code: public static String createLookupKey(String realm, String userid) {
          String key = null;
          if (realm != null && userid != null) {
              key = realm + KEY_SEPARATOR + userid;
          }
          return key;
      }
docs: Creates a key to be used with the AuthCacheService.
  The parameters must not be null, otherwise a null key is returned.
  
  @param realm
  @param userid
  @return