code: private static boolean hasValidSubscriptionType(RosterPacket.Item item) {
          switch (item.getItemType()) {
              case none:
              case from:
              case to:
              case both:
                  return true;
              default:
                  return false;
          }
      }
docs: Ignore ItemTypes as of RFC 6121, 2.1.2.5.
  
  This is used by {@link RosterPushListener} and {@link RosterResultListener}.