This document discusses Chapter 2 of an introduction to Java programming textbook. It covers elementary programming concepts such as writing simple Java programs to perform computations, using variables to store data, numeric data types, operators, and literals. The chapter objectives are to learn how to solve practical problems by working with Java primitive data types, variables, input/output, and other basic programming elements.

1. Chapter 2 Elementary Programming
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
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1

2. Motivations
In the preceding chapter, you learned how to
create, compile, and run a Java program. Starting
from this chapter, you will learn how to solve
practical problems programmatically. Through
these problems, you will learn Java primitive data
types and related subjects, such as variables,
constants, data types, operators, expressions, and
input and output.
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2

3. Objectives
 To write Java programs to perform simple computations (§2.2).
 To obtain input from the console using the Scanner class (§2.3).
 To use identifiers to name variables, constants, methods, and classes (§2.4).
 To use variables to store data (§§2.5–2.6).
 To program with assignment statements and assignment expressions (§2.6).
 To use constants to store permanent data (§2.7).
 To name classes, methods, variables, and constants by following their naming conventions (§2.8).
 To explore Java numeric primitive data types: byte, short, int, long, float, and double (§2.9.1).
 To perform operations using operators +, -, *, /, and % (§2.9.2).
 To perform exponent operations using Math.pow(a, b) (§2.9.3).
 To write integer literals, floating-point literals, and literals in scientific notation (§2.10).
 To write and evaluate numeric expressions (§2.11).
 To obtain the current system time using System.currentTimeMillis() (§2.12).
 To use augmented assignment operators (§2.13).
 To distinguish between postincrement and preincrement and between postdecrement and
predecrement (§2.14).
 To cast the value of one type to another type (§2.15).
 To describe the software development process and apply it to develop the loan payment program
(§2.16).
 To represent characters using the char type (§2.17).
 To represent a string using the String type (§2.18).
 To obtain input using the JOptionPane input dialog boxes (§2.19).
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4. Introducing Programming with an
Example
Listing 2.1 Computing the Area of a Circle
This program computes the area of the circle.
ComputeArea
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5. animation
Trace a Program Execution
public class ComputeArea { allocate memory
/** Main method */ for radius
public static void main(String[] args) {
double radius; radius no value
double area;
// Assign a radius
radius = 20;
// Compute area
area = radius * radius * 3.14159;
// Display results
System.out.println("The area for the circle of radius " +
radius + " is " + area);
}
}
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
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6. animation
Trace a Program Execution
public class ComputeArea {
/** Main method */ memory
public static void main(String[] args) {
double radius; radius no value
double area; area no value
// Assign a radius
radius = 20;
allocate memory
// Compute area for area
area = radius * radius * 3.14159;
// Display results
System.out.println("The area for the circle of radius " +
radius + " is " + area);
}
}
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
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7. animation
Trace a Program Execution
public class ComputeArea { assign 20 to radius
/** Main method */
public static void main(String[] args) {
double radius; radius 20
double area; area no value
// Assign a radius
radius = 20;
// Compute area
area = radius * radius * 3.14159;
// Display results
System.out.println("The area for the circle of radius " +
radius + " is " + area);
}
}
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
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8. animation
Trace a Program Execution
public class ComputeArea {
/** Main method */ memory
public static void main(String[] args) {
double radius; radius 20
double area; area 1256.636
// Assign a radius
radius = 20;
compute area and assign it
// Compute area
to variable area
area = radius * radius * 3.14159;
// Display results
System.out.println("The area for the circle of radius " +
radius + " is " + area);
}
}
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
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9. animation
Trace a Program Execution
public class ComputeArea {
/** Main method */ memory
public static void main(String[] args) {
double radius; radius 20
double area; area 1256.636
// Assign a radius
radius = 20;
// Compute area
area = radius * radius * 3.14159; print a message to the
console
// Display results
System.out.println("The area for the circle of radius " +
radius + " is " + area);
}
}
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10. Reading Input from the Console
