27.2. Aggregation and Composition

Aggregation is a kind of association used to model whole-part relationships between things. The whole is called the composite.

For instance, physical assemblies are organized in aggregation relationships, such as a Hand aggregates Fingers.

Aggregation in the UML

Aggregation is shown in the UML with a hollow or filled diamond symbol at the composite end of a whole-part association (see Figure 27.5).

Figure 27.5. Aggregation notation.

Aggregation is a property of an association role.[1]

[1] Recall that each end of an association is a role, and that a UML role has various properties, such as multiplicity, name,

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