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An association in a UML class diagram can be an aggregation or composition in order to define a "part-of" relationship. An aggregation is depicted with a white diamond on one end of the relationship, a composition with a black diamond. The diamond is placed on the association end that is connected to the owning element, the "whole". The opposite side is connected to the "part" element.

Although not common, it is also possible to use aggregations and compositions when defining association relationships in a UML use case diagram.

Steps:

Choose one of the following options:

  • In the diagram, click the end of the association relationship that is connected to the owning element. Click once to make it an aggregation, click twice to make it a composition.



  • Open the association properties pop-up window, and on one of the end tabs, in aggregation, select "share" to make it an aggregation, or select "composite" to make it a composition.