Beginning Java Objects: Chapter 2
Abstraction and Modeling
Generalization Through Abstraction
If we eliminate enough detail from an abstraction, it becomes generic enough to apply to a wide range
of specific situations or instances. Such generic abstractions can often be quite useful. For example, a
diagram of a generic cell in the human body might include only a few features of the structures that are
found in an actual cell:
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
Even though our brains are adept at abstracting concepts such as road maps and landscapes, that still
leaves us with hundreds of thousands, if not millions, of separate abstractions to deal with over our
lifetimes. To cope with this aspect of complexity, human beings systematically arrange information into
categories according to established criteria; this process is known as classification.
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:
temporarily ignoring the rest of the hierarchy. By doing so, we automatically reduce the number of
concepts that we mentally need to 'juggle' at any one time to a manageable subset of the overall
abstraction hierarchy; in the simplistic example above, we are now dealing with only four concepts
rather than the original 13. No matter how complex an abstraction hierarchy grows to be, it needn't
overwhelm us if it is properly organized.
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