Chapter 5 - Methods Prentice Hall, Inc. All rights reserved.

3 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 2003 Prentice Hall, Inc. All rights reserved.

5 Math-Cla ss 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 2003 Prentice Hall, Inc. All rights reserved.

6 Methods Decla ra tions 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 2003 Prentice Hall, Inc. All rights reserved.

7 import java.awt.container; import javax.swing.*; public class SquareIntegers extends JApplet { public void init() { JTextArea outputarea = new JTextArea(); Container container = getcontentpane(); container.add( outputarea ); int result; String output = ""; for ( int counter = 1; counter <= 10; counter++ ) { result = square( counter ); output += "The square of " + counter + " is " + result + "\n"; }

8 outputarea.settext( output ); } public int square( int y ) { return y * y; } }

Method Decla ra tions (cont.) 9 General format of method declaration: return-value-type method-name( parameter1, parameter2,, parametern ) { declarations and statements } Method can also return values: return expression; 2003 Prentice Hall, Inc. All rights reserved.

import java.awt.container; 10 import javax.swing.*; public class MaximumTest extends JApplet { // initialize applet by obtaining user input and creating GUI public void init() { // obtain user input String s1 = JOptionPane.showInputDialog( "Enter first floating-point value" ); String s2 = JOptionPane.showInputDialog( "Enter second floating-point value" ); String s3 = JOptionPane.showInputDialog( "Enter third floating-point value" ); // convert user input to double values double number1 = Double.parseDouble( s1 ); double number2 = Double.parseDouble( s2 ); double number3 = Double.parseDouble( s3 ); double max = maximum( number1, number2, number3 ); // method call // create JTextArea to display results JTextArea outputarea = new JTextArea(); // display numbers and maximum value outputarea.settext( "number1: " + number1 + "\nnumber2: " + number2 + "\nnumber3: " + number3 + "\nmaximum is: " + max );

11 Container container = getcontentpane(); container.add( outputarea ); } public double maximum( double x, double y, double z ) { return Math.max( x, Math.max( y, z ) ); } }

Argument Promotion 12 Coercion of arguments Forcing arguments to appropriate type to pass to method e.g., System.out.println( Math.sqrt( 4 ) ); Evaluates Math.sqrt( 4 ) Then evaluates System.out.println() Promotion rules Specify how to convert types without data loss 2003 Prentice Hall, Inc. All rights reserved.

13 Type Valid promotions double None float double long float or double int long, float or double char int, long, float or double short int, long, float or double byte short, int, long, float or double boolean None (boolean values are not considered to be numbers in Java) 2003 Prentice Hall, Inc. All rights reserved.

Java API Packages 14 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; 2003 Prentice Hall, Inc. All rights reserved.

Ra ndom-number Genera tion 15 Java random-number generators Math.random() ( int ) ( Math.random() * 6 ) Produces integers from 0-5 Use a seed for different random-number sequences 2003 Prentice Hall, Inc. All rights reserved.

1 // Fig. 6.7: RandomIntegers.java 2 // 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 times 13 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 output 19 20 // if counter divisible by 5, append newline to String output 21 if ( counter % 5 == 0 ) 22 output += "\n"; 23 24 } // end for 25 Produce integers in range 1-6 Math.random returns doubles. We cast the double as an int 16 2003 Prentice Hall, Inc. All rights reserved.

26 JOptionPane.showMessageDialog( null, output, 27 "20 Random Numbers from 1 to 6", 28 JOptionPane.INFORMATION_MESSAGE ); 29 30 System.exit( 0 ); // terminate application 31 32 } // end main 33 34 } // end class RandomIntegers 17 RandomIntegers. java 2003 Prentice Hall, Inc. All rights reserved.

1 // Fig. 6.8: RollDie.java 2 // Roll a six-sided die 6000 times. 3 import javax.swing.*; 4 5 public class RollDie { 6 7 public static void main( String args[] ) 8 { 9 int frequency1 = 0, frequency2 = 0, frequency3 = 0, 10 frequency4 = 0, frequency5 = 0, frequency6 = 0, face; 11 12 // summarize results 13 for ( int roll = 1; roll <= 6000; roll++ ) { 14 face = 1 + ( int ) ( Math.random() * 6 ); 15 16 // determine roll value and increment appropriate counter 17 switch ( face ) { 18 19 case 1: 20 ++frequency1; 21 break; 22 23 case 2: 24 ++frequency2; 25 break; 26 27 case 3: 28 ++frequency3; 29 break; 30 Produce integers in range 1-6 Increment appropriate frequency counter, depending on randomly generated number 18 2003 Prentice Hall, Inc. All rights reserved.

