Digital root recursive function

Haven't any experience writing recursive functions.(Generally tend to avoid them - not sure if that's a negative). What do you think about the following?

This is also my first time using JUnit, I'd appreciate input if I'm doing something wrong there.

Challenge: Create a digital Root Function.

Specifications:

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

Given n, take the sum of the digits of n.

If the resulting value has two digits, continue reducing until a single-digit number is produced.

This is only applicable to the natural numbers.

Solution:

public static int getDigitalRoot(int num) {
        if (Integer.toString(num).length() == 1) { return num; }
        int result = 0;

        for (char c : Integer.toString(num).toCharArray()) {
            result += Character.getNumericValue(c);
        }
        
        return getDigitalRoot(result);    
    }

Tests:

@Test
    public void Tests() {
      assertEquals(6 , Utilities.getDigitalRoot(942));
      assertEquals(7, Utilities.getDigitalRoot(16));
    }

With a bit of knowledge of modular math then you can know that for numbers > 0 getDigitalRoot(n)== n%9 except when dividable by 9 then it is 9.

This simplifies the algorithm a bit.

public static int getDigitalRoot(int n) {
    if(n==0) 
        return 0;
    if(n%9 == 0) 
        return 9;
    return n%9;
}

If you still want the looped version you can do a variation; instead of using another variable to hold the result and retrying until smaller than 10 just do this:

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

It works for roughly the same reason as the first modulo operation.

StackExchange Code Review Q#79361, answer score: 8