Difference between ++ and +1

Question

When I had startIndex++ in the last line of the while loop it caused an infinite loop, but using startIndex + 1, the program worked fine. I was wondering why this is so?

public static int numOccurrences(String src, String q) {

        int startIndex = src.indexOf(q);
        int counter = 0;

        while (startIndex != -1) {
            counter++;
            startIndex = src.indexOf(q, startIndex + 1);
        }

        return counter;
    }

Answer 1

Consider what happens - if indexOf fails, it returns -1 . If you had a blind startIndex++ after that, then your startIndex would become 0 , the loop would wrap around, and 0 != -1 is TRUE, continuing the loop. You'd get another -1 , increment it to 0 , and around and around you go - search, fail, increment, loop/repeat.

Answer 2

The expression startIndex++ increments the value of the variable startIndex , but it returns the old value of startIndex . After evaluating the expression which contains startIndex++ you assign the result to startIndex . That means that in your case that increment is discarded and startIndex++ and startIndex will have the exact same result: the same occurrence will be found each time, and you have an infinite loop.

The expression startIndex + 1 , on the other hand, does not change the value of startIndex , but evaluates to the integer following the value of startIndex . Now the indexOf starts searching after the found occurrence, so you do not have an infinite loop.