Use case generalization versus extension

Question

UML Use Case Diagrams allow for two seemingly equivalent ways to show that a given use case might be realised in several different ways namely use case generalizations as op

Answer 1

When modelling with an extend relationship, the extending use case (pay via xxx) is executed when the extended use case (make payment) is at a precise location (given by the extension point, payment type) if the condition for such extend relationship holds (e.g. for "Pay via Paypal", the condition would be payment_type=PAYPAL). In this model, "Pay via Paypal" does only deals with the details of completing a payment transaction with Paypal, while "Make Payment" specifies all the behaviors that are independent of the payment method (such as computing the total amount, and saving the result of the transaction once that it was performed).

On the other hand, generalization (which is defined not only for use cases, but also for any classifier) is a broader concept, since it do not detail (at the diagram level) the details of when and how the specialized behaviors are executed. If "Pay via Paypal" were a specialization of "Make Payment" it would redefine the behavior of "Make Payment", which could be substantially different from the behavior of "Pay via credit card".

Being an extending use case does not means that the alternatives have to be hardcoded. Indeed, your first example is also a valid implementation of a extend relationship, since pd->pay(pd) will invoke different behaviors depending on the selected payment type. Actually, use case diagram models what a system is supposed to do, while low-level implementation details are better specified in activity diagrams.