@Override
  public List<String> indexKeys(String indexName, String indexKey) {
    lock.lock();
    try {
      if (!this.indexers.containsKey(indexName)) {
        throw new IllegalArgumentException(String.format("index %s doesn't exist!", indexName));
      }
      Map<String, Set<String>> index = this.indices.get(indexName);
      Set<String> set = index.get(indexKey);
      List<String> keys = new ArrayList<>(set.size());
      for (String key : set) {
        keys.add(key);
      }
      return keys;
    } finally {
      lock.unlock();
    }
  }
@ Override public List < String > indexKeys ( String indexName , String indexKey ) { lock . lock ( ) ; try { if ( ! this . indexers . containsKey ( indexName ) ) { throw new IllegalArgumentException ( String . format ( "index %s doesn't exist!" , indexName ) ) ; } Map < String , Set < String > > index = this . indices . get ( indexName ) ; Set < String > set = index . get ( indexKey ) ; List < String > keys = new ArrayList < > ( set . size ( ) ) ; for ( String key : set ) { keys . add ( key ) ; } return keys ; } finally { lock . unlock ( ) ; } }
Index keys list.

@param indexName the index name
@param indexKey the index key
@return the list