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Lecture 4: Chapter 6 - MethodsOutline Introduction Program Modules in Java Math-Class Methods Method Declarations Java API Packages Random-Number Generation Scope of Declarations Methods of Class JApplet Method Overloading Example Using Recursion: The Fibonacci Series

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Introduction

• Modules– Small pieces of a problem

– Facilitate design, implementation, operation and maintenance of large programs

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Program Modules in Java

• Modules in Java– Methods

– Classes

• Java API provides several modules• Programmers can also create modules

– e.g., programmer-defined methods

• Methods– Invoked by a method call

– Returns a result to calling method (caller)

– Similar to a boss (caller) asking a worker (called method) to complete a task

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Math-Class Methods

• Class java.lang.Math– Provides common mathematical calculations

– Calculate the square root of 900.0:• Math.sqrt( 900.0 )

– Method sqrt belongs to class Math• Dot (.) allows access to method sqrt

– The argument 900.0 is located inside parentheses

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Method Description Example abs( x ) absolute value of x (this method also has float, int and long versions) abs( 23.7 ) is 23.7

abs( 0.0 ) is 0.0 abs( -23.7 ) is 23.7

ceil( x ) rounds x to the smallest integer not less than x ceil( 9.2 ) is 10.0 ceil( -9.8 ) is -9.0

cos( x ) trigonometric cosine of x (x is in radians) cos( 0.0 ) is 1.0 exp( x ) exponential method ex exp( 1.0 ) is 2.71828

exp( 2.0 ) is 7.38906 floor( x ) rounds x to the largest integer not greater than x floor( 9.2 ) is 9.0

floor( -9.8 ) is -10.0 log( x ) natural logarithm of x (base e) log( Math.E ) is 1.0

log( Math.E * Math.E ) is 2.0 max( x, y ) larger value of x and y (this method also has float, int and long

versions) max( 2.3, 12.7 ) is 12.7 max( -2.3, -12.7 ) is -2.3

min( x, y ) smaller value of x and y (this method also has float, int and long versions)

min( 2.3, 12.7 ) is 2.3 min( -2.3, -12.7 ) is -12.7

pow( x, y ) x raised to the power y (xy) pow( 2.0, 7.0 ) is 128.0 pow( 9.0, 0.5 ) is 3.0

sin( x ) trigonometric sine of x (x is in radians) sin( 0.0 ) is 0.0 sqrt( x ) square root of x sqrt( 900.0 ) is 30.0

sqrt( 9.0 ) is 3.0 tan( x ) trigonometric tangent of x (x is in radians) tan( 0.0 ) is 0.0 Fig. 6.2 Math-class methods.

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Methods Declarations

• Methods– Allow programmers to modularize programs

• Makes program development more manageable

• Software reusability

• Avoid repeating code

– Local variables• Declared in method declaration

– Parameters• Communicates information between methods via method calls

Outline7

SquareIntegers.java

Line 21Declare result to store square of number

Line 26Method init invokes method square

Line 26Method square returns int that result stores

1 // Fig. 6.3: SquareIntegers.java2 // Creating and using a programmer-defined method.3 import java.awt.Container;4 5 import javax.swing.*;6 7 public class SquareIntegers extends JApplet {8 9 // set up GUI and calculate squares of integers from 1 to 1010 public void init()11 {12 // JTextArea to display results 13 JTextArea outputArea = new JTextArea();14 15 // get applet's content pane (GUI component display area)16 Container container = getContentPane(); 17 18 // attach outputArea to container19 container.add( outputArea ); 20 21 int result; // store result of call to method square22 String output = ""; // String containing results23 24 // loop 10 times25 for ( int counter = 1; counter <= 10; counter++ ) {26 result = square( counter ); // method call27 28 // append result to String output 29 output += "The square of " + counter + " is " + result + "\n";30 31 } // end for

Declare result to store square of number

Method square returns int that result stores

Method init invokes method square (next slide)

Outline8

SquareIntegers.java

Line 38y is the parameter of method square

Line 40Method square returns the square of y

32 33 outputArea.setText( output ); // place results in JTextArea34 35 } // end method init36 37 // square method declaration 38 public int square( int y ) 39 { 40 return y * y; // return square of y41 42 } // end method square 43 44 } // end class SquareIntegers

y is the parameter of method square

Method square returns the square of y

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Method Declarations (cont.)

