What is the purpose of using interface when its methods don't have any implementation we have to override them every time?

Question

interface Interface {
    void m1();
}

class Child implements Interface {
    public void m1() {
        System.out.println("Child.....");
    }
}

public class InterfaceDemo {
    public static void main(String[] args) {
        Child c = new Child();
        c.m1();
        Interface i = new Child();
        i.m1();
    }
}   

Answer 1

This is useful when you have several classes implementing same interface. It allows to use polymorphism. You can also use abstract classes to implement some common functionality. And starting Java 8 you can provide default implementation in interfaces themselves.

interface Shape {
  void draw();
  double getSquare();
}

class Circle implements Shape {
  public void draw() {}
  public double getSquare() {return 4 * PI * r * r;}
}

class Square implements Shape {
  public void draw() {}
  public double getSquare() {return w * w;}
}

class Main {
  public static void main(String[] args) {
    for (Shape s : Arrays.asList(new Circle(), new Square(), new Square(), new Circle())) {
      s.draw(); //draw a shape. In this case it doesn't matter what exact shapes are in collection since it is possible to call interface method
    }
  } 
}