Why do most system architects insist on first programming to an interface?

Question

Almost every Java book I read talks about using the interface as a way to share state and behaviour between objects that when first \"constructed\" did not seem to share a r

Answer 1

Why extends is evil. This article is pretty much a direct answer to the question asked. I can think of almost no case where you would actually need an abstract class, and plenty of situations where it is a bad idea. This does not mean that implementations using abstract classes are bad, but you will have to take care so you do not make the interface contract dependent on artifacts of some specific implementation (case in point: the Stack class in Java).

One more thing: it is not necessary, or good practice, to have interfaces everywhere. Typically, you should identify when you need an interface and when you do not. In an ideal world, the second case should be implemented as a final class most of the time.