Thread: what is the use of interfaces and abstract classes?

Interfaces are a way to give a data type to an object. Java is strict on data typing compared to c.
They allow the compiler to verify that a class object has the method(s) required for that data type.
A common used interface is a listener. The compiler guarantees that any object that says it is a listener will have all the required methods defined by the interface.

public class Main {
 
 
 
    public static void main(String[] args) {
 
          shape circleshape=new circle();
 
             circleshape.Draw();
    }
}
 
 interface shape
 {
     public   String baseclass="shape";
 
     public void Draw();
 
 
 }
 class circle implements shape
 {
 
    public void Draw() {
        System.out.println("Drawing Circle here");
    }
 
 
 }

both programs do the same!

public class HelloWorld {
 
 
 
    public static void main(String[] args) {
 
          circle circleshape=new circle();
 
             circleshape.Draw();
    }
}
 
 
 class circle 
 {
 
    public void Draw() {
        System.out.println("Drawing Circle here");
    }
 
 
 }

What's the point?

Where is the method that takes an argument of type shape?

void SomeMethod(shape aShape) {
}

You could NOT pass an object of the circle class from your second code example to this method. It is not a shape.

Is interface shape can used as a data-type?
and where as a circle can not be use as a data type when since it is not implemented shape?

Both shape and circle are data types.
And so are Main and HelloWorld data types.