Java Hashmap: How to get key from value?

Question

If I have the value \"foo\", and a HashMap ftw for which ftw.containsValue(\"foo\") returns true, how can I

Answer 1

Yes, you have to loop through the hashmap, unless you implement something along the lines of what these various answers suggest. Rather than fiddling with the entrySet, I'd just get the keySet(), iterate over that set, and keep the (first) key that gets you your matching value. If you need all the keys that match that value, obviously you have to do the whole thing.

As Jonas suggests, this might already be what the containsValue method is doing, so you might just skip that test all-together, and just do the iteration every time (or maybe the compiler will already eliminate the redundancy, who knows).

Also, relative to the other answers, if your reverse map looks like

Map>

you can deal with non-unique key->value mappings, if you need that capability (untangling them aside). That would incorporate fine into any of the solutions people suggest here using two maps.