In this code, error showing "division by zero", for the condition inside the while loop. Why is this happening?

> Here when I add System.out.println(j) inside the loop, the initial number is random, and the next outputs are huge values from 400000
>Value of j is Initialized to 1, does while loop counts all int values unless mentioned?

public static void main(String[] args) {
    int num = 31;
    int j = 1, c = 0;
    while (num != 0) {
        //System.out.println(j);
        if (num % j == 0) {
            c += j;
        }
        j++;
    }
    System.out.println(c);
}

The only way that the loop could be exited in normal circumstances, is if num became 0. But you never reassign num to any other number, so it is always 31, so the loop won't end (in a regular manner).

What happens though is in your loop you're incrementing j on every pass. j is declared an int. ints have maximum values defined, after which they go to minimum value, so from maximum (positive) number, after another increase you get a minimum (negative) number. Then you count up till you reach 0. And then the exception is thrown as the application tries to divide by 0.
(see here: https://stackoverflow.com/questions/5131131/what-happens-when-you-increment-an-integer-beyond-its-max-value)

The reason you see 400000 as the first displayed value is because the loops go so fast, that the console is not able to handle it gracefully, and you see just some of the outputs displayed.

Not sure what you're trying to achieve, but in order for this program to work differently, either change the loop condition or add a break statement inside - as mentioned in the comments.