Digital root is the recursive sum of all the digits in a number.

Given n, take the sum of the digits of n. If that value has more than one digit, continue reducing in this way until a single-digit number is produced. The input will be a non-negative integer.
Examples

16  -->  1 + 6 = 7

942 --> 9 + 4 + 2 = 15 --> 1 + 5 = 6
132189 --> 1 + 3 + 2 + 1 + 8 + 9 = 24 --> 2 + 4 = 6
493193 --> 4 + 9 + 3 + 1 + 9 + 3 = 29 --> 2 + 9 = 11 --> 1 + 1 = 2

Solution in java

    public static int digital_root(int n) {
        int r = n;
        while (r > 9) {
            r = String.valueOf(r)
                    .chars()
                    .map(Character::getNumericValue)
                    .sum();
        }
        return r;
    }

Reference:https://www.codewars.com/kata/541c8630095125aba6000c00/train/java