public class Test
{
        public static void main(String[] args)
        {
                abc a = new abc();
                dog b = new dog();
                cat d = new cat();
                animal c = a.mno(b);
                animal w = a.mno(d);
        }
}

abstract class animal
{
        public abstract void move ();
        public void move2 () {
                System.out.println("Animal moving");

        }
}
class dog extends animal
{
        public void bark()
        {
                System.out.println("ruff");
        }

        @Override
        public void move() {
                System.out.println("Dog moving");
        }
}

class cat extends animal
{
        public void bark()
        {
                System.out.println("meow");
        }

        @Override
        public void move() {
                System.out.println("Cat moving");
        }
}

class abc
{
        public animal mno(animal a) {
              //  a.bark();
              //  causes error because parent Animal has
              //  no idea of bark in child classes

                /*
                 * One way to think about variables would be to think of them as pointers to Objects. Here a reference of Animal(parent class)
                 * can refer to an object of Cat / Dog (child class). The only restriction is that you can only invoke those methods on the reference that
                 * are in the parent class. When a method is invoked, the reference merely invokes the method of the same name in the object it points to.
                 * So the reference pointer doesn't need to distinguish between Dog/Cat object so long as the object is a child of the reference type and
                 * has implemented(overridden) the method being invoked.
                 *
                 * Think of it as INVOKING THE METHOD IN THE OBJECT THE REFERENCE POINTS TO
                 * and not INVOKING THE METHOD IN THE REFERENCE ITSELF.
                 */
                a.move();

                // every object inherits the definition from the parent class.
                a.move2();
                return a;
        }
}
/*
 * OUTPUT:
 * Dog moving
 * Animal moving
 * Cat moving
 * Animal moving
 */