Topic 3 Part 1 Method

7/30/2019 Topic 3 Part 1 Method

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OBJECT ORIENTED

PROGRAMMING IN JAVAMethod in Java Program


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COURSE LEARNING OUTCOMES :

Upon completion of this course, the studentsshould be able to:

1. Describe the features and basic concepts of Javalanguage.

1. Write, compile Java source code and interpret Java bytecode using Java Development Kit (JDK).

1. Implement the key of OOP concepts: classes and objects,inheritance and polymorphism.

1. Incorporate exception handling in Java Programming.


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OBJECTIVES :

1. Create classes in Java program

1. Identify built-in classes in Java program

1. Create an object in Java program

1. Explain the concepts of accessing objects via referencevariables

2. Explain the concepts of accessing objects data andmethod

3. Explain the scope of variables in classes


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Preview Questions

State TRUE or FALSE1. A class can contain ONLY ONE method

2. The basic form of method must return astring

3. Worker methods can also caller methods4. Main method accepts an array of strings

5. A methods can only have one parameter

6. Methods cannot have the same name as the

class.7. Methods can be declares outisde a class as

a standalone method.


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Define Method

In a class is contained within one or moremethods.

The syntax of all method declarations is :

[modifiers] return_type method_identifier([arguments])

{

//method code block

}


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Where : [modifiers] represent several Java

technology keywords that modify the waymethods are used. Modifiers are OPTIONAL

return_type is the type of value return from amethod that can be used elsewhere in theprogram. Methods can return only ONE item(literal value, variable, object reference and soon). If nothing is be returned, the keyword voidmust be specified as the return type.

Method_identifier is the name of themethods


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The ([arguments]) represent a list of

variables whose values are passed to the methodfor use by the method. Arguments are OPTIONAL(indicated by the square brackets); many methoddo not accept arguments.

Themethod_code_block is a sequence of

statements that the method performs. A widevariety of tasks can take place in the code block orbody of a method.


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Basic Form of a Method

The basic form of a method accepts no arguments andreturns nothing.

The following display method from Shirt class is a basic

method :public void displayInformation() {

System.out.println(Shirt ID : + shirtID);

System.out.println(Shirt description : +

description);

System.out.println(Color Code: + colorCode);

System.out.println(Shirt Price: + price);

System.out.println(Quantity in stock: +

quantityInStock);

} // end of display method

This method prints several lines of information.


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Invoking a Method From a Different

Class To invoke/execute a method in a different class,

use the dot (.) operator with an object referencevariable to do to access the public variables ofan object.

E.g :public class ShirtTest {

public static void main(String args[]) {

Shirt myShirt;

myShirt = new Shirt();

myShirt.displayInformation();

}

}


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In this e.g, an object reference variable callmyShirt is declared and initialized to aShirt object (on line 3 and 4).

ThemyShirt object reference variable then

invokes the display method within theShirt object (line 6).


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Calling and Worker Methods

In the previous example, the ShirtTest classcall the display method from within anothermethod (main method).

Therefore, themain method is referred to as the

calling method because it is invoking or callinganother method to do some work.

Conversely, the display method is referred to as

the worker method because itdoes some work for

the main method. Many methods are both calling and worker

methods because they not only do some work, butcall other methods too.


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When a calling method calls a workermethod, the calling method stops executionuntil the worker method is done.

After the worker method has completed,program flow is said to return to the pointafter the method invocation in the callingmethod (return to line 6 in theShirtTest.java class)


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Invoking a Method in the Same Class

Calling a method in the same class is easy,justinclude the name of the worker methodand its arguments, if any.


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Guidelines for Invoking Methods

1. There is no limit to the number of method callsthat a calling method can make.

2. The calling method and the worker method canbe in the same class or in a different class.

3. The way to invoke the worker method isdifferent, depending on whether it is in the sameclass or a different class from the calling method.

4. You can invoke methods in any order. Methods do

not need to be completed in the order in whichthey are listed in the class where they aredeclared (the class containing the workermethods)


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Passing Arguments and Returning

Values Methods can be invoked by a calling method

with a list of arguments (variables or valuesto be used by the worker method).

Additionally, methods can return a value tothe calling method that can be used in thecalling method.


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123

910..

45678

Object 1 Object 2

V1

V2

Caller method Walker method

Value 1 beingpassed from

object 1 toobject 2

Object2returns value2 to object 1


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Declare Methods With Arguments

public void setFloor(int desiredFloor){

while (currentFloor!=desiredFloor){

if (currentFloor


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The main Method

Main method also accepts arguments, one or more (an array)

of references to String objects

public static void main(String args[])

Why must the main method accepts these arguments?

The Java programming language requires that the

main method be written so that it can accepts

values when you execute the program from the

command line.


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Invoking Methods With Arguments

To pass arguments from one method to another, include the

arguments in the parentheses of the method call.

List the arguments in the same order in which they are listed

in the declaration of the worker method and pass all

required arguments.

The compiler checks to see if the type, order and number of

the parameters passed matches the type, order and numberof the parameters a mehod accepts


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Declare Methods With Return Value

Syntax :

[modifier] data_type

identifier(parameters_list)

{//line of code}

E.g :

public int sum(int numOne, int numTwo)

{//line of code}

A method can return only ONE value but can

accept multiple arguments


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Returning a Value

To return a value from a method, use the

return keyword.

