out.writeInt(delegate.size());
      for (Entry<K, V> entry : delegate.entrySet()) {
        out.writeObject(entry.getKey());
        out.writeObject(entry.getValue());
      }
      out.writeObject(null); // terminate entries
    }

    @J2ktIncompatible // java.io.ObjectInputStream
    MapMaker readMapMaker(ObjectInputStream in) throws IOException {
      int size = in.readInt();
      return new MapMaker()