Boolean values. Java has three Bool-
ean operators:  && ,  || , and  ! .
G-2  Glossary
Boolean type  A type with two possible values:  true  and  false .
Boundary test case  A test case involving values that are at the outer boundary of the set of
legal values. For example, if a method is expected to work for all nonnegative integers, then 0
is a boundary test case.
Bounds error  Trying to access an array element that is outside the legal range.
break statement  A statement that terminates a loop or  switch  statement.
Breakpoint  A point in a program, specified in a debugger, at which the debugger stops exe-
cuting the program and lets the user inspect the program state.
Buffer  A temporary storage location for holding values that have been produced (for exam-
ple, characters typed by the user) and are waiting to be consumed (for example, read a line at
a time).
Bug  A programming error.
Byte  A number made up of eight bits. Essentially all currently manufactured computers use
a byte as the smallest unit of storage in memory.
Bytecode  Instructions for the Java virtual machine.
Call stack  The ordered set of all methods that currently have been called but not yet termi-
nated, starting with the current method and ending with  main .
Callback  A mechanism for specifying a block of code so it can be executed at a later time.
Case sensitive  Distinguishing upper- and lowercase characters.
Cast  Explicitly converting a value from one type to a different type. For example, the cast
from a floating-point number  x  to an integer is expressed in Java by the cast notation  (int)x .
catch clause  A part of a  try  block that is executed when a matching exception is thrown by
any statement in the  try  block.
Central processing unit (CPU)  The part of a computer that executes the machine
instructions.
Character  A single letter, digit, or symbol.
Checked exception  An exception that the compiler checks. All checked exceptions must
be declared or caught.
Class  A programmer-defined data type.
Code coverage  A measure of the amount of source code that has been executed during
 testing.
Cohesive  A class is cohesive if its features support a single abstraction.
Collection  A data structure that provides a mechanism for adding, removing, and locating
elements.
Collaborator  A class on which another class depends.
Command line  The line the user types to start a program in DOS or UNIX or a command
window in Windows. It consists of the program name followed by any necessary arguments.
Comment  An explanation to help the human reader understand a section of a program;
ignored by the compiler.
Compiler  A program that translates code in a high-level language (such as Java) to machine
instructions (such as bytecode for the Java virtual machine).
Compile-time error  An error that is detected when a program is compiled.
Component  See User-interface component
 Glossary G-3
Composition  An aggregation relationship where the aggregated objects do not have an
existence independent of the containing object.
Computer program  A sequence of instructions that is executed by a computer.
Concatenation  Placing one string after another to form a new string.
Concrete class  A class that can be instantiated.
Console program  A Java program that does not have a graphical window. A console pro-
gram reads input from the keyboard and writes output to the terminal screen.
Constant  A value that cannot be changed by a program. In Java, constants are defined with
the reserved word  final .
Constructor  A sequence of statements for initializing a newly instantiated object.
Construction  Setting a newly allocated object to an initial state.
Container  A user-interface component that can hold other components and present them
together to the user. Also, a data structure, such as a list, that can hold a collection of objects
and present them individually to a program.
Content pane  The part of a Swing frame that holds the user-interface components of the
frame.
Coupling  The degree to which classes are related to each other by dependency.
CRC card  An index card representing a class that lists its responsibilities and collaborating
classes.
De Morgan’s Law  A law about logical operations that describes how to negate expressions
formed with and and or operations.
Debugger  A program that lets a user run another program one or a few steps at a time, stop
execution, and inspect the variables in order to analyze it for bugs.
Dependency  The uses relationship between classes, in which one class needs services pro-
vided by another class.
Directory  A structure on a disk that can hold files or other directories; also called a folder.
Documentation comment  A comment in a source file that can be automatically extracted
into the program doc umentation by a program such as  javadoc .
Dot notation  The notation object.method(arguments) or object.variable used to invoke a
method or access a vari able.
Doubly-linked list  A linked list in which each link has a reference to both its predecessor
and successor links.
Dynamic method lookup  Selecting a method to be invoked at run time. In Java, dynamic
method lookup consid ers the class of the implicit parameter object to select the appropriate
method.
Editor  A program for writing and modifying text files.
Encapsulation  The hiding of implementation details.
Enumeration type  A type with a finite number of values, each of which has its own sym-
bolic name.
Escape character  A character in text that is not taken literally but has a special meaning
when combined with the character or characters that follow it. The  \  character is an escape
character in Java strings.
Escape sequence  A sequence of characters that starts with an escape character, such as  \n 
or  \" .
G-4  Glossary
Event  See  User-interface event
Event class  A class that contains information about an event, such as its source.
Event adapter  A class that implements an event listener interface by defining all methods
to do nothing.
Event handler  A method that is executed when an event occurs.
Event listener  An object that is notified by an event source when an event occurs.
Event source  An object that can notify other classes of events.
Exception  A class that signals a condition that prevents the program from continuing nor-
mally. When such a con dition occurs, an object of the exception class is thrown.
Exception handler  A sequence of statements that is given control when an exception of a
particular type has been thrown and caught.
Explicit parameter  A parameter of a method other than the object on which the method is
invoked.
Expression  A syntactical construct that is made up of constants, variables, method calls,
and the operators combining them.
Extension  The last part of a file name, which specifies the file type. For example, the
extension .java  denotes a Java file.
Fibonacci numbers  The sequence of numbers 1, 1, 2, 3, 5, 8, 13, . . . , in which every term is
the sum of its two predecessors.
File  A sequence of bytes that is stored on disk.
finally clause  A part of a  try  block that is executed no matter how the  try  block is exited.
Flag  See Boolean type
Floating-point number  A number that can have a fractional part.
Folder  See Directory
Font  A set of character shapes in a particular style and size.
Frame  A window with a border and a title bar.
Garbage collection  Automatic reclamation of memory occupied by objects that are no
longer referenced.
Generic class  A class with one or more type parameters.
Graphics context  A class through which a programmer can cause shapes to appear on a
window or off-screen bit map.
grep  The “global regular expression print” search program, useful for finding all strings
matching a pattern in a set of files.
GUI (Graphical User Interface)  A user interface in which the user supplies inputs through
graphical components such as buttons, menus, and text fields.
Hard disk  A device that stores information on rotating platters with magnetic coating.
Hardware  The physical equipment for a computer or another device.
Hash code  A value that is computed by a hash function.
Hash collision  Two different objects for which a hash function computes identical values.
Hash function  A function that computes an integer value from an object in such a way that
different objects are likely to yield different values.
 Glossary G-5
Hash table  A data structure in which elements are mapped to array positions according to
their hash function val ues.
Heapsort algorithm  A sorting algorithm that inserts the values to be sorted into a heap.
High-level programming language  A programming language that provides an abstract
view of a computer and allows programmers to focus on their problem domain.
IDE (Integrated Development Environment)  A programming environment that
includes an editor, compiler, and debugger.
Immutable class  A class without a mutator method.
Implementing an interface  Implementing a class that defines all methods specified in the
interface.
Implicit parameter  The object on which a method is invoked. For example, in the call
x.f(y) , the object  x  is the implicit parameter of the method  f .
Importing a class or package  Indicating the intention of referring to a class, or all classes
in a package, by the simple name rather than the qualified name.
Inheritance  The is-a relationship between a more general superclass and a more specialized
subclass.
Initialize  Set a variable to a well-defined value when it is created.
Inner class  A class that is defined inside another class.
Instance method  A method with an implicit parameter; that is, a method that is invoked
on an instance of a class.
Instance of a class  An object whose type is that class.
Instance variable  A variable defined in a class for which every object of the class has its
own value.
Instantiation of a class  Construction of an object of that class.
Integer  A number that cannot have a fractional part.
Integer division  Taking the quotient of two integers and discarding the remainder. In Java
the  /  symbol denotes integer division if both arguments are integers. For example,  11/4  is  2 ,
not  2.75 .
Interface type  A type with no instance variables, only abstract methods and constants.
Iterator  An object that can inspect all elements in a container such as a linked list.
javadoc  The documentation generator in the Java SDK. It extracts documentation com-
ments from Java source files and produces a set of linked HTML files.
Lexicographic ordering  Ordering strings in the same order as in a dictionary, by skipping
all matching charac ters and comparing the first non-matching characters of both strings. For
example, “orbit” comes before “orchid” in lexicographic ordering. Note that in Java, unlike a
dictionary, the ordering is case sensitive:  Z  comes before  a .
Library  A set of precompiled classes that can be included in programs.
Linear search  Searching a container (such as an array or list) for an object by inspecting
each element in turn.
Linked list  A data structure that can hold an arbitrary number of objects, each of which is
stored in a link object, which contains a pointer to the next link.
Literal  A notation for a fixed value in a program, such as –2, 3.14, 6.02214115E23,  "Harry" ,
or  'H' .
G-6  Glossary
Local variable  A variable whose scope is a block.
Logging  Sending messages that trace the progress of a program to a file or window.
Logical operator  See Boolean operator .
Logic error  An error in a syntactically correct program that causes it to act differently from
its specification. (A form of run-time error.)
Loop  A sequence of instructions that is executed repeatedly.
Loop and a half  A loop whose termination decision is neither at the beginning nor at the
end.
Machine code  Instructions that can be executed directly by the CPU.
Magic number  A number that appears in a program without explanation.
main method  The method that is first called when a Java application executes.
Map  A data structure that keeps associations between key and value objects.
Memory location  A value that specifies the location of data in computer memory.
Merge sort  A sorting algorithm that first sorts two halves of a data structure and then
merges the sorted subarrays together.
Method  A sequence of statements that has a name, may have parameter variables, and may
return a value. A method can be invoked any number of times, with different values for its
parameter variables.
Modifier  A reserved word that indicates the accessibility of a feature, such as   private  or
public .
Modulus  The  %  operator that computes the remainder of an integer division.
Mock object  An object that is used during program testing, replacing another object and
providing similar behav ior. Usually, the mock object is simpler to implement or provides bet-
ter support for testing.
Mutator method  A method that changes the state of an object.
Mutual recursion  Cooperating methods that call each other.
Name clash  Accidentally using the same name to denote two program features in a way
that cannot be resolved by the compiler.
Nested loop  A loop that is contained in another loop.
Networks  An interconnected system of computers and other devices.
new operator  An operator that allocates new objects.
Newline  The  '\n' character, which indicates the end of a line.
No-argument constructor  A constructor that takes no arguments.
Null reference  A reference that does not refer to any object.
Number literal  A fixed value in a program this is explicitly written as a number, such as –2
or 6.02214115E23.
Object  A value of a class type.
Object-oriented programming  Designing a program by discovering objects, their
prop erties, and their relationships.
Object reference  A value that denotes the location of an object in memory. In Java, a
variable whose type is a class contains a reference to an object of that class.
 Glossary G-7
Off-by-one error  A common programming error in which a value is one larger or smaller
than it should be.
Opening a file  Preparing a file for reading or writing.
Operating system  The software that launches application programs and provides services
(such as a file system) for those programs.
Operator  A symbol denoting a mathematical or logical operation, such as  +  or  && .
Operator associativity  The rule that governs in which order operators of the same
precedence are executed. For example, in Java the  - operator is left-associative because
a-b-c  is interpreted as  (a-b)-c , and  =  is right-asso ciative because  a=b=c  is interpreted
as  a=(b=c) .
Operator precedence  The rule that governs which operator is evaluated first. For exam-
ple, in Java the  &&  opera tor has a higher precedence than the  ||  operator. Hence  a||b&&c  is
interpreted as  a||(b&&c) . (See Appendix B.)
Overloading  Giving more than one meaning to a method name.
Overriding  Redefining a method in a subclass.
Package  A collection of related classes. The  import  statement is used to access one or more
classes in a package.
Panel  A user-interface component with no visual appearance. It can be used to group other
components.
Parallel arrays  Arrays of the same length, in which corresponding elements are logically
related.
Parameter passing  Specifying expressions to be arguments for a method when it is called.
Parameter variable  A variable of a method that is initialized with a value when the method
is called.
Partially filled array  An array that is not filled to capacity, together with a companion
variable that indicates the number of elements actually stored.
Permutation  A rearrangement of a set of values.
Polymorphism  Selecting a method among several methods that have the same name on the
basis of the actual types of the implicit parameters.
Primitive type  In Java, a number type or  boolean .
Priority queue  An abstract data type that enables efficient insertion of elements and effi-
cient removal of the smallest element.
Programming  The act of designing and implementing computer programs.
Project  A collection of source files and their dependencies.
Prompt  A string that tells the user to provide input.
Pseudocode  A high-level description of the actions of a program or algorithm, using a mix-
ture of English and informal programming language syntax.
Pseudorandom number  A number that appears to be random but is generated by a math-
ematical formula.
Public interface  The features (methods, variables, and nested types) of a class that are
accessible to all clients.
Queue  A collection of items with “first in, first out” retrieval.
G-8  Glossary
Quicksort  A generally fast sorting algorithm that picks an element, called the pivot, parti-
tions the sequence into the elements smaller than the pivot and those larger than the pivot, and
then recursively sorts the subsequences.
Reader  In the Java input/output library, a class from which to read characters.
Recursion  A method for computing a result by decomposing the inputs into simpler values
and applying the same method to them.
Recursive method  A method that can call itself with simpler values. It must handle the
simplest values without calling itself.
Redirection  Linking the input or output of a program to a file instead of the keyboard or
display.
Reference  See Object reference
Regression testing  Keeping old test cases and testing every revision of a program against
them.
Regular expression  A string that defines a set of matching strings according to their con-
tent. Each part of a reg ular expression can be a specific required character; one of a set of per-
mitted characters such as  [abc] , which can be a range such as  [a-z] ; any character not in a set
of forbidden characters, such as  [^0-9] ; a repetition of one or more matches, such as  [0-9]+ , or
zero or more, such as  [ACGT] ; one of a set of alternatives, such as  and|et|und ; or various other
possibilities. For example,  “[A-Za-z][0-9]+”  matches  “Cloud9”  or  “007”  but not  “Jack” .
Relational operator  An operator that compares two values, yielding a Boolean result.
Reserved word  A word that has a special meaning in a programming language and there-
fore cannot be used as a name by the programmer.
Return value  The value returned by a method through a  return  statement.
Reverse Polish notation  A style of writing expressions in which the operators are written
following the oper ands, such as 234*+ for 2+3*4 .
Roundoff error  An error introduced by the fact that the computer can store only a finite
number of digits of a floating-point number.
Run-time error  An error in a syntactically correct program that causes it to act differently
from its specification.
Run-time stack  The data structure that stores the local variables of all called methods as a
program runs.
Scope  The part of a program in which a variable is defined.
Secondary storage  Storage that persists without electricity, e.g., a hard disk.
Selection sort  A sorting algorithm in which the smallest element is repeatedly found and
removed until no ele ments remain.
Sentinel  A value in input that is not to be used as an actual input value but to signal the end
of input.
Sequential search  See Linear search
Set  An unordered collection that allows efficient addition, location, and removal of elements.
Shadowing  Hiding a variable by defining another one with the same name.
Shallow copy  Copying only the reference to an object.
Shell script  A file that contains commands for running programs and manipulating files.
Typing the name of the shell script file on the command line causes those commands to be
executed.
 Glossary G-9
Shell window  A window for interacting with an operating system through textual
commands.
Short-circuit evaluation  Evaluating only a part of an expression if the remainder cannot
change the result.
Side effect  An effect of a method other than returning a value.
Software  The intangible instructions and data that are necessary for operating a computer
or another device.
Source code  Instructions in a programming language that need to be translated before exe-
cution on a computer.
Source file  A file containing instructions in a programming language such as Java.
Stack  A data structure with “last-in, first-out” retrieval. Elements can be added and
removed only at one position, called the top of the stack.
Stack trace  A printout of the call stack, listing all currently pending method calls.
State  The current value of an object, which is determined by the cumulative action of all
methods that were invoked on it.
State diagram  A diagram that depicts state transitions and their causes.
Statement  A syntactical unit in a program. In Java a statement is either a simple statement,
a compound statement, or a block.
Static method  A method with no implicit parameter.
Static variable  A variable defined in a class that has only one value for the whole class, and
which can be accessed and changed by any method of that class.
String  A sequence of characters.
Subclass  A class that inherits variables and methods from a superclass but may also add
instance variables, add methods, or redefine methods.
Substitution principle  The principle that a subclass object can be used in place of any
superclass object.
Superclass  A general class from which a more specialized class (a subclass) inherits.
Swing  A Java toolkit for implementing graphical user interfaces.
Symmetric bounds  Bounds that include the starting index and the ending index.
Syntax  Rules that define how to form instructions in a particular programming language.
Syntax diagram  A graphical representation of grammar rules.
Syntax error  An instruction that does not follow the programming language rules and is
rejected by the compiler. (A form of compile-time error.)
Tab character  The  '\t'  character, which advances the next character on the line to the next
one of a set of fixed positions known as tab stops.
Ternary operator  An operator with three arguments. Java has one ternary operator,
a?b:c .
Test suite  A set of test cases for a program.
Text file  A file in which values are stored in their text representation.
Throw an exception  Indicate an abnormal condition by terminating the normal control
flow of a program and transferring control to a matching  catch  clause.
throws clause  Indicates the types of the checked exceptions that a method may throw.
G-10  Glossary
Token  A sequence of consecutive characters from an input source that belongs together for
the purpose of analyz ing the input. For example, a token can be a sequence of characters other
than white space.
Trace message  A message that is printed during a program run for debugging purposes.
try block  A block of statements that contains exception processing clauses. A  try  block
contains at least one  catch  or  finally  clause.
Turing machine  A very simple model of computation that is used in theoretical computer
science to explore computability of problems.
Two-dimensional array  A tabular arrangement of elements in which an element is speci-
fied by a row and a col umn index.
Type  A named set of values and the operations that can be carried out with them.
Type parameter  A parameter in a generic class or method that can be replaced with an
actual type.
Unary operator  An operator with one argument.
Unchecked exception  An exception that the compiler doesn’t check.
Unicode  A standard code that assigns code values consisting of two bytes to characters used
in scripts around the world. Java stores all characters as their Unicode values.
Unified Modeling Language (UML)  A notation for specifying, visualizing, constructing,
and documenting the artifacts of software systems.
Uninitialized variable  A variable that has not been set to a particular value. In Java, using
an uninitialized local variable is a syntax error.
Unit testing  Testing a method by itself, isolated from the remainder of the program.
URL (Uniform Resource Locator)  A pointer to an information resource (such as a web
page or an image) on the World Wide Web.
User-interface component  A building block for a graphical user interface, such as a but-
ton or a text field. User-interface components are used to present information to the user and
allow the user to enter information to the pro gram.
User-interface event  A notification to a program that a user action such as a key press,
mouse move, or menu selection has occurred.
Variable  A symbol in a program that identifies a storage location that can hold different
values.
Virtual machine  A program that simulates a CPU that can be implemented efficiently on
a variety of actual machines. A given program in Java bytecode can be executed by any Java
virtual machine, regardless of which CPU is used to run the virtual machine itself.
void  A reserved word indicating no type or an unknown type.
Walkthrough  A step-by-step manual simulation of a computer program.
White-box testing  Testing methods by taking their implementations into account, in con-
trast to black-box test ing; for example, by selecting boundary test cases and ensuring that all
branches of the code are covered by some test case.
White space  Any sequence of only space, tab, and newline characters.
Wrapper class  A class that contains a primitive type value, such as  Integer .
Writer  In the Java input/output library, a class to which characters are to be sent.
I-1
Symbols
; (semicolon)
ending  if statements, 184–185
ending method statements, 12
omitting, 14
+ (plus sign)
addition operator, 38
concatenation operator, 157
++ (plus signs), increment, 140
. (dot), name syntax, 403–404
( ) (parentheses), in arithmetic operations,
139, 144–145
{ } (braces)
enclosing blocks of code, 205–206
lining up code, 184
&& (ampersands), and operator, 213–214, 216,
217
* (asterisk), multiplication operator, 38, 139
\ (backslash), escape sequence, 158, 517
$ (dollar sign), in variable names, 39
= (equal sign), assignment operator, 40–41
== (equal signs), relational operator
comparing object references, 190
comparing strings, 189, 192–193
syntax, 187t
testing for  null , 191
! (exclamation point), not operator, 215
!= (exclamation point, equal), relational
operator, 187t
> (greater than), relational operator, 187t
>= (greater than, equal), relational operator,
187t
< (less than), relational operator, 187t
<= (less than, equal), relational operator, 187t
- (minus sign), subtraction operator, 38
-- (minus signs), decrement, 140
% (percent sign), modulus, 140–141
" (quotation marks), string delimiters, 12
' (single quote), character literal delimiter,
158
/ (slash), division operator, 38, 139
/*...*/ (slash asterisk...), comment
delimiters, 40
/** (slash, asterisks), comment indicator, 90
// (slashes), comment delimiter, 40
_ (underscore), in variable names, 39
|| (vertical bars), or operator, 213–214, 216,
217
< > (brackets), diamond syntax, 356
A
abs method,  java.lang.Math class, 142t, A-22
Abstract classes, 441–442
Access specifiers, 83
Accessor methods, 50–51, 383–384
acos method,  java.lang.Math class, A-23
actionPerformed method,  java.awt.event.
ActionListener interface, 491, 493, A-15
Actor classes, 380
add method
java.awt.Container class, A-13
java.math.BigDecimal class, 138, A-27
java.math.BigInteger class, 138, A-27
java.util.ArrayList<E> class, 348, A-28
java.util.Collection<E> interface, A-29
java.util.ListIterator<E> interface, A-32
javax.swing.ButtonGroup class, A-35
javax.swing.JMenu class, A-37
javax.swing.JMenuBar class, A-38
addActionListener method,  javax.swing.
AbstractButton class, 486–492, A-35
addChangeListener method,  javax.swing.
JSlider class, A-39
addFirst method,  java.util.LinkedList<E>
class, A-31
addItem method,  javax.swing.JComboBox class,
A-36
addKeyListener method,  java.awt.Component
class, 501, A-12
addLast method,  java.util.LinkedList<E>
class, A-31
addMouseListener method,  java.awt.Component
class, 497, A-12
Address.java class, 579–580
Adleman, Leonard, 533
Aggregation, 564–565, 568–569
Algorithms. See also Loops, common
algorithms.
characteristics of, 17
combining and adapting, 331–336
definition, 17
Index
I-2  Index
Algorithms (continued)
developing, 336–339
evolving into programs, 19–20
example, 18
executable, 17
full code examples, 333, 339
pseudocode for, 19–20
reusing, 331–336
terminating, 17
for tiling a floor, 22–23
unambiguous, 17
Altair 8800, 406
Ampersands ( && ), and operator, 213–214,
216, 217
Andreesen, Marc, 454
Anonymous classes, 482–483
API (application programming interface),
52–55
append method,  javax.swing.JTextArea class,
A-39
Apple II, 406
Applets, definition, 6
Application development, examples. See
“Hello, World” program; Printing
invoices (sample program).
Application programming interface (API),
52–55
AreaMeasurer.java class, 480
Arguments
definition, 13
method, 44–45
object construction, 49
Ariane rocket incident, 544
Arithmetic operations. See also Data types;
specific operations.
combining with assignment, 145
computation overflow, 133
hand tracing, 154–156
integer division, 140–141, 144
modulus, 140–141
powers, 141–142
remainders, 140–141
roots, 141–142
rounding, 133–134
roundoff errors, 134
unintended integer division, 144
Arithmetic operators. See also specific
operators.
* (asterisk), multiplication, 139
- (minus sign), subtraction, 38
-- (minus signs), decrement, 140
% (percent sign), modulus, 140–141
+ (plus sign), addition, 38
++ (plus signs), increment, 140
/ (slash), division, 139
definition, 139
expressions, 139
parentheses, 139, 144–145
precedence, 139
ARPANET, 454
Array lists. See also  java.util.ArrayList<E>
class.
versus arrays, 354, 355t
auto-boxing, 351
constructor, 348
converting from array algorithms, 352
copying, 350
declaring, 348–349
definition, 347
diamond syntax, 356
enhanced  for loop, 349–350
full code example, 353
generic class, 348
inserting elements, 352
iterating through, 349–350
overview, 348–349
removing matches, 352–353
size, determining, 356
syntax, 347
type parameter, 348
wrapper classes, 351, 351t
ArrayList method,  java.util.ArrayList<E>
class, A-28
Arrays.
versus array lists, 354, 355t
bounds errors, 314, 318
companion variable, 316
current size, 316
declaring, 312–315, 315t
definition, 312
fixed length, 315
full code example, 317
How To example, 334–336
initialization, 312–314
iterating through, 321–322
length, determining, 316–317
matrices, 340–346
with methods, 316
multidimensional, 347
of objects, 318
overview, 312–314
parallel, 318–319
partially filled, 316–317
printing, 344–345
printing element separators, 330
references, 315
 Index I-3
sequences of related values, 318
size, determining, 356
sorting, 331
specifying elements of, 312–314
syntax, 313
underestimating data size, 331
unfilled, 318
uninitialized, 318
variable row lengths, 345–346
world population table (Worked
Example), 345
Arrays, common algorithms
animation, 325
averaging elements, 323
converting to array lists, 352
copying arrays, 327–328
element position, finding, 324
element separators, 323–324
filling, 322
growing arrays, 327–328
inserting elements, 325
maximum value, finding, 323
minimum value, finding, 323
printing element separators, 330
reading input, 328–330
removing elements, 324–325
rolling dice (Worked Example), 336
searching for values, 324
summing elements, 323
swapping elements, 326
Arrays, two-dimensional
accessing elements, 341–342
accessing rows and columns, 342–343
animation, 343
declaring, 340–341
definition, 340
locating neighboring elements, 342
syntax, 341
Artificial intelligence, 221
ASCII encoding, 518–519
asin method,  java.lang.Math class, A-23
Assertions, 543–544
Assignment
= (equal sign), assignment operator, 40–41
changing variable values, 40–41
combining with arithmetic operations,
145
versus declaration, 42–43
full code example, 41
syntax, 41
Association, 568–569
Associativity, Java operators, A-5
Asterisk ( * ), multiplication operator, 38, 139
Asymmetric bounds, 260
atan method,  java.lang.Math class, A-23
atan2 method,  java.lang.Math class, A-23
ATM simulation (Worked Example), 581
Attributes, UML, 567
Auto-boxing, 351
Averages
array elements, 323
computing with loops, 272
B
Backing up files, 11
Backslash ( \ ), escape sequence, 158, 517
BankAccount.java class, 96–97
BankAccountTester.java class, 103
Batch files, 358–359
Berners-Lee, Tim, 454
BigDecimal type, 138
BigInteger type, 138
binarySearch method
java.util.Arrays class, A-29
java.util.Collections class, A-30
Black-box testing, 210
Blocks of code
definition, 205
enclosing with braces, 205–206
BlueJ environment, 56–57
Booch, Grady, 382
boolean data type, 132t
Boolean method,  java.lang.Boolean class, A-20
Boolean operators
&& (ampersands), and operator, 213–214,
216, 217
! (exclamation point), not operator, 215
|| (vertical bars), or operator, 213–214,
216, 217
De Morgan’s Law, 217
full code example, 214
inverting conditions, 215
precedence, 214
short-circuit evaluation of, 217
Boolean variables, in loops, 266
booleanValue method,  java.lang.Boolean class,
A-20
BorderLayout method,  java.awt.BorderLayout
class, A-11–12
Boundary test cases, 210–211
Bounds errors, arrays, 314, 318
Bounds for loops, choosing, 260
I-4  Index
Braces ( { } )
enclosing blocks of code, 205–206
lining up code, 184
Brackets ( < > ), diamond syntax, 356
break statement, 267–268
Breakpoints, 287–289
Buffer overrun attacks, 320
Bugs, historical first, 291. See also
Debuggers; Debugging.
Button-press events, 491
Buttons (user interface), 490–492
ButtonViewer.java class, 487
byte data type, 132t
C
Calendar programs, calculating elapsed days
(Worked Example), 57
Callbacks, 477–481
Call-by-reference, 386–390
Call-by-value, 386–390
Calling methods, 34–35
Camel case, 39
CarComponent.java class, 114
Car.java class, 114–115
Cars
autonomous vehicles, 221
drawing, 112–115
CarViewer.java class, 115
Case sensitivity
constants, 135
definition, 9
errors caused by, 16
misspelling words, 16
variable names, 39
CashRegister.java class, 136–137
CashRegisterTester.java class, 137–138
cast operator
converting floating-point numbers to
integer, 142–143
full code example, 142
syntax, 143
Casting from interfaces to classes, 472–473
catch clause, 536–537, 542–543
Catching exceptions, 536–537
ceil method,  java.lang.Math class, 142t, A-23
Central processing unit (CPU), 3
char data type, 132t
Character encodings, 518–519
Character patterns. See Regular expressions.
Characters. See also Strings.
classifying, 520–521
definition, 156, 158
international alphabets, 163
reading, 520
reading from a string, 522
versus strings, 158–159
Unicode, 158–159, 163
charAt method,  java.lang.String class, 158
Checked exceptions, 537–539
ChoiceQuestion.java class, 433–434
Circles, drawing, 66–67
Class files, 10
Class relationships
aggregation, 564–565, 568–569
association, 568–569
composition, 568–569
coupling, 563–564
dependency, 563–564
full code example, 565
“has a.” See Aggregation.
inheritance, 565–566
“is a.” See Inheritance.
“knows about.” See Dependencies,
classes.
multiplicities, 568–569
recording, sample program, 572–573
Classes. See also Subclasses; Superclasses;
specific classes.
abstract, 441–442
actors, 380
anonymous, 482–483
candidates for, 561
cohesion, 381–382, 385
collaborators, 562
concrete, 441–442
consistency, 385
CRC (class-responsibility-collaboration)
method, 561–563, 566–567
declaring, 12
definition, 12, 35–36
dependencies, 382–383, 385
discovering, 380–381, 561–563
for drawing shapes, 112–116
extending. See Inheritance.
final, 442
identifying, 560–561
immutable, 383–384
importing from packages, 54
inner, 481–482
naming conventions, 39
nouns as, 560
public. See Public interfaces, classes.
 Index I-5
public interfaces, 43–44
testing, 56
utility, 380
Classes, common patterns
collecting values, 391–392
counters, 391
full code example, 394
managing object properties, 392
modeling moving objects, 393–395
modeling objects with distinct states,
392–393
summing totals, 390–391
Classes, implementing
constructors, 94–95
How To, 98–101
instance variables, 93
for a menu (Worked Example), 101
methods, 95–97
Class-responsibility-collaboration (CRC)
cards. See CRC (class-responsibility-
collaboration) cards.
ClickListener.java class, 486
clone method,  java.lang.Object class, A-25
close method
java.io.InputStream class, A-19
java.io.OutputStream class, A-19
java.io.PrintStream class, A-19
java.util.Scanner class, A-34
close method,  java.io.PrintWriter class
automatic invocation, 544
closing text files, 515–516
description, A-19
Code coverage, 210
Cohesion
classes, 381–382, 385
methods, 381–382, 385
Collaborators, 562
Collecting values, common class patterns,
391–392
Color method,  java.awt.Color class, A-12
Colors, 68, 68t
Command line arguments, 527–529
Comments
/*...*/ (slash asterisk...), delimiters, 40
// (slashes), delimiter, 40
converting to documentation. See  javadoc
program.
definition, 40
public interfaces to classes, 89–92
on variables, 40
Companion variable, 316
compare method,  java.util.Comparator<T>
interface, A-30
compareTo method
comparing objects, 473–474
comparing strings, 189
java.lang.Comparable<T> interface, A-21
java.lang.String class, A-25
Comparing
adjacent values with loops, 275–276
floating-point numbers, 188
full code example, 191
lexicographic ordering, 189
object contents, 190, 473–474. See also
compareTo method;  equals method.
object references, 190
rectangles, 190
relational operators, 186–188, 216
strings, 188–189, 192–193
testing for  null , 190–191
Compilation process, 10
animation, 10
Compilers, 6
Compile-time errors, 15
Composition, 568–569
Computation overflow, 133
Computer programs, definition, 2. See
also Java programs; Programming;
Software.
Computer viruses, 320
Computers
anatomy of, 3–5
common uses for, 5
description, 2
history of. See History of computers.
Concatenating strings, 157
Concrete classes, 441–442
Conditional operators, 185
Consistency, classes and methods, 385
Console input, 157
Constants
case sensitivity, 135
declaring, 136
definition, 134
interface types, 470
naming conventions, 135
syntax, 136
constants reserved word, 470
Constructing objects. See Object
construction.
Constructors
calling one from another, 112
I-6  Index
Constructors (continued)
declaring as  void , 92–93
implementing classes, 94–95
versus methods, 87
naming, 88
no-argument, 88
specifying, 87–88
specifying for public interfaces, 87–88
Containers, as event listeners, 493
contains method,  java.util.Collection<E>
interface, A-29
continue statements, 267–268
Converting. See also  cast operator.
array lists from array algorithms, 352
classes to interface variables, 471
comments to documentation, 90–92
floating-point numbers to integer,
142–143
integers to floating-point, 142–143
interface variables from classes, 471
numbers from strings, 522–523
strings from objects. See  toString method.
strings to integers. See  parseInt method.
strings to numbers, 162, 522–523. See also
parseInt method.
Copying. See also  java.lang.Cloneable
interface.
array lists, 350
arrays, 327–328
files. See Backing up files.
objects, 475–477. See also  clone method.
copyOf method,  java.util.Arrays class,
327–328, A-29
cos method,  java.lang.Math class, 142t, A-23
Count-controlled loops, 254
Counter.java class, 84, 85–86
Counters
common class patterns, 391
in loops, 272–273
Counting loop iterations, 260
CountryValue.java class, 531–532
Coupling, 563–564
CPU (central processing unit), 3
CRC (class-responsibility-collaboration)
cards
discovering classes, 570–572
in program design, 566–567
sample program, 561–563
Credit card processing (Worked Example),
279
currentTimeMillis method,  java.lang.System
class, A-26
CYC project, 221
D
Dangling  else problem, 204–205
DARPA, 454
DARPA Urban Challenge, 221
Data types. See also Arithmetic operations;
specific types.
boolean , 132t
byte , 132t
char , 132t
double , 132t, 133–134
float , 132t
int , 132t
long , 132t, 133
number literals, 133
numbers with fractions. See Double data
type; Floating-point numbers.
numbers without fractions. See  int type.
short , 132t
testing for, 451–453. See also  instanceof
operator.
DataAnalyzer.java class, 545–546
Databases, privacy issues, 580
Data.java class, 468, 480
DataSetReader.java class, 547–549
Date method,  java.util.Date class, A-30
De Morgan, Augustus, 217
De Morgan’s Law, 217
Debuggers
breakpoints, 287–289
definition, 286
inspecting variables, 287–289
overview, 286–289
single stepping, 287–289
stepping into/over, 288
Debugging code
the first bug, 291
How To, 289–290
Worked Example, 291
Decisions. See also Comparing;  if
statements.
