code: public int addKey(String key) {
          JdkUtils.requireNonNull(key);
          int nextIndex = keys.size();
          final Integer mapIndex = JdkUtils.putIfAbsent(keys, key, nextIndex);
          return mapIndex == null ? nextIndex : mapIndex;
      }
docs: Add the key and return it's index code. If the key already is present, the previous
  index code is returned and no insertion is done.
  
  @param key key to add
  @return index of the key