State diagram (UML)

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Example UML State diagram.

The state diagram in the Unified Modeling Language is essentially a Harel statechart with standardized notation[1] ,[2] which can describe many systems, from computer programs to business processes. In UML 2 the name has been changed to State Machine Diagram. The following are the basic notational elements that can be used to make up a diagram:

  • Filled circle, pointing to the initial state
  • Hollow circle containing a smaller filled circle, indicating the final state (if any)
  • Rounded rectangle, denoting a state. Top of the rectangle contains a name of the state. Can contain a horizontal line in the middle, below which the activities that are done in that state are indicated
  • Arrow, denoting transition. The name of the event (if any) causing this transition labels the arrow body. A guard expression may be added before a "/" and enclosed in square-brackets ( eventName[guardExpression] ), denoting that this expression must be true for the transition to take place. If an action is performed during this transition, it is added to the label following a "/" ( eventName[guardExpression]/action ).
  • Thick horizontal line with either x>1 lines entering and 1 line leaving or 1 line entering and x>1 lines leaving. These denote join/fork, respectively.

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