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Activity Diagram

Activity diagrams are constructed from a limited repertoire of shapes, connected with arrows. The most important shape types:
  • rounded rectangles represent activities;
  • diamonds represent decisions;
  • bars represent the start (split) or end (join) of concurrent activities;
  • a black circle represents the start (initial state) of the workflow;
  • an encircled black circle represents the end (final state).
Arrows run from the start towards the end and represent the order in which activities happen. Hence they can be regarded as a form of flowchart. Typical flowchart techniques lack constructs for expressing concurrency. However, the join and split symbols in activity diagrams only resolve this for simple cases; the meaning of the model is not clear when they are arbitrarily combined with decisions or loops. 

While in UML 1.x, activity diagrams were a specialized form of state diagrams, in UML 2.x, the activity diagrams were re-formalized to be based on Petri net-like semantics, increasing the scope of situations that can be modeled using activity diagrams. These changes cause many UML 1.x activity diagrams to be interpreted differently in UML 2.x