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,

Get Applying UML and Patterns: An Introduction to Object-Oriented Analysis and Design and the Unified Process, Second Edition now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.