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Aggregation — Definition and Notation

publish date: 2026/06/08 08:10:24.651727 UTC

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The diagram below shows an aggregation association in a UML class diagram.

In UML, the diamond shape on an aggregation association is placed next to which class, and what does aggregation mean?

Diamond next to the part class; aggregation means the part class is abstract

Diamond next to the whole class; aggregation means that one object (the whole) is composed of other objects (the parts)

Diamond next to the part class; aggregation means the part class has no operations

Diamond in the middle of the line; aggregation means both classes are optional

Correct Answer

Diamond next to the whole class; aggregation means that one object (the whole) is composed of other objects (the parts)

Explanation

In UML aggregation, the diamond shape is added next to the class that represents the whole. Aggregation means that one object (the whole) is composed of other objects (the parts). For example, Patient Record (the whole) is an aggregate of Patient and Consultation (the parts). The diamond is placed at the Patient Record end of each line to show it is the whole.

Reference

Software Engineering, Ian Sommerville, 9th edition

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