What causes this recursive function to crash compared to another nearly identical one? [duplicate]

Question

This causes a Stack Overflow error. I just need help explaining why this one crashes compared to the correct one which is similar. I have used the debugger, but it is still unclear to me.

public static void main(String[] args) {
    countForwards(5);
}
public static void countForwards( int num ) {
    if (num >= 0){
        countForwards(num--);
    }
    System.out.print(num + " ");
}

I know this is the solution but I don't understand why it is different

public static void countForwards( int num ) {
    if (num >= 0){
        countForwards(num - 1);
    }
    System.out.print(num + " ");
}

Answer 1

countForwards(num--) passes the original value of num to the recursive call, which means the recursion never ends.

countForwards(--num) would allow the recursion to end.

After seeing all the traffic this question got, I thought it would be worth it to expand the answer a little.

As paxdiablo commented, even though countForwards(--num) allows the recursion to terminate, it behaves differently than countForwards(num-1).

Both variants would cause the following series of recursive calls :

countForwards(5)
countForwards(4)
countForwards(3)
countForwards(2)
countForwards(1)
countForwards(0)
countForwards(-1)

but they will output a different series of numbers when the recursion unwinds :

num - 1	-- num
-1	-1
0	-1
1	0
2	1
3	2
4	3
5	4

The reason for the difference is that num-1 doesn't change the value of num while --num decrements num.