Chapter 25 - Beyond C & C++: Operators, Methods, and Arrays in Java

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Chapter 25 - Beyond C & C++:

Operators, Methods, and Arrays in Java

Outline

25.1 Introduction

25.2 Primitive Data Types and Keywords 25.3 Logical Operators

25.4 Method Definitions 25.5 Java API Packages

25.6 Random Number Generation 25.7 Example: A Game of Chance 25.8 Methods of Class JApplet

25.9 Declaring and Allocating Arrays 25.10 Examples Using Arrays

25.11 References and Reference Parameters

25.12 Multiple-Subscripted Arrays

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25.1Introduction

• In this chapter

– Differences between C, C++, and Java – Java's logical operators and methods

– Packages that comprise Applications Programming Interface (API)

– Craps simulator

– Random numbers in Java – Arrays in Java

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25.2Primitive Data Types and Keywords

• Primitive data types

– char, byte, short, int, long, float, double, boolean

– Building blocks for more complicated types

• All variables must have a type before being used

• Strongly typed language

– Primitive types portable, unlike C and C++

• In C/C++, write different versions of programs – Data types not guaranteed to be identical

– ints may be 2 or 4 bytes, depending on system

• WORA - Write once, run anywhere

– Default values

• boolean gets false, all other types are 0

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25.2Primitive Data Types and Keywords (II)

Type Size in

bits Values Standard

boolean 8 true or false

char 16 ’\u0000’ to ’\uFFFF’ (ISO Unicode character set) byte 8 –128 to +127

short 16 –32,768 to +32,767 int 32 –2,147,483,648 to

+2,147,483,647

long 64 –9,223,372,036,854,775,808 to

+9,223,372,036,854,775,807 float 32 –3.40292347E+38 to

+3.40292347E+38

(IEEE 754 floating point) double

64 –

1.79769313486231570E+308 to

+1.79769313486231570E+30 8

(IEEE 754 floating point)

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25.2Primitive Data Types and Keywords (III)

• Keywords

– Reserved names, cannot be used as identifiers – Used to implement features

J ava Keywords

abstract boolean break byte case catch char class continue default

do double else extends false

final finally float for if

implements import instanceof int interface

long native new null package

private protected public return short static super switch synchronized this throw throws transient true try void volatile while

Keywords that are reserved but not used by Java

const goto

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25.3Logical Operators

• Logical operators

– Form complex conditions and control structures – Logical AND (&&)

• true if both conditions true

– Logical OR (||)

• true if either condition true

• true if both conditions true (inclusive)

• If left condition true, skips right condition

– Boolean logical AND (&) , boolean logical inclusive OR (|)

• Act like counterparts, but always evaluate both expressions

• Useful if expression performs action:

birthday == true | ++age >= 65

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25.3Logical Operators (II)

• Logical Operators (continued)

– Boolean logical exclusive OR (^)

• true if exactly one condition true

• false if both conditions true

– Logical NOT (negation)

• Unary operator (one operand)

– All other logical operators binary (two operands)

• Reverses condition

• If true, returns false

• If false, returns true

• != - "does not equal"

if (grade != sentinelValue)

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25.3Logical Operators (III)

• More GUI Classes (javax.swing)

– JTextArea

• Create an area where text can be displayed

• Provide (rows, columns) to constructor to specify size JTextArea myArea; //declares object type

myArea = new JTextArea( 17, 20 ); //initialize

– myArea.setText( myString );

• Sets the text of myArea to myString

– JScrollPane

• Creates a window that can scroll JScrollPane myScroller =

new JScrollPane ( myArea );

• Declaration and initialization, allows myArea have scrolling

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25.3Logical Operators (IV)

• More GUI classes

– showMessageDialog(null, myScroller, titleString, type);

• Second argument indicates that myScroller (and attached myArea) should be displayed in message dialog

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Outline

1. Class definition 1.1 Initialize

objects

(JTextArea, JScrollPane)

