Aggregation

An aggregation is a special type of association. If inheritance is considered to be the is-a relationship, aggregation can be considered to be the HAS-A relationship.

Aggregation is used to describe a relation between two or more classes, when one class contains the other from a logical point of view, but instances of the contained class can live independently of the first class, outside of its context, or can be shared among other classes. For example, a Department HAS-A Teacher; additionally, every Teacher must belong to Department, but if a Department ceases to exist, a Teacher can still be active as shown in the following diagram: