How to select a random key from a HashMap in Java?

Question

I\'m working with a large ArrayList>, and I would repeatedly need to select a random key from a random HashMap (and do some stuff with it).

Answer 1

How about wrapping HashMap in another implementation of Map? The other map maintains a List, and on put() it does:

if (inner.put(key, value) == null) listOfKeys.add(key);

(I assume that nulls for values aren't permitted, if they are use containsKey, but that's slower)