1. Create a Scanner object
Scanner input = new Scanner(System.in);
2. Use the methods next(), nextByte(), nextShort(),
nextInt(), nextLong(), nextFloat(), nextDouble(), or
nextBoolean() to obtain to a string, byte, short, int, long,
float, double, or boolean value. For example,
System.out.print("Enter a double value: ");
Scanner input = new Scanner(System.in);
double d = input.nextDouble();
ComputeAreaWithConsoleInput ComputeAverage
Run Run
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11. Identifiers
 An identifier is a sequence of characters that consist of
letters, digits, underscores (_), and dollar signs ($).
 An identifier must start with a letter, an underscore (_),
or a dollar sign ($). It cannot start with a digit.
– An identifier cannot be a reserved word. (See Appendix A,
“Java Keywords,” for a list of reserved words).
 An identifier cannot be true, false, or
null.
 An identifier can be of any length.
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12. Variables
// Compute the first area
radius = 1.0;
area = radius * radius * 3.14159;
System.out.println("The area is “ +
area + " for radius "+radius);
// Compute the second area
radius = 2.0;
area = radius * radius * 3.14159;
System.out.println("The area is “ +
area + " for radius "+radius);
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13. Declaring Variables
int x; // Declare x to be an
// integer variable;
double radius; // Declare radius to
// be a double variable;
char a; // Declare a to be a
// character variable;
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14. Assignment Statements
x = 1; // Assign 1 to x;
radius = 1.0; // Assign 1.0 to radius;
a = 'A'; // Assign 'A' to a;
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15. Declaring and Initializing
in One Step
 int x = 1;
 double d = 1.4;
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16. Named Constants
final datatype CONSTANTNAME = VALUE;
final double PI = 3.14159;
final int SIZE = 3;
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17. Naming Conventions
 Choose meaningful and descriptive names.
 Variables and method names:
– Use lowercase. If the name consists of several
words, concatenate all in one, use lowercase
for the first word, and capitalize the first letter
of each subsequent word in the name. For
example, the variables radius and area, and
the method computeArea.
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
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18. Naming Conventions, cont.
 Class names:
– Capitalize the first letter of each word in
the name. For example, the class name
ComputeArea.
 Constants:
– Capitalize all letters in constants, and use
underscores to connect words. For
example, the constant PI and
MAX_VALUE
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19. Numerical Data Types
Name Range Storage Size
byte –27 to 27 – 1 (-128 to 127) 8-bit signed
short –215 to 215 – 1 (-32768 to 32767) 16-bit signed
int –231 to 231 – 1 (-2147483648 to 2147483647) 32-bit signed
long –263 to 263 – 1 64-bit signed
(i.e., -9223372036854775808 to 9223372036854775807)
float Negative range: 32-bit IEEE 754
-3.4028235E+38 to -1.4E-45
Positive range:
1.4E-45 to 3.4028235E+38
double Negative range: 64-bit IEEE 754
-1.7976931348623157E+308 to -4.9E-324
Positive range:
4.9E-324 to 1.7976931348623157E+308
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20. Numeric Operators
Name Meaning Example Result
+ Addition 34 + 1 35
- Subtraction 34.0 – 0.1 33.9
* Multiplication 300 * 30 9000
/ Division 1.0 / 2.0 0.5
% Remainder 20 % 3 2
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21. Integer Division
+, -, *, /, and %
5 / 2 yields an integer 2.
5.0 / 2 yields a double value 2.5
5 % 2 yields 1 (the remainder of the division)
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22. Remainder Operator
Remainder is very useful in programming. For example, an
even number % 2 is always 0 and an odd number % 2 is always
1. So you can use this property to determine whether a number
is even or odd. Suppose today is Saturday and you and your
friends are going to meet in 10 days. What day is in 10
days? You can find that day is Tuesday using the following
expression:
Saturday is the 6th day in a week
A week has 7 days
(6 + 10) % 7 is 2
The 2nd day in a week is Tuesday
After 10 days
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23. Problem: Displaying Time
Write a program that obtains hours and
minutes from seconds.
DisplayTime Run
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24. NOTE
Calculations involving floating-point numbers are
approximated because these numbers are not stored
with complete accuracy. For example,
System.out.println(1.0 - 0.1 - 0.1 - 0.1 - 0.1 - 0.1);
displays 0.5000000000000001, not 0.5, and
System.out.println(1.0 - 0.9);
displays 0.09999999999999998, not 0.1. Integers are
stored precisely. Therefore, calculations with integers
yield a precise integer result.
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25. Exponent Operations
System.out.println(Math.pow(2, 3));
// Displays 8.0
System.out.println(Math.pow(4, 0.5));
// Displays 2.0
System.out.println(Math.pow(2.5, 2));
// Displays 6.25
