JAVA - OBJECT ORIENTED PROGRAMMING PRINCIPLES

ENCAPSULATION

· Combining of data members and related member functions in single unit(class)

· Hiding data and member functions from other classes of program

· Access specifiers can be used to control access to these class members

Example

class Student

{

int Id;

String Name;

Void Display()

{

…..

}

}

INHERITANCE

· Creating a new class by using features of existing class

· Deriving a child class by inheriting features of parent class

· By writing extends keyword followed by parent classname beside the child class, we inherit parent class features into child class

Use: Code reusability

Simple/single inheritance

Class A

{

int id;

}

Class B extends A

{

Void display()

{

S.o.p(id);

}

}

Types of inheritance (supported using classes):

Single inheritance

A

|

B

Multilevel inheritance

A

|

B

|

C

Hierarchical inheritance (tree like structure)

A

|B |C

|X |y |I |j

Types of inheritance(supported using interfaces):

Multiple inheritance

A B

| |

C

Hybrid inheritance

A B

| |

C

| |

D

Polymorphism

Existing in many forms

Poly-many; morphism-forms

Types:

Static/compile time

Overloading

Dynamic /runtime

Overriding

Static/compile time

Method overloading

When in a class, method with same name exists multiple but with different type of parameters

Example:

class A

{

void display()

{

System.out.println("welcome");

}

void display(int a)

{

System.out.println("a "+a);

}

void display(char grade)

{

System.out.println("grade "+grade);

}

}

Method overriding

In inheritance, when child class redefines parent class method its called overriding

Class A

{

Void display()

{

S.o.p(“hi”);

}

}

Class B extends A

{

@Override

Void display()

{

S.o.p(“welcome”);

}

}

Abstraction

Showing necessary details and hiding functionality

Abstraction is achieved in java using

Abstract class

· Class that contains at least one abstract method is abstract class

· Abstract method is a method without definition

· The abstract method is implemented /given definition by child class of abstract class.

· Abstract class cannot be instantiated/cannot have object creation.

· Abstract class must be declared abstract using abstract keyword.

abstract Class Z

{

abstract Void display1(); //abstract method

Void display2() //concrete method

{

S.o.p(“hi”);

}

}

Class A extends Z

{

@Override

void display1()

{

S.o.p(“implemented by A”);

}

}

Interface

Pure abstract class

Interface interface_name

{

Non abstract methods

}

interface Z

{

Void display1(); //abstract method

default Void display2() //concrete method

{

S.o.p(“hi”);

}

}

Class A implements Z

{

@Override

void display1()

{

S.o.p(“implemented by A”);

}

}

IMPORTANT KEYWORDS

super

static

this

super

super KEYWORD USED IN INHERITANCE

· to refer to parent class in child class, super keyword is used.

Class A

{

Int id=100;

}

Class B extends A

{

Int id=200;

Void display()

{

S.o.p(super.id);

S.o.p(id);

}

}

Static:

· Static members get memory only once in program

· The same memory is used throughout the program

· Variables and methods in java can be declared as static

· Static members are not bounded with objects

Static variable

static int a=100;

static method

static void display()

{

}

this

acts as current class object

to refer to class members in same class, this keyword acts as class object.

When the instance variable and local variable have same name, then this keyword is used with class member i.e. to refer to instance member this keyword is used.

class A

{

int id1=100;

void display1()

{

int id1=200;

System.out.println("id1: "+ id1);

System.out.println("id1: "+ this.id1);

}

}