Use case diagrams represent use cases, actors, and relationships between use cases and actors. UML defines three types of links between actors and use cases:
The participation of the actor is signaled by a solid line between the actor and the use case. This is the only relationship between actors and use cases.
A uses relationship between use cases means that an instance of the source use case also includes the behavior described by the target use case.
Representation of the uses relationship using a stereotyped generalization relationship
An extends relationship between use cases means that the source use case extends the behavior of the destination use case.
Representation of the extends relationship using a stereotyped generalization relationship
The diagram below gives an implementation example of the various relationships between use cases. Money transfer by computer is an extension of the transfer operation performed at the bank lobby. In both cases, the customer must be identified.
Issues regarding the benefits of use cases, and how to identity, construct and refine them, are covered in much greater depth in the next chapter.
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