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A recurring request asks to see a complete example of using interfaces and abstract classes. Based on the feedback, the original answer proved a bit too theoretical, so in this follow-up Java Q&A;, I will bring the discussion down to earth by presenting a framework that employs both interfaces and abstract classes.

To lay the groundwork necessary for a clear discussion, I review Java class and interface constructs from both type and implementation-centric viewpoints. As detailed in my earlier article, "Thanks Type and Gentle Class" (JavaWorld, January 2001), classes and interfaces establish a system's type hierarchy, whereas only classes establish the implementation hierarchy. Classes divide into two types: concrete and abstract.

In Java, under what circumstances would you use abstract classes instead of interfaces? When you declare a method as abstract, can other nonabstract methods access it? In general, could you explain what abstract classes are and when you might use them?

It's really a tight-knit family thing. In the Java programming language, people can always use inner classes, which is a short hand of using composition, to get around multiple inheritance of implementation. If it feels more like a small family, go for the abstract class.

A more important issue is versioning. There are two schools of thought about virtual methods. The academic school of thought says, "Everything should be virtual, because I might want to override it someday." The pragmatic school of thought, which comes from building real applications that run in the real world, says, "We've got to be real careful about what we make virtual."

In the previous column, you started your exploration of interfaces. In this installment, you delve deeper in interfaces and explain how they relate to abstract classes.