Ivor Horton's Beginning Java™ 2, JDK™ 5th Edition

6.11. Designing Classes

A basic problem in object-oriented programming is deciding how the classes in your program should relate to one another. One possibility is to create a hierarchy of classes by deriving classes from a base class that you have defined and adding methods and data members to specialize the subclasses. The Animal class and the subclasses derived from it are an example of this. Another possibility is to define a set of classes that are not hierarchical, but that have data members that are themselves class objects. A Zoo class might well have objects of types derived from Animal as members, for example. You can have class hierarchies that contain data members that are class objects—you already have this with the classes derived from Animal since they have members of type String. The examples so far have been relatively clear-cut as to which approach to choose, but it is not always so evident. Quite often you will have a choice between defining your classes as a hierarchy and defining classes that have members that are class objects. Which is the best approach to take?

Like almost all questions of this kind, there are no clear-cut answers. If object-oriented programming were a process that you could specify by a fixed set of rules that you could just follow blindly, you could get the computer to do it. There are some guidelines though, and some contexts in which the answer may be more obvious.

Aside from the desirability of reflecting real-world relationships between ...