Extend a Use Case to add alternative courses of action to a Use Case scenario.
For the Make Reservation Use Case, assume that a customer has good credit. You can extend the Make ReservationUse Case with another Use Case, called Reject Customer Because of Bad Credit.
1 From the Draw toolbar, select the oval Use Case symbol. Place a Use Case symbol on the diagram underneath Make Reservation, name it Reject Customer Because of Bad Credit, and press Enter.
2 From the Draw toolbar, select the Extends line drawing tool and draw an Extends line from the Reject Customer Because of Bad Credit Use Case to the Make Reservation Use Case. You do not need to name the line. When drawn, the <<extends>> stereotype shows on the line.
Specify the Extension Point
Open the definition of the extends line between Reject Customer Because of Bad Credit and Make Reservation. Double-click on the line or right-click on it and select Edit.
1 For the Extension Point property, click Choices to get a list of steps of the extended Use Case.
2 Drag-and-drop the step Check Customer Credit into the Extension Point field. Click OK to close the definition.
When to Stop Decomposing Use Cases
General Use Case theory recommends that at least one Use Case be prepared for each significantly different kind of scenario instance. Each scenario shows a different sequence of interactions between actors and the system, with all decisions definite.
When you have arrived at the lowest Use Case level, you can create a child Sequence diagram (and accompanying Collaboration diagram) for the Use Case. With these Sequence and Collaboration diagrams, you can model the implementation of the scenario.
