10 - UML Activity Diagrams

Summary

UML activity diagrams are the object-oriented equivalent of flow charts and data-flow diagrams from structured development (Gane and Sarson 1979). In UML 1.x, UML activity diagrams were a specialization of UML state machine diagrams, although in UML 2.x they are full-fledged artifacts. UML activity diagrams are used to explore the logic of

1. ■ a complex operation,

2. ■ a complex business rule,

3. ■ a single use case,

4. ■ several use cases,

5. ■ a business process,

6. ■ concurrent processes,

7. ■ software processes.

General Guidelines

Place the Starting Point in the Top Left Corner

A starting point is modeled with a filled circle, using the same notation that UML state chart diagrams use. Every UML activity diagram should have a starting point, and placing it at the top left corner reflects the way that people in Western cultures begin reading. Figure 41, depicting the business process of enrolling in a university, takes this approach. Another good option is to place it at the top center of the diagram.

Include an Ending Point

An ending point is modeled with a filled circle with a border around it. Some people's style is to make ending points optional—sometimes an activity is simply a dead end—but if this is the case, then there is no harm in indicating that the only activity edge (formerly known as a transition) is to an ending point. That way, when someone else reads your diagram, they know that you have considered how to exit these activities.