• General format of method declaration:

return-value-type method-name( parameter1, parameter2, …, parameterN ){ declarations and statements}

• Method can also return values: return expression;

Outline10

Maximum.java

Lines 13-18User inputs three Strings

Lines 21-23Convert Strings to doubles

Line 25Method init passes doubles as arguments to method maximum

1 // Fig. 6.4: MaximumTest.java2 // Finding the maximum of three floating-point numbers.3 import java.awt.Container;4 5 import javax.swing.*;6 7 public class MaximumTest extends JApplet {8 9 // initialize applet by obtaining user input and creating GUI10 public void init()11 {12 // obtain user input13 String s1 = JOptionPane.showInputDialog(14 "Enter first floating-point value" );15 String s2 = JOptionPane.showInputDialog(16 "Enter second floating-point value" );17 String s3 = JOptionPane.showInputDialog(18 "Enter third floating-point value" );19 20 // convert user input to double values21 double number1 = Double.parseDouble( s1 );22 double number2 = Double.parseDouble( s2 );23 double number3 = Double.parseDouble( s3 );24 25 double max = maximum( number1, number2, number3 ); // method call26 27 // create JTextArea to display results28 JTextArea outputArea = new JTextArea();29 30 // display numbers and maximum value 31 outputArea.setText( "number1: " + number1 + "\nnumber2: " + 32 number2 + "\nnumber3: " + number3 + "\nmaximum is: " + max );33

User inputs three Strings

Convert Strings to doubles

Method init passes doubles as arguments to

method maximum

Outline11

Maximum.java

Line 46Method maximum returns value from method max of class Math

34 // get applet's GUI component display area35 Container container = getContentPane();36 37 // attach outputArea to Container c38 container.add( outputArea );39 40 } // end method init41 42 // maximum method uses Math class method max to help 43 // determine maximum value 44 public double maximum( double x, double y, double z )45 { 46 return Math.max( x, Math.max( y, z ) ); 47 48 } // end method maximum 49 50 } // end class Maximum

Method maximum returns value from method max of class Math

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Java API Packages

• Packages

– Classes grouped into categories of related classes

– Promotes software reuse

– import statements specify classes used in Java programs

• e.g., import javax.swing.JApplet;

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Package Description java.applet The Java Applet Package contains the Applet class and several interfaces that enable applet/browser interaction

and the playing of audio clips. In Java 2, class javax.swing.JApplet is used to define an applet that uses the Swing GUI components.

java.awt The Java Abstract Window Toolkit Package contains the classes and interfaces required to create and manipulate GUIs in Java 1.0 and 1.1. In Java 2, the Swing GUI components of the javax.swing packages are often used instead.

java.awt.event The Java Abstract Window Toolkit Event Package contains classes and interfaces that enable event handling for GUI components in both the java.awt and javax.swing packages.

java.io The Java Input/Output Package contains classes that enable programs to input and output data (see Chapter 17, Files and Streams).

java.lang The Java Language Package contains classes and interfaces (discussed throughout this text) that are required by many Java programs. This package is imported by the compiler into all programs.

java.net The Java Networking Package contains classes that enable programs to communicate via networks (see Chapter 18, Networking).

java.text The Java Text Package contains classes and interfaces that enable a Java program to manipulate numbers, dates, characters and strings. The package provides many of Java’s internationalization capabilities that enable a program to be customized to a specific locale (e.g., an applet may display strings in different languages, based on the user’s country).

java.util The Java Utilities Package contains utility classes and interfaces, such as date and time manipulations, random-number processing capabilities with class Random, storing and processing large amounts of data and breaking strings into smaller pieces called tokens with class StringTokenizer (see Chapter 20; Data Structures, Chapter 21, Java Utilities Package and Bit Manipulation; and Chapter 22, Collections).

javax.swing The Java Swing GUI Components Package contains classes and interfaces for Java’s Swing GUI components that provide support for portable GUIs.

javax.swing.event The Java Swing Event Package contains classes and interfaces that enable event handling for GUI components in package javax.swing.

