What is aggregation?
John Moore Aggregation is a special kind of association that specifies a whole/part relationship between the aggregate (whole) and a component part. When compared to an ordinary association, some distinguishing characteristics of aggregation include:
- aggregation is an asymmetric relationship
- aggregation is a transitive relationship
- aggregation implies stronger coupling
- behavior (copy, delete, etc.) is normally propagated across an aggregation.
The decision to use aggregation is largely a matter of judgement. James Rumbaugh suggests that, when in doubt, use an ordinary association.