Digging into Use Case Relationships

Use case diagrams are one of the key concepts in the Unified Modeling Language, but their semantics and notation have some gaps that lead to frequent misunderstandings among practitioners, even about very basic questions. In this paper we address some issues regarding the relationships in which use cases may take part. The Include and Extend relationships between two use cases have presently an inconsistent definition, since they are represented as stereotyped dependencies, but they are not true dependencies in the metamodel. Besides, the direction of the dependency arrow in the Extend relationship can be misleading, unnatural and difficult to understand for the common practitioner. Finally, we show also some conceptual problems regarding the included or extending use cases, which in our opinion are not true use cases.