Question

I’m looking for a smart and fast way of getting only the values, of multiple arraylists (stored in a hashmap), that are in ALL of the other arraylists.

E.g.
[a] = 1, 2, 3, 4, 5
[b] = 1, 3
[c] = 3

Result = 3

What is the fastest way to achieve this in Java?

Answer 1

In Google Guava:

  // assuming you have List<List<?>> lists that is non-empty
  Set<?> result = Sets.newLinkedHashSet(lists.get(0));
  for (int i = 1; i < lists.size(); i++) {
    result.retainAll(ImmutableSet.copyOf(lists.get(i)));
  }

  return result;