Abstract Class

Abstract class is a restricted class that cannot be used to create objects (to access it, it must be inherited from another class). Similar to abstract class there is something called Abstract method. Abstract method can only be created inside Abstract class and it does not have a body. The body of the Abstract method will be provided by the child class. A normal class cannot have abstract methods but an abstract class can have both abstract and regular methods.

Syntax
abstract class className {
    //Fields, Methods and Abstract methods
}

NOTE
In order to declare Abstract class we need to use Abstarct keyword before class, In order to create Abstarct method we need to use Abstract keyword before method and that method should not have a body. If a child class inheriting abstract class , that child class must override all the abstract method present inside Abstract class.

CODE

EG 1 . Creating a Abstract class called Shape and Inheriting to Class Rectangle.

abstract class Shape {
    //Abstract methods
    public abstract double Area();
    public abstract double Volume();
    public abstract double Perimeter();
}

class Rectangle : Shape {
    public double length, width, height;
    public override double Area() {
        return length * width;
    }
    public override double Volume() {
        return Area() * height;
    }
    public override double Perimeter() {
        return 2 * (length+width);
    }
}

class Program {
    static void Main(string[] args) {
        //Upcasting
        Shape s1 = new Rectangle();
    }
}

CODE EXPLANATION
In the above code, we created a abstract class called Shape. Inside Shape abstract class we created 3 abstract methods called Area, Volume and Perimeter. Now we created a new normal class called Rectangle where we inherited Abstract class Shape. The moment we inherit abstract class shape to normal class Rectangle, system will start to show error because all the abstract method present inside abstract class must be overridden inside child class Rectangle. Even if we miss one abstract method to override, system will throw compile time error. In abstraction Upcasting plays a major role. Inside main method we used upcasting. We created a new instance of shape called S1. Using s1 if we invoke Area method system will go to its referred child class Rectangle and its Area method will be executed.


Where to use Abstract class?
The purpose of an abstract class is to provide a common definition of a parent class that multiple child classes can share.
Let's say you need to create Dog, Cat, Hamster and Fish objects. They possess similar properties like color, size, and number of legs as well as behavior so you create an Animal Parent class. However, what color is an Animal? How many legs does an Animal object have? In this case, it doesn't make much sense to instantiate an object of type Animal but rather only its child classes.

Exercises
1. Create an abstract class called Shape with 4 abstract methods Area(), Perimeter(), Volume() and Weight(). Now create 3 child classes called Circle, Triangle and Rhombus inherit Shape abstract class.

(Click Here, For more exercises)