After reading the most excellent book "Head First Design Patterns", I began proselytizing to my colleagues the benefits of patterns and design principles. While extolling the virtues of my favorite pattern - Strategy Pattern - I was asked a question which gave me pause. Strategy, of course, uses inheritance and composition and I was on one of my tirades about "program to an interface (or supertype) not an implementation", when a colleague asked "why use an abstract base class instead of a concrete class?".
I could only come up with "well you force your subclasses to implement abstract methods and prevent them from instantiating the ABC". But to be honest the question caught me off gaurd. Are these the only benefits of using an abstract base class over a concrete class at the top of my hierarchy?
If you need specific methods to be implemented, then use an Interface. If there is shared logic that can be pulled out, use an abstract base class. If the base set of functionality is complete on its own, then you can use a concreate class as the base. An abstract base class, and an Interface cannot be instantiated directly, and that is one of the advantages. If you can use a concrete type, then you need to do override methods, and that has a "code smell" to it.