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Defi nitions
An aggregation denotes an association end as an aggregate and describes a
whole-part hierarchy.
A composition denotes an association end and describes a whole-part
hierarchy, where the composite is existentially responsible for its parts.
156 CHAPTER 3 UML??”Unifi ed Modeling Language
The meanings of aggregations and compositions can be best explained by using
a simple example. Figure 3.14 represents one aggregation and one composition.
Both are denoted by a rhombus at the association end. The composition as the
stronger form has a filled rhombus.
The example class model describes that a house consists of between one and
an arbitrary number of rooms, and that a room can contain an arbitrary number of
pieces of furniture.
The aggregation underlines the whole-part relationship. The room is the whole
and the pieces of furniture are the parts. However, the room is not existentially
responsible for these pieces of furniture.
The relationship from the house to its rooms is different. The house is the
whole and responsible for its parts, the rooms.


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