Boolean operations, 213–218
conditional operators, 185
Declaring
array lists, 348–349
arrays, 312–315, 315t
classes, 12
constants, 136
 Index I-7
instance variables, 82–83, 108
methods, 47
variables, 36–38, 42–43
Definite loops, 254
Denver’s luggage handling system, 195
Dependencies
classes, 382–383, 385, 563–564
methods, 382–383, 385
Dialog boxes
file choosing, 517–518
full code example, 518
input, 162–163
output, 162–163
showInputDialog method,  javax.swing.
JOptionPane class, A-38
showMessageDialog method,  javax.swing.
JOptionPane class, A-38
showOpenDialog method,  javax.swing.
JFileChooser class, A-37
showSaveDialog method,  javax.swing.
JFileChooser class, A-37
Diamond syntax, 356
Die.java class, 283–284
DieSimulator.java class, 284
Dijkstra, Edsger, 210
Dimension method,  java.awt.Dimension class,
A-13
Directories. See Folders.
Discovering classes, 380–381, 561–563,
570–572
divide method,  java.math.BigInteger class,
A-27
“Division by zero” errors, 15
do loops, 262–263
Documentation
API (application programming interface),
52–55
converting comments to. See  javadoc
program.
online help, 55
Documenting, methods, 573–575
Dollar sign ( $ ), in variable names, 39
Dongles, 253
Dot (.), name syntax, 403–404
double data type
definition, 132t
for financial calculations, 134
overflow, 133
precision, 133
Double method,  java.lang.Double class, A-21
doubleValue method,  java.lang.Double class,
A-21
Dr. Java environment, 56
draw method,  java.awt.Graphics2D class,
64–65, A-14
Drawing. See also Graphical applications;
specific shapes.
a car, 112–115
circles, 66–67
colors, 68
on a component, 62–65
ellipses, 66–67
a face, 69–70
fills, 68
a flag (How To), 116–119
graphical shapes (How To), 116–119
lines, 67
rectangles, 61–65
shape classes, 112–116
drawLine method,  java.awt.Graphics class,
A-13
drawString method,  java.awt.Graphics2D class,
67, A-14
Dynamic method lookup, 438
E
E constant,  java.lang.Math class, 135
Earthquake descriptions
full code example, 198
Loma Prieta quake, 196
Richter scale, 196t
Editing pictures (Worked Example), 57
Editors, definition, 8
Electronic voting machines, 104
ElevatorSimulation2.java class, 219–220
ElevatorSimulation.java , 182–183
Ellipse2D.Double method,  java.awt.geom.
Ellipse2D.Double class, A-16
Ellipses, drawing, 66–67
else statements, dangling  else problem,
204–205
Empty string versus null reference, 191
Empty strings, 156
EmptyFrameViewer.java class, 62
Encapsulation, instance variables, 84–86
Encryption algorithms, 533
Enhanced  for loop
array lists, 349–350
arrays, 321–322
I-8  Index
ENIAC (electronic numerical integrator and
computer), 5
Enumeration types, 206–207.
EOFException method,  java.io.EOFException ,
A-18
Equal sign ( = ), assignment operator, 40–41
Equal signs ( == ), relational operator
comparing object references, 190
comparing strings, 189, 192–193
syntax, 187t
testing for  null , 191
equals method
comparing objects, 450–451
inheritance, 454
java.awt.Rectangle class, 190
java.lang.Object class, A-25
java.lang.String class, 188–190, A-25
equalsIgnoreCase method,  java.lang.String
class, A-25
Error handling, input errors. See also
Exception handling.
code sample, 545–549
overview, 545
Error messages
logging, 212–213
reading exception reports, 162
stack trace, 162
trace messages, 212–213
Errors. See also specific errors.
arrays, 318
compile-time, 15
dangling  else problem, 204–205
declaring constructors as  void , 92
declaring instance variables in local
variables, 108
“Division by zero,” 15
exceptions, 15
full code example, 15
ignoring parameter values, 98
logic, 15
in loops, 247–249
misspelling words, 16
roundoff, 134, 188–189
run-time, 15
syntax, 15
unbalanced parentheses, 144
uninitialized object references, 109
unintended integer division, 144
unnecessary instance variables, 108–109
Errors, diagnosing, encapsulation, 85
ESA (European Space Agency), 544
Escape sequences, 158
Event handling, user interface events, 485
Event listeners
calling listener methods, 490
containers as, 493
definition, 485
event source, 485
forgetting to attach, 493
inner classes for, 487–489
Event-controlled loops, 254
Events
button press, 491
definition, 485
timer, 494–496
Exception handlers, 536–537
Exception handling. See also Error handling.
animation, 537
catch clause, 536–537, 542–543
catching exceptions, 536–537
checked exceptions, 537–539
definition, 534
designing exception types, 540–541
exception handlers, 536–537
finally clause, 539–540, 542–543
full code example, 539
internal errors, 537
squelching exceptions, 542
throw early, catch late, 542
throwing exceptions, 534–535, 543
throws clause, 538–539
try blocks, 536–537
try statement, 536–537, 542–543
unchecked exceptions, 537
Exception reports, 162
Exceptions, definition, 15
Exclamation point, equal ( != ), relational
operator, 187t
Exclamation point ( ! ), not operator, 215
Executable algorithms, 17
exists method,  java.io.File class, A-18
exit method,  java.lang.System class, A-26
exp method,  java.lang.Math class, 142t, A-23
Explicit parameters, 110
Expressions, 139, 145
Extending classes. See Inheritance.
extends reserved word, 427
F
Face, drawing, 69–70
FaceComponent.java class, 69–70
Fifth-Generation Project, 221
 Index I-9
File class, 514
File dialog boxes
choosing files from a list, 517–518
full code example, 518
File method,  java.io.File class, A-18
File names, as string literals, 517
FileInputStream method,  java.
io.FileInputStream class, A-18
FileNotFoundException , 515–516, 537
FileOutputStream method,  java.
io.FileOutputStream class, A-18
Files, 10–11. See also Folders.
fill method,  java.awt.Graphics2D class, 68,
A-14
Filling arrays, 322
Fills, drawing, 68
Final classes, 442
finally clause, 539–540, 542–543
Financial calculations, data type for, 134
First kit computer, 406
Fixed length arrays, 315
Flags. See Boolean operators.
Flags, drawing, 116–119
float data type, 132t
Floating-point numbers
assigning to integer variables, 134
comparing, 188
converting to integer, 142–143. See also
cast operator.
precision, 133–134
Floating-point values, mixing with integer,
139
floor method,  java.lang.Math class, 142t,
A-23
Flowcharting, loops, 263
Flowcharts, 207–210. See also Storyboards.
FlowLayout method,  java.awt.FlowLayout class,
A-13
Folders, 10. See also Files.
Font method,  java.awt.Font class, A-13
for loops, 254–259, 261
Format flags, 524t
format method,  java.lang.String class, A-26
Format specifiers, 148–150, 525
Format types, 525t
Formatting output
format flags, 524t
format specifiers, 148–150, 525
format types, 525t
full code example, 525
overview, 524
forName method,  java.lang.Class class, A-21
Frame windows, 61–62, 65
G
Garbage collection, 107
Generic class, array lists, 348
get method
java.util.ArrayList<E> class, 348, A-28
java.util.Calendar class, A-29
java.util.Map<K, V> interface, A-32
getCenterX method,  java.awt.geom.
RectangularShape class, A-17
getCenterY method,  java.awt.geom.
RectangularShape class, A-17
getFirst method,  java.util.LinkedList<E>
class, A-31
getGlobal method,  java.util.logging.Logger
class, 213, A-35
getHeight method
java.awt.Component class, A-12
java.awt.geom.RectangularShape class, A-17
java.awt.Rectangle class, 50–51, A-14
getKeyStrokeForEvent method,  javax.swing.
KeyStroke class, 500–501, A-39
getMaxX method,  java.awt.geom.
RectangularShape class, A-17
getMaxY method,  java.awt.geom.
RectangularShape class, A-17
getMessage method,  java.lang.Throwable class,
541, A-27
getMinX method,  java.awt.geom.
RectangularShape class, A-17
getMinY method,  java.awt.geom.
RectangularShape class, A-17
getSelectedFile method,  javax.swing.
JFileChooser class, 518, A-37
getSelectedItem method,  javax.swing.
JComboBox class, A-36
getSource method,  java.util.EventObject
class, A-30
getText method,  javax.swing.text.
JTextComponent class, A-40
getValue method,  javax.swing.JSlider class,
A-39
getWidth method
java.awt.Component class, A-12
java.awt.geom.RectangularShape class, A-17
java.awt.Rectangle class, 50–51, A-14
I-10  Index
getX method
java.awt.event.MouseEvent class, 498, A-16
java.awt.geom.Point2D class, A-17
java.awt.Rectangle class, 50–51, A-14
getX1 method,  java.awt.geom.Line2D class,
A-16
getX2 method,  java.awt.geom.Line2D class,
A-16
getY method
java.awt.event.MouseEvent class, 498, A-16
java.awt.geom.Point2D class, A-17
java.awt.Rectangle class, 50–51, A-14
getY1 method,  java.awt.geom.Line2D class,
A-16
getY2 method,  java.awt.geom.Line2D class,
A-16
Gosling, James, 6
Graphical applications. See also Pictures;
specific shapes.
displaying components in frames, 65
drawing on a component, 62–65
frame windows, 61–62
Greater than, equal ( >= ), relational operator,
187t
Greater than ( > ), relational operator, 187t
GregorianCalendar method,  java.util.
GregorianCalendar class, A-31
grep program, 526
GridLayout method,  java.awt.GridLayout class,
A-14
grow method,  java.awt.Rectangle class, A-15
Gutenberg project, 454
H
Hand tracing
animation, 251
arithmetic operations, 154–156
description, 203–204
loops, 249–253
objects, 105–107
Hardware, definition, 2
“Has-a” relationships. See Aggregation.
HashMap method,  java.util.HashMap<K, V>
class, A-31
HashSet method,  java.util.HashSet<E> class,
A-31
hasNext method
java.util.Iterator<E> interface, A-31
java.util.Scanner class, A-34
hasNextDouble method,  java.util.Scanner
class, 219, A-34
hasNextInt method,  java.util.Scanner class,
218–219, A-34
hasNextLine method,  java.util.Scanner class,
A-34
hasPrevious method,  java.util.
ListIterator<E> interface, A-32
“Hello, World” program
analyzing, 12–14
source code, 12
writing, 8–11
HelloPrinter.java class, 12
Help, online, 55. See also Documentation.
High-level languages, 6
History of computers
Altair 8800, 406
Apple II, 406
corporate monopolies, 60
first kit computer, 406
hardware evolution, 5
IBM, 60
microprocessors, 406
Microsoft, 60
personal computing, 406
Univac Corporation, 60
VisiCalc, 406
Hoff, Marcian E., 406
Houston, Frank, 359
I
IBM, history of computers, 60
if statements. See also  switch statements.
animation, 196, 200
dangling  else problem, 204–205
definition, 180
duplicate code in branches, 186
ending with a semicolon, 184–185
flowchart for, 181
implementing (How To), 193–195
input validation, 218–220
multiple alternatives, 196–199
nesting, 200–203
sample program, 182–183
syntax, 182
IllegalArgumentException method,  java.lang.
IllegalArgumentException , 534–535,
A-21
ImageIcon method,  javax.swing.ImageIcon
class, A-35
Immutable classes, 383–384
Implementation, classes. See Classes,
implementing.
implements reserved word, 466–468
 Index I-11
Implicit parameters, 110
import directive, 398–399, 401
Importing
classes from packages, 54
packages, 401
in method,  java.lang.System class, 147
Income tax computation, 200–203
Income tax rate schedule, 200t
Indefinite loops, 254
Indenting code, with tabs, 185
IndexOutOfBoundsException , 537
Infinite loops, 248
INFO message level, 213
info method,  java.util.logging.Logger class,
213, A-35
Information hiding. See Encapsulation.
