How to adjust tax to some tax rate

Question

In my application I have some List with tax rates getTaxRates(). The problem is that user can put some value which should not be possible. That means tax rates can be taken only from this list E.g. 7%, 9% and 21% and if user puts 7.5% then it should be saved 9%, if he puts 4% then 7%. The problem is the tax rate is flexible and I don't know what is the value- it can the list of 5 values e.g. 2%, 5%, 19%, 21% or 5%, 9%, 16%. How to implement this condition?

For know I have only the method which can see if the value is in range but no clue how to implement the condition checking the values which will come later from the user of my application settings.

private static boolean inRange(List<Integer> list, int min, int max) {
    return list.stream().allMatch(i -> i >= min && i <= max);
}

Answer 1

There are multiple ways to approach this problem.

Here is one possible way using a List and BigDecimal to get you started:

public static BigDecimal findRate(List<BigDecimal> allowedRates, BigDecimal inputRate) {
    return allowedRates.stream()
            .min(Comparator.comparing(a -> inputRate.setScale(0, RoundingMode.HALF_UP)
                    .subtract(a).abs()))
            .orElseThrow(IllegalArgumentException::new);
}

Here is a sample Junit 5 test. You can check to see whether your particular requirements are met.

List<BigDecimal> ALLOWED_RATES = List.of(new BigDecimal("7.0"),
        new BigDecimal("9.0"),
        new BigDecimal("21.0"));

@ParameterizedTest(name = "#{index}: Testing {0}")
@DisplayName("Find the appropriate tax rate")
@CsvSource({
        "7.0, 7.0",
        "9.0, 9.0",
        "21.0, 21.0",
        "10.0, 9.0",
        "16.0, 21.0",
        "22.0, 21.0",
        "0, 7.0"
})
public void testFindTaxRate(BigDecimal input, BigDecimal expected) {
    BigDecimal actual = MyTaxRateCalculator.findRate(ALLOWED_RATES, input);
    Assertions.assertEquals(expected, actual);
}

In your production code, the method doesn't need to be static. It does make this unit test a little less verbose for this answer.

You may wish to sort, remove duplicates, or perform error-checking as needed based on your requirements on your allowed tax rate list. This method will also work fine if you use change the implementation from a List to a Set that maintains order if that works better for you. Using a Set is really nice, but it tends to require changes elsewhere in the system, and sometimes many IMO.

Finally, be careful which type you use for this.

UPDATE:

If you are looking to always choose the higher bracket given a choice between the two nearest values in your allowed list, then your code could become something like:

public static BigDecimal findRate(List<BigDecimal> allowedRates, BigDecimal inputRate) {

    BigDecimal lastBracket = allowedRates.stream().reduce((a, b) -> b).orElse(null);
    if (inputRate.compareTo(lastBracket) >= 0) {
        return lastBracket;
    }

    return allowedRates.stream()
            .filter(rate -> rate.compareTo(inputRate) >= 0)
            .findFirst()
            .orElseThrow(IllegalArgumentException::new);
}

And your test could be:

List<BigDecimal> ALLOWED_RATES = List.of(new BigDecimal("7.0"),
     new BigDecimal("9.0"),
     new BigDecimal("21.0"));

@ParameterizedTest(name = "#{index}: testing {0}")
@DisplayName("Find the appropriate tax rate")
@CsvSource({
        "7.0, 7.0",
        "9.0, 9.0",
        "21.0, 21.0",
        "10.0, 21.0",
        "16.0, 21.0",
        "22.0, 21.0",
        "0, 7.0"
})
public void testFindRate(BigDecimal input, BigDecimal expected) {
    BigDecimal actual = MyTaxRateCalculator.findRate(ALLOWED_RATES, input);
    Assertions.assertEquals(expected, actual);
}

Answer 2

You can use TreeSet for this kind of implementation. TreeSet provides ceiling() method. This method will return the least element in the set greater than or equal to the given element, or null if there is no such element.

Implementation:

private static Float getTaxRates(TreeSet<Float> taxRates, Float taxRate) {
    return taxRates.ceiling(taxRate);
}