code: public T friends_areFriends(Collection<Integer> userIds1, Collection<Integer> userIds2)
      throws FacebookException, IOException {
      if (userIds1 == null || userIds2 == null || userIds1.isEmpty() || userIds2.isEmpty()) {
        throw new IllegalArgumentException("Collections passed to friends_areFriends should not be null or empty");
      }
      if (userIds1.size() != userIds2.size()) {
        throw new IllegalArgumentException(String.format("Collections should be same size: got userIds1: %d elts; userIds2: %d elts",
                                                         userIds1.size(), userIds2.size()));
      }
  
      return this.callMethod(FacebookMethod.FRIENDS_ARE_FRIENDS,
                             new Pair<String, CharSequence>("uids1", delimit(userIds1)),
                             new Pair<String, CharSequence>("uids2", delimit(userIds2)));
    }
docs: Retrieves whether pairs of users are friends.
  Returns whether the first user in <code>userIds1</code> is friends with the first user in
  <code>userIds2</code>, the second user in <code>userIds1</code> is friends with the second user in
  <code>userIds2</code>, etc.
  @param userIds1
  @param userIds2
  @return T
  @throws IllegalArgumentException if one of the collections is null, or empty, or if the
  collection sizes differ.
  @see <a href="http://wiki.developers.facebook.com/index.php/Friends.areFriends">
  Developers Wiki: Friends.areFriends</a>