1.2 Declare String 1.3 Append logical operator

statements

1 // Fig. 25.7: LogicalOperators.java 2 // Demonstrating the logical operators 3 import javax.swing.*;

4

5 public class LogicalOperators {

6 public static void main( String args[] ) 7 {

8 JTextArea outputArea = new JTextArea( 17, 20 );

9 JScrollPane scroller = new JScrollPane( outputArea );

10 String output = "";

11

12 output += "Logical AND (&&)" +

13 "\nfalse && false: " + ( false && false ) + 14 "\nfalse && true: " + ( false && true ) + 15 "\ntrue && false: " + ( true && false ) + 16 "\ntrue && true: " + ( true && true );

17

18 output += "\n\nLogical OR (||)" +

19 "\nfalse || false: " + ( false || false ) + 20 "\nfalse || true: " + ( false || true ) + 21 "\ntrue || false: " + ( true || false ) + 22 "\ntrue || true: " + ( true || true );

23

24 output += "\n\nBoolean logical AND (&)" +

25 "\nfalse & false: " + ( false & false ) + 26 "\nfalse & true: " + ( false & true ) + 27 "\ntrue & false: " + ( true & false ) + 28 "\ntrue & true: " + ( true & true );

29

30 output += "\n\nBoolean logical inclusive OR (|)" +

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Outline

2. Add output to outputArea

2.1

showMessageDialog (notice second

argument)

2.2 System.exit required for programs with GUIs

35

36 output += "\n\nBoolean logical exclusive OR (^)" + 37 "\nfalse ^ false: " + ( false ^ false ) + 38 "\nfalse ^ true: " + ( false ^ true ) + 39 "\ntrue ^ false: " + ( true ^ false ) + 40 "\ntrue ^ true: " + ( true ^ true );

41

42 output += "\n\nLogical NOT (!)" + 43 "\n!false: " + ( !false ) + 44 "\n!true: " + ( !true );

45

46 outputArea.setText( output );

47 JOptionPane.showMessageDialog( null, scroller,

48 "Truth Tables", JOptionPane.INFORMATION_MESSAGE );

49 System.exit( 0 );

50 } 51 }

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Outline

Program Output

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25.4Method Definitions

• Method definition format

return-value-type method-name( parameter-list ) {

declarations and statements }

– Method-name: any valid identifier

– Return-value-type: data type of the result

• void - method returns nothing

• Can return at most one value

– Parameter-list: comma separated list, declares parameters

• Method call must have proper number and type of parameters

– Declarations and statements: method body (block)

• Variables can be declared inside blocks (can be nested)

• Method cannot be defined inside another function

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25.4Method Definitions (II)

• Program control

– When method call encountered

• Control transferred from point of invocation to method

– Returning control

• If nothing returned: return;

– Or until reaches right brace

• If value returned: return expression;

– Returns the value of expression – Example user-defined method:

public int square( int y ) {

return y * y }

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25.4Method Definitions (III)

• Calling methods

– Three ways

• Method name and arguments

– Can be used by methods of same class – square( 2 );

• Dot operator - used with objects

– g.drawLine( x1, y1, x2, y2 );

• Dot operator - used with static methods of classes – Integer.parseInt( myString );

– More Chapter 26

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25.4Method Definitions (IV)

• More GUI components

– Content Pane - on-screen display area

• Attach GUI components to it to be displayed

• Object of class Container (java.awt)

– getContentPane

• Method inherited from JApplet

• Returns reference to Content Pane

Container c = getContentPane();

– Container method add

• Attaches GUI components to content pane, so they can be displayed

• For now, only attach one component (occupies entire area)

• Later, learn how to add and layout multiple components c.add( myTextArea );

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Outline

1. init

1.1 Initialize variables

1.2 JTextArea object

1.3 Container object

1.4 Attach outputArea to Content Pane 2. Call method 2.1 Set text of output

1 // Fig. 25.8: SquareInt.java

2 // A programmer-defined square method 3 import java.awt.Container;

4 import javax.swing.*;

5

6 public class SquareInt extends JApplet { 7 public void init()

8 {

10

12

13 // get the applet's GUI component display area 14 Container c = getContentPane();

15

16 // attach outputArea to Container c 17 c.add( outputArea );

18

19 int result;

20

21 for ( int x = 1; x <= 10; x++ ) { 22 result = square( x );

23 output += "The square of " + x + 24 " is " + result + "\n";

25 } 26

28 }

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Outline

3. Method definition

Program Output

31 public int square( int y ) 32 {

33 return y * y;

34 } 35 }

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25.5Java API Packages

• As we have seen

– Java has predefined, grouped classes called packages – Together, all the packages are the Applications

Programming Interface (API)

– Fig 25.10 has a list of the packages in the API

• Import

– Import statements specify location of classes

– Large number of classes, avoid reinventing the wheel

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25.6Random Number Generation

• Math.random()

– Returns a random double,greater than or equal to 0.0, less than 1.0

• Scaling and shifting

n = a + (int) ( Math.random() * b ) n = random number

a = shifting value b = scaling value

In C we used %, but in Java we can use * For a random number between 1 and 6,

n = 1 + (int) ( Math.random() * 6 )

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25.7Example: A Game of Chance

• Redo "craps" simulator from Chapter 5

• Rules

– Roll two dice

• 7 or 11 on first throw, player wins

• 2, 3, or 12 on first throw, player loses

• 4, 5, 6, 8, 9, 10 - value becomes player's "point"

– player must roll his point before rolling 7 to win

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25.7Example: A Game of Chance (II)

• User input

– Till now, used message dialog and input dialog

• Tedious, only show one message/ get one input at a time

– Now, we will use event handling for more complex GUI

• extends keyword

– Class inherits data and methods from another class – A class can also implement an interface

• Keyword implements

• Interface - specifies methods you must define in your class

• Event handling

– Event: user interaction (i.e., user clicking a button) – Event handler: method called in response to an event

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25.7Example: A Game of Chance (III)

• Interface ActionListener

– Requires that you define method actionPerformed

• actionPerformed is the event handler

• Class JTextField

– Can display or input a line of text

• Class JButton

– Displays a button which can perform an action if pushed – Method addActionListener( this );

• Specifies this applet should listen for events from the JButton object

– Each component must know which method will handle its events

• Registering the event handler

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25.7Example: A Game of Chance (IV)

• Class JButton (continued)

– We registered this applet with our JButton

• The applet "listens" for events from the

– actionPerformed is the event handler

• Event-driven programming

– User's interaction with GUI drives program

• final

– Declares a variable constant

• Cannot be modified

• Must be initialized at declaration

• const int MYINT = 3;

• Use all uppercase for final variables

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25.7Example: A Game of Chance (V)

• Methods of class Container

– Recall that the Content Pane is of class Container – Method setLayout

• Define layout managers (determine position and size of all components attached to container)

• FlowLayout - Most basic layout manager

– Items placed left to right in order added to container – When end of line reached, continues on next line

c = getContentPane();

c.setLayout( new FlowLayout() );

Initialized with object of class FlowLayout

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Outline

1. import

2. Define class (implements) 2.1 Initialize variables and objects

2.2 setLayout

1 // Fig. 25.13: Craps.java 2 // Craps

3 import java.awt.*;

4 import java.awt.event.*;

6

7 public class Craps extends JApplet implements ActionListener { 8 // constant variables for status of game

9 final int WON = 0, LOST = 1, CONTINUE = 2;

10

11 // other variables used in program

12 boolean firstRoll = true; // true if first roll 13 int sumOfDice = 0; // sum of the dice

14 int myPoint = 0; // point if no win/loss on first roll 15 int gameStatus = CONTINUE; // game not over yet

16

17 // graphical user interface components

18 JLabel die1Label, die2Label, sumLabel, pointLabel;

19 JTextField firstDie, secondDie, sum, point;

20 JButton roll;

21

22 // setup graphical user interface components 23 public void init()