Fig. 6.6 Java API packages (a subset).

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6.7 Random-Number Generation

• Java random-number generators

– Math.random()

• ( int ) ( Math.random() * 6 )

– Produces integers from 0 - 5

– Use a seed for different random-number sequences

Outline15

RandomIntegers.java

Line 16Produce integers in range 1-6

Line 16Math.random returns doubles. We cast the double as an int

1 // Fig. 6.7: RandomIntegers.java2 // Shifted, scaled random integers.3 import javax.swing.JOptionPane;4 5 public class RandomIntegers {6 7 public static void main( String args[] )8 {9 int value;10 String output = "";11 12 // loop 20 times13 for ( int counter = 1; counter <= 20; counter++ ) {14 15 // pick random integer between 1 and 6 16 value = 1 + ( int ) ( Math.random() * 6 );17 18 output += value + " "; // append value to output19 20 // if counter divisible by 5, append newline to String output21 if ( counter % 5 == 0 )22 output += "\n";23 24 } // end for25

Produce integers in range 1-6

Math.random returns doubles. We cast the double as an int

Outline16

RandomIntegers.java

26 JOptionPane.showMessageDialog( null, output, 27 "20 Random Numbers from 1 to 6", 28 JOptionPane.INFORMATION_MESSAGE );29 30 System.exit( 0 ); // terminate application31 32 } // end main33 34 } // end class RandomIntegers

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Scope of Declarations

• Scope– Portion of the program that can reference an entity by its

name

– Basic scope rules• Scope of a parameter declaration

• Scope of a local-variable declaration

• Scope of a method or field of a class

Outline18

Scoping.java

Line 11field x

Line 26Local variable x

Line 28Method start uses local variable x

1 // Fig. 6.10: Scoping.java2 // A scoping example.3 import java.awt.Container;4 5 import javax.swing.*;6 7 public class Scoping extends JApplet {8 JTextArea outputArea;9 10 // field that is accessible to all methods of this class11 int x = 1; 12 13 // create applet's GUI14 public void init()15 {16 outputArea = new JTextArea();17 Container container = getContentPane(); 18 container.add( outputArea );19 20 } // end method init21 22 // method start called after init completes; start calls23 // methods useLocal and useField24 public void start()25 {26 int x = 5; // local variable in method start that shadows field x27 28 outputArea.append( "local x in start is " + x );29

Field x has class scope

Local variable x has block scope

Method start uses local variable x

Outline19

Scoping.java

Line 42Recreate variable x and initialize it to 25

Lines 40-50Method useLocal uses local variable x

30 useLocal(); // useLocal has local x31 useField(); // useInstance uses Scoping's field x32 useLocal(); // useLocal reinitializes local x33 useField(); // Scoping's field x retains its value34 35 outputArea.append( "\n\nlocal x in start is " + x );36 37 } // end method start38 39 // useLocal creates and initializes local variable x during each call40 public void useLocal()41 {42 int x = 25; // initialized each time useLocal is called43 44 outputArea.append( "\n\nlocal x in useLocal is " + x +45 " after entering useLocal" );46 ++x;47 outputArea.append( "\nlocal x in useLocal is " + x +48 " before exiting useLocal" );49 50 } // end method useLocal51

Re-create variable x and initialize it to 25

Method useLocal uses local variable x

Outline20

Scoping.java

Lines 53-61Method useField uses field x

52 // useField modifies Scoping's field x during each call53 public void useField()54 {55 outputArea.append( "\n\nfield x is " + x +56 " on entering useField" );57 x *= 10;58 outputArea.append( "\nfield x is " + x +59 " on exiting useField" );60 61 } // end method useInstance62 63 } // end class Scoping

Method useField uses field x

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Methods of Class JApplet

• Java API defines several JApplet methods– Defining methods of Fig. 6.11 in a JApplet is called

overriding those methods.

