Question

I just wanted to create a little Java-Puzzle, but I puzzled myself. One part of the puzzle is:

What does the following piece of code do:

public class test {
    public static void main(String[] args) {
        int i = 1;
        i += ++i + i++ + ++i;

        System.out.println("i = " + i);
    }
}

It outputs 9.

My (at least partly) wrong explanation:

I’m not quite sure, but I think the term after i += gets evaluated like this:

So

int i = 1;
i += ++i + i++ + ++i;

is the same as

int i = 1;
i += ((++i) + (i++)) + (++i);

This gets evaluated from left to right (See Pre and postincrement java evaluation).

The first ++i increments i to 2 and returns 2. So you have:

i = 2;
i += (2 + (i++)) + (++i);

The i++ returns 2, as it is the new value of i, and increments i to 3:

i = 3;
i += (2 + 2) + ++i;

The second ++i increments i to 4 and returns 4:

i = 4;
i += (2 + 2) + 4;

So you end up with 12, not 9.

Where is the error in my explanation? What would be a correct explanation?

Answer 1

i += ++i + i++ + ++i; is the same as i = i + ++i + i++ + ++i;

The right-hand side is calculated from left-to-right, yielding i = 1 + 2 + 2 + 4; (which yields i = 9).