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Does updating grades extend a feature that shows grades? I am not sure if the notation is correct, so I am hoping you guys will know it.
No, it does not extend. Updating grades is a standard CRUD operation.
You may think of extension use cases as an Interface with the behaviors of the context that it presents. Programming to the interface reduces dependency on implementation specifics and makes code more reusable and flexible to changes.
In UML modeling, you can use an extend relationship to specify that one use case (extension) extends the behavior of another use case (base).
Suppose we sell products. Customers can set requirements to a degree like setting size or color. So, such requirements add extra flow to the use case. In such scenarios, extend relationship can be used to pull the alternative flows into a new use case.
Updating grades do not extend show grades.