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11 Methods

CE00858-1: Fundamental Programming Techniques

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Objectives

In this session, we will:• introduce methods and look at the main() method• implement methods and call them• pass values into methods• return values from methods

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Methods

• named block of code that can be invoked one or more times in an application• performs a well defined task• has no side effects• is easily understood

• advantages:• can focus on what a block of code does, not how• can call method as many times as needed• reduces length of code• simplifies testing

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Methods in Java

• method header:• returnType:

• type of data that is returned by method• void if method doesn’t return anything

• methodName • parameters enclosed by brackets:

• information needed by method • brackets must be included even if no parameters needed

• method body:• enclosed by braces• 1 or more statements to be executed

returnType methodName (parameters) {

statements;}

method header

method body

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main() method

• all Java applications must have a main method• automatically invoked when program run• can include calls to other methods

public class MyClass{

public static void main (String [] args) { //method body

//calls to other methods}

}

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Structure of a class

public class MyClass{

public static void main (String [] args) { //method body for main

} public static void myMethod1 () {

//method body for myMethod1 } public static void myMethod2() {

//method body for myMethod2 }}

• class may have many methods

Method implementation

• if a program is to be implemented using methods, need to:• select meaningful name for method• consider values needed by method to perform its

task• consider values returned by method• analyse operations performed inside method body

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Method example – greet()

• problem:• a method is required to output a greeting to the user

on two separate lines

• method:• method name: greet• method parameters: none• method returns: nothing• analysis for method body: 2 output statements to

greet userpublic static void greet (){ System.out.println("Hello and welcome"); System.out.println("How are you?");}

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Calling methods example – GreetUser

• once a method has been written it can be called as many times as required by specifying its name and any parameters required

public class GreetUser{

public static void main (String [] args){

greet (); greet ();

}

public static void greet (){ System.out.println("Hello and welcome");

System.out.println("How are you?");}

}

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greet() method with parameters

• problem:• the greet() method is to be modified so that a name

is passed as a parameter to be output

• method:• method name: greet• method parameters: name – String to be output• method returns: nothing• analysis for method body: 2 output statements to

greet userpublic static void greet (String name){ System.out.println("Hello " + name + " and welcome"); System.out.println("How are you?");}

parameter

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Calling methods with parameters

• if the method has parameters, these must be given a value when the method is called

import java.util.*;public class GreetUser{

public static void main (String [] args){

Scanner kybd = new Scanner(System.in);String myName = kybd.next();

greet (myName);String yourName = kybd.next();

greet (yourName);}

public static void greet (String name){

System.out.println("Hello " + name + " and welcome"); System.out.println("How are you?");

}}

value for parametervalue for parameter

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Call by value

• when a value is passed into a method, a copy of it is taken and used within the method

• after the method finishes, changes to the value are not kept

11 Methods

public class Swap{

public static void main (String [] args){

int x = 3;int y = 4;System.out.println("x is: " + x + ", y is: " + y);swap(x, y);System.out.println("x is: " + x + ", y is: " + y);

}public static void swap (int a, int b){

int temp = a;a = b;b = temp;

}}

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Swap.java

what is output?

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greet() method with return value

• problem:• the greet() method is to be modified so that a name

is read inside the method, the greeting is output and the name is returned to the calling program

• method:• method name: greet• method parameters: none• method returns: String name• analysis for method body:

• create Scanner• prompt user for name• input name • 2 output statements to greet user• return name

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public static String greet (){

Scanner kybd = new Scanner(System.in);System.out.print("Enter name: ");

String name = kybd.next();

System.out.println("Hello " + name + " and welcome"); System.out.println("How are you?");

return name;}

return type

return statement

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Calling methods with return values

• if the method returns a value the calling program should do something with that value:• output it• store it• use it in a comparison

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import java.util.*;public class GreetUser{

public static void main (String [] args){

String aName = greet ();if (aName.equals("Elizabeth")){

System.out.println("Are you the queen?");}

}

public static String greet (){

Scanner kybd = new Scanner(System.in);System.out.print("Enter name: ");

String name = kybd.next();

System.out.println("Hello " + name + " and welcome");

System.out.println("How are you?");

return name;}

}

GreetUser.java

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Analysis and methods

• when faced with a specification the designer does not consider how the solution will be implemented

• the use of methods is an implementation decision which should be taken after analysis

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Method example – Underline

• analysis:• output a row of 10 *• output a row of 15 @

• implementation• as same actions will be done, but with different values, this

is a candidate for implementation using methods and parameters

• method name: underline()• method parameters:

• ch – character to be output• times – integer number of times character is to be output

• method doesn't return a value

• problem:• a program is required to output a row of 10 stars,

followed by a row of 15 @ symbols

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underline() method analysis

• analysis for method body:• what data is used?

• ch: character, passed as parameter• times: integer, passed as parameter

• what operations are performed?• iteration needed as ch is output several times

• what operations are done once before the loop?• none

• how many times is loop repeated?• times = 1 to times

• what operations are done inside the loop?• output ch

• what operations are done after the loop?• output new line

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//passing parameters to methodspublic class Underline{

public static void main (String [] args){ underline('*', 10); underline('@', 15);}

public static void underline (char ch, int times){ for (int i = 0; i < times; i++) {

System.out.print(ch); } System.out.println();

}}

Underline.java

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Method example – Largest4

• analysis• what data is used?

• num1, num2, num3 and num4: integers input by user• what operations are performed?

• determine larger of num1 and num2• determine larger of num3 and num4• determine larger of two previous results• output largest number

• problem:• output the largest of 4 numbers input by the user

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larger() method analysis

• implementation• as same actions will be done, but with different

values, this is a candidate for implementation using methods and return values

• method name: larger()• method parameters:

• a, b – integers to compare• method returns:

• larger integer

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larger() method analysis cont.

• method body analysis• what data is used?

• a: integer passed as parameter• b: integer passed as parameter

• what operations are performed?• selection needed as a larger dealt with differently to b

larger• what operations are done before the selection?

• none• what operations are done if a larger?

• store a• what operations are done if b larger?

• store• what operations are done after the selection?

• return larger

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//program to show return valuesimport java.util.*;public class Largest4{

public static void main (String [] args){

//input data Scanner kybd = new Scanner(System.in);System.out.print("Enter 4 numbers: ");int num1 = kybd.nextInt();int num2 = kybd.nextInt();int num3 = kybd.nextInt();int num4 = kybd.nextInt();

//determine larger of num1 and num2//determine larger of num3 and num4//determine larger of two previous results//output largestSystem.out.println("Largest is: " + larger(larger(num1, num2), larger(num3,

num4)));}

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Largest4.java

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public static int larger(int a, int b){

int largerNum;if (a > b){

largerNum = a;}else{ largerNum = b;}

return largerNum;}

}

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Summary

In this session we have:• looked at implementing methods in Java• seen how methods are called from within the main()

method• passed parameters into methods • returned results from methods

In the next session we will:• look at making methods even more flexible