Software Architect's Handbook

Association

An association is a broad term that refers to a semantic relationship between classes. If one class uses another class (unidirectional), or two classes use each other (bidirectional), they have a relationship. Relationships between classes are represented by an association, which is shown on a class diagram as a solid line:

When there are no arrowheads at the end of the line, the navigability of the association is unspecified. However, in diagrams where arrows are used for one-way navigable associations, a line with no arrows is assumed to represent bidirectional navigability. In the preceding example, which would mean that both ...