Relationships in Use Case Diagrams
There are three basic types of relationships used in use case diagrams:
This first, generalization, has already been discussed in the context of
Actors. Usually, the use of the
generalization relationship is restricted to actors. A generalization is
denoted by a solid line with an open arrow at one end.
The second relationship is the association. Associations are typically
used between actors and use cases to denote an actor performing actions
in a particular use case. An association is denoted by a solid line, such
as the association in Figure 1 from the Customer actor to the
Buy Part use case. Occasionally, you may see an open arrow on
an association. Such an association is said to be "navigable", in otherwords
messages only go one way. This is very rare in a use case diagram.
The third relationship is a dependency. Dependencies are typically used
to relate two use cases. It is rare that dependencies are not stereotyped
as illustrated in figure 1. Dependencies are denoted by a dashed line with
an open arrow.
There is a fourth kind of relationship for realizations, this releationship
is discussed on its own in another section.
Figure 1 - Use Case Relationships