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

Honestly your argument is rather moot. What's happening here is an implicit conversion to an IVehicle. You and your friend seem to be arguing about whether it's better to do it immediately (as per the first code listing), or later on (when you call the method, as per the second code listing). Either way, it's going to be implicitly converted to an IVehicle, so the real question is -- do you need to deal with a Car, or just a Vehicle? If all you need is an IVehicle, the first way is perfectly fine (and preferable if at a later point you want to transparently swap out a car for a bike). If you need to treat it like a car at other points in your code, then just leave it as a car.