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As usual were going to illustrate these kind of diagrams by
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using an example. In particular we are going to describe the state
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transition diagram for part of our example, for part of
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our course management system. And we're going to focus on the course
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offering class. When the class is created, it enters the
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initialization state, in which the activity performed is to initialize the
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course. At this point, a simple case is the case in
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which the class is cancelled, so there is a cancelled event.
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And if that happens, the class is simply cancelled. So it
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gets into this final state, which is the state cancelled. And
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the activity in this case is to notify the registered students.
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Obviously if this is the flow there will be no registered students.
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However something else can happen when we are in this initial
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state. So what can happen is that a student can register, so
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in this case the add student event is triggered. And the
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corresponding action is to set the count, in this case it would
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be the count of students for the course offering, to one. And
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there will be a change of state and the course offering will
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get into this open state. And the action that will be performed
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on entry will be to register the student. At this point more
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students may register. So other student events might occur and notice that we
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have a curve here that tells us this event will trigger this
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transition only if the count is less than 10. So we're assuming
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that we're not going to have more than 10 students just for lack
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of a better number in our course offering. So if that
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happens, if the count is less than ten so then the count
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is incremented so the increment count action takes place. And the
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system goes back into the open state, and the new student
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is registered. Now here we have an interesting transition, because there's
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no event triggering the transition, but simply the fact that the
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count is equal to 10. So you can imagine this as
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being a transition that is always enabled so can always be triggered,
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but will be guarded. By the fact that the count has
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to be exactly ten. So basically this transition will take place
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only when enough students are added, such we get to count
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them. Being incremented and being equal to ten and then the transition
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will occur. And we will get into the closed state, in
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which the class is no longer open because there are enough students
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registered. And at this point, what will happen is that the
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course will be finalized. So there will be this activity which performs
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some operation that is needed to finalize the course. Another possibility
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is that when we are in the open state, the course
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is cancelled. And if the course is cancelled, in this case,
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we go again to the cancel state. But here, the activity
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of notifying registered students makes more sense. Because we will have
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at least one registered student in this state, and therefore we'll
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need to notify such student that the course offering has been
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cancelled. Finally, is it also possible also to cancel a course
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after it has been closed? And in this case again, the
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same thing will happen. The class will reach the cancelled state and
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all the students, in this case ten students, that are registered
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for the course will be notified that the course has been cancelled.
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So, if we look at this state transition diagram, you can
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see that it's pretty easy to see what the evolution of objects
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of this class can be. How they can go from their initial
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state to various final states depending on what are the external events
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that reach the system.
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