24 {

25 Container c = getContentPane();

26 c.setLayout( new FlowLayout() );

27

28 die1Label = new JLabel( "Die 1" );

29 c.add( die1Label );

30 firstDie = new JTextField( 10 );

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Outline

2.3 add

components to GUI

2.4

addActionListener 3. Define

actionPerformed (event handler)

31 firstDie.setEditable( false );

32 c.add( firstDie );

33

34 die2Label = new JLabel( "Die 2" );

35 c.add( die2Label );

36 secondDie = new JTextField( 10 );

37 secondDie.setEditable( false );

38 c.add( secondDie );

39

40 sumLabel = new JLabel( "Sum is" );

41 c.add( sumLabel );

42 sum = new JTextField( 10 );

43 sum.setEditable( false );

44 c.add( sum );

45

46 pointLabel = new JLabel( "Point is" );

47 c.add( pointLabel );

48 point = new JTextField( 10 );

49 point.setEditable( false );

50 c.add( point );

51

52 roll = new JButton( "Roll Dice" );

53 roll.addActionListener( this );

54 c.add( roll );

55 } 56

57 // call method play when button is pressed 58 public void actionPerformed( ActionEvent e ) 59 {

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Outline

3.1 Define play 3.2 switch

61 } 62

63 // process one roll of the dice 64 public void play()

65 {

66 if ( firstRoll ) { // first roll of the dice 67 sumOfDice = rollDice();

68

69 switch ( sumOfDice ) {

70 case 7: case 11: // win on first roll 71 gameStatus = WON;

72 point.setText( "" ); // clear point text field 73 break;

74 case 2: case 3: case 12: // lose on first roll 75 gameStatus = LOST;

76 point.setText( "" ); // clear point text field 77 break;

78 default: // remember point 79 gameStatus = CONTINUE;

80 myPoint = sumOfDice;

81 point.setText( Integer.toString( myPoint ) );

82 firstRoll = false;

83 break;

84 } 85 }

86 else {

87 sumOfDice = rollDice();

88

89 if ( sumOfDice == myPoint ) // win by making point 90 gameStatus = WON;

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Outline

3.3 Define rollDice

91 else

92 if ( sumOfDice == 7 ) // lose by rolling 7 93 gameStatus = LOST;

94 } 95

96 if ( gameStatus == CONTINUE ) 97 showStatus( "Roll again." );

98 else {

99 if ( gameStatus == WON )

100 showStatus( "Player wins. " +

101 "Click Roll Dice to play again." );

102 else

103 showStatus( "Player loses. " +

104 "Click Roll Dice to play again." );

105

106 firstRoll = true;

107 } 108 } 109

110 // roll the dice

111 public int rollDice() 112 {

113 int die1, die2, workSum;

114

115 die1 = 1 + ( int ) ( Math.random() * 6 );

116 die2 = 1 + ( int ) ( Math.random() * 6 );

117 workSum = die1 + die2;

118

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Outline

Program Output

121 sum.setText( Integer.toString( workSum ) );

122

123 return workSum;

124 } 125}

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25.8Methods of Class JApplet

• Methods of Class JApplet

– init, start, stop, paint, destroy – Called automatically during execution

– By default, have empty bodies

– Must define yourself, using proper first line

• Otherwise, will not be called automatically

• See Figure 25.14 for proper first lines

• Method repaint

– Dynamically change appearance of applet

– Cannot call paint (do not have a Graphics object) – repaint(); calls update which passes Graphics

object for us

• Erases previous drawings and calls paint

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25.8Methods of Class JApplet (II)

First line of JApplet methods (descriptions Fig. 25.14)

public void init() public void start()

public void paint( Graphics g ) public void stop()

public void destroy()

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25.9Declaring and Allocating Arrays

• Arrays

– Specify type, use new operator – Two steps:

int c[]; //declaration

c = new int[ 12 ]; //initialization

– One step:

int c[] = new int[12];

