The Activity Diagram

The activity diagram is the “new kid on the block,” a special form of state diagram. Each state is considered an activity state that executes and then progresses to the next activity state. An activity diagram models workflows, computations, and complex operation steps.

Activity diagrams also are beneficial during the use-case definition process. Pathways within the use-cases can be easily modeled with an activity diagram (rather than an alternative outline format). Figure 7-13 shows an activity diagram for the checkCredit() operation in the Remulak Productions Order class.

The activity diagram resembles a flowchart; notice that it includes a decision diamond. The presence of merge and splitter bars makes it very convenient to model concurrent activities and synchronization points.

Activity diagrams focus on the what of the flow, not the how. For example, the activity state of “Prepare Credit Message” on Figure 7-13 tells us what to do, but not how to do it.