Inheritance. See also Polymorphism.
animation, 432
class relationships, 565–566
definition, 422
full code example, 429, 447
indicating, 427
versus interfaces, 469
purpose of, 426
super reserved word, 435–436
Inheritance hierarchy
developing (How To), 443–447
Worked Example, 448
Initialization
arrays, 312–314
static methods, 399
Initialization, variables
assignment statements, 41
definition, 37
local, 107
static, 399
Initials.java class, 160
Inner classes
definition, 481
as event listeners, 487–489
instance variables, 488
local variables, 488
overview, 481–483
Input. See also  java.util.Scanner class;
Output.
from a console, 157
definition, 4
dialog boxes, 162–163
full code example, 162
from a keyboard, 147–148
prompting for, 147–148
reading, 147–148
strings, 157
Input statements, syntax, 147
Input validation, 218–220
InputStreamReader method,  java.
io.InputStreamReader class, A-19
Instance methods, 145–146
Instance variables
access specifiers, 83
declaring, 82–83
declaring in local variables, 108
definition, 82
encapsulation, 84–86
full code example, 85
implementing classes, 93
return values, 84
syntax, 83
type, specifying, 83
unnecessary, 108–109
instanceof operator, 451–453
Instantiating, interface types, 470
int type, 38, 132t
Integer division, 140–141, 144
Integer method,  java.lang.Integer class, A-22
Integer values, mixing with floating-point,
139
Integers
computation overflow, 133
converting to floating-point, 142–143
definition, 37
Integrated development environment, 8–9
Intel Corporation, 146
Interface types
constants, declaring, 470
defining, 464–466
definition, 464
implementing, 466–468
implements reserved word, 466–468
instantiating, 470
public methods, 470
Interface variables
casting from interfaces to classes, 472–473
converting from classes, 471
full code example, 473
invoking methods, 471–472
Interfaces. See also specific interfaces.
for callbacks, 477–481
versus inheritance, 469
modifying method parameter types,
489–490
Worked Example: number sequences, 473
I-12  Index
International alphabets, 163
Internet, history of, 454
Internet Explorer, 454
intersection method,  java.awt.Rectangle
class, A-15
intValue method,  java.lang.Integer class,
A-22
Investment.java class, 245–246, 257–258
InvestmentRunner.java class, 246, 258
InvestmentViewer1.java class, 488–489
InvestmentViewer2.java class, 492
Invoice.java class, 576–578
InvoicePrinter.java class, 576
“Is-a” relationships. See Inheritance.
isDigit method,  java.lang.Character class,
521, A-20
isEditable method
javax.swing.JComboBox class, A-36
javax.swing.text.JTextComponent class,
A-40
isLetter method,  java.lang.Character class,
521, A-20
isLowerCase method,  java.lang.Character
class, 521, A-20
isSelected method,  javax.swing.
AbstractButton class, A-35
isUpperCase method,  java.lang.Character
class, 521, A-20
isWhiteSpace method,  java.lang.Character
class, classifying characters, 521
iterator method,  java.util.Collection<E>
interface, A-30
J
Jacobson, Ivar, 382
Java language. See also Applets;
Programming.
definition, 2
high-level languages, 6
integrated development environment, 8–9
platform independence, 7
portability, 7
versions, 7t
Java library, definition, 7. See also Packages.
Java operators, summary of, A-5–6t
Java programs. See also specific elements.
class files, 10
compilation process, 10
compilers, 6
getting started. See “Hello, World”
program.
machine code, 6
source code, 10
source files, 10
java.awt package, A-11–15
java.awt.BorderLayout class, method
summary, A-11–12. See also specific
methods.
java.awt.Color class
colors, specifying, 68
method summary, A-12. See also specific
methods.
java.awt.Component class
graphical frames, 61
method summary, A-13. See also specific
methods.
java.awt.Container class, method summary,
A-13. See also specific methods.
java.awt.Dimension class, method summary,
A-13. See also specific methods.
java.awt.event package, A-15–16
java.awt.event.ActionListener interface,
method summary, A-15. See also
specific methods.
java.awt.event.KeyEvent class
description, A-15
processing keystrokes, 500–501
java.awt.event.KeyListener interface, method
summary, A-15–16
java.awt.event.MouseEvent class, method
summary, A-16. See also specific
methods.
java.awt.event.MouseListener interface,
method summary, A-16. See also
specific methods.
java.awt.FlowLayout class, method summary,
A-13. See also specific methods.
java.awt.Font class, method summary, A-13.
See also specific methods.
java.awt.Frame class
frame titles, 61
method summary, A-13. See also specific
methods.
java.awt.geom package, A-16–17
java.awt.geom.Ellipse2D.Double class
drawing an ellipse, 66–67
method summary, A-16. See also specific
methods.
java.awt.geom.Line2D class, method summary,
A-16. See also specific methods.
 Index I-13
java.awt.geom.Line2D.Double class
drawing lines, 66–67
method summary, A-17. See also specific
methods.
java.awt.geom.Point2D class, method
summary, A-17. See also specific
methods.
java.awt.geom.Point2D.Double class
method summary, A-17. See also specific
methods.
points, specifying, 67
java.awt.geom.RectangularShape class, method
summary, A-17. See also specific
methods.
java.awt.Graphics class
coloring shapes, 68
method summary, A-13. See also specific
methods.
java.awt.Graphics2D class
drawing on components, 63–65
drawing strings, 67
filling shapes, 68
method summary, A-14. See also specific
methods.
java.awt.GridLayout class, method summary,
A-14. See also specific methods.
java.awt.Rectangle class
drawing rectangles, 48–51
method summary, A-14–15. See also
specific methods.
moving rectangles, 51, 55–56
sizing rectangles, 53–54
java.awt.Shape interface, A-15
javadoc program
/** (slash, asterisks), comment indicator,
90
converting comments to documentation,
90–92
documenting methods, 573–575
full code example, 91
overview, 92
parameter variables, 90
java.io package, A-18–20
java.io.EOFException class, method summary,
A-18. See also specific methods.
java.io.File class, method summary, A-18.
See also  java.util.Scanner class;
specific methods.
java.io.FileInputStream class, method
summary, A-18. See also specific
methods.
java.io.FileNotFoundException , summary,
A-18
java.io.FileOutputStream class, method
summary, A-18. See also specific
methods.
java.io.InputStream class, method summary,
A-19. See also specific methods.
java.io.InputStreamReader class, method
summary, A-19. See also specific
methods.
java.io.IOException
checked exceptions, 537–538
reading web pages, 517
summary, A-19
java.io.OutputStream class, method summary,
A-19. See also specific methods.
java.io.PrintStream class
formatting output, 148–150
method summary, A-19–20. See also
specific methods.
java.io.PrintWriter class, method summary,
A-19–20. See also specific methods.
java.lang package, A-20–27
java.lang.Boolean class, method summary,
A-20. See also specific methods.
java.lang.Character class, method summary,
A-20. See also specific methods.
java.lang.Class class, method summary,
A-21. See also specific methods.
java.lang.Cloneable interface, summary, A-21
java.lang.CloneNotSupportedException , A-21
java.lang.Comparable<T> interface
full code example, 474
method summary, A-21. See also specific
methods.
overview, 473–477
java.lang.Double class
converting strings to numbers, 162
method summary, A-21. See also specific
methods.
java.lang.Error class, summary, A-21
java.lang.IllegalArgumentException
method summary, A-21. See also specific
methods.
unchecked exceptions, 537
java.lang.IllegalStateException , summary,
A-22
java.lang.Integer class
converting strings to numbers, 162
method summary, A-22. See also specific
methods.
I-14  Index
java.lang.Integer class (continued)
minimum/maximum values, getting, 132
java.lang.InterruptedException , A-22
java.lang.Math class, method summary, 142,
A-22–24. See also specific methods.
java.lang.NullPointerException , 543, A-24
java.lang.NumberFormatException , 537, A-25
java.lang.Object class, method summary,
A-25. See also specific methods.
java.lang.RuntimeException , 537, A-25
java.lang.String class
charAt method, 158
compareTo method, 189
equals method, 188–190
length method, 43–44, 156
method summary, A-25–26. See also
specific methods.
replace method, 46
substring method, 159–160
toUpperCase method, 43–44
java.lang.System class
in method, 147
method summary, A-26–27. See also
specific methods.
java.lang.Throwable class, method summary,
A-27. See also specific methods.
java.math package, A-27
java.math.BigDecimal class
add method, 138
method summary, A-27
multiply method, 138
subtract method, 138
java.math.BigInteger class
add method, 138
multiply method, 138
subtract method, 138
java.math.BigInteger class, method summary,
A-27
java.net package, A-28
java.net.URL class
method summary, A-28.
reading web pages, 517
java.util package, A-28–34
java.util.ArrayList<E> class
add method, 348
get method, 348
method summary, A-28. See also specific
methods.
remove method, 349
set method, 348–349
size method, 348
java.util.Arrays class
copyOf method, 327–328
method summary, A-29. See also specific
methods.
toStrings method, 323–324
java.util.Calendar class, method summary,
A-29. See also specific methods.
java.util.Collection<E> interface, method
summary, A-29–30. See also specific
methods.
java.util.Collections class, method
summary, A-30. See also specific
methods.
java.util.Comparator<T> interface, method
summary, A-30. See also specific
methods.
java.util.Date class, method summary, A-30.
See also specific methods.
java.util.EventObject class, method
summary, A-30. See also specific
methods.
java.util.GregorianCalendar class, method
summary, A-31. See also specific
methods.
java.util.HashMap<K, V> class, method
summary, A-31. See also specific
methods.
java.util.HashSet<E> class, method summary,
A-31. See also specific methods.
java.util.InputMismatchException , A-31
java.util.Iterator<E> interface, method
summary, A-31. See also specific
methods.
java.util.LinkedList<E> class, method
summary, A-31–32. See also specific
methods.
java.util.List<E> interface, method
summary, A-32. See also specific
methods.
java.util.ListIterator<E> interface, method
summary, A-32. See also specific
methods.
java.util.logging package, A-34–35
java.util.logging.Level class, A-34
java.util.logging.Logger class
getGlobal method, 213
info method, 213
method summary, A-35. See also specific
methods.
setLevel method, 213
 Index I-15
java.util.Map<K, V> interface, method
summary, A-32–33. See also specific
methods.
java.util.NoSuchElementException
catching exceptions, 537
summary, A-33
java.util.PriorityQueue<E> class, method
summary, A-33. See also specific
methods.
java.util.QueueMap<E> interface, method
summary, A-33. See also specific
methods.
java.util.Random class
nextDouble method, 283
nextInt method, 283
java.util.Random class, method summary,
A-33. See also specific methods.
java.util.Scanner class. See also  java.io.File
class.
hasNextDouble method, 219
hasNextInt method, 218–219
method summary, A-33–34. See also
specific methods.
next method, 157
nextDouble method, 148
nextInt method, 147
java.util.Set<E> interface, A-34
java.util.TreeMap<K, V> class, method
summary, A-34. See also specific
methods.
java.util.TreeSet<E> class, method summary,
A-34. See also specific methods.
javax.swing package, A-35–40
javax.swing.AbstractButton class, method
summary, A-35. See also specific
methods.
javax.swing.ButtonGroup class, method
summary, A-35. See also specific
methods.
javax.swing.event package, A-40
javax.swing.event.ChangeEvent class, A-40
javax.swing.event.ChangeListener interface,
method summary, A-40. See also
specific methods.
javax.swing.ImageIcon class, method
summary, A-35. See also specific
methods.
javax.swing.JButton class
listening for events, 486–489
method summary, A-35. See also specific
methods.
javax.swing.JCheckBox class, method
summary, A-36. See also specific
methods.
javax.swing.JComboBox class, method
summary, A-36. See also specific
methods.
javax.swing.JComponent class
drawing on components, 63–65
method summary, A-36. See also specific
methods.
javax.swing.JFileChooser class
file dialog boxes, 517–518
method summary, A-36–37. See also
specific methods.
javax.swing.JFrame class
method summary, A-37. See also specific
methods.
setDefaultCloseOperation method, 61
javax.swing.JLabel class
labelling buttons, 490–492
method summary, A-37. See also specific
methods.
javax.swing.JMenu class, method summary,
A-37. See also specific methods.
javax.swing.JMenuBar class, method summary,
A-38. See also specific methods.
javax.swing.JMenuItem class, method
summary, A-38. See also specific
methods.
javax.swing.JOptionPane class
method summary, A-38. See also specific
methods.
showInputDialog method, 162
showMessageDialog method, 163
javax.swing.JPanel class
description, A-38
grouping components in panels, 490–492
javax.swing.JRadioButton class, method
summary, A-38. See also specific
methods.
javax.swing.JScrollPane class, method
summary, A-38. See also specific
methods.
javax.swing.JSlider class, method summary,
A-39. See also specific methods.
javax.swing.JTextArea class, method
summary, A-39. See also specific
methods.
javax.swing.JTextField class, method
summary, A-39. See also specific
methods.
