Concurrent Modification Exception

Question

I'm trying to make an applet of Asteroid objects that float across the screen. If two asteroids collide then the one with the lesser speed is supposed to break up into two smaller asteroids. Once an asteroid is size 1 it should vanish.

When I try to compare two asteroids I get a ConcurrentModificationException and I'm not sure why.

private volatile Collection<Asteroid> belt;
private void handleCollisions() {


    Collection<Asteroid> psuedoBelt = belt;
    Iterator<Asteroid> one;
    Iterator<Asteroid> two;

    for (one = psuedoBelt.iterator(); one.hasNext();) {
        Asteroid aOne = one.next();
        for (two = psuedoBelt.iterator(); two.hasNext();) {

            Asteroid aTwo = two.next();
            if (aOne.collidesWith(aTwo)) {
                if (aOne.getSpeed() > aTwo.getSpeed()) {
                    Collection<Asteroid> split = aTwo.split();
                    two.remove();
                    for (Iterator<Asteroid> three = split.iterator(); three
                            .hasNext();) {
                        psuedoBelt.add(three.next());
                    }
                } else {
                    Collection<Asteroid> split = aOne.split();
                    one.remove();
                    for (Iterator<Asteroid> three = split.iterator(); three
                            .hasNext();) {
                        psuedoBelt.add(three.next());
                    }
                }
            }
        }
    }

    belt = psuedoBelt;

}

Did you do any research? Look to the right of the page. – Sotirios Delimanolis Feb 08 '14 at 23:44 — Sotirios Delimanolis, Feb 08 '14 at 23:44
http://stackoverflow.com/a/1496221/1376108 – Muzaffer Feb 08 '14 at 23:46 — Muzaffer, Feb 08 '14 at 23:46

score 1 · Answer 1 · answered Feb 08 '14 at 23:48

1

First, you create an iterator :

for (one = psuedoBelt.iterator(); one.hasNext();) {

Then, a second one, on the same collection :

    for (two = psuedoBelt.iterator(); two.hasNext();) {

Then, you remove an item with your second iterator :

    two.remove();

The problem is that when you remove the item from the collection, the first iterator (one) is not aware about this removal.

Hence, on one.next(), it detects that the collection has been modified and raises this exception.

There are 2 solutions to that :

Try to use only one iterator OR
Keep a list of items to remove and remove them after iterating on your collection.

answered Feb 08 '14 at 23:48

Arnaud Denoyelle

29,980
16
92
148

What if the first Iterator's object has the lesser speed? – TemporaryFix Feb 08 '14 at 23:49
1

@Programatic I don't understand your question : the iterators are not executed concurrently so the speed has no impact on the execution order. – Arnaud Denoyelle Feb 08 '14 at 23:52

Concurrent Modification Exception

1 Answers1