Aggregation is a general term for any whole-part relationship between objects. Its symbol is a white diamond, like this:

Figure 5.5. The representation of aggregation in the UML

This diagram can be read as “an Army is an aggregate of one or more soldiers and one or more weapons”. The aggregation is expressing a loose whole-part relationship between the army and its constituents, where the constituents are not necessarily members of the aggregate. For example, if the army as an entity ceased to exist there would still be soldiers (they would simply be unemployed). This topic has lead to some very heated debate. In practice the distinction between a computer system which works, and a system which does not, is highly unlikely to hinge on whether the designers used aggregations or named associations. An aggregation can usually be replaced by an association called “has” without much loss of clarity.

It is not clear that composition is more expressive than a simple association labelled “contains”. Composition expresses a whole-part relationship where the “parts” are definitely contained within the “whole”.

Figure 5.6. The representation of composition in the UML

This example can be read as “a book consists of one or more pages, and one binding”. Note the distinctions between this and the previous example. First, the binding and pages physically comprise the book; it is impossible for the binding and pages to be in one place and book in another. Second, destruction of the book would imply destruction of the binding and pages.