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The very last diagram that I want to discuss is

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the state transition diagram. The state transition diagram is defined for

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each relevant class in the system and basically shows the

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possible live history of a given class or object. So what

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does it mean to describe the life history? It means

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that it describes the possible states of the class as defined

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by the values of the class attributes. And it also describes

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the events that cause a transition from one state to another.

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Finally, it describes the actions that result from a state

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change. So if you put all of this together you

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can see how this can represent the whole history of

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the class, from its creation to its destruction. So let me

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discuss the transition diagram in more detail, and also provide

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information about the notation used to represent them. We have

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states, that are represented by ovals with a name. And

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we have transitions marked by the event that triggers the transition.

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What transitions indicate is the passage from one state to

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another state as the consequence of some external stimuli. Notice

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that not all events will cause a state transition. So

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for example, some events might be consumed within a single state.

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And we'll get to that in a second. But in

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most cases, an event will trigger some state transition. Events

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may also produce actions and they may also have attributes

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which are analogous to parameters in a method call. And Boolean

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conditions that guard the state transition that is prevented

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from happening in the case the conditions are not

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satisfied. States may also be associated with activities and

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actions. Specifically, activities are operations performed by an object

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when it is in a given state and that

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takes time to complete. Actions, conversely, just like the

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actions corresponding to an event are instantaneous operations that

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are performed by an object. And can be triggered

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on entry. So, when the object reaches a given state,

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when the object exits that state, and also when a

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specific event occurs. And in this case, this notation is

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basically a shortcut for any event that will cause a

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state transition that will bring the object back into the

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same state. Since we have several actions and activities, it

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is probably worthwhile clarifyinig the ordering of such actions and

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activities. So the way in which these actions and activities

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occur is, first of all, we have the actions on the

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incoming transition, so this is performed first. Then if there is an

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entry action that is the next action that would be performed, then

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we have activity and event actions as appropriate. And finally exit actions.