I-16  Index
javax.swing.Keystroke class, method
summary, A-39. See also specific
methods.
javax.swing.text package, A-40
javax.swing.text.JTextComponent class,
method summary, A-40. See also
specific methods.
javax.swing.Timer class
method summary, A-40. See also specific
methods.
processing timer events, 494–496
JButton method,  javax.swing.JButton class,
490–492, A-35
JCheckBox method,  javax.swing.JCheckBox
class, A-36
JComboBox method,  javax.swing.JComboBox
class, A-36
JFileChooser method,  javax.swing.
JFileChooser class, A-36
JLabel method,  javax.swing.JLabel class, A-37
JMenu method,  javax.swing.JMenu class, A-37
JMenuBar method,  javax.swing.JMenuBar class,
A-38
JMenuItem method,  javax.swing.JMenuItem
class, A-38
JRadioButton method,  javax.swing.
JRadioButton class, A-38
JScrollPane method,  javax.swing.JScrollPane
class, A-38
JSlider method,  javax.swing.JSlider class,
A-39
JTextArea method,  javax.swing.JTextArea
class, A-39
JTextField method,  javax.swing.JTextField
class, A-39
K
Kahn, Bob, 454
Keyboard input, 147–148
keyPressed method, 504, A-15
keyReleased method, 500–501, A-15
keySet method,  java.util.Map<K, V> interface,
A-32
keyTyped method, 501, A-15
“Knows-about” relationships. See
Dependencies, classes.
L
Labels (user interface), 490
LargestInArray class, 329–330
Lenat, Douglas, 221
length method,  java.lang.String class, 43–44,
156, A-26
Less than, equal ( <= ), relational operator, 187t
Less than ( < ), relational operator, 187t
Lexicographic ordering, 189
Library. See Java library.
Licklider, J.C.R., 454
Line breaks, 158
Line2D.Double method,  java.awt.geom.Line2D.
Double class, A-17
LineItem.java class, 578
Lines (graphic), drawing, 67
Lines (of text), reading, 521–522
Listeners. See Event listeners.
listIterator method,  java.util.List<E>
interface, A-32
Literals, string, 156
Local variables
declaring instance variables in, 108
definition, 107
duplicate names, 206
full code example, 107
garbage collection, 107
initializing, 107
log method,  java.lang.Math class, 142t, A-23
log10 method,  java.lang.Math class, 142t,
A-23
Logging messages, 212–213
Logic errors, 15
Loma Prieta earthquake, 196
long data type, 132t, 133
Loop and a half problem, 266–267
Loops
animation, 251, 255
asymmetric bounds, 260
Boolean variables, 266
bounds, choosing, 260
break statements, 267–268
common errors, 247–249
continue statements, 267–268
count-controlled, 254
counting iterations, 260
credit card processing (Worked Example),
279
definite, 254
definition, 242
do loops, 262–263
enhanced  for loop, 321–322
event-controlled, 254
 Index I-17
flowcharting, 263
full code example, 262, 321–322
hand tracing, 249–253
indefinite, 254
infinite, 248
loop and a half problem, 266–267
for loops, 254–259, 261
manipulating pixel images (Worked
Example), 282
nesting, 279–282
off-by-one errors, 248–249
post-test, 262
pre-test, 262
redirecting input/output, 266
sentinel values, 263–266
while loops, 242–247
writing (How To), 276–279
Loops, common algorithms
averages, computing, 272
comparing adjacent values, 275–276
counters, 272–273
finding first match, 273
full code example, 275
maximum/minimum values, finding, 274
prompting for first match, 274
totals, computing, 272
Luggage handling system, 195
M
Machine code, 6
Magic numbers, 139
main method
command line arguments, 527–529
definition, 12
Managing object properties, common class
patterns, 392
Matrices, 340–346
max method,  java.lang.Math class, 142t, A-24
Maximum value, finding in arrays, 323
Maximum/minimum values, finding with
loops, 274
MAX_VALUE method,  java.lang.Integer class,
132
MeasurableTester.java class, 468
Measurer.java class, 479
MeasurerTester.java class, 480–481
Medals.java class, 344–345
Menus, implementing a class for, 101
Messages. See Error messages.
Method summary,  java.lang.Math class, 142
Methods. See also specific methods; specific
methods.
accessing data without modifying. See
Accessor methods.
accessor, 50–51
arguments, 44–45, 319–320
call-by-reference, 386–390
call-by-value, 386–390
calling, 13, 34–35
versus constructors, 87
declaring, 47
definition, 35
documenting, 573–575
duplicate names. See Overloading.
final, 442
full code example, 47, 51
implementing classes, 95–97
instance, 145–146
invoking on numbers, 145–146
modifying data. See Mutator methods.
mutator, 50–51
naming conventions, 39
overloading, 47
passing information to. See Parameters;
this reference.
private implementation, 44
public interfaces, classes, 86–87
public interfaces to classes, 43–44
return values, 45–46
static, 145–146
in UML diagrams, 567
verbs as, 560
Methods, designing
accessors, 383–384
cohesion, 381–382, 385
consistency, 385
dependencies, 382–383, 385
mutator, 383–384
public interfaces, 381–382
side effects, 384–385
Methods, static
accessing instance variables, 398
definition, 396–397
full code example, 397
initialization, 399
minimizing use of, 397–398
static imports, 398–399
Microprocessors, 406
Microsoft
history of computers, 60
product schedules, 212
min method,  java.lang.Math class, 142t, A-24
Minimum value, finding, arrays, 323
I-18  Index
Minus sign ( - ), subtraction operator, 38
Minus signs ( -- ), decrement, 140
MIN_VALUE method,  java.lang.Integer class,
132
Misspelling words, 16
Mock objects, 483–484
mod method,  java.math.BigInteger class, A-27
Modeling
moving objects, common class patterns,
393–395
objects with distinct states, common class
patterns, 392–393
Modulus, 140–141
Monte Carlo method, 285–286
MonteCarlo.java , 285–286
Morris, Robert, 320
Mosaic, 454
mouseClicked method, A-16
mouseEntered method, A-16
mouseExited method, A-16
mousePressed method, A-16
mouseReleased method, A-16
Multidimensional arrays, 347
Multiplicities, 568–569
multiply method
java.math.BigDecimal class, 138, A-27
java.math.BigInteger class, 138, A-27
Murphy, Walter, 580
Mutator methods, 50–51, 383–384
Mycin program, 221
N
\n newline character, 158
Name clashes, packages, 401–402
Naming conventions
classes, 39
constants, 135
constructors, 88
local variables, 206
methods, 39
packages, 401–402
variables, 39, 43
Naughton, Patrick, 6
Nesting
if statements, 200–203
loops, 279–282
Networks, definition, 4
next method,  java.util.Iterator<E> interface,
A-31
next method,  java.util.Scanner class
consuming white space, 519–520, 523–524
description, A-34
reading strings from the console, 157
reading words, 519–520
nextDouble method,  java.util.Random class,
283, A-33
nextDouble method,  java.util.Scanner class
consuming white space, 523–524
description, A-34
reading floating-point numbers, 148
nextInt method,  java.util.Random class
description, A-33
random numbers, 283
nextInt method,  java.util.Scanner class
consuming white space, 523–524
description, A-34
reading input, 147
screening integers, 523
nextLine method,  java.util.Scanner class
description, A-34
reading lines, 521–522
Nicely, Thomas, 146
No-argument constructors, 88
Nouns as classes, 560
null , testing for, 190–191
Null reference versus empty string, 191
Number literals, 133
Number sequences (Worked Example), 473
Number variables versus object variables, 58
Numbers
big, 138
constants, 134–138
converting from strings, 522–523
data types, 132–134
with fractions. See Floating-point
numbers.
without fractions. See  int type.
invoking methods on, 145–146
magic, 139
primitive types, 132
O
Object class. See also Superclasses.
definition, 448
equals method, 450–451
instanceof operator, 451–452
toString method, 448–449, 453
Object construction
arguments, 49
definition, 49
invoking a constructor like a method, 49
 Index I-19
overview, 48–50
syntax, 49
Object references
definition, 58
full code example, 59
number variables versus object variables,
58
overview, 57–60
uninitialized, 109
Object variables versus number variables, 58
Objects
arrays of, 318
comparing. See  equals method.
definition, 34–35
full code example, 106
hand tracing, 105–107
testing. See Mock objects.
OFF message level, 213
Off-by-one errors, 248–249
Online help, 55. See also Documentation.
openStream method,  java.net.URL class, A-28
Operators. See specific operators.
out object,  System class, 13–14, 35
Output. See also Input.
definition, 4
dialog boxes, 162–163
format specifiers, 148–150
formatting, 148–150
full code example, 162
line breaks, 158
Overflow, computation
definition, 133
double data type, 133
Overloading methods, 47
Overloading subclasses, accidentally, 435
Overriding methods
forcing, 441–442
preventing, 442
superclass, 431, 441–442
P
Packages. See also specific packages.
.  (dot), name syntax, 403–404
accessing, 404
API documentation, 54–55
definition, 400
description, 8
importing, 401
importing classes from, 54
Java library, 7
name clashes, 401–402
naming conventions, 401–402
organizing classes in, 400–401
programming with (How To), 404–405
source files, 402–403
summary of, 400t
syntax, 402
paintComponent method,  javax.swing.
JComponent class, 63–65, A-36
Panels (user interface), 490–492
Parallel arrays, 318–319
@param tag, 90
Parameter variables
ignoring, 98
javadoc program, 90
Parameters
explicit, 110
full code example, 111
implicit, 110
this reference, 109–111
Parentheses ( ( ) ), in arithmetic operations,
139, 144–145
parseDouble method,  java.lang.Double class
converting strings to numbers, 162,
522–523
description, A-21
parseDouble method,  java.lang.Integer class,
162
parseInt method,  java.lang.Integer class
converting strings to numbers, 162,
522–523
method summary, A-22
Partially filled arrays, 316–317
Patent, definition, 533
peek method,  java.util.QueueMap<E> interface,
A-33
Pentium floating-point bug, 146
Percent sign ( % ), modulus, 140–141
Peripheral devices, 4. See also specific
devices.
Personal computing, 406
PGP (Pretty Good Privacy) encryption, 533
PI constant,  java.lang.Math class, 135
Pictures, editing (Worked Example), 57. See
also Drawing; Graphical applications.
Piracy, software, 253
Pixel images, manipulating with loops
(Worked Example), 282
Platform independence, 7
Plus sign ( + )
addition operator, 38
concatenation operator, 157
I-20  Index
Plus signs ( ++ ), increment, 140
Point2D.Double method,  java.awt.geom.
