code: private Object readResolve() throws InvalidObjectException {
          try {
              return WeekFields.of(firstDayOfWeek, minimalDays);
          } catch (IllegalArgumentException iae) {
              throw new InvalidObjectException("Invalid serialized WeekFields: " + iae.getMessage());
          }
      }
docs: Return the singleton WeekFields associated with the
  {@code firstDayOfWeek} and {@code minimalDays}.
  @return the singleton WeekFields for the firstDayOfWeek and minimalDays.
  @throws InvalidObjectException if the serialized object has invalid
  values for firstDayOfWeek or minimalDays.