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 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).
Aggregation is a property of an association role.
 Recall that each end of an association is a role, and that a UML role has various properties, such as multiplicity, name,