System.out.println(Math.pow(2.5, -2));
// Displays 0.16
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26. Number Literals
A literal is a constant value that appears directly
in the program. For example, 34, 1,000,000, and
5.0 are literals in the following statements:
int i = 34;
long x = 1000000;
double d = 5.0;
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27. Integer Literals
An integer literal can be assigned to an integer variable as
long as it can fit into the variable. A compilation error
would occur if the literal were too large for the variable to
hold. For example, the statement byte b = 1000 would
cause a compilation error, because 1000 cannot be stored
in a variable of the byte type.
An integer literal is assumed to be of the int type, whose
value is between -231 (-2147483648) to 231–1
(2147483647). To denote an integer literal of the long
type, append it with the letter L or l. L is preferred because
l (lowercase L) can easily be confused with 1 (the digit
one).
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28. Floating-Point Literals
Floating-point literals are written with a decimal
point. By default, a floating-point literal is treated
as a double type value. For example, 5.0 is
considered a double value, not a float value. You
can make a number a float by appending the letter f
or F, and make a number a double by appending
the letter d or D. For example, you can use 100.2f
or 100.2F for a float number, and 100.2d or 100.2D
for a double number.
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29. Scientific Notation
Floating-point literals can also be specified in
scientific notation, for example, 1.23456e+2,
same as 1.23456e2, is equivalent to 123.456, and
1.23456e-2 is equivalent to 0.0123456. E (or e)
represents an exponent and it can be either in
lowercase or uppercase.
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30. Arithmetic Expressions
3 4x 10 ( y 5)( a b c) 4 9 x
9( )
5 x x y
is translated to
(3+4*x)/5 – 10*(y-5)*(a+b+c)/x + 9*(4/x + (9+x)/y)
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31. How to Evaluate an Expression
Though Java has its own way to evaluate an
expression behind the scene, the result of a Java
expression and its corresponding arithmetic expression
are the same. Therefore, you can safely apply the
arithmetic rule for evaluating a Java expression.
3 + 4 * 4 + 5 * (4 + 3) - 1
(1) inside parentheses first
3 + 4 * 4 + 5 * 7 – 1
(2) multiplication
3 + 16 + 5 * 7 – 1
(3) multiplication
3 + 16 + 35 – 1
(4) addition
19 + 35 – 1
(5) addition
54 - 1
(6) subtraction
53
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32. Problem: Converting Temperatures
Write a program that converts a Fahrenheit degree
to Celsius using the formula:
5
celsius ( 9 )( fahrenheit 32 )
Note: you have to write
celsius = (5.0 / 9) * (fahrenheit – 32)
FahrenheitToCelsius Run
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33. Problem: Displaying Current Time
Write a program that displays current time in GMT in the
format hour:minute:second such as 1:45:19.
The currentTimeMillis method in the System class returns
the current time in milliseconds since the midnight, January
1, 1970 GMT. (1970 was the year when the Unix operating
system was formally introduced.) You can use this method
to obtain the current time, and then compute the current
second, minute, and hour as follows.
Elapsed
time
ShowCurrentTime
Time
Unix Epoch Current Time
01-01-1970
00:00:00 GMT
System.currentTimeMills() Run
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34. Shortcut Assignment Operators
Operator Example Equivalent
+= i += 8 i = i + 8
-= f -= 8.0 f = f - 8.0
*= i *= 8 i = i * 8
/= i /= 8 i = i / 8
%= i %= 8 i = i % 8
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35. Increment and
Decrement Operators
Operator Name Description
++var preincrement The expression (++var) increments var by 1 and evaluates
to the new value in var after the increment.
var++ postincrement The expression (var++) evaluates to the original value
in var and increments var by 1.
--var predecrement The expression (--var) decrements var by 1 and evaluates
to the new value in var after the decrement.
var-- postdecrement The expression (var--) evaluates to the original value
in var and decrements var by 1.
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36. Increment and
Decrement Operators, cont.
int i = 10; Same effect as
int newNum = 10 * i++; int newNum = 10 * i;
i = i + 1;
int i = 10; Same effect as
int newNum = 10 * (++i); i = i + 1;
int newNum = 10 * i;
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37. Increment and
Decrement Operators, cont.
Using increment and decrement operators makes
expressions short, but it also makes them complex and
difficult to read. Avoid using these operators in expressions
that modify multiple variables, or the same variable for
multiple times such as this: int k = ++i + i.
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38. Assignment Expressions and
Assignment Statements
Prior to Java 2, all the expressions can be used as
statements. Since Java 2, only the following types of
expressions can be statements:
variable op= expression; // Where op is +, -, *, /, or %
++variable;
variable++;
--variable;
variable--;
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39. Numeric Type Conversion
Consider the following statements:
byte i = 100;
long k = i * 3 + 4;
double d = i * 3.1 + k / 2;
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40. Conversion Rules
When performing a binary operation involving two
operands of different types, Java automatically
converts the operand based on the following rules:
1. If one of the operands is double, the other is
converted into double.
2. Otherwise, if one of the operands is float, the other is
converted into float.
3. Otherwise, if one of the operands is long, the other is
converted into long.
4. Otherwise, both operands are converted into int.
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41. Type Casting
Implicit casting
double d = 3; (type widening)
Explicit casting
int i = (int)3.0; (type narrowing)
int i = (int)3.9; (Fraction part is
truncated)
What is wrong? int x = 5 / 2.0;
range increases
byte, short, int, long, float, double
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42. Problem: Keeping Two Digits After
Decimal Points
Write a program that displays the sales tax with two
digits after the decimal point.
SalesTax Run
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43. Casting in an Augmented Expression
In Java, an augmented expression of the form x1 op=
x2 is implemented as x1 = (T)(x1 op x2), where T is
the type for x1. Therefore, the following code is
correct.
int sum = 0;
sum += 4.5; // sum becomes 4 after this statement
sum += 4.5 is equivalent to sum = (int)(sum + 4.5).
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44. Software Development Process
Requirement
Specification
System
Analysis
System
Design
Implementation
Testing
Deployment
Maintenance
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45. Requirement Specification
A formal process that seeks to understand
Requirement
Specification
the problem and document in detail what
the software system needs to do. This
System phase involves close interaction between
Analysis
users and designers.
System
Design
Implementation
Testing
Most of the examples in this book are simple,
and their requirements are clearly stated. In Deployment
the real world, however, problems are not
well defined. You need to study a problem Maintenance
carefully to identify its requirements.
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46. System Analysis
Requirement
Specification Seeks to analyze the business
process in terms of data flow, and
System
Analysis to identify the system’s input and
output.
System
Design
Implementation
Testing
Part of the analysis entails modeling
the system’s behavior. The model is
Deployment
intended to capture the essential
elements of the system and to define
Maintenance
services to the system.
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47. System Design
Requirement
Specification
The process of designing the
system’s components.
System
Analysis
System
Design
Implementation
Testing
This phase involves the use of many levels
Deployment
of abstraction to decompose the problem into
manageable components, identify classes and
interfaces, and establish relationships among Maintenance
the classes and interfaces.
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48. IPO
Requirement
Specification
System
Analysis Input, Process, Output
System
Design
Implementation
Testing
The essence of system analysis and design is input,
process, and output. This is called IPO. Deployment
Maintenance
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49. Implementation
Requirement The process of translating the
Specification
system design into programs.
System Separate programs are written for
Analysis
each component and put to work
System together.
Design
Implementation
Testing
This phase requires the use of a
programming language like Java. Deployment
The implementation involves
coding, testing, and debugging. Maintenance
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50. Testing
Requirement
Specification Ensures that the code meets the
requirements specification and
System
Analysis weeds out bugs.
System
Design
Implementation
Testing
An independent team of software
engineers not involved in the design Deployment
and implementation of the project
usually conducts such testing. Maintenance
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51. Deployment
Requirement
Specification Deployment makes the project
available for use.
System
Analysis
System
Design
Implementation
Testing
For a Java applet, this means
installing it on a Web server; for a Deployment
Java application, installing it on the
client's computer. Maintenance
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52. Maintenance
Requirement
Specification Maintenance is concerned with
changing and improving the
System
Analysis product.
System
Design
Implementation
Testing
A software product must continue to
perform and improve in a changing Deployment
environment. This requires periodic
upgrades of the product to fix newly Maintenance
discovered bugs and incorporate changes.
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52

