* Registers this extractor with the ExtractorFactory.
     * @param keyList The list of keys to register this extractor under.
     */
    public void register(final List<String> keyList) {
        if (keyList == null || keyList.isEmpty()) {
            throw new IllegalArgumentException("keyList must not be null or empty.");
        }
        getExtractorFactory().addExtractor(keyList, this);
    }

    /**

     *
     * @param keyList the list of keys to associate with the extractor
     * @param extractor the extractor to add
     * @throws CrawlerSystemException if the key list is null or empty, or if the extractor is null
     */
    public void addExtractor(final List<String> keyList, final Extractor extractor) {
        if (keyList == null || keyList.isEmpty()) {

  }

  @Override
  ImmutableSet<K> createKeySet() {
    @SuppressWarnings("unchecked")
    ImmutableList<K> keyList =
        (ImmutableList<K>)
            new RegularImmutableMap.KeysOrValuesAsList(alternatingKeysAndValues, keyOffset, size);
    return new RegularImmutableMap.KeySet<>(this, keyList);
  }

  @Override
  boolean isPartialView() {
    return false;
  }