Point2D.Double class, A-17
Polymorphism. See also Inheritance.
animation, 438
definition, 438
dynamic method lookup, 438, 440
overview, 437–438
PopulationDensity.java class, 532
Portability, 7
Post-test loops, 262
pow method,  java.lang.Math class, 142t, A-24
Powers, calculating, 141–142
PowerTable.java class, 280–282
Precedence
arithmetic operations, 139
Boolean operators, 214
Java operators, A-5
Precision
double data type, 133
floating-point numbers, 133–134
Pre-test loops, 262
previous method,  java.util.ListIterator<E>
interface, A-32
Primitive number types, 132
Print commands, full code example, 14
print method
java.io.PrintStream class, 14, A-19
java.io.PrintWriter class, A-19
printf method
java.io.PrintStream class, 148–150, A-20
java.io.PrintWriter class, 514–516, A-20
Printing
array element separators, 330
arrays, 344–345
numerical values, 13
starting a new line, 14
Printing invoices (sample program)
CRC cards, 570–572
discovering classes, 570–572
documenting methods, 573–575
implementation, 575–580
overview, 569
recording class relationships, 572–573
requirements gathering, 569–570
sample code, 576–580
UML diagrams, 572–573
println method
java.io.PrintStream class, 14, A-19–20
java.io.PrintWriter class, 514–516,
A-19–20
printStackTrace method,  java.lang.Throwable
class, 537, A-27
PrintStream method
java.io.PrintStream class, A-19
java.io.PrintWriter class, A-19
PrintWriter class
character encoding, 519
constructing with a string, 517
writing text files, 514–516
PrintWriter method
java.io.PrintStream class, A-19
java.io.PrintWriter class, A-19
PriorityQueue method,  java.util.
PriorityQueue<E> class, A-33
Privacy issues, databases, 580
Private method implementation, 44
Problem statements, converting to
pseudocode (How To), 151–154
Product.java class, 578–579
Program development, examples. See
“Hello, World” program; Printing
invoices.
Programming. See also Applets; Java
language.
compilers, 6
definition, 2
getting started. See “Hello, World”
program.
high-level languages, 6
machine code, 6
scheduling time for, 212
Prompting
for first match, with loops, 274
for input, 147–148
protected access feature, 442–443
Pseudocode
for algorithms, 19–20
writing (How To), 151–154
Pseudorandom numbers, 284
Public interfaces, classes
class declaration, 89
commenting, 89–92
constructors, specifying, 87–88
definition, 89
methods, specifying, 86–87
overview, 43–44
syntax, 89
uses for, 89
Public methods, interface types, 470
put method,  java.util.Map<K, V> interface,
A-32
 Index I-21
Pyramids, computing volume and surface
area (Worked Example), 154
Q
QuestionDemo1.java class, 425
QuestionDemo2.java class, 432–433
QuestionDemo3.java class, 438–439
Question.java class, 424
Quotation marks ( " ), string delimiters, 12
R
Radiation therapy incidents, 359
Random method,  java.util.Random class, A-33
Random numbers
finding approximate solutions, 285–286
generating, 283–284
Monte Carlo method, 285–286
pseudorandom numbers, 284
read method,  java.io.InputStream class, A-19
Reading input. See also Text files, reading
and writing; Writing output.
into arrays, 328–330
characters, 520
characters from a string, 522
classifying characters, 520–521
converting strings to numbers, 522–523
error handling, 545–549
lines, 521–522
mixed input types, 523–524
from a prompt, 147–148
text files, 514–516
validating numbers, 523
white space, consuming, 519–520
words, 519–520
Rectangle method,  java.awt.Rectangle class,
A-15
RectangleComponent.java class, 64–65, 494–495
RectangleFrame.java class, 495–496
Rectangles
comparing, 190
drawing, 61–65
RectangleViewer.java class, 64–65, 496
Regression testing, 356–358
Regular expressions, 526
Relational operators, 186–188, 216
Remainders, calculating, 140–141
remove method
java.util.ArrayList<E> class, 349, A-28
java.util.Collection<E> interface, A-30
java.util.Iterator<E> interface, A-31
java.util.Map<K, V> interface, A-33
java.util.PriorityQueue<E> class, A-33
removeFirst method,  java.util.LinkedList<E>
class, A-32
removeLast method,  java.util.LinkedList<E>
class, A-32
repaint method,  java.awt.Component class
method summary, A-12. See also specific
methods.
repainting components, 495–496
Repainting graphic components, 495–496,
A-12
replace method,  java.lang.String class, 46,
A-26
Requirements, gathering, 569–570
Reserved words, as variable names, 39
@return tag, 90
Return values
instance variables, 84
methods, 45–46
Reusing algorithms. See Interface types.
Richter scale, 196t
Rivest, Ron, 533
Rocket incident, 544
Rolling dice (Worked Example), 336
Roots, calculating, 141–142
round method
definition, 142t
description, A-24
rounding floating-point numbers, 142
Rounding, 133–134
Roundoff errors, 134, 188–189
RSA encryption, 533
Rumbaugh, James, 382
Run-time errors, 15
S
Safety, Java, 7
Scanner class
character encoding, 519
constructing with a string, 517
reading text files, 514–516
Scanner method,  java.util.Scanner class, A-33
Scheduling time for programming, 212
ScoreTester.java class, 357
Semicolon ( ; )
ending  if statements, 184–185
ending method statements, 12
omitting, 14
I-22  Index
Sentinel values, 263–266
SentinelDemo.java class, 264–265
set method
java.util.ArrayList<E> class, 348–349,
A-28
java.util.ListIterator<E> interface, A-32
setBorder method,  javax.swing.JComponent
class, A-36
setColor method,  java.awt.Graphics class, 68,
A-14
setDefaultCloseOperation method,  javax.
swing.JFrame class, 61, A-37
setEditable method
javax.swing.JComboBox class, A-36
javax.swing.text.JTextComponent class,
A-40
setFocusable method,  java.awt.Component
class, 501
setFont method,  javax.swing.JComponent class,
A-36
setJMenuBar method,  javax.swing.JFrame class,
A-37
setLayout method,  java.awt.Container class,
A-13
setLevel method,  java.util.logging.Logger
class, 213, A-35
setLine method,  java.awt.geom.Line2D class,
A-16
setLocation method
java.awt.geom.Point2D class, A-17
java.awt.Rectangle class, A-15
setPreferredSize method,  java.awt.Component
class, A-12
setSelected method,  javax.swing.
AbstractButton class, A-35
setSize method
java.awt.Component class, 61, A-12
java.awt.Rectangle class, 53–54, A-15
setText method,  javax.swing.text.
JTextComponent class, A-40
setTitle method,  java.awt.Frame class, 61,
A-13
setVisible method,  java.awt.Component class,
61, A-12
Shamir, Adi, 533
Shape classes, 112–116
Shell scripts, 358–359
Shipping costs, computing (full code
example), 209
Short circuiting Boolean evaluation, 217
short data type, 132t
showInputDialog method,  javax.swing.
JOptionPane class, 162, A-38
showMessageDialog method,  javax.swing.
JOptionPane class, 163, A-38
showOpenDialog method,  javax.swing.
JFileChooser class, 518, A-37
showSaveDialog method,  javax.swing.
JFileChooser class, 518, A-37
Side effects, 384–385
Simulation programs, 283
sin method,  java.lang.Math class, 142t, A-24
size method
java.util.ArrayList<E> class, 348, A-28
java.util.Collection<E> interface, A-30
Slash, asterisks ( /** ), comment indicator, 90
Slash (/), division operator, 38, 139
Slash asterisk... ( /*...*/ ), comment
delimiters, 40
Slashes (//), comment delimiter, 40
Software, definition, 2. See also
Programming.
Software piracy, 253
sort method
java.util.Arrays class, A-29
java.util.Collections class, A-30
Sorting, arrays, 331
Source code, 10
Source files, 10
Spaces in
comparisons, 189
expressions, 145
input. See White space.
variable names, 39
Spaghetti code, 208–209
Spelling errors, 16
sqrt method,  java.lang.Math class, 142t, A-24
Stack trace, 162
start method,  javax.swing.Timer class, A-40
stateChanged method,  javax.swing.event.
ChangeListener interface, A-40
Statements, punctuating, 12, 14
Static imports, 398–399
Static methods
accessing instance variables, 398
definition, 396–397
full code example, 397
initialization, 399
minimizing use of, 397–398
 Index I-23
overview, 145–146
static imports, 398–399
Stepping through code, 287–289
stop method,  javax.swing.Timer class, A-40
Storage devices, 3. See also specific devices.
Storyboards, 269–271. See also Flowcharts.
String class, 35–36
String type, 156
Strings. See also Characters; Substrings.
versus characters, 158–159
comparing, 188–189
concatenating, 157
converting from objects. See  toString
method.
converting to integers. See  parseInt
method.
converting to numbers, 522–523. See also
parseInt method.
counting positions in, 158–159
definition, 13, 156
empty, 156
escape sequences, 158
input, 157
joining. See Concatenating.
length, computing, 156
literals, 156
reading characters from, 522
reading from the console, 157
String class. See  java.lang.String class.
Unicode characters, 158–159
Subclasses. See also Classes; Superclasses.
accidental overload, 435
calling superclass methods, 431–432
constructors, 436
declaration, example, 428–429
definition, 422
implementing, 426–430
inherited methods, 431
overriding superclass methods, 431
replicating instance variables, 430
substitution principle, 422–423
versus superclasses, 430
substring method,  java.lang.String class,
159–160, A-26
Substrings. See also Strings.
extracting, 159–160
extracting (Worked Example), 195
subtract method
java.math.BigDecimal class, 138, A-27
java.math.BigInteger class, 138, A-27
Summing
array elements, 323
totals, common class patterns, 390–391
super reserved word, 435–436
Superclasses. See also Classes;  Object class;
Subclasses.
definition, 422
versus subclasses, 430
substitution principle, 422–423
Surface area of a pyramid, computing
(Worked Example), 154
Swapping, array elements, 326
switch statements, 199. See also  if statements.
Syntax errors, 15
System class, 10, 35
T
Tabs, indenting code, 185
tan method,  java.lang.Math class, 142t, A-24
Tax rates, 200t
TaxCalculator.java class, 202–203
TaxReturn.java class, 200–202
TCP/IP (Transmission Control Protocol/
Internet Protocol), 454
Teller machine simulation (Worked
Example), 581
Terminating algorithms, 17
Test cases, selecting, 210–211
Test suites, 356
Tester classes, 102–103
Testing
black-box, 210
boundary cases, 210–211
classes, 56
code coverage, 210
for  null , 190–191
objects. See Mock objects.
programs, 55–56
regression, 356–358
unit, 102–103
unit test frameworks, 407–408
white-box, 210
Text files, reading and writing. See also
Reading input; Writing output.
How To, 530–532
overview, 514–516
Therac-25 incidents, 359
this reference, 109–111
Throw early, catch late, 542
throw statement, 534–535
Throwable method,  java.lang.Throwable class,
A-27
I-24  Index
Throwing exceptions, 534–535, 542, 543
throws clause, 538–539
Tiling a floor, algorithm for, 22–23
Timer events, 494–496
Timer method,  javax.swing.Timer class, A-40
toDegrees method,  java.lang.Math class, 142t,
A-24
toLowerCase method,  java.lang.String class,
A-26
toRadians method,  java.lang.Math class, 142t,
A-24
toString method
java.lang.Integer class, A-22
java.lang.Object class, A-25
java.util.Arrays class, A-29
toString method,  java.util.Arrays class
inheritance, 453
inserting element separators, 323–324
overriding, 448–449
Total.java class, 515–516
Totals, computing with loops, 272
toUpperCase method,  java.lang.String class,
43–44, A-26
Trace messages, 212–213
Tracing. See Hand tracing.
Transistors, 3
translate method,  java.awt.Rectangle class,
51, 55–56, A-15
Travel time computation (Worked Example),
156
TreeMap method,  java.util.TreeMap<K, V>
class, A-34
TreeSet method,  java.util.TreeSet<E> class,
A-34
trim method, 522
Trimming white space, 522
Try blocks, 536–537
try statement, 536–537, 542–543
Two-dimensional arrays. See Arrays, two-
dimensional.
