Stop copying code
you don't understand.

// Same method, different behaviour — that's Polymorphism
abstract class Shape {
  abstract double area();
  void describe() {
    System.out.println("Area = " + area());
  }
}

class Circle extends Shape {
  double radius;
  Circle(double r) { this.radius = r; }
  double area() { return Math.PI * radius * radius; }
}

// Magic: same call → different result
Shape s = new Circle(7.0);
s.describe();  // Area = 153.93