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Method When the method is called and its purpose public void init()

This method is called once by the applet container when an applet is loaded for execution. It performs initialization of an applet. Typical actions performed here are initializing fields, creating GUI components, loading sounds to play, loading images to display (see Chapter 19, Multimedia) and creating threads (see Chapter 16, Multithreading).

public void start()

This method is called after the init method completes execution. In addition, if the browser user visits another Web site and later returns to the HTML page on which the applet resides, method start is called again. The method performs any tasks that must be completed when the applet is loaded for the first time and that must be performed every time the HTML page on which the applet resides is revisited. Typical actions performed here include starting an animation (see Chapter 19) and starting other threads of execution (see Chapter 16).

public void paint( Graphics g )

This drawing method is called after the init method completes execution and the start method has started. It is also called every time the applet needs to be repainted. For example, if the user covers the applet with another open window on the screen and later uncovers the applet, the paint method is called. Typical actions performed here involve drawing with the Graphics object g that is passed to the paint method by the applet container.

public void stop()

This method is called when the applet should stop executing—normally, when the user of the browser leaves the HTML page on which the applet resides. The method performs any tasks that are required to suspend the applet’s execution. Typical actions performed here are to stop execution of animations and threads.

public void destroy()

This method is called when the applet is being removed from memory—normally, when the user of the browser exits the browsing session (i.e., closes all browser windows). The method performs any tasks that are required to destroy resources allocated to the applet.

Fig. 6.11 JApplet methods that the applet container calls during an applet’s execution.

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Method Overloading

• Method overloading– Several methods of the same name

– Different parameter set for each method• Number of parameters

• Parameter types

Outline24

MethodOverload.java

Lines 22-29Method square receives an int as an argument

1 // Fig. 6.12: MethodOverload.java2 // Using overloaded methods3 import java.awt.Container;4 5 import javax.swing.*;6 7 public class MethodOverload extends JApplet {8 9 // create GUI and call each square method10 public void init()11 {12 JTextArea outputArea = new JTextArea();13 Container container = getContentPane();14 container.add( outputArea );15 16 outputArea.setText( "The square of integer 7 is " + square( 7 ) +17 "\nThe square of double 7.5 is " + square( 7.5 ) );18 19 } // end method init20 21 // square method with int argument 22 public int square( int intValue ) 23 { 24 System.out.println( "Called square with int argument: " +25 intValue ); 26 27 return intValue * intValue; 28 29 } // end method square with int argument 30

Method square receives an int as an argument

Outline25

MethodOverload.java

Lines 32-39Overloaded method square receives a double as an argument

31 // square method with double argument 32 public double square( double doubleValue ) 33 { 34 System.out.println( "Called square with double argument: " +35 doubleValue ); 36 37 return doubleValue * doubleValue; 38 39 } // end method square with double argument 40 41 } // end class MethodOverload

Called square with int argument: 7Called square with double argument: 7.5

Overloaded method square receives a double as an argument

Outline26

MethodOverload.java

Lines 8 and 15Compiler cannot distinguish between methods with identical names and parameter sets

Fig. 6.17 Compiler error messages generated from overloaded methods with identical parameter lists and different return types.

1 // Fig. 6.13: MethodOverload.java2 // Overloaded methods with identical signatures.3 import javax.swing.JApplet;4 5 public class MethodOverload extends JApplet {6 7 // declaration of method square with int argument8 public int square( int x )9 {10 return x * x;11 }12 13 // second declaration of method square 14 // with int argument causes syntax error15 public double square( int y ) 16 { 17 return y * y; 18 } 19 20 } // end class MethodOverload

MethodOverload.java:15: square(int) is already defined in MethodOverload public double square( int y ) ^1 error

Compiler cannot distinguish between methods with identical names and

parameter sets

Outline27

FactorialTest.java

Line 21Invoke method factorial

1 // Fig. 6.15: FactorialTest.java2 // Recursive factorial method.3 import java.awt.*;4 5 import javax.swing.*;6 7 public class FactorialTest extends JApplet {8 JTextArea outputArea;9 10 // create GUI and calculate factorials of 0-1011 public void init()12 {13 outputArea = new JTextArea();14 15 Container container = getContentPane();16 container.add( outputArea );17 18 // calculate the factorials of 0 through 1019 for ( long counter = 0; counter <= 10; counter++ )20 outputArea.append( counter + "! = " +21 factorial( counter ) + "\n" );22 23 } // end method init24

