How to sum digits of an integer in java?

A simple solution using streams:

int n = 321;
int sum = String.valueOf(n)
    .chars()
    .map(Character::getNumericValue)
    .sum();

You can do it using Recursion

//Sum of digits till single digit is obtained
public int sumOfDigits(int num) 
        {
            int sum = 0;

            while (num > 0)
            {
                sum = sum + num % 10;
                num = num / 10;
            }

            sum = (sum <10) ? sum : sumOfDigits(sum);

            return sum;
        }

Recursions are always faster than loops!

Shortest and best:

public static long sumDigits(long i) {
    return i == 0 ? 0 : i % 10 + sumDigits(i / 10);
}

public static void main(String[] args) {
        int num = 321;
        int sum = 0;
        while (num > 0) {
            sum = sum + num % 10;
            num = num / 10;
        }
        System.out.println(sum); 
}

Output

6