BigDecimal division, rounding and sums

Question

I am working with a BigDecimal value that represents a currency amount. I need to split this amount into 6 rates with the first 5 rates being rounded up to 5, the 6th rate will be the rest.

BigDecimal numberOfRates = new BigDecimal("6");
BigDecimal currencyAmount = new BigDecimal("650.30");
BigDecimal rate = currencyAmount.divide(numberOfRates);
rate = //rate rounded up to closest multiple of 5
BigDecimal lastRate = currencyAmount.subtract(rate.multiply(new BigDecimal("5"));

My 2 questions are:

• How do I round to the closest multiple of 5 (or any other int)?
• Will the 6 rates summed up always give the original currencyAmount or are there issues with precision due to the division?

(This is a simplified example, I do realize that the last rate could be negative with this setup.)

* Updated Question *

Expected result with the example is:

• Rates 1-5: 110.00
• Rate 6: 100.30 And in this case: 5*110.00 + 100.30 = 650.30

By using the indicated approaches will it always be the case that the sum of all rates equals the initial amount?

Answer 1

Use the standard approach of divide, round, multiply

private static BigDecimal round(BigDecimal input, int multiple) {
    return input.divide(new BigDecimal(multiple))
            .setScale(0, RoundingMode.CEILING)
            .multiply(new BigDecimal(multiple));
}

for (double i = 0; i < 10; i += 0.9) {
    System.out.println(String.format("%.1f => %s", i, round(new BigDecimal(i), 5)));
}

Output

0.0 => 0
0.9 => 5
1.8 => 5
2.7 => 5
3.6 => 5
4.5 => 5
5.4 => 10
6.3 => 10
7.2 => 10
8.1 => 10
9.0 => 10
9.9 => 10