Inheritance in Java
Published: 2021-10-25 15:05:04

Inheritance is an important pillar of OOP(Object-Oriented Programming). It is the mechanism in java by which one class is allowed to inherit the features(fields and methods) of another class. 

Important terminology: 

Super Class: The class whose features are inherited is known as superclass(or a base class or a parent class).
Sub Class: The class that inherits the other class is known as a subclass(or a derived class, extended class, or child class). The subclass can add its own fields and methods in addition to the superclass fields and methods.
Reusability: Inheritance supports the concept of “reusability”, i.e. when we want to create a new class and there is already a class that includes some of the code that we want, we can derive our new class from the existing class. By doing this, we are reusing the fields and methods of the existing class.
How to use inheritance in Java

The keyword used for inheritance is extends. 

Syntax : 



class derived-class extends base-class  
{  
   //methods and fields  
}  
Example:
 In the below example of inheritance, class Bicycle is a base class, class MountainBike is a derived class that extends Bicycle class and class Test is a driver class to run program. 

Java
// Java program to illustrate the
// concept of inheritance
 
// base class

class Bicycle {

    // the Bicycle class has two fields

    public int gear;

    public int speed;
 

    // the Bicycle class has one constructor

    public Bicycle(int gear, int speed)

    {

        this.gear = gear;

        this.speed = speed;

    }
 

    // the Bicycle class has three methods

    public void applyBrake(int decrement)

    {

        speed -= decrement;

    }
 

    public void speedUp(int increment)

    {

        speed += increment;

    }
 

    // toString() method to print info of Bicycle

    public String toString()

    {

        return ("No of gears are " + gear + "\n"

                + "speed of bicycle is " + speed);

    }
}
 
// derived class

class MountainBike extends Bicycle {
 

    // the MountainBike subclass adds one more field

    public int seatHeight;
 

    // the MountainBike subclass has one constructor

    public MountainBike(int gear, int speed,

                        int startHeight)

    {

        // invoking base-class(Bicycle) constructor

        super(gear, speed);

        seatHeight = startHeight;

    }
 

    // the MountainBike subclass adds one more method

    public void setHeight(int newValue)

    {

        seatHeight = newValue;

    }
 

    // overriding toString() method

    // of Bicycle to print more info

    @Override public String toString()

    {

        return (super.toString() + "\nseat height is "

                + seatHeight);

    }
}
 
// driver class

public class Test {

    public static void main(String args[])

    {
 

        MountainBike mb = new MountainBike(3, 100, 25);

        System.out.println(mb.toString());

    }
}
Output
No of gears are 3
speed of bicycle is 100
seat height is 25
In the above program, when an object of MountainBike class is created, a copy of all methods and fields of the superclass acquire memory in this object. That is why by using the object of the subclass we can also access the members of a superclass.