stream.writeObject(countMap);
  }

  @J2ktIncompatible // serialization
  private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
    stream.defaultReadObject();
    @SuppressWarnings("unchecked") // reading data stored by writeObject
    ConcurrentMap<E, Integer> deserializedCountMap =
        (ConcurrentMap<E, Integer>) requireNonNull(stream.readObject());