protected static function getListStrategy($listType) {
    // get list strategies
    if (self::$listStrategies == null) {
      self::$listStrategies = ObjectFactory::getInstance('listStrategies');
    }

    $strategy = null;

    // search strategy
    if (isset(self::$listStrategies[$listType])) {
      $strategy = self::$listStrategies[$listType];
    }
    else {
      throw new ConfigurationException('No ListStrategy implementation registered for '.$listType);
    }
    return $strategy;
  }