Chapter-5-JAVA PROGRAMMING USING GUI-III

8/8/2019 Chapter-5-JAVA PROGRAMMING USING GUI-III

1/27

CREATED BY: PRIYANKA PAREEK

1Created By: PRIYANKA PAREEK1/19/2011


2/27

INTRODUCTION

Program: is a set of instructions given tocomputer. These instructions initiate some

action and sometimes called as executable

instructions. In Java programs instructions are specified

through Methods or functions.

Methods or Function is a sequence of somedeclaration statements and executable

statements.



3/27

Amethod is a separatepiece of codethat

can be called by a main program or anyother method to perform some specific

function.

The following are characteristics of methods:

It can return one or no values

It may accept as many parameters it needs or no

parameter at all. Parameters are also called

function arguments. After the method has finished execution, it goes

back to the method that called it.



4/27

WHY METHODS?

To Cope With Complexity

Hiding Details

Reuse



5/27

METHOD/ FUNCTION DEFINITION

A method or a function is a sequence of

statements that carry out specific task(s)

Amethod or function must be defined before itis used anywhere in the program:

[access- specifier ] [modifier] return-type

method name (parameter list)

{

body of the method

}



6/27

Where

access specifier : can be either public or

protected or private. Default is friendly modifier: can be one of: final, native,

synchronized, transient, volatile.

return-type: specifies the type of value that thereturn statement of the method returns. If no

value being returned, it should be void.

Method-name: valid java identifier.

Parameter list: it is a comma-separated list of

variables of a method referred to as its arguments

or parameters.1/19/2011 Created By: PRIYANKA PAREEK 6


7/27

METHOD PROTOTYPE AND SIGNATURE:

AMethod Prototypeis the first line of the

method definition that tells the program about the

type of the value returned by the method and the

number and type of arguments.

Method Signature: it basically refers to the

number and types of arguments. It is a part of the

method prototype.

1/19/2011 Created By: PRIYANKA PAREEK 7


8/27

USE OF KEYWORD VOID

Methods not returning in any value are declared void

methods i.e., having void s their return type.

syntax: void methods-name(parameter list);

example:void setLabelText( )

{

jLabel.setText(### Thank U###);

}



9/27

9

Methods Methods in Java define the behavior of the class.

Methods are similar to procedures or subroutines in otherlanguages.

The real benefits of object orientation come from hiding theimplementation of a class behind its operations.

Methods access the internal implementation details of a class

that are hidden from other objects. Hiding data behind methods is so fundamental to object

orientation it has a name - encapsulation.

Methods have zero or more parameters.

A method can have a return value.

A method's statements appear in a block of curly braces { and }that follow the method's signature.

public void speak () {

System.out.println("Hey Barney ...");

}Created By: PRIYANKA PAREEK1/19/2011


10/27

10

INVOKING A METHOD

Provide an object reference and the method name,separated by a dot (.):

fred.speak();

Parameters are passed to methods as a comma-separated list.

A method can return a single value as a result, suchas an int:

public int add (int a, int b) {

return a+b;

}

A method can return no result:public void setFirstName (String firstName) {

this.firstName = firstName;

}

Created By: PRIYANKA PAREEK1/19/2011


11/27


12/27

Example: to print cube of a given number using

a method:

private voidComputeActionPerformed(java.awt.event.ActionEvent

evt) {

double n =Double.parseDouble(jTextField1.getText());

double res = CalcCube(n);

jLabel3.setText("" +res); }

double CalcCube(double b){

double cu = b * b * b;

return cu; }1/19/2011 Created By: PRIYANKA PAREEK 12


13/27

ACTUAL AND FORMAL PARAMETERS

Actual parameters: are

the parameters

appearing in the method

call statement.

Formal parameters:

are the ones that appear

in method definition.

Example:

Formal Parameters:

int mult (int x, int y) {

return x* y;

}

Actual parameters:

int length = 10;

int width =5;

Int area = mult(length,

width);1/19/2011Created By: PRIYANKA PAREEK

13


14/27

Arguments to Methods

Arguments to methods can be:

of primitive data types i.e. char, byte, short, int,

long, float, double, boolean.

of reference data types i.e. objects or arrays

A method is invoked in two manners: Call by

Value and Call by Reference. These two

ways are also called as Pass By Value and Passby Reference.



15/27

Call by Value (or Pass By Value )

When a pass-by-value occurs, the method makes a copy ofthe value of the variable passed to the method. The

method cannot accidentally modify the original argument

even if it modifies the parameters during calculations.

In the given example, we called the method test and

passed the value ofias parameter. The value of i is copied

to the variable of the methodj. Sincej is the variable

changed in the test method, it will not affect the variable

value ifi in main since it is a different copy of the

variable.

15

Created By: PRIYANKA PAREEK1/19/2011


16/27

Call by Value (or Pass By Value )



17/27

Call by Reference(or Pass by Reference)

When a pass-by-reference occurs, the reference toan object is passed to the calling method. This

means that, the method makes a copy of the

reference of the variable passed to the method.However, unlike in pass-by-value, the method

can modify the actual object that the reference is

pointing to, since, although different references

are used in the methods, the location of the datathey are pointing to is the same.



18/27

Call by Reference(or Pass by Reference)



19/27


Figure : Pass-by-referenceexample

A common misconception about pass-by-reference in Java is when

creating a swap method using Java references. Take note that

Java manipulates objects 'by reference, but it passes object

references to methods 'by value.'"As a result, you cannot write a

standard swap method to swap objects.


20/27

Returning from a method

A method returns value through return statement. A method terminates when either a return

statement is encountered or the last statement in

the method is executed. Areturn statement is used to terminate a method

whether or not it returns a value.



21/27


22/27

RETURNING VALUES

They may of three types of methods in Java:Computational Methods: these are the methods

that calculate or compute some value and returned

the computed value. Example: Math.sqrt () and

Math.cos( ) are computational methods.

Computational methods always return a computed

result.

ManipulativeMethods: these are the methodsthat manipulate information and return a success or

failure code.


23/27

Procedural methods: these are the methods that

perform an action and have no explicit returnvalue. Example: System.out.println() method.


24/27

SCOPE REVISITED

The scopedetermines where in the programthe variable is accessible. The scope also

determines the lifetime of a variable or how

long the variable can exist in memory. The

scope is determined by where the variable

declaration is placed in the program.

Def: The program part(s) in which a particular

piece of code or a data value(e.g.. Variable)

can be accessed is known as the piece-of-

codes or variables Scope.


25/27

CONSTRUCTORS C

onstructors: a member method with thesame name as its class is called Constructor

and it is used to initialize the objects of that

class type with a legal initial value.

Example:

class A( ){

int a;

float b;public A( ){

a = 0;

b = 0.0; } }


26/27


27/27

Documents

Chapter-5-JAVA PROGRAMMING USING GUI-III