Use Case Diagrams

A use case diagram names who uses the system and what each of them can do. It contains no classes, no methods and no data. It is drawn before any of that exists.

Two words carry it. An actor is a role that uses the system, drawn as a stick figure. A use case is one thing an actor can achieve, drawn as an oval with a verb phrase inside it.

The test for a use case is that it produces a result the actor cares about. "Borrow a book" is a use case. "Click the search button" is not; it is a step inside one.

The four parts of the picture

**The system boundary