How am I supposed to loop repeatedly over a "function" in Java?

I have written code which takes in a number and multiplies its digits together:

public static int findSum(int k) {
    int sum = 0;
    while (k != 0) {
        sum = sum + ((k % 10) * (k % 10));
        k = k / 10;
    }
    return sum;
}

public static void main(String[] args) {
    System.out.println(findSum(713));
}

My code returns 19 when my input is 82, because 1² + 9² is 82.

However, I want to modify my code so that my code would continue to square each digit and add them together until there is only one non-zero digit left.

So basically, if my input is 19, the code would return 1 because:

• 1² + 9² = 82
• from which digits 8 and 2 would do: 8² + 2² = 68,
• from which digits 6² + 8² = 100,
• from which 1² + 0² + 0² = 1.

What changes to my code should I make in order for it to work?

You could call your method recursivly by checking if sum > 9

public static int findSum(int k)
{
    int sum = 0;
    while (k != 0)
    {
        sum = sum + ((k % 10)*(k % 10));
        k = k/10;
    }
    return sum > 9 ? findSum(sum) : sum;
}