Types of data. See Data types.
U
UML (Unified Modeling Language)
attributes, 567
definition, 382
methods, 567
in program design (How To), 566–567
relationship symbols, 566t
sample program, 572–573
The Unified Modeling Language User
Guide, 569
Unambiguous algorithms, 17
Unchecked exceptions, 537
Undeclared variables, 42
Underscore ( _ ), in variable names, 39
Unfilled arrays, 318
Unicode, 158–159, 163
Unicode encoding, 519, A-1–3t
The Unified Modeling Language User
Guide, 569
Uninitialized arrays, 318
Uninitialized variables, 42
Unintended integer division, 144
union method,  java.awt.Rectangle class, A-15
Unit testing, 102–103
Univac Corporation, history of computers,
60
URL method,  java.net.URL class, A-28
useDelimiter method,  java.util.Scanner class
description, A-34
reading characters, 520
User interface components. See also specific
components.
buttons, 490–492
labels, 490
panels, 490–492
repainting, 495–496
UTF-8 encoding, 519
Utility classes, 380
V
Validating numeric input, 523
Variables. See also Instance variables; Local
variables.
assigning values to, 40–41
comments, 40
declaration versus assignment, 42–43
declaring, 36–38
definition, 37
full code example, 41
initializing, 37, 41
inspecting during debugging, 287–289
in  for loop headers, 261
lifetime of, animation, 107
naming conventions, 39, 43
object versus number, 58
scope, minimizing, 205–206
syntax, 37
 Index I-25
undeclared, 42
uninitialized, 42
Variables, static
definition, 395–396
full code example, 397
initialization, 399
static imports, 398–399
Verbs as methods, 560
Vertical bars  (|| ), or operator, 213–214, 216,
217
Viruses, 320
VisiCalc, 406
void reserved word
declaring constructors, 92–93
definition, 47
Volume of a pyramid, computing (Worked
Example), 154
Volume.java class, 150
Voting machines, 104
W
Web pages, reading
full code example, 517
sample code, 517
while loops, 242–247
White space
consuming, 519–520, 523
trimming, 522
White-box testing, 210
Wilkes, Maurice, 291
Words, reading, 519–520
World population table (Worked Example),
345
World Wide Web, 454
Wrapper classes, 351, 351t
write method,  java.io.OutputStream class,
A-19
writeObject method,  java.io.OutputStream
class, A-19
Writing output, 524–526. See also Reading
input; Text files, reading and writing.
Writing text files, 514–516
Z
Zimmerman, Phil, 533
C-1
Preface
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IllustratIon CredIts
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Chapter 3
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Chapter 7
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Worked Example 7.1, WE1 (top): © ktsimage/iStockphoto.
Worked Example 7.1, WE1 (bottom): © Ryan Ruffatti/iStockphoto.
Chapter 8
Page 379, 380: © Ivan Stevanovic/iStockphoto.
Page 381, 408: © Sergey Ivanov/iStockphoto.
Page 384, 408: © manley099/iStockphoto.
Page 385, 408: AP Photo/Frank Franklin II.
Page 386: Frank Rosenstein/Digital Vision/Getty Images, Inc.
Page 391, 409: © paul prescott/iStockphoto.
Page 393, 409: © John Alexander/iStockphoto.
Page 395, 409: © Diane Diederich/iStockphoto.
Page 401, 404, 409: © Don Wilkie/iStockphoto
Page 406: Reprint Courtesy of International Business Machines Corporation, © International
Business Machines Corporation.
Page 413: © DNY59/iStockphoto.
Page 415: © pixhook/iStockphoto.
Page 417 (top): © ThreeJays/iStockphoto.
Page 417 (bottom): © Maria Toutoudaki/iStockphoto.
Chapter 9
Page 421, 422: © Jason Hosking/Photodisc/Getty Images, Inc.
Page 422, 456: © Richard Stouffer/iStockphoto (vehicles);
© Ed Hidden/iStockphoto (motorcycle); © YinYang/iStockphoto (car);
© Robert Pernell/iStockphoto (truck); Media Bakery (sedan);
Cezary Wojtkowski/Age Fotostock America (SUV).
Page 423: © paul kline/iStockphoto.
Page 427, 456: Media Bakery.
Page 438, 456: © Alpophoto/iStockphoto.
Page 448: © Sean Locke/iStockphoto.
Page 450: © Ken Brown/iStockphoto.
Page 459: © Pali Rao/iStockphoto.
Worked Example 9.1, WE1: © Sean Locke/iStockphoto.
Chapter 10
Page 463, 464 (top): © supermimicry/iStockphoto.
Page 464, 502: © Oxana Oleynichenko/iStockphoto.
Page 466: © gregory horler/iStockphoto.
Page 473 (top), 503: © Andrew Rich/iStockphoto.
Page 473 (middle): © Norebbo/iStockphoto.
Page 474: © Janis Dreosti/iStockphoto.
Page 475: © Alex Gumerov/iStockphoto.
Page 478: © Dan Herrick/iStockphoto.
Page 481, 503: © maureenpr/iStockphoto.
Page 483: © Don Nichols/iStockphoto.
Page 485: © Seriy Tryapitsyn/iStockphoto.
C-6 Illustration Credits
Page 491, 503: © Eduard Andras/iStockphoto.
Page 494, 504: © jeff giniewicz/iStockphoto.
Page 497, 504: © james Brey/iStockphoto.
Page 501: © Dmitry Shironosov/iStockphoto.
Page 502: Courtesy of Richard Stallman.
Page 509: © KathyMuller/iStockphoto.
Worked Example 10.1, WE1: © Norebbo/iStockphoto.
Chapter 11
Page 513, 514 (top): James King-Holmes/Bletchley Park Trust/Photo Researchers, Inc.
Page 527, 550: © xyno/iStockphoto.
Page 530: © Oksana Perkins/iStockphoto.
Page 532: © Nancy Ross/iStockphoto.
Page 533: (mobile phone) © Anna Khomulo/iStockphoto.
Page 535, 550: © Lisa F. Young/iStockphoto.
Page 537, 550: © Andraz Cerar/iStockphoto.
Page 538, 550: © tillsonburg/iStockphoto.
Page 539, 550: © archives/iStockphoto.
Page 544: © AP/Wide World Photos.
Page 552: © Chris Price/iStockphoto.
Page 556: © Chris Dascher/iStockphoto.
Worked Example 11.1, WE1: © Nancy Ross/iStockphoto.
Chapter 12
Page 559, 560: © Petrea Alexandru/iStockphoto.
Page 561, 581: © Oleg Prikhodko/iStockphoto.
Page 563: © visual7/iStockphoto.
Page 565, 581: © bojan fatur/iStockphoto.
Page 570: © Scott Cramer/iStockphoto.
Page 580: © Greg Nicholas/iStockphoto
Page 581 (top): © Mark Evans/iStockphoto.
Worked Example 12.1, WE1: © Mark Evans/iStockphoto.
Chapters Available on the Web
Chapter 13 
Page 587, 588 (top): © Nicolae Popovici/iStockphoto.
Page 588, 620: © Davis Mantel/iStockphoto.
Page 593, 620: © Nikada/iStockphoto.
Page 596, 620: © gerenme/iStockphoto.
Page 598, 621: © pagadesign/iStockphoto.
Page 603, 621: © Jeanine Groenwald/iStockphoto.
Page 606: Science Photo Library/Photo Researchers, Inc.
Page 614, 621: © Lanica Klein/iStockphoto.
Chapter 14 
Page 629, 630 (top): © Volkan Ersoy/iStockphoto.
Page 630, 662: © Zone Creative/iStockphoto.
Page 640, 662: © Kirby Hamilton/iStockphoto.
Illustration Credits C-7
Page 641, 662: © Rich Legg/iStockphoto.
Page 647: © Christopher Futcher/iStockphoto.
Page 652: Topham/The Image Works.
Page 653–657, 662, 669 (lightbulbs): © Kraska/iStockphoto.
Chapter 15 
Page 671, 672: © nicholas belton/iStockphoto.
Page 673 (top left): © Filip Fuxa/iStockphoto.
Page 673 (top center), 704: © parema/iStockphoto.
Page 673 (top right): © Vladimir Trenin/iStockphoto.
Page 673 (books, bottom), 704: © david franklin/iStockphoto.
Page 675, 704: © andrea laurita/iStockphoto.
Page 680: © Denis Vorob’yev/iStockphoto.
Page 681, 704: © Alfredo Ragazzoni/iStockphoto.
Page 682, 704: © Volkan Ersoy/iStockphoto.
Page 688, 704: © Tom Hahn/iStockphoto.
Page 689: © Ermin Gutenberger/iStockphoto.
Page 690, 704: © one clear vision/iStockphoto.
Page 692 (top), 705: © John Madden/iStockphoto.
Page 692 (left): © budgetstockphoto/iStockphoto.
Page 693, 702, 705: Photodisc/Punchstock.
Page 695: © paul kline/iStockphoto.
Page 697, 705: © Jorge Delgado/iStockphoto.
Page 700: © Skip ODonnell/iStockphoto.
Page 703 (top): Courtesy of Nigel Tout.
Page 708: © martin mcelligott/iStockphoto.
Page 710: © Luis Carlos Torres/iStockphoto.
Input
Scanner in = new Scanner(System.in);
//  Can also use new Scanner(new File("input.txt"));
int n = in.nextInt();
double x = in.nextDouble();
String word = in.next();
String line = in.nextLine();
while (in.hasNextDouble())
{
double x = in.nextDouble();
Process x
}
Linked Lists, Sets, and Iterators
LinkedList<String> names = new LinkedList<String>();
names.add("Bob"); //  Adds at end
ListIterator<String> iter = names.listIterator();
iter.add("Ann"); //  Adds before current position
String name = iter.next(); //  Returns  "Ann"
iter.remove(); //  Removes  "Ann"
Set<String> names = new HashSet<String>();
names.add("Ann"); //  Adds to set if not present
names.remove("Bob"); //  Removes if present
Iterator<String> iter = names.iterator();
while (iter.hasNext())
{
Process iter.next()
}
Arrays
int[] numbers = new int[5];
int[] squares = { 0, 1, 4, 9, 16 };
int[][] magicSquare =
{
{ 16, 3, 2, 13},
{ 5, 10, 11, 8},
{ 9, 6, 7, 12},
{ 4, 15, 14, 1}
};
for (int i = 0; i < numbers.length; i++)
{
numbers[i] = i * i;
}
for (int element : numbers)
{
Process element
}
System.out.println(Arrays.toString(numbers));
//  Prints [0, 1, 4, 9, 16]
Element
Element type type Length
All elements are zero.
Maps
Map<String, Integer> scores = new HashMap<String, Integer>();
scores.put("Bob", 10);
Integer score = scores.get("Bob");
for (String key : scores.keySet())
{
Process key  and scores.get(key)
}
Key Value
type type
Returns  null if key not present
Output
System.out.print("Enter a value: ");
System.out.println("Volume: " + volume);
System.out.printf("%-10s %10d %10.2f", name, qty, price);
PrintWriter out = new PrintWriter("output.txt");
out.close();
Left-justified string Integer Floating-point number
Field width Precision
Does not advance to new line.
Use  + to concatenate values.
Remember to close output file.
Use  print / println / printf
to write output to file.
Array Lists
ArrayList<String> names = new ArrayList<String>();
names.add("Ann");
names.add("Cindy"); // [Ann, Cindy], names.size() is now 2
names.add(1, "Bob"); // [Ann, Bob, Cindy]
names.remove(2); // [Ann, Bob]
names.set(1, "Bill"); // [Ann, Bill]
String name = names.get(0); //  Gets  "Ann"
System.out.println(names); //  Prints  [Ann, Bill]
Element type
Use wrapper type,
Integer ,  Double , etc.,
for primitive types.
Add elements to the end
Initially empty