code: public void empty() {
      for (Entry<String, String> entry : idToVersion.entrySet()) {
        expirationVersion.remove(entry.getValue());
        versionToItem.remove(entry.getValue());
        if (collectionCachePath != null
            && !collectionCachePath.resolve(entry.getValue()).toFile().delete()) {
          log.debug("couldn't delete " + entry.getValue());
        }
      }
      idToVersion.clear();
    }
docs: Empty.