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5. Activity and State Diagrams
Previously we have seen how the interaction diagrams demonstrate the behavior of several system objects while executing a single use case. When you want to display the order of events on a broader scale then you should use activity and state diagrams.
An activity can be defined as the execution of a task whether it be a physical activity or the simple execution of code. Simply put, the activity diagram depicts the sequence of activities. Like any simple flow chart, activity diagrams have the support for conditional behavior, but also have added support for parallel execution as well.
Start: each activity diagram has only one start (symbol above) at which the sequence of the actions begins.
End: each activity diagram has only one finish at which the sequence of actions ends
Activity: activities are to be connected together by transitions. Transitions are actually directed arrows which are flowing from the previous activity to the next activity. They can be optionally accompanied by a textual label of the form:
The guard is considered to be a conditional expression which when true indicates that the transition has taken place. The label is optional and is represented in free form.
To show conditional behavior you can use a branch and a merge. The top diamond is a branch and can have only one transition flowing into it but any number of mutually exclusive transitions flowing out. So, the guards on the outgoing transitions must resolve themselves in order that only one is followed. The merge is used to finish the conditional behavior. There can be any number of incoming transitions, and only one outgoing transition.
To show the parallel behavior you can use a fork and a join. The fork(placed at top) has only one transition entering and any number of transitions which are exiting, all of which will be taken in action. The join(placed at the bottom) represents the end of the parallel behavior and has any number of transitions entering while there is only one leaving.