Java BigDecimal: How to set scale only if it's greater than certain precision point?

Question

I have a use case where I want to set scale only if the number's precision is bigger than certain number. In other words, I want to set scale to 4 if the precision is 5 but leave the number as it is if precision is less than 4.

This is the expected result I am expecting ->

123.123 => 123.123 -> leave the number as it is because precision is less than 4.

123.123456 => 123.1235 -> scale is set, and rounded up.

123 => 123 -> leave the number as it is, since precision is less than 4.

0.1234445 => 0.1234 -> scale is set, and rounded down.

How can I achieve this result using Java's BigDecimal?

Answer 1

It seems like the question you really are asking is "How do I truncate the decimal portion of a BigDecimal so that it has at most 4 numbers to the right of the decimal point?"

First, I think you should ensure you understand what scale and precision mean with regards to a BigDecimal.

If you run this small program:

public static void main(String[] args) {
    List<BigDecimal> decimals = new LinkedList<>();
    decimals.add( new BigDecimal(".123456"));
    decimals.add( new BigDecimal("1.23456"));
    decimals.add( new BigDecimal("12.3456"));
    decimals.add( new BigDecimal("123.456"));
    decimals.add( new BigDecimal("1234.56"));
    decimals.add( new BigDecimal("12345.6"));
    decimals.add( new BigDecimal("123456"));

    for(BigDecimal bd : decimals){
        System.out.println(bd.toPlainString() + ". scale = " + bd.scale() + ", precision = " + bd.precision());
    }
}

You'll see the following output:

0.123456. scale = 6, precision = 6 
1.23456. scale = 5, precision = 6
12.3456. scale = 4, precision = 6
123.456. scale = 3, precision = 6
1234.56. scale = 2, precision = 6
12345.6. scale = 1, precision = 6
123456. scale = 0, precision = 6

You should now see that what you want to do is query and then adjust the scale of the number.

static BigDecimal doIt(BigDecimal input){
    if(input.scale() > 4){
        return input.setScale(4, RoundingMode.HALF_DOWN);
    }
    else{
        return input;
    }
}

A little test program shows the output is what you requested:

public static void main(String[] args) {
    List<BigDecimal> decimals = new LinkedList<>();
    decimals.add(new BigDecimal("123.123"));
    decimals.add(new BigDecimal("123.123456"));
    decimals.add(new BigDecimal("123"));
    decimals.add(new BigDecimal("0.1234445"));


    for (BigDecimal bd : decimals) {
        System.out.println(bd.toPlainString() + " ---> " + doIt(bd));
    }
}

Output

123.123 ---> 123.123
123.123456 ---> 123.1235
123 ---> 123
0.1234445 ---> 0.1234