For example, the following code returns the

current value held in the sum variable.public int sum(int numOne, int numTwo) {

int result=numOne + numTwo;

return result;}


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Advantages of Method Use

1. Make program more readable and easier to maintain

Easy to configure out what a program is doing if the code is dividedamong several different methods with names that match thebehavior of the methods

2. Make development and maintenance quicker.

You can choose to create and test the program one method at a timeto ensure that the program, as a whole will work when it is finished.

3. Central to reusable software

The Java technology class libraries have many classes withmethods that you can use over and over again in yourprogram.

4. Allow separate objects to cmmunicate and to distributethe workperformed by the program

A method in one object can invoke a method in another object. Theobject can pass the method information and receive a return value.


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Passing Parameters

There are two ways of passing parameters

to a method.

They are:

Pass-by-value

Pass-by-reference


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Pass by Value

When parameters are passed by value, only

the value of the variable is passed to the

method.

Any change made to the variable in thecalled method will not affect the variable in

the calling method.


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public class value{public static void main(String args[]){int A=5,B=7;System.out.println("Before calling the method:");

System.out.println("A = " + A);System.out.println("B = " + B);change(A,B);System.out.println("After calling the method:");System.out.println("A = " + A);System.out.println("B = " + B);System.out.println("\nThis is pass by value");}

static void change(int A,int B){A=2;B=3;}

}


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When the Function is called

1001 A 1050 B

1100 A 1155 B

5 7

5 7

After returning to main

1001 A 1050 B

1100 A 1155 B

5 7

2 3

Before calling the Function

1001 A 1050 B

5 7

Memory

Address

Variable

Name

Value


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Pass by Reference

When parameters are passed by reference,

the address of the variable is passed to the

method.

Any change made to this variable in thecalled method will affect the variable in the

calling method.


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public class Reference{public static void main(String args[]){int[] arr={5,7};

System.out.println("Before calling the method:");

System.out.println("arr[0] = " + arr[0]);System.out.println("arr[1] = " + arr[1]);

change(arr);System.out.println("After calling the method:");System.out.println("arr[0] = " + arr[0]);System.out.println("arr[1] = " + arr[1]);System.out.println("This is pass by reference");

}

static void change(int num[]){num[0]=2;num[1]=3;

}}


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When the Function is called

1001 arr[0] 1050 arr[1]

num[0] num[1]

5 7

Before calling the Function

1001 arr[0] 1050 arr[1]

5 7

Memory

Address

Array Name

Value

After returning to main

1001 arr[0] 1050 arr[1]

num[0] num[1]

2 3


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Creating static Methods and Variables

Methods and variables that are unique to an instance

are called instance methods and instance variable

Using methods thst do not require object

instantiation, such as main method. These are called

class methods or static method. You can invoke themwithout creating an objectfirst.

Java programming language allows you to create

static variables or class variables, which you can use

without creating an object.

The main method is a static method that you have

already used. You do not have to create object

instance to use the main method.


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Declare Static Method

Declare using static keyword.

E.g :

static Properties getProperties()


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Invoking static Method

Static or class methods are not part of any

object instance (just the class), you should

not use an object reference variable to

invoke them.

Instead, use the class name. the syntax for

invoking a static method is :

Classname.method();

Shirt.getDetail();


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Declare static Variable

Use the static keyword to declare that there

can only be one copy of the variable in

memory associated with a class, not a copy

for each object instance.

E.g :

static double saleTax=8.25;


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Accessing static Variable

Use the class name to access a static variable.

The syntax for invoking a static variable is :

Classname.variable;

For example, to access the value the static variablefor PI in the Math class :

double myPI;

myPI=Math.PI;

Variables can have both the static and final modifierto indicate that there is only one copy of the variableand that the contents of the variable cannot bechange. The PI variable in the Math class is a staticfinal variable.


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Polymorphism

Polymorphism is the existence of an entityin many forms. In object oriented

programming language it refers to the

ability of the objects to behave differently

based on the input given.

Polymorphism is implemented in Java usingtechniques:

Method Overloading

Constructor Overloading.


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Method Overloading

Two or more methods having the samename but different signature.

Methods which have same name but

different parameters are called asoverloaded methods.

The parameters in overloaded methodsshould differ in at least one of the following:

Number of parameters

Data type of the parameters


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Observe the program

class Poly

{


{

Over obj=new Over();obj.display();

obj.display(2);

obj.display("Kuala lumpur is theGarden city of lights",3);

}

}


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class Over

{

void display()

{

System.out.println("Melaka is ahistorical state");

}


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void display(int i)

{

int j=0;

while(j


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void display(String str,int i)

{

for (int j=1;j


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void display(int i)

{

int j=0;

while(j


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Constructor Overload

The concept of having two or moreconstructors with different signature in the

same class is called Constructor Overloading.

Two or more constructors in a class withsame name and different parameters are

called as overloaded constructors.


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Observe the program

class c_o_load

{


{

volume vol1=new volume(4);

volume vol2=new volume(4,6);

vol1.sphere();

vol2.cylinder();

}

}


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class volume{

double r,h;

volume(double r)

{

this.r=r;

}

volume(double r,double h)

{

this.r=r;

this.h=h;

}


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void sphere()

{

double vol;

vol=4/3*3.14*r*r*r;

System.out.println("The volume ofSphere is " + vol);

}


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void cylinder()

{

double vol;

vol=3.14*r*r*h;

System.out.println("The volume ofcylinder is " + vol);

}

}//end of class

Topic 3 Part 1 Method

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