[xyrouter]

> though in this case the easy and correct thing for the interviewer to do is to say "OK, how is Collections.shuffle implemented?"

And very likely that's what really happened. I have been in such interviews and it is sometimes hard to guess whether the interviewer wants the practical answer (calling a library function) from engineering standpoint or the conceptual answer (demonstrating that I can implement the inner workings of the function) from CS standpoint. Often I would just ask a follow up question to clarify exactly at which level of abstraction does the interviewer want my solution to be in?

In situations where I offer a solution that does not match the interviewer's expectation, they clarify the expectations.