& its Importance.
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The state diagram depicts the change of an object through a time period. Based up on the events that occur, the state diagram displays how the object can change from start to finish.
States can be represented as a rounded rectangle with the name of the state displayed as shown above. You can optionally include an activity which represents a longer running task during that state.
Transitions connect these states together. These represent the events which cause the object to change from one state to another. The guard clause of the label is mutually exclusive and must either be true or false. Actions represent the tasks which run so as to cause the transitions.
Actions are varied from activities in the way that their actions cannot be interrupted, while an activity can be interrupted by the incoming event. Both can actually display an operation on the object which is being studied. For example, an operation which sets an attribute would be considered an action, while a long calculation might be considered as an activity. The specific separation between the two depends actually on the object and the system which is being studied.
Like the activity diagrams, state diagrams have only one start and one end from which the state transitions can start and end respectively.
Putting it all Together
Activity diagrams are used to depict the work flow in parallel and also conditionally. They are useful while working out the order and the concurrency of a sequential algorithm and that too while analyzing the steps which are there in a business process and while working with threads.
State diagrams display the change of an object over time and are useful when an object exhibits interesting or unusual behavior - such as that of a user interface component.
As always, these diagrams should be used only so as to serve a purpose. You should not feel that a state diagram should be drawn for every system object and an activity diagram for every system process. You should use them only here they add value to your design. You may choose not to include these diagrams in your system design, and your work may well be considered to be complete and useful. The purpose of Visual Case Tool and of these diagrams is to help you to do your job. So if the diagram becomes too complicated and confusing, you and those working with you may lose focus of the task at hand.