Invoke method factorial

Outline28

FactorialTest.java

Lines 29-30Test for base case (method factorial can solve base case)

Line 34Else return simpler problem that method factorial might solve in next recursive call

25 // recursive declaration of method factorial 26 public long factorial( long number ) 27 { 28 // base case 29 if ( number <= 1 ) 30 return 1; 31 32 // recursive step 33 else 34 return number * factorial( number - 1 );35 36 } // end method factorial 37 38 } // end class FactorialTest

Test for base case (method factorial can solve base case)

Else return simpler problem that method factorial might solve

in next recursive call

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Example Using Recursion: The Fibonacci Series

• Fibonacci series– Each number in the series is sum of two previous numbers

• e.g., 0, 1, 1, 2, 3, 5, 8, 13, 21…

fibonacci(0) = 0 fibonacci(1) = 1fibonacci(n) = fibonacci(n - 1) + fibonacci( n – 1 )

Outline30

FibonacciTest.java

1 // Fig. 6.16: FibonacciTest.java2 // Recursive fibonacci method.3 import java.awt.*;4 import java.awt.event.*;5 6 import javax.swing.*;7 8 public class FibonacciTest extends JApplet implements ActionListener {9 JLabel numberLabel, resultLabel;10 JTextField numberField, resultField;11 12 // set up applet’s GUI13 public void init()14 {15 // obtain content pane and set its layout to FlowLayout16 Container container = getContentPane();17 container.setLayout( new FlowLayout() );18 19 // create numberLabel and attach it to content pane20 numberLabel = new JLabel( "Enter an integer and press Enter" );21 container.add( numberLabel );22 23 // create numberField and attach it to content pane24 numberField = new JTextField( 10 );25 container.add( numberField );26 27 // register this applet as numberField’s ActionListener28 numberField.addActionListener( this ); 29

Outline31

FibonacciTest.java Line 43Method actionPerformed is invoked when user presses EnterLine 45We use long, because Fibonacci numbers become large quicklyLines 48-53Pass user input to method fibonacci

30 // create resultLabel and attach it to content pane31 resultLabel = new JLabel( "Fibonacci value is" );32 container.add( resultLabel );33 34 // create numberField, make it uneditable35 // and attach it to content pane36 resultField = new JTextField( 15 );37 resultField.setEditable( false );38 container.add( resultField );39 40 } // end method init41 42 // obtain user input and call method fibonacci43 public void actionPerformed( ActionEvent event )44 { 45 long number, fibonacciValue;46 47 // obtain user’s input and convert to long48 number = Long.parseLong( numberField.getText() );49 50 showStatus( "Calculating ..." ); 51 52 // calculate fibonacci value for number user input53 fibonacciValue = fibonacci( number ); 54 55 // indicate processing complete and display result56 showStatus( "Done." ); 57 resultField.setText( Long.toString( fibonacciValue ) );58 59 } // end method actionPerformed60

Method actionPerformed is invoked when user presses Enter

We use long, because Fibonacci numbers

become large quickly

Pass user input to method fibonacci

Outline32

FibonacciTest.java

Lines 65-66Test for base case (method fibonacci can solve base case)

Lines 69-70Else return simpler problem that method fibonacci might solve in next recursive call

61 // recursive declaration of method fibonacci 62 public long fibonacci( long n ) 63 { 64 // base case 65 if ( n == 0 || n == 1 ) 66 return n; 67 68 // recursive step 69 else 70 return fibonacci( n - 1 ) + fibonacci( n - 2 );71 72 } // end method fibonacci 73 74 } // end class FibonacciTest

Else return simpler problem that method fibonacci might solve

in next recursive call

Test for base case (method fibonacci can solve base case)

Outline33

FibonacciTest.java

Outline34

FibonacciTest.java