Inheritance

Inheritance is a mechanism where we can inherit all the fields and methods of one class to another. It is an important part of OOPs (Object Oriented programming system). When we inherit from an existing class, we can reuse methods and fields of the parent class. Moreover, we can add new methods and fields in our current class also.

Syntax
class Class2 : Class1 {
    //Fields and methods
}

In the above syntax, we can see 2 classes called Class1 and Class2. Class1 is present in the right hand side(RHS) of : and Class2 is present in the left hand side(LHS). This RHS and LHS classes are having multiple names. We'll study what are some other terms used for these classes below

RHS class - Parent class, Base class, Super class
LHS class - Child class, Inherited class, Derived class, Sub class

CODE

EG 1 . Creating a class called Rectangle2D and inheriting to Rectangle3D class

class Rectangle2D {
    //Protected members can be only accessed inside its child class.
    protected double length, width;
    public double Area() {
        return length * width;
    }
    public double Perimeter() {
        return 2 * (length+width);
    }
}

//Inheriting Rectangle2D to Rectangle3D
class Rectangle3D : Rectangle2D {
    public double height;
    public double Volume() {
        return length * width * height;
    }
}

CODE EXPLANATION
In the above code, we created a class called Rectangle2D which is going to have fields and methods of 2 Dimensional Rectangle. We created one more class for 3 dimensional rectangle called Rectangle3D, Here whatever fields and methods present inside Rectangle2D class is going to present inside Rectangle3D class also. Hence we inherited Rectangle2D to Rectangle3D. Now Rectangle3D class will possess all the fields and methods present inside Rectangle2D class. In Rectangle2D class we created 2 protected access modifier variables called Length and width. Those 2 modifiers can be only accessed inside the child classes where Rectangle2D is the Parent class.


Why we use inheritance?
It is used when we create a class and we want to reuse some of the methods or properties from existing class.