In Java, how to achieve this type of number rounding?

Question

I am writing a small physics app. What I am planning to do is to make number rounding. The issue is that it is not a fixed rounding, but rather a variable rounding that depends on the value of the decimal digits. I will give an explanation for the issue.

• I always need to keep the whole integer part (if any) and the first five decimal digits (if any).

• half up rounding is always used.

  21.1521421056 becomes 21.15214
  34.1521451056 becomes 34.15215

• If the result consists of only decimal digits then:

  • If the first five digits include non zero digits then keep them.

    0.52131125 becomes 0.52131
    0.21546874 becomes 0.21547
    0.00120012 becomes 0.0012

  • If the first five digits are all zero digits 0.00000 then go down to first five digits that include non zero digits.

    0.0000051234 becomes 0.0000051234
    0.000000000000120006130031 becomes 0.00000000000012001

I need to play this rounding while working with BigDecimal because it is a requirement for my needs.

Answer 1

I think this will work, based on experimentation, if I understand correctly what you want. If d is a BigDecimal that contains the number:

BigDecimal rounded = d.round(new MathContext
    (d.scale() - d.precision() < 5 
        ? d.precision() - d.scale() + 5 
        : 5));

Answer 2

Is this, what you are looking for?

public static void main(String[] args){
     double d = 0.000000000000120006130031;

     System.out.println(round(d, 5));
 }

 private static double round(double d, int precision) {
    double factor = Math.pow(10D, precision);
    int value = (int)d;
    double re = d-value;


    if (re * factor <= 0.1 && d != 0) {
        while (re * factor <= 0.1) {
            factor *= 10;
        }
        factor *= Math.pow(10D, precision);
    }
    re = ((int)(re*factor))/factor+value;

    return re;
}

(sorry, it's a little quick & dirty, but you can improve it, if you want)

EDIT: make it <= in the conditions, this should work better