Use Case Diagram

The use case model has three components. The first component, and the only one actually defined by the UML, is the use case diagram. To supplement the diagram, it is almost always necessary to add a use case narrative. Once a use case is fully described, it is possible to dissect it into scenarios, or test cases. Remember that although diagrams are extremely valuable and provide substantial information, they nearly always require some form of explanation to put them into context and ensure their proper interpretation.

There are four modeling elements which make up the use case diagram:

1. system,
2. actors,
3. use cases,
4. and associations.

Each association becomes a dialog that must be explained in a use case narrative. Each narrative in turn provides a set of scenarios that function as test cases when evaluating the design and implementation of the use case.
In the next lesson, you will learn about the use case system element.

The UML provides use case diagram notation to illustrate the names of use cases can actors, and the relationships between them (see Figure 3.3)

Use case diagrams and use case relationships are secondary in use case work. Use cases are text documents. Doing use case work means to write text.
A common sign of a novice (or academic) use-case modeler is a preoccupation with use case diagrams and use case relationships, rather than writing text. World-class use case experts such as Anderson, Fowler, Cockburn, among others, downplay use case diagrams and use case relationships, and instead focus on writing. With that as a caveat, a simple use case diagram provides a succinct visual context diagram for the system, illustrating the external actors and how they use the system.
Suggestion: Draw a simple use case diagram in conjunction with an actor-goal list.
A use case diagram is an excellent picture of the system context; it makes a good context diagram, that is, showing the boundary of a system, what lies outside of it, and how it gets used. It serves as a communication tool that summarizes the behavior of a system and its actors.