code: @Override
    public List<String> listKeys() {
      lock.lock();
      try {
        List<String> keys = new ArrayList<>(this.items.size());
        for (Map.Entry<String, ApiType> entry : this.items.entrySet()) {
          keys.add(entry.getKey());
        }
        return keys;
      } finally {
        lock.unlock();
      }
    }
docs: List keys.
  
  @return the list