How to format decimal number precision

secret super star :

How to format the decimal number precision like given below:

double d = 1/3 returns 0.3333333333333333 and mathematically 3 is repeated infinitely.

d = ((double)3)/ (double)41; returns 0.07317073170731707 and here 07317 is repeated.

Now, the ask to format the output like below

0.3333333333333333 should be formatted to 0.(3) as 3 is repeated.

Similarly 0.07317073170731707 should be formatted to 0.(07317) as 07317 repeated

I had looked into DecimalFormat class but I am able to format only number of precisions.

public static void main(String[] args) {
        DecimalFormat formatter = new DecimalFormat("#0.'('##')'");

        double d = ((double)1)/ (double)3;
        System.out.println("formatted "+formatter.format(d));
        System.out.println(d);


        d = ((double)1)/ (double)2;
        System.out.println("formatted "+formatter.format(d));
        System.out.println(d);


        d = ((double)3)/ (double)41;
        System.out.println("formatted "+formatter.format(d));
        System.out.println(d);

    }

Output:

formatted 0.33()
0.3333333333333333
formatted 0.5()
0.5
formatted 0.07()
0.07317073170731707

Is there any built-in class in Java available to achieve the same?

floxbr :

As pointed out in the comments, if you only have a String representation, to begin with, there is no way to tell if there is actually some repeating part to it or if the precision simply is not high enough.

However, from your sample, it seems that you actually want to find the repeating part of fractions where you know both the numerator and denominator as an exact integer. That is not too hard of a problem; for example, you can look at https://codereview.stackexchange.com/questions/87522/decimal-expansion-of-a-rational-number

=== Update ===

I was working on a class representing a rational number with numerator and denominator as BigIntegers (and assumed to be reduced as far as possible). There I implemented a method to produce a representation as decimal String and came up with the following:

public String toDecimalString() {

    var result = new StringBuilder();

    var remainders = new HashMap<BigInteger, Integer>();

    if(!isNonNegative()) result.append("-");
    var divAndRem = numerator.abs().divideAndRemainder(denominator);
    result.append(divAndRem[0].toString());

    if(isWholeNumber()) return result.toString();

    result.append(".");

    int currentPosition = result.length();
    remainders.put(divAndRem[1], currentPosition);

    while(!divAndRem[1].equals(BigInteger.ZERO)) {
        divAndRem = divAndRem[1].multiply(BigInteger.TEN).divideAndRemainder(denominator);
        result.append(divAndRem[0].toString());
        if(remainders.containsKey(divAndRem[1])) {
            int periodStart = remainders.get(divAndRem[1]);
            return result.substring(0, periodStart)+"["+result.substring(periodStart)+"]";
        }
        remainders.put(divAndRem[1], ++currentPosition);
    }

    return result.toString();

}