Understanding abstract classes

In Chapter 9, we covered the abstract keyword. Let’s review some key points that we discussed. An abstract method is exactly that – it is abstract. It has no code. It doesn’t even have curly braces – {}. This is typically a design decision. The class containing the abstract method wants subclasses to provide the code. This means that the class itself is “incomplete” and therefore any class defining an abstract method must itself be abstract. Any subclass of the abstract class must either override the abstract method or declare that it too is abstract. The compiler will complain otherwise.

However, the inverse is not the case – an abstract class need not have any abstract methods at all. Again, this is a design decision. Since the class is marked as abstract, it is considered “incomplete” (even though it may contain code for all the methods). This prevents objects based on abstract classes...