– Primitive elements initialized to zero or false

• Non-primitive references are null

– Multiple declarations:

String b[] = new String[ 100 ], x[] = new String[ 27 ];

Also:

double[] array1, array2;

Put brackets after data type

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25.10 Examples Using Arrays

• new

– Dynamically creates arrays

• Method length

– Returns length of the array myArray.length

• Initializer lists

int myArray[] = { 1, 2, 3, 4, 5 };

• new operator not needed, provided automatically

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Outline

1. Initialize array 1.1 Loop and

generate random numbers (note scaling)

1.2 Append output 1.3 Initialize

outputArea 2.

showMessageDialog

1 // Fig. 25.21: RollDie.java

2 // Roll a six-sided die 6000 times 3 import javax.swing.*;

4

5 public class RollDie {

6 public static void main( String args[] ) 7 {

8 int face, frequency[] = new int[ 7 ];

10

11 for ( int roll = 1; roll <= 6000; roll++ ) { 12 face = 1 + ( int ) ( Math.random() * 6 );

13 ++frequency[ face ];

14 } 15

16 output += "Face\tFrequency";

17

18 for ( face = 1; face < frequency.length; face++ ) 19 output += "\n" + face + "\t" + frequency[ face ];

20

23

24 JOptionPane.showMessageDialog( null, outputArea, 25 "Rolling a Die 6000 Times",

26 JOptionPane.INFORMATION_MESSAGE );

27

28 System.exit( 0 );

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Outline

Program Output

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25.11 References and Reference Parameters

• Passing arguments to methods

– Call-by-value: pass copy of argument

– Call-by-reference: pass original argument

• Improve performance, weaken security

• In Java, cannot choose how to pass arguments

– Primitive data types passed call-by-value

– References to objects passed call-by-reference

• Original object can be changed in method

– Arrays in Java treated as objects

• Passed call-by-reference

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25.12 Multiple-Subscripted Arrays

• Multiple-Subscripted Arrays

– Represent tables

• Arranged by m rows and n columns (m by n array)

• Can have more than two subscripts

– Java does not support multiple subscripts directly

• Create an array with arrays as its elements

• Array of arrays

• Declaration

– Double brackets

int b[][];

b = new int[ 3 ][ 3 ];

• Declares a 3 by 3 array

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25.12 Multiple-Subscripted Arrays (II)

• Declaration (continued)

– Initializer lists

int b[][] = { { 1, 2 }, { 3, 4 } };

– Each row can have a different number of columns:

int b[][];

b = new int[ 2 ][ ]; // allocate rows

b[ 0 ] = new int[ 5 ]; // allocate columns for row 0 b[ 1 ] = new int[ 3 ]; // allocate columns for row 1

1 2 3 4

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Outline

1. Define class

1.1 init

1.2 Initialize double-scripted arrays

1 // Fig. 25.23: InitArray.java

2 // Initializing multidimensional arrays 3 import java.awt.Container;

5

6 public class InitArray extends JApplet { 7 JTextArea outputArea;

8

9 // initialize the applet 10 public void init()

11 {

12 outputArea = new JTextArea();

13 Container c = getContentPane();

14 c.add( outputArea );

15

16 int array1[][] = { { 1, 2, 3 }, { 4, 5, 6 } };

17 int array2[][] = { { 1, 2 }, { 3 }, { 4, 5, 6 } };

18

19 outputArea.setText( "Values in array1 by row are\n" );

20 buildOutput( array1 );

21

22 outputArea.append( "\nValues in array2 by row are\n" );

23 buildOutput( array2 );

24 } 25

26 public void buildOutput( int a[][] ) 27 {

28 for ( int i = 0; i < a.length; i++ ) { 29

30 for ( int j = 0; j < a[ i ].length; j++ )

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Outline

Program Output

31 outputArea.append( a[ i ][ j ] + " " );

32

33 outputArea.append( "\n" );

34 } 35 } 36 }