Polymorphism and Interfaces in Java (can polymorphism be used to implement interfaces…why?)

Question

In the real world what do people use this for (to solve what types of problems)? Can I see some example code of these working together? All I can find is code about cats and

Answer 1

Map peopleByName = new HashMap();

If, down the road, I decide the memory overhead of HashMap is too much, I can re-do this as a TreeMap, and live with the slightly more expensive lookup times

Map peopleByName = new TreeMap();

Because peopleByName is a Map, not a TreeMap or a HashMap, all my calls are guaranteed to work on either map regardless of implementation.

This is best illustrated with the following example

public class CatsAndDogsDrinkingMilkAndCoffee {

   // what, no? :-(

}