Why would you declare an Interface and then instantiate an object with it in Java?

Question

A friend and I are studying Java. We were looking at interfaces today and we got into a bit of an discussion about how interfaces are used.

The example code my frie

Answer 1

Declaring interfaces and instantiating them with objects allows for a powerful concept called polymorphism.

List list = new ArrayList();
list.add(new Car());
list.add(new Bike());

for (int i = 0; i < list.size(); ++i)
    list.get(i).doSomeVehicleAction();   // declared in IVehicle and implemented differently in Car and Bike

To explicitly answer the question: You would use an interface declaration (even when you know the concrete type) so that you can pass multiple types (that implement the same interface) to a method or collection; then the behavior common to each implementing type can be invoked no matter what the actual type is.