code: @Override
      public RoundedMoney add(MonetaryAmount amount) {
          MoneyUtils.checkAmountParameter(amount, currency);
          if (amount.isZero()) {
              return this;
          }
          return new RoundedMoney(number.add(amount.getNumber().numberValue(BigDecimal.class)), currency,
                  rounding).with(rounding);
      }
docs: Arithmetic Operations