runtime polymorphism using interfaces in Java

(Source - Stack overflow) Interfaces allow you to use classes across different hierarchies polymorphically. For example, say you have the following interface: public interface Movable {     void move(); }   Any number of classes, across class hierarchies could implement Movable in their own specific way, yet still be used by some caller in a uniform way. So if you have the following two classes: public class Car extends Vehicle implements Movable {     public void move() {        //implement move, vroom, vroom!     } } public class Horse extends Animal implements Movable {     public void move() {        //implement move, neigh!     } } From the perspective of the caller, it's just a Movable   Movable movable = ...; movable.move();  //who am I?