Generating all combinations of multiple lists

// Adapted from https://stackoverflow.com/a/23870892/5602641

public static <T> Collection<List<T>> permutations(List<Collection<T>> collections) {
  if (collections == null || collections.isEmpty()) {
    return Collections.emptyList();
  } else {
    Collection<List<T>> output = new LinkedList<T>();
    recursePermutations(collections, output, 0, new LinkedList<T>());
    return output;
  }
}

private static <T> void recursePermutations(List<Collection<T>> source, Collection<List<T>> output, int depth, List<T> current) {
  if (depth == source.size()) {
    output.add(current);
    return;
  }

  Collection<T> currentCollection = source.get(depth);
  for (T element : currentCollection) {
    List<T> copy = new LinkedList<T>(current);
    copy.add(element);
    recursePermutations(source, output, depth+1, copy);
  }
}