An aggregation relationship is a specialized form of association in which a whole is related to its part(s). Aggregation is known as a “part-of” or containment relationship. The UML notation for an aggregation relationship is an association with a diamond next to the class denoting the aggregate (whole), as shown in Figure 6-3.
The following tests may be used to determine if an association should be an aggregation:
Is the phrase “part of” used to describe the relationship?
Are some operations on the whole automatically applied to its parts? For example, delete a course ...