code: public OptionalInt getLastPage() {
          if(total.isPresent()) {
              return of(calcLastPage(total.getAsInt(), pageSize));
          } else {
              return empty();
          }
      }
docs: Returns number of the last page, or {@code Optional.empty()} if {@link #getTotal()} is unknown / empty.
  
  @return the number of the last page.