code: private static byte[] serializeEmptyHashKeys() {
      try {
        // we don't synchronize here: the worst thing that go wrong here is repeated assignment to the same value
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        Encoder encoder = new BinaryEncoder(bos);
        encoder.writeInt(0);
        return bos.toByteArray();
      } catch (IOException e) {
        throw new RuntimeException("encoding empty hash keys went wrong - bailing out: " + e.getMessage(), e);
      }
    }
docs: many entries will have no hash keys. Serialize that once and for good