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javapolymorphism

Why do we use Interface reference types in Java?

I'm about to take the final exam in my very first object-oriented programming class and I still don't understand something about the concept of polymorphism.

Let's say I have an abstract class "Vehicle" and this class has one subclass named "Aircraft". My question is, what is the different between these two codes ? 

Aircraft Jetplane = new Aircraft();

and

Vehicle Jetplane = new Aircraft();
Solution

  • In the second, then Jetplane could be anything else that inherits from Vehicle, not just an Aircraft. For example, you could have something like

    Vehicle veh = null;
if (someCondition)
    veh = new Aircraft();
else
    veh = new Boat();

    That can't be done in the first sample, because a Boat is not an Aircraft.