OOP and Composition: “Has-a” Relationships

The notion of composition was introduced in Chapter 25. From a programmer’s point of view, composition involves embedding other objects in a container object, and activating them to implement container methods. To a designer, composition is another way to represent relationships in a problem domain. But, rather than set membership, composition has to do with components—parts of a whole.

Composition also reflects the relationships between parts, called a “has-a” relationships. Some OOP design texts refer to composition as aggregation (or distinguish between the two terms by using aggregation to describe a weaker dependency between container and contained); in this text, a “composition” simply refers to a collection of embedded objects. The composite class generally provides an interface all its own and implements it by directing the embedded objects.