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Activity diagrams
According to the UML4 specification, UML activity diagrams are a special case of a state machine in which each state represents an ongoing activity and transitions to the next state occur when activities are completed. For most people, activity diagrams represent little more than a sophisticated version of traditional flowcharts. As a result, activity diagrams have been the subject of much debate in the UML world. Activity diagrams can be used to describe business processes, user interaction sequences (use cases), or the implementation of a particular operation. Figure 2 shows a rather trivial activity diagram describing the check-out process in a basic supermarket. Swimlanes shows who is responsible for what activities, and sync bars show forks and joints that allow activities to be performed simultaneously or at least in any particular sequence.