Question

I’m creating a Swing application to make a Game with. It creates images in random locations off the screen and when they leave the screen I would like to remove them. Please take a look at the code snippet:

public void checkTrolls(){ //CAUSES EXCEPTION ERROR WHEN SPRITE EXIT SCREEN
    for(AutomatedSprite a : trolls){
        if(a.getX() < 0 - a.getImage().getWidth())
            trolls.remove(a);
        if(a.getY() < 0 - a.getImage().getWidth())
            trolls.remove(a);
        if(a.getX() > 800)
            trolls.remove(a);
        if(a.getY() > 600)
            trolls.remove(a);
    }
}

@Override
public void run() {
    long beforeTime, timeDiff, sleep;

    beforeTime = System.currentTimeMillis();

    while(true){
        dodger.update(); //update sprite
        if(trolls.size() != 6){
            trolls.add(new AutomatedSprite("images/troll_face.png"));
        }
        for(Sprite troll : trolls){
            troll.update();         //UPDATES MY SPRITES
        }
        checkTrolls();             //CHECKS TROLLS EXITING THE SCREEN
        repaint();

        for(Sprite troll : trolls){
            System.out.println("X: " + troll.getX());
            System.out.println("Y: " + troll.getY());
        }

        timeDiff = System.currentTimeMillis() - beforeTime;
        sleep = timeDiff - DELAY;

        if(sleep < 0)
            sleep = 5;

        try {
            Thread.sleep(sleep);
        } catch (InterruptedException e) { e.printStackTrace(); }

        beforeTime = System.currentTimeMillis();
    }
}

trolls is a Vector of AutomatedSprites, when they leave the screen I get a ConcurrentModificationException, apparently I can’t remove the instances from my vector.

So it seems that I can’t remove anything from the vector while the thread is updating all my sprites, is there a way to pause my thread so I can remove the sprite?

P.S: here is the entire class in case I missed something: Pastebin

Answer 1

You cannot remove from a collection while iterating over it, this is true for a single threaded environment as well as a multithreaded one. syncrhonized will still cause the issue, Thread.sleep too. Use and Iterator and remove that way.

public void checkTrolls(){
    for(Iterator<AutomatedSprite> itr = trolls.iterator(); itr.hasNext();){
        AutomatedSpring nextElemnt = itr.next();
        if(youShouldRemoveTheSprite){
            itr.remove();
        }
    }
}

So here you use the Iterator supplied by your trolls collection. And you are asking the Iterator to safely remove the object from the collection.

Now if you are executed checkTrolls with multiple threads then you will need to synchronize. You can do that like this

public synchronized void checkTrolls(){ ...

Edit based on your recent comment/link.

It isn’t so much you assigning the Iterator.next() to a variable, its that you are invoking iterator.next() many times. Each time you invoke next() you are moving the iterator to the List’s next element. So at the end of one loop iteration you move the iterator to the 6’th element in the list. If you were indexing it instead it would look like:

   for(Iterator<AutomatedSprite> itr = trolls.iterator(); itr.hasNext(); ){
       if(trolls.get(0).getX() < 0 - trolls.get(1).getImage().getWidth() ||      
                      trolls.get(2).getY() < 0 - trolls.get(3).getImage().getWidth() ||
                       trolls.get(4).getX() > 800 ||
                       trolls.get(5).getY() > 600){
               trolls.remove(trolls.get(5));
       }

Note for this example: indexing at 0,1,2,3,4… is only for demonstration, in practice it would be i = 0; start for loop trolls.get(i++).getX(), trolls.get(i++).getY() and so forth. If your list was 10,000 you would eventually get a NoSuchElementException

So for example, if you only have 3 trolls, once you get to the 4th itr.next() you’ll get a NoSuchElementException. For that reason you’ll want to store the next() element in a variable and work on that variable so the itr.hasNext(); returns correctly and the itr.remove() too works correctly.