53. Problem:
Computing Loan Payments
This program lets the user enter the interest
rate, number of years, and loan amount, and
computes monthly payment and total
payment.
loanAmount monthlyInterestRate
monthlyPayment
1 1
numberOfYears 12
(1 monthlyInterestRate)
ComputeLoan Run
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53

54. Character Data Type
Four hexadecimal digits.
char letter = 'A'; (ASCII)
char numChar = '4'; (ASCII)
char letter = 'u0041'; (Unicode)
char numChar = 'u0034'; (Unicode)
NOTE: The increment and decrement operators can also be used
on char variables to get the next or preceding Unicode character.
For example, the following statements display character b.
char ch = 'a';
System.out.println(++ch);
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55. Unicode Format
Java characters use Unicode, a 16-bit encoding scheme
established by the Unicode Consortium to support the
interchange, processing, and display of written texts in the
world’s diverse languages. Unicode takes two bytes,
preceded by u, expressed in four hexadecimal numbers
that run from 'u0000' to 'uFFFF'. So, Unicode can
represent 65535 + 1 characters.
Unicode u03b1 u03b2 u03b3 for three Greek
letters
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56. Problem: Displaying Unicodes
Write a program that displays two Chinese
characters and three Greek letters.
DisplayUnicode Run
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56

57. Escape Sequences for Special Characters
Description Escape Sequence Unicode
Backspace b u0008
Tab t u0009
Linefeed n u000A
Carriage return r u000D
Backslash u005C
Single Quote ' u0027
Double Quote " u0022
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58. Appendix B: ASCII Character Set
ASCII Character Set is a subset of the Unicode from u0000 to u007f
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59. ASCII Character Set, cont.
ASCII Character Set is a subset of the Unicode from u0000 to u007f
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59

60. Casting between char and
Numeric Types
int i = 'a'; // Same as int i = (int)'a';
char c = 97; // Same as char c = (char)97;
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61. Problem: Monetary Units
This program lets the user enter the amount in
decimal representing dollars and cents and output
a report listing the monetary equivalent in single
dollars, quarters, dimes, nickels, and pennies.
Your program should report maximum number of
dollars, then the maximum number of quarters,
and so on, in this order.
ComputeChange Run
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
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61

