code: public CmsResourceFilter addRequireFolder() {
  
          CmsResourceFilter extendedFilter = (CmsResourceFilter)clone();
  
          extendedFilter.m_onlyFolders = Boolean.TRUE;
          extendedFilter.updateCacheId();
  
          return extendedFilter;
      }
docs: Returns an extended filter that requires all returned resources to be folders.<p>
  
  @return an extended filter that requires all returned resources to be folders