Model Reference Properties

Reference properties as unidirectional associations

A reference property, like the one in the example below, can be modeled in a UML class diagram. In the diagram, the reference property will appear in the form of an association end owned by its domain class. This is visualized with the help of a small filled circle also called a dot. To perform this visualization, we must connect the domain class and the range class of the reference property with an association line, placing an ownership dot at the end of this line at the range class side and annotating this association end with the property name and a multiplicity symbol. In this way, we get a unidirectional association with a source class that’s the property’s domain and a target class that’s the property’s range.

The fact that an association end is owned by the class at the other end implies that the association end represents a reference property. This is visually expressed by the association end ownership dot at the chair association end in the example shown below. In the example on the right, the represented reference property is Committee::chair and has ClubMember as the range. Such an association, with only one association end ownership dot, is considered unidirectional because it allows navigation (or object access) in one direction only. The direction goes from the class at the opposite side of the dot (the source class) to the class where the dot is placed (the target class). The the two diagrams shown below express essentially equivalent models: