CLASSES, OBJECTS &

METHODS

TOPICS TO COVER

Introduction.

Defining a class.

Creating Objects.

Accessing Class Members.

INTRODUCTION Underlying structure of each JAVA programs is

CLASSES.

CLASS

DATA ITEMS

FUNCTIONS

OBJECTS basic program

Components

CREATE

METHODS OBJECTS

FIELDS

METHODS

CREATE

DEFINING A CLASS A class is a user-defined data type.

Variables and Functions can be created within class

SYNTAX EXAMPLE

class classname { field declaration; method declaration; }

class area { int side; int length; }

Declaring variables

Instance variables are declared exactly as LOCAL variables

INSATNCE VARIABLES

METHOD DECLARATION Without methods class has NO LIFE.

Since objects created by such class cannot respond to any messages.

Thus, methods are necessary for MANIPULATING DATA.

SYNTAX EXAMPLE

type method-name(parameter list)

{

}

Type of the value the method returns. It can be

void, int, float, double

class area { int side; int length; void get(int s, int l) { side = s; length = l; } }

EXAMPLE FOR CREATING CLASSES

Design a class Account that stores customer name, account number, and type of account. Include necessary methods to achieve following tasks:-

Deposit money.

Display balance.

Permit withdrawal and update balance.

An object in JAVA is essentially a block of memory that contains

space to store all the instance variables.

Creating an object also refers to INSTANTIATING AN OBJECT.

Objects in JAVA are created using new. The new operator dynamically

allocates memory for an object an returns a reference to it.

SYNTAX:-

class area { int side; int length; void get(int s, int l) { side = s; length = l; }

classname objectname;

objectname = new classname();

area a1;

a1 = new area();

area a1 = new area();

combined

Allocates at run-time null

a1

Indicates that it does not point

to any object

a1

Values are to be assigned to variables in order to use them in

our programs.

Since we are outside the class, we cannot access the instance

variables and methods directly.

Object and dot operator are used to do this.

SYNTAX:-

objectname.variablename = value;

objectname.methodname(parameter-list);

area a1 = new area();

a1.side = 10;

a1. get (10, 15);

class area { int side; int length; void get(int s, int l) { side = s; length = l; }

a1.tri();

a1.square();

}

Constructors

Method Overloading

Constructor Overloading

Nesting of methods

JAVA allows objects to initialize themselves when they are

created CONSTRUCTOR

PROPERTIES:-

• Initializes an object immediately upon creation.

• Same name as the class in which it resides and syntactically similar

to a method.

• Is called automatically after the object is created.

• Does not have return type.

EXAMPLE

NO RETURN TYPE

OUTPUT

Methods have same name, but different parameter list .

Is used when objects are required to perform similar tasks but

using different input parameters.

Also known as POLYMORPHISM.

Here, the aim is to provide several method definitions all with

same name, but different parameter lists.

The difference may either in number or type of arguments

Method’s return type does not play any role in this

In addition to overloading methods, you can also

overload constructor method

EXAMPLE

A method of a class can be called only by an object of that class using dot operator.

A method can be called by using only its name by another method of the same class

NESTING OF METHODS

STATIC MEMBERS

Is used to define a member that is common to all objects and accessed

without using a particular object.

Thus member belongs to the class as a whole rather than the objects

created from the class.

Used when we want to have a variable common to all instances of a

class.

SYNTAX:- static int count; static int max(int x, int y)

STATIC MEMBERS

Class variables and Class methods

Referred to as

EXAMPLE

RESTRICTIONS FACED BY STATIC METHODS:-

They can only call other static methods.

They can only access static data.

They cannot refer to this or super in any way.

STATIC

METHODS ARE

CALLED USING

CLASS NAME

USING OBJECTS as PARAMETERS

We know how to pass simple types as parameters to methods.

It is possible, correct and common to pass OBJECTS to methods.

EXAMPLE

CALL by VALUE vs. CALL by REFERENCE

CALL BY VALUE CALL BY REFERANCE

This method copies the value of an argument into the formal parameter of the subroutine

In this method, reference to an argument is passed to the parameter.

Does not access actual argument

This reference is used to access the actual argument.

Thus, changes made to parameter of the subroutine have no effect on the argument.

Thus, changes made to parameter will have an effect on the argument.

EXAMPLE

CALL by VALUE:- Simple type arguments are passed

to methods.

CALL by REFERENCE:- Objects are passed to methods

REMEMBER

RECURSION

JAVA supports recursion.

Recursion is the process of defining something in

terms of itself.

A method that calls itself is said to be recursive.

VISIBILITY CONTROL

Visibility modifiers are used to restrict the access to certain variables and methods from outside the class.

Also known as ACCESS MODIFIERS.

Visibility Labels

PUBLIC PRIVATE

PROTECTED

VISIBILITY CONTROL (Contd..)

PUBLIC Visible to entire class in which it is defined and All the class Outside

PRIVATE Enjoys highest degree of protection. Accessible only with their own class. Cannot be inherited, thus not accessible in sub-class.

Friendly When no access modifier is specified then the default version of public accessibility is known as “FRIENDLY”

PUBLIC

Friendly

Makes fields visible in all classes, regardless of their packages

Makes fields visible only in the same package.

PROTECTED Its level lies between PUBLIC ACCESS & FRIENDLY ACCESS. Makes the fields visible not only to all classes and subclasses in same package but also to subclasses in other packages