Change is used to indicate that some part of a model represents a difference in the system or its environment from one time to another. In UML 2, change is expressed by a difference in state at different times.
Q: Change is not an explicit concept in UML 1, any more than possibility was.
A: Right. In UML 1, change is represented, for example, by Flow relationships. And crops up in every kind of behavior specification.
In the semantics included in the UML action semantics submission, change was represented by Change. (The UML action semantics was adopted without a semantics.)
Q: What is it that changes in a model?
A: For example, an action changes an object; that means the state of that object immediately before that action is not the same as the state of that object immediately after that action. An action can also create objects, establish associations, change associations, and also end objects and associations.
Q: That seems to require the concept, time.
A: That's right. In a model that specifies change, there are at least two times.
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