code: private static Double calculateMean(Money minMoney, Money maxMoney) {
      if (minMoney == null || maxMoney == null) {
        return null;
      }
      return calculateMean(minMoney.getMicroAmount(), maxMoney.getMicroAmount());
    }
docs: Returns the mean of the {@code microAmount} of the two Money values if neither is null, else
  returns null.