31 case 4: 32 ++frequency4; 33 break; 34 35 case 5: 36 ++frequency5; 37 break; 38 39 case 6: 40 ++frequency6; 41 break; 42 43 } // end switch 44 45 } // end for 46 47 JTextArea outputarea = new JTextArea(); 48 49 outputarea.settext( "Face\tFrequency" + "\n1\t" + frequency1 + 50 "\n2\t" + frequency2 + "\n3\t" + frequency3 + 51 "\n4\t" + frequency4 + "\n5\t" + frequency5 + 52 "\n6\t" + frequency6 ); 53 54 JOptionPane.showMessageDialog( null, outputarea, 55 "Rolling a Die 6000 Times", JOptionPane.INFORMATION_MESSAGE ); 56 57 System.exit( 0 ); // terminate application 58 59 } // end main 60 61 } // end class RollDie 2003 Prentice Hall, Inc. All rights reserved. 19

20 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 label in a labeled break or continue statement Scope of a local-variable declaration that appears in the initialization section of a for statement s header Scope of a method or field of a class 2003 Prentice Hall, Inc. All rights reserved.

1 // Fig. 6.10: Scoping.java 2 // 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 class 11 int x = 1; 12 13 // create applet's GUI 14 public void init() 15 { 16 outputarea = new JTextArea(); 17 Container container = getcontentpane(); 18 container.add( outputarea ); 19 20 } // end method init 21 22 // method start called after init completes; start calls 23 // methods uselocal and usefield 24 public void start() 25 { Field x has class scope Local variable x has block scope Method start uses local variable x 26 int x = 5; // local variable in method start that shadows field x 27 28 outputarea.append( "local x in start is " + x ); 29 21 2003 Prentice Hall, Inc. All rights reserved.

30 uselocal(); // uselocal has local x 31 usefield(); // useinstance uses Scoping's field x 32 uselocal(); // uselocal reinitializes local x 33 usefield(); // Scoping's field x retains its value 34 35 outputarea.append( "\n\nlocal x in start is " + x ); 36 37 } // end method start 38 39 Re-create variable x and // uselocal creates and initializes initialize local it to 25 variable x during each call 40 public void uselocal() 41 { 42 int x = 25; // initialized each time uselocal is called 43 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 uselocal 51 Method uselocal uses local variable x 22 2003 Prentice Hall, Inc. All rights reserved.

52 // usefield modifies Scoping's field x during each call 53 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 useinstance 62 63 } // end class Scoping Method usefield uses field x 23 2003 Prentice Hall, Inc. All rights reserved.

Methods of Cla ss JApplet 24 Method public void init() When the method is called and its purpose 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. 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. public void This drawing method is called after the init method completes execution and the start method has paint( Graphics started. It is also called every time the applet needs to be repainted. For example, if the user covers g ) 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() public void destroy() 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. 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.

1 // Fig. 6.12: MethodOverload.java 2 // Using overloaded methods 3 import java.awt.container; 4 5 import javax.swing.*; 6 7 public class MethodOverload extends JApplet { 8 9 // create GUI and call each square method 10 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 init Method square receives an int as an argument 20 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 26 2003 Prentice Hall, Inc. All rights reserved.

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 Overloaded method square receives a double as an argument 27 Called square with int argument: 7 Called square with double argument: 7.5 2003 Prentice Hall, Inc. All rights reserved.

1 // Fig. 6.13: MethodOverload.java 2 // 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 argument 8 public int square( int x ) 9 { 10 return x * x; 11 } 12 13 // second declaration of method square 14 // with int argument causes syntax error 15 public double square( int y ) 16 { 17 return y * y; 18 } 19 20 } // end class MethodOverload Compiler cannot distinguish between methods with identical names and parameter sets 28 MethodOverload.java:15: square(int) is already defined in MethodOverload public double square( int y ) ^ 1 error 2003 Prentice Hall, Inc. All rights reserved.

29 Recursion Recursive method Calls itself (directly or indirectly) through another method Method knows how to solve only a base case Method divides problem Base case Simpler problem Method now divides simpler problem until solvable Recursive call Recursive step 2003 Prentice Hall, Inc. All rights reserved.

30 5! 5! Final value = 120 5! = 5 * 24 = 120 is returned 5 * 4! 5 * 4! 4! = 4 * 6 = 24 is returned 4 * 3! 4 * 3! 3! = 3 * 2 = 6 is returned 3 * 2! 3 * 2! 2! = 2 * 1 = 2 is returned 2 * 1! 1 2 * 1! 1 1 returned (a) Sequence of recursive calls. (b) Values returned from each recursive call. 2003 Prentice Hall, Inc. All rights reserved.

1 // Fig. 6.15: FactorialTest.java 2 // 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 0-10 11 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 10 19 for ( long counter = 0; counter <= 10; counter++ ) 20 outputarea.append( counter + "! = " + 21 factorial( counter ) + "\n" ); 22 23 } // end method init 24 Invoke method factorial 31 2003 Prentice Hall, Inc. All rights reserved.

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 32 2003 Prentice Hall, Inc. All rights reserved.

33 Recursion vs. Itera tion Iteration Uses repetition structures (for, while or do while) Repetition through explicitly use of repetition structure Terminates when loop-continuation condition fails Controls repetition by using a counter Recursion Uses selection structures (if, if else or switch) Repetition through repeated method calls Terminates when base case is satisfied Controls repetition by dividing problem into simpler one 2003 Prentice Hall, Inc. All rights reserved.

34 Recursion vs. Itera tion (cont.) Recursion More overhead than iteration More memory intensive than iteration Can also be solved iteratively Often can be implemented with only a few lines of code 2003 Prentice Hall, Inc. All rights reserved.