Introduction to Class

Object-Oriented Programming

Module Introduction

1. Classes and Objects

2. Instance variables

3. Methods

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#1 – Classes and Objects

Class declaration

Constructors

Creating objects

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Creating Class

The template or blueprint from which objects are actually made.

Syntax:class ClassName{

//definition of class}

All code that you write in Java is inside a class.

Classes can be built by extending other classes

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Constructor - how to instance an object?

Same name as the class.

A class can have more than one constructor.

A constructor can take zero, one, or more parameters.

Has no return value.

Always called with the new operator.

Use for initializing variables and invoking any methods that may be for initialization.

If a class does not have an explicit constructor, a default constructor (no-argument) is provided.

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Name of class

1. Use Naming rules

2. Always captain first letter

3. Use Noun for naming

Data member

• What data does it need to know?

Constructor

• Define the way to instance object

1. Same name as the

class name

2. Like function in C but

have no return value

Methods

• What can it do?

1. Like function in C

2. Use Verb for naming

3. Always lower first letter

Dissecting the Rectangle class

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Creating objects (instance object)

An object must be created before using in a program.

1. Declare a variable to store the object reference.1. Objects can only be manipulated via references

2. Creating an object.1. Using the new operator in conjunction with a call to a constructor

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#2 – Data Member

Instance Variables

Class Variables

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Instance variables

The values of those variables define the state of object.

All instances of a class have the same instance variables May have different values

inside them.

Has an “access modifier” associated with it.

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Example of Instance variables

recA

length = 3.0width = 4.0

recB

length = 6.4width = 4.7

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Class Variables (Static Variables)

Variable that is accessed without using an object of a class.

Declare using the static keyword.

Only one copy of a static variable is shared by all the objects of the class. Change in the value of static

variable is reflected by all the objects of the class.

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Example of Static Variables

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#3 - Methods

Definition

Instance Method

Calling method and Passing Arguments by Value.

Calling method and Passing Arguments by Reference.

Static Methods

Variable Argument Methods

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A method is defined as the actual implementation of an operation on an object.

Syntax:

access_specifier modifier datatype method_name (parameter_list)

{//body of the method

}

The method name should begin with a lowercase verb and follow with a proper noun.

Method

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Instance Method

A function defined in a class

Invoked by an instance object and can access instance variables

Provide a mechanism for accessing the private data stored in an object

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Invoking Methods

Using the '.' operator

Syntax: <the object reference> ‘.’ <the method to be invoked>

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Calling method and Passing Arguments by Value.

Value from calling method is passed as an argument to the called method.

Any change made to that passed value in the called method will not modify the value in the calling method.

Variables of primitive types (int, float ..) are passed by value.

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Example of Calling method and Passing Arguments by Value.

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Calling method and Passing Arguments by Reference.

Called method to change the value of the parameter passed to if from the calling method.

When references are passed as parameters, the caller can change the values stored but not the reference variables.

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Example of Calling method and Passing Arguments by Reference

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Static Methods

Methods that do not operate on objects. Can be called without creating any objects of the class.

Call static method, supply the name of the class.

Declared using the static keyword.

Can directly refer only to static variables and other static methods of the class.

Cannot refer to non-static methods and variables

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Example of Static Methods

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static methods usage

When a method doesn't need to access the object state

When a method only needs to access static fields of the class.

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Variable Argument Methods

Allow calling a method with variable number of arguments.

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That’s about all for today!

Thank you all for your attention and patient !

Classes and Objects

Instance variables

Methods