62. Trace ComputeChange
Suppose amount is 11.56
int remainingAmount = (int)(amount * 100); remainingAmount 1156
// Find the number of one dollars
int numberOfOneDollars = remainingAmount / 100;
remainingAmount
remainingAmount = remainingAmount % 100;
initialized
// Find the number of quarters in the remaining amount
int numberOfQuarters = remainingAmount / 25;
remainingAmount = remainingAmount % 25;
// Find the number of dimes in the remaining amount
int numberOfDimes = remainingAmount / 10;
remainingAmount = remainingAmount % 10;
// Find the number of nickels in the remaining amount
int numberOfNickels = remainingAmount / 5;
remainingAmount = remainingAmount % 5;
// Find the number of pennies in the remaining amount
int numberOfPennies = remainingAmount;
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
62

63. animation
Trace ComputeChange
Suppose amount is 11.56
int remainingAmount = (int)(amount * 100); remainingAmount 1156
// Find the number of one dollars
int numberOfOneDollars = remainingAmount / 100; numberOfOneDollars 11
remainingAmount = remainingAmount % 100;
// Find the number of quarters in the remaining amount numberOfOneDollars
int numberOfQuarters = remainingAmount / 25; assigned
remainingAmount = remainingAmount % 25;
// Find the number of dimes in the remaining amount
int numberOfDimes = remainingAmount / 10;
remainingAmount = remainingAmount % 10;
// Find the number of nickels in the remaining amount
int numberOfNickels = remainingAmount / 5;
remainingAmount = remainingAmount % 5;
// Find the number of pennies in the remaining amount
int numberOfPennies = remainingAmount;
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
63

64. animation
Trace ComputeChange
Suppose amount is 11.56
int remainingAmount = (int)(amount * 100); remainingAmount 56
// Find the number of one dollars
int numberOfOneDollars = remainingAmount / 100; numberOfOneDollars 11
remainingAmount = remainingAmount % 100;
// Find the number of quarters in the remaining amount
int numberOfQuarters = remainingAmount / 25; remainingAmount
remainingAmount = remainingAmount % 25; updated
// Find the number of dimes in the remaining amount
int numberOfDimes = remainingAmount / 10;
remainingAmount = remainingAmount % 10;
// Find the number of nickels in the remaining amount
int numberOfNickels = remainingAmount / 5;
remainingAmount = remainingAmount % 5;
// Find the number of pennies in the remaining amount
int numberOfPennies = remainingAmount;
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
64

65. animation
Trace ComputeChange
Suppose amount is 11.56
int remainingAmount = (int)(amount * 100); remainingAmount 56
// Find the number of one dollars
int numberOfOneDollars = remainingAmount / 100; numberOfOneDollars 11
remainingAmount = remainingAmount % 100;
// Find the number of quarters in the remaining amount
int numberOfQuarters = remainingAmount / 25; numberOfOneQuarters 2
remainingAmount = remainingAmount % 25;
// Find the number of dimes in the remaining amount numberOfOneQuarters
int numberOfDimes = remainingAmount / 10; assigned
remainingAmount = remainingAmount % 10;
// Find the number of nickels in the remaining amount
int numberOfNickels = remainingAmount / 5;
remainingAmount = remainingAmount % 5;
// Find the number of pennies in the remaining amount
int numberOfPennies = remainingAmount;
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
65

66. animation
Trace ComputeChange
Suppose amount is 11.56
int remainingAmount = (int)(amount * 100); remainingAmount 6
// Find the number of one dollars
int numberOfOneDollars = remainingAmount / 100; numberOfOneDollars 11
remainingAmount = remainingAmount % 100;
// Find the number of quarters in the remaining amount
int numberOfQuarters = remainingAmount / 25; numberOfQuarters 2
remainingAmount = remainingAmount % 25;
// Find the number of dimes in the remaining amount
int numberOfDimes = remainingAmount / 10; remainingAmount
remainingAmount = remainingAmount % 10; updated
// Find the number of nickels in the remaining amount
int numberOfNickels = remainingAmount / 5;
remainingAmount = remainingAmount % 5;
// Find the number of pennies in the remaining amount
int numberOfPennies = remainingAmount;
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
66

