Reviews : Java Books :
Beginning Java Objects : Chapters 2 and 3

Buy this book Title: Beginning Java Objects ISBN: 1861004176 US Price: $ 39.99 Canadian Price: C$ 59.95 UK Price: £ 28.99 Publication Date: November 2000 Pages: 800 © Wrox Press Limited, US and UK.

Composite Classes

Whenever we have a class, such as Student or Professor, in which one or more of the attributes are themselves references to other objects, we refer to the class as a composite class. The number of levels to which objects can be conceptually bundled inside one another is endless, and so composite classes enable us to model very sophisticated real world concepts. As it turns out, most 'interesting' classes are composite classes.

With composite class instances, it appears as though we are nesting objects one inside the other:

Object nesting does indeed sometimes make sense: namely, if an object 'A' doesn't have a life of its own, and only exists for the purpose of serving enclosing object 'B'.

• Think of your brain, for example, as an object that exists only within the context of your body (another object).
• As an example of object nesting relevant to the SRS, let's consider a grade book used to track student performance in a particular course. If we were to define a GradeBook class, and then create GradeBook objects as attributes — one per Course object — then it might be reasonable for each GradeBook object to exist wholly within its associated Course object. No other objects would need to communicate with the GradeBook directly; if a Student object wished to ask a Course object what grade the Student has earned, the Course object might internally consult its embedded GradeBook object, and simply hand a letter grade back to the Student.

However, as we saw in the sample Student and Professor classes above, we often encounter a situation where an object 'A' needs to refer to an object 'B', object 'B' needs to refer back to 'A', and both objects need to be able to respond to requests independently of each other. In such a case, we cannot physically embed one object wholly inside the other; this would lead to infinitely recursive nesting.

Handles come to the rescue! In reality, we are typically not storing whole objects as attributes inside of other objects, but rather references to objects. When an attribute of an object 'A' is defined in terms of an object reference 'B', the two objects exist separately in memory, and simply have a convenient way of finding one another whenever it is necessary for them to interact. Think of yourself as an object, and your cellular phone number as your handle. Other people — 'objects' — can reach you to speak with you whenever they need to, even though they don't know where you are physically located, using your phone number.

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