State diagram

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State Diagram

State Diagrams show the different states of an Object during its life and the stimuli that cause the Object to change its state.

State Diagrams view Objects as state machines or finite automates that can be in one of a set of finite states and that can change its state via one of a finite set of stimuli. For example an Object of type NetServer can be in one of following states during its life:

  • Ready
  • Listening
  • Working
  • Stopped

and the events that can cause the Object to change states are

  • Object is created
  • Object receives message listen
  • A Client requests a connection over the network
  • A Client terminates a request
  • The request is executed and terminated
  • Object receives message stop


States are the building block of State Diagrams. A State belongs to exactly one class and represents a summary of the values the attributes of a class can take. A UML State describes the internal state of an object of one particular class

Note that not every change in one of the attributes of an object should be represented by a State but only those changes that can significantly affect the workings of the object

There are two special types of States: Start and End. They are special in that there is no event that can cause an Object to return to its Start state, in the same way as there is no event that can possible take an Object out of its End state once it has reached it.

State diagram with Umbrello
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