Inheritance in Java

DEFINITION OF INHERITANCE :

Inheritance is one of the cornerstones of Object Oriented Programming (OOP) which means to inherit the properties of one class and extends according to the need. The two keywords used in inheritance are extends (for class) and implements (for interface).

Inheritance allows the creation of hierarchical classifications. In inheritance a general class is used to define the common methods and variables that can be used in other class. The class which inherits the general class will get all the public methods and variables present in the general class. Now, general class is called super class or base class and the class which inherits its properties is knows as sub class or child class.

To understand the concept of inheritance, consider an example :

Suppose you want to calculate the area of a square, a rectangle, a circle having same radius as length of square than by using the same length available in the base class Shape you can easily extends that class to Square class or rectangle class or circle class by the use of extends keyword.

 class Shape  
 {  
      //define a length which is public and can be used in square,   
      //rectangle and circle classes  
      public int length = 10;  
 }  
   
 //inherit the properties of Shape class with the use of extends keyword  
   
 class Square extends Shape  
 {  
      void area()  
      {  
           //calculate area of square  
           int area = length * length;  
   
           //print it on the screen  
           System.out.println("Area of square : "+area);  
      }  
 }  
   
 class Rectangle extends Shape  
 {  
      void area()  
      {  
           //define a breadth  
           int breadth = 7;  
           //calculate area of rectangle  
           int area = length * breadth;  
           //print on the screen  
           System.out.println("Area of rectangle : "+area);  
      }  
 }  
   
 class Circle extends Shape  
 {  
      void area()  
      {  
           //calculate area of circle using length of the shape class as radius  
           float area = 3.14f * length * length;  
           //print on the screen  
           System.out.println("Area of circle : "+area);  
      }  
 }  
   
 //make a entry class which contains main method  
 class InheritanceDemo  
 {  
      public static void main(String[] args)  
      {  
           //object of child class square  
           Square sq = new Square();  
           //object of child class rectangle  
           Rectangle rec = new Rectangle();  
           //object of child class circle  
           Circle cir = new Circle();  
             
           //call the area methods of all child class to   
           //get the area of different objects  
           sq.area();  
           rec.area();  
           cir.area();  
      }  
 }  

Output :

Why we use Inheritance?

Inheritance is used because it provides the Reusability of the code like in the above example we used the same length from the Shape class thrice in different three classes.

Types of Inheritance in Java :

There are three general types of inheritance in java. These are :

1.  Simple Inheritance : In simple inheritance there is one base class and a single child class which extends the properties of base class.

     2. Multilevel Inheritance : In multilevel inheritance there is single parent and a child class and a further sub child class which inherit properties of child class.

Hierarchical Inheritance : In this type of inheritance many child classes inherit the properties of the same child class like the example used in this blog post.

Note : Multiple Inheritance in which a class inherits properties of more than one class is not present in Java to remove the ambiguity in the code.