code: public byte[] toByteArray(final ArrayOfItemsSerDe<? super T> serDe) {
      if (itemsSeen_ == 0) {
        // null class is ok since empty -- no need to call serDe
        return toByteArray(serDe, null);
      } else {
        return toByteArray(serDe, data_.get(0).getClass());
      }
    }
docs: Returns a byte array representation of this sketch. May fail for polymorphic item types.
  
  @param serDe An instance of ArrayOfItemsSerDe
  @return a byte array representation of this sketch