Answer 1

● here 'num = num%10;' will never make num to 0, so resulting in StackOverflowError [Logical Error made in code]

//here is rough code (changes made by assuming U wish to add digits of number) :

public class Main {
public static int temp = 0;
public static void main(String[] args) {
System.out.println(sum(123));
}
public static int sum(int num){
if (num == 0){
return temp;
}
temp = temp + num % 10;
num = num / 10;
return sum(num);
}

}