java - If - else doesn't work - where could be mistake

Question

world! :) It is my third programming day (please, be lenient...) Code was intended to find out whether the sum (when it reaches 1 digit only) of digits of number 123456789 can be divided by 9. I cannot find how to make second 'else' work - any help and explanation why so would be highly appreciated:

package lesson3;

public class Task6 {
    
public static void main(String[] args) {

        int n=123456789;
        System.out.println(n);
        do{
            private static int sumDigits(int n)
            int s=sumDigits(n); 
            if (s < 10) ;
            {
                if (s == 9) System.out.println("divides by 9");
                else System.out.println("doesn't divide by 9");
                break;
            }

           else n = s;

        }while (true);

    }

question from:https://stackoverflow.com/questions/65874801/if-else-doesnt-work-where-could-be-mistake

Answer 1

1. The function, sumDigits should return the sum of digits of the parameter, n. For this, you can add each digit of n to a variable, sum initialized with the value, 0. The value of x % 10 gives you the last digit of x. Every time you add the last digit of n to sum, divide it by 10 to make it one digit less from the end.
2. You need a loop to pass the sum of digits to sumDigits until the sum of digits become a single digit. I suggest you use a do-while loop which guarantees its body to be executed at least once.
3. At the end of the loop mentioned in point#2, you will have the sum of digits as a single digit. The final thing that you need to do now is to check if this sum is divisible by 9.

Note that if x % y == 0, then x is divisible by y.

Demo:

public class Main {
    public static void main(String[] args) {
        int n = 123456789;
        int singleDigitSum = 0;
        int divisibleBy = 9;

        do {
            singleDigitSum = sumDigits(n);
            n = singleDigitSum;
        } while (singleDigitSum > 9);

        if (singleDigitSum % divisibleBy == 0) {
            System.out.println("The single digit sum is divisible by " + divisibleBy);
        } else {
            System.out.println("The single digit sum is divisible by " + divisibleBy);
        }
    }

    private static int sumDigits(int n) {
        int sum = 0;
        while (n > 0) {
            sum += n % 10;
            n /= 10;
        }
        return sum;
    }
}

Output:

The single digit sum is divisible by 9

Note: Java does not allow defining a method/function inside another method/function. You have done this mistake by defining the function, sumDigits inside the method, main.