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.
Abstraction
Just as with inheritance, composition provides a mechanism for building objects. In fact, I would argue that there are only two ways to build classes ...
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