About Question enthuware.ocpjp.v7.2.1292 :

[boyonsea]

Why cant "Queue container = new PriorityQueue();" be used?

[admin]

The first sentence of the explanation explains exactly what you are asking. I am quoting it here for your convenience,

Since the return type of getRequestToProcess() method is Request, and the return statement does not have a cast, it means that container must be a typed generic collection that returns Request objects only.

[unvector]

If there would be such answer:

Queue<Request> container = new PriorityQueue<Request>();

Would it be correct? it would compile, but PriorityQueue would throw exception while adding an element to nonempty queue.

[admin]

Yes, it would have been valid. Yes, it is possible for an exception to be thrown if the code is not used correctly but that is irrelevant for the question.

HTH,
Paul.