code: private void writeObject (ObjectOutputStream out)
          throws IOException
      {
          out.defaultWriteObject();
          for (int idx = 0, pos = 0; pos < _size; idx++) {
              int value = _buckets[idx];
              if (value != _sentinel) {
                  out.writeInt(value);
                  pos++;
              }
          }
      }
docs: Custom serializer.