code: @Override
    public void write(DataOutput out) throws IOException {
      super.write(out);
      
      // Write out the number of entries in the map
      
      out.writeInt(instance.size());
      
      // Then write out each key/value pair
      
      for (Map.Entry<WritableComparable, Writable> e: instance.entrySet()) {
        out.writeByte(getId(e.getKey().getClass()));
        e.getKey().write(out);
        out.writeByte(getId(e.getValue().getClass()));
        e.getValue().write(out);
      }
    }
docs: {@inheritDoc}