UML METHODOLOGY - Step 9: State Transition diagrams

11.1. State transition diagrams

State transition diagrams show the life history of a given class, the events that cause a transition from one state to another, and the actions that result from a state change. This life cycle is expressed in terms of the different states that the various objects can find themselves and the events that cause the transitions of their states.

A State is a situation during the life of an object, which satisfies some condition, performs some activity, or waits for some event, in other words is one of the possible conditions in which the object can exist. An object has a limited number of possible states and can be in only one state at a time (see fig. 18).

Nested states.

A Transition is a change from an originating state to a successor state as a result of some stimulus. Transitions may be automatic (may not be interrupted) and they may be labeled with an event, condition, and/or action. The syntax for a transition label has three parts, all of which are optional: event [ guard condition ] [1] / action [2] (see fig. 18).

Events maybe of several kinds [Detsis, 2000]:

A designated condition becoming true (when keyword; e.g. when (speed > 50 mph))
Receipt of an explicit signal from one object to another
Receipt of a call for an operation by an object
Passage of a designated period of time after a designated event (often the entry of the current state) (after keyword; e.g. after (10 minutes))

Actions and activies are implemented the same but are treated differently. Actions are associated with transitions and are considered to occur quickly and are not interrupted, while activities are associated with states and can take longer.

Figure 18 Components of a state transition diagram

Figure 19 shows guarded transitions. A guard condition is a conditional expression in terms of attribute values that prevents a transition from occurring unless the condition is true. In the following fig. the state moves from OK to Overdrawn if [balance < amount] which is the guard condition. The transitions that start and end in the same state are called self-transitions.

Figure 19 Example state transition diagram

UML also defines concurrent state transition diagrams (see e.g. Fowler, 2000a). In the concurrent sections of such a state diagram the given object is in two different states, one from each diagram. When the object leaves the concurrent states, it is in only a single state. Concurrent state transition diagrams are useful when a given object has sets of independent behaviours.

[1] A Boolean expression that is used to make a transition conditional, i.e. the transition occurs if and only if the guard condition is true.
[2] Behaviour that accompanies a state transition and is performed by an object while in a given state.

11. Step 9: Object view

11.1. State transition diagrams