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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.
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