The Object-Oriented Thought Process, Fourth Edition

Composition Relationships

We have already seen that composition represents a part of a whole. Although the inheritance relationship is stated in terms of is-a, composition is stated in terms of has-a. We know intuitively that a car “has-a” steering wheel (see Figure 9.2).

Figure 9.2. A composition relationship.

Is-a and Has-a

Please forgive my grammar: For consistency, I will stick with “has a engine,” even though “has an engine” is grammatically correct. I do this because I want to simply state the rules as “is-a” and “has-a.”

The reason to use composition is that it builds systems by combining less complex parts. This is a common way for ...