code: private String getUserKey( AppInfo appInfo, String userId )
      {
          if ( userId == null ) {
              throw new IllegalArgumentException( "userId should not be null" );
          }
  
          return String.format( "%s%s", getAppPrefix( appInfo ), userId );
      }
docs: Builds the key of a credentials according to a given appInfo and userId
  
  @param appInfo The application informations
  @param userId The user identifier
  @return The user key