UML Association

There are other forms of containment that do not have whole / part implications (aggregation).

There is not likely to be much difference in the way that the two relationships - Aggregation or Association - are implemented. That is, it would be very difficult to look at the code and determine whether a particular relationship ought to be aggregation or association. For this reason, it is pretty safe to ignore the aggregation relationship altogether. As the amigos said in the UML 0.8 document: “...if you don’t understand don’t use it.”

Aggregation and Association both correspond to the Has-by-reference relationship from the Booch-94 notation.

For example, Each Window refers back to its parent Frame. This is not aggregation since it is not reasonable to consider a parent Frame to be part of a child Window. We use the association relationship to depict this.

It is sometimes more expressive to show the attributes as associations: