UML is revealed to contain three different defects concerning the use-case class that were buried in OOSE and handed over to it. These defects are: 1) the use-case class and its instance are unusually defined, 2) a conjecture that is against the definition of the class is introduced without any reasons, and 3) the execution procedure of a use-case instance does not actually work because of some flaws concerning the execution control. These defects have been causing unnecessary confusion in UML's specification of the use-case class. An object-oriented real-world model is built that represents a typical situation of using a use case in the analysis and design states, and another definition of the use-case class is constructed that successfully solves the problems.
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