This is an intermediate question, probably, but strikes me as belonging in advanced because complexity is involved.
MultiMap maps a key to a List or Vector which would mean that you would call get Iterator on the the return from a key lookup. You would then have to walk the value with the iterator, counting and
testing and so on. I don't see where an Iterator is going to give out a count(), accessing the list or vector before walking the iterator would tell how many elements are in the List or Vector