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.

Generalization Through Abstraction

This overly simplified diagram doesn't look like a real nerve cell, or a real muscle cell, or a real blood cell; and yet, it can still be used in an educational setting to describe certain aspects of the structure and function of all of these cell types — namely, those features that the various cell types have in common.

The simpler an abstraction — that is, the fewer features it presents — the more general it is, and the more versatile it is in describing a variety of real-world situations. The more complex an abstraction, the more restrictive it is, and thus the fewer situations it is useful in describing.

Organizing Abstractions Into Classification Hierarchies

For example, science categorizes all natural objects as belonging to either the animal, plant, or mineral kingdom. In order for a natural object to be classified as an animal, it must satisfy the following rules:

• It must be a living being.
• It must be capable of spontaneous movement.
• It must be capable of rapid motor response to stimulation.

The rules for what constitute a plant, on the other hand, are different:

• It must be a living being (same as for an animal).
• It must lack an obvious nervous system.
• It must possess cellulose cell walls.

Given clear-cut rules such as these, placing an object into the appropriate category, or class, is rather straightforward. We can then 'drill down', specifying additional rules which differentiate various types of animal, for example, until we've built up a hierarchy of increasingly more complex abstractions from top to bottom. A simple example of an abstraction hierarchy is shown at the right.

When thinking about an abstraction hierarchy such as the one shown above, we mentally step up and down the hierarchy, automatically zeroing in on only the single layer or subset of the hierarchy (known as a subtree) that is important to us at a given point in time. For example, we may only be concerned with mammals, and so can focus on the mammalian subtree:

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