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State Transition Diagram

Shows the dynamic behavior of classes


The state of an object represents the cumulative results of its behavior. At any givn point in time, the state of an object encompasses all of its properties (the object's attributes and relationships) and the values of each of these properties.

The ability to nest states gives depth to state transition diagrams. A state transition diagram is used to show the state space of a given class, the events (messages) that trigger a transition from one state to another, and the actions that result from a state change.
The notation is borrowed from Harel.

Harel, D. 1987. Statecharts: A Visual Formalism for Complex Systems. Science of Computer Programming vol. 8.

The notation:

  • State icon:

    A (unique) name is required for each state; for certain states it could be useful to indicate some of the actions associated with:

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  • History:

    The state goes into his lastly assumed state upon entry. This is indicated with adorning the state icon with:

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  • Nesting:

    State transitions to and from substates are represented with a stubbed arrow:

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As previously said, actions may be associated with states. In particular one may specify actions to be carried out upon entry or exit of a state.

Each state transition connects two states. A state may have transition to itself, and many transition can depart from the same state, although each transition must be unique, i.e. there cannot be any circumstance (event) able to trigger more than one state transition from the same state.

In every state transition diagram there must be exactly one default start state, which is designated by writing a transition from a filled circle.

It is also possible to designate a stop state by drawing a transition to a filled circle within a larger unfilled circle (not shown in the example).


Author: Massimo Marino
Last updated: Jenny Williams

Last modification: 29 January 2001
Last significant update: 26 April 1996