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Object-Oriented Analysis and Design 331

11

In the figure, you see that a Geranium is a special kind of Flower. If you and I agree to

draw our inheritance (generalization/specialization) diagrams like this, we’ll understand

each other perfectly. Over time, we’ll probably want to model lots of complex relation-

ships, and so we’ll develop our own complicated set of diagramming conventions and

rules.

Of course, we’ll need to explain our conventions to everyone else with whom we work,

and each new employee or collaborator will have to learn our conventions. We might

interact with other companies that have their own conventions, and we’ll need to

allow time to negotiate a common convention and to compensate for the inevitable

misunderstandings.

It would be more convenient if everyone in the industry agreed on a common modeling

language. (For that matter, it would be convenient if everyone in the world agreed on a

single spoken language, but one thing at a time.)

The lingua francaof software analysis and design is UML—the Unified Modeling

Language. The job of the UML specification is to answer questions such as, “How

should we draw an inheritance relationship?” The geranium drawing shown in Figure

11.1 would be drawn in UML as shown in Figure 11.2.

FIGURE11.2
UML drawing of
specialization.

Flower

Geranium

In UML, classes are drawn as rectangles, and inheritance is drawn as a line with an

arrowhead. Interestingly, the arrowhead points from the more specialized class to the

more general class. The direction of the arrow can be counterintuitive, but it doesn’t mat-

ter; as long as we all agree, then after we learn the representation, we can communicate.

The details of the UML are rather straightforward. The diagrams generally are not hard

to use or to understand, and you’ll learn about them as they are presented. Although it is

possible to write a whole book on the UML, the truth is that 90 percent of the time, you

use only a small subset of the UML notation, and that subset is easily learned.

Software Design: The Process ............................................................................

Theprocessof object-oriented analysis and design is much more complex and important

than the modeling language. So, of course, it is ironic that you hear much less about it.