7.3 UML Diagrams for Object-Oriented Designs
7.3.1 Use Case Diagrams
A use case model, Figure 7.20, consists of actors, use cases, and use case descriptions:
Figure 7.20: The components of the use case model
Its purpose, in simple terms, is to show why systems are used and who uses them.
Each system has its own model, with actors depicting users (more correctly, roles performed by users). The reasons why these actors are using the system are shown as a set of use cases within the system boundary. Two simple examples are given in Figure 7.21. Each system is drawn as a rectangular box, with the relevant use cases shown as ellipses inside them (the box, representing the system boundary, is optional). Outside the system, the boundary is the actors, connected via lines to (associated with) the use cases. In (a), both the navigator actor and the pilot actor interact with the NAV/WAS system in the same way; they use it to find out what the navigation waypoints...
