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An activity diagram is a special case of a statechart diagram (statechart diagram: A representation of a state machine, attached to a class or method, that describes the response of the class to outside stimuli.) in which all of the states (state: The period of time during which an actor exhibits a particular behavior.) are action states and the flow of control (floating stencil: A stencil that appears in the size and location you choose. By default, stencils are docked on the left side of the drawing window. You can make stencils float, or you can dock them on the right side of the drawing window.) is triggered by the completion of actions in the source state.
Related to a specific class (class: In a static structure diagram, a set of objects with similar structure, behavior, and relationships. Classes are declared in class (static structure) diagrams and represent concepts in the systems being modeled.) or use case (use case: In a use case diagram, a representation of a set of events that occurs when an actor uses a system to complete a process. Normally, a use case is a relatively large process, not an individual step or transaction.), an activity diagram describes the internal behavior of a method. Use an activity diagram to represent a flow driven by internally generated actions. Use a statechart diagram (statechart diagram: A representation of a state machine, attached to a class or method, that describes the response of the class to outside stimuli.) to represent a flow in response to external events.
Activity diagrams encourage you to notice and document parallel and concurrent activities. This makes them excellent tools for modeling workflow, analyzing use cases, and dealing with multi-threaded applications.