Use cases were invented by Ivar Jacobson2. They describe the behavior of a system from a user's standpoint by using actions and reactions. They allow the definition of the system's boundary, and the relationships between the system and the environment.
2Jacobson I. 1992, Object-Oriented Software Engineering, A Use Case Driven Approach, Addison-Wesley.
Use cases address a lack of support in the early object-oriented methods, such as OMT-1 and Booch'91, which did not supply any technique for determining requirements. In this sense, use cases associated with object-oriented techniques provide a complete approach for the whole project lifecycle, from specification to implementation.
A use case corresponds to a specific kind of system use. It is an image of a system's functionality, which is triggered in response to the stimulation of an external actor.
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