Composition

It is natural to think of objects as containing other objects. A television set contains a tuner and video display. A computer contains video cards, keyboards, and drives. Although the computer can be considered an object unto itself, the drive is also considered a valid object. In fact, you could open up the computer and remove the drive and hold it in your hand. Both the computer and the drive are considered objects. It is just that the computer contains other objects ”such as drives .

In this way, objects are often built, or composed , from other objects ”this is composition.

Has-a Relationships

Although an inheritance relationship is considered an is-a relationship for reasons already discussed, a composition relationship is termed a has-a relationship. Using the example in the previous section, a television has-a tuner and has-a video display. A television is obviously not a tuner, so there is no inheritance relationship. In the same vein, a computer has-a video card, has-a keyboard, and has-a disk drive. The topics of inheritance, composition, and how they relate to each other is covered in great detail in Chapter 7, "Mastering Inheritance and Composition."