67. The String Type
The char type only represents one character. To represent a string
of characters, use the data type called String. For example,
String message = "Welcome to Java";
String is actually a predefined class in the Java library just like the
System class and JOptionPane class. The String type is not a
primitive type. It is known as a reference type. Any Java class can
be used as a reference type for a variable. Reference data types
will be thoroughly discussed in Chapter 8, “Objects and Classes.”
For the time being, you just need to know how to declare a String
variable, how to assign a string to the variable, and how to
concatenate strings.
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
67

68. String Concatenation
// Three strings are concatenated
String message = "Welcome " + "to " + "Java";
// String Chapter is concatenated with number 2
String s = "Chapter" + 2; // s becomes Chapter2
// String Supplement is concatenated with character B
String s1 = "Supplement" + 'B'; // s1 becomes SupplementB
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
68

69. Debugging
Logic errors are called bugs. The process of finding and
correcting errors is called debugging. A common approach
to debugging is to use a combination of methods to narrow
down to the part of the program where the bug is located.
You can hand-trace the program (i.e., catch errors by
reading the program), or you can insert print statements in
order to show the values of the variables or the execution
flow of the program. This approach might work for a
short, simple program. But for a large, complex program,
the most effective approach for debugging is to use a
debugger utility.
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
69

70. Debugger
Debugger is a program that facilitates debugging.
You can use a debugger to
Execute a single statement at a time.
Trace into or stepping over a method.
Set breakpoints.
Display variables.
Display call stack.
Modify variables.
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
70

71. JOptionPane Input
This book provides two ways of obtaining input.
1. Using the Scanner class (console input)
2. Using JOptionPane input dialogs
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
71

72. Getting Input from Input Dialog Boxes
String input = JOptionPane.showInputDialog(
"Enter an input");
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
72

73. Getting Input from Input Dialog Boxes
String string = JOptionPane.showInputDialog(
null, “Prompting Message”, “Dialog Title”,
JOptionPane.QUESTION_MESSAGE);
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
73

74. Two Ways to Invoke the Method
There are several ways to use the showInputDialog method. For
the time being, you only need to know two ways to invoke it.
One is to use a statement as shown in the example:
String string = JOptionPane.showInputDialog(null, x,
y, JOptionPane.QUESTION_MESSAGE);
where x is a string for the prompting message, and y is a string for
the title of the input dialog box.
The other is to use a statement like this:
JOptionPane.showInputDialog(x);
where x is a string for the prompting message.
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
74

75. Converting Strings to Integers
The input returned from the input dialog box is a string. If
you enter a numeric value such as 123, it returns “123”.
To obtain the input as a number, you have to convert a
string into a number.
To convert a string into an int value, you can use the
static parseInt method in the Integer class as follows:
int intValue = Integer.parseInt(intString);
where intString is a numeric string such as “123”.
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
75

76. Converting Strings to Doubles
To convert a string into a double value, you can use the
static parseDouble method in the Double class as follows:
double doubleValue =Double.parseDouble(doubleString);
where doubleString is a numeric string such as “123.45”.
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
76

77. Problem: Computing Loan Payments
Using Input Dialogs
Same as the preceding program for computing loan
payments, except that the input is entered from the
input dialogs and the output is displayed in an
output dialog.
loanAmount monthlyInterestRate
1 1
numberOfYears 12
(1 monthlyInterestRate)
ComputeLoanUsingInputDialog Run
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
77

78. Companion
Website Debugging in NetBeans
Supplement II.E, Learning Java Effectively with
NetBeans
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
78

79. Companion
Website Debugging in Eclipse
Supplement II.G, Learning Java Effectively with
NetBeans
Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
79