ConcurrentModificationException errors

[tdegroot96]

So I’m working on a little side-scrolling shooter and got some stuff working,
but now I got stuck in a problem. Whenever I want to remove something
from the bulletList, I get the ConcurrentModificationException. Also when
I want to shoot multiple bullets really fast, it crashes as well. I thought I
was doing this the right way, but it seems I’m doing something wrong.
bulletList declaration:

private List<Bullet> bulletList = new ArrayList<Bullet>();

Iterator:

public void tick() {
		// tickCount++;

		p.tick();
		player.tick();
		
		Iterator<Bullet> it = bulletList.iterator();
		while (it.hasNext()) {
			Bullet bullet = it.next();
			bullet.tick();
			
			if (!bullet.living) {
				bulletList.remove(bullet);
			}
		}
	}

Bullet.tick()

public void tick() {
		time--;
		if (time > 0) {
			moveBullet();
		} else {
			die();
		}
		System.out.println("Bullet Tick! Time: " + time);
	}

Die would only set the boolean living to false.
Does anyone know what I’m doing wrong?
Any help would be highly appreciated!

[65K]

it.remove()

[tdegroot96]

Alright, that was really helpful! It works now, but still I have the ConcurrentModificationException when I try to shoot really fast.

[jonjava]

It’s because you’re trying to remove items while you’re looping through the list.

@princec

Though it has to be said, considering the number of things that might be found in an ArrayList, removing things from the head of the queue is really often so fast you’d never ever notice it. It depends on how many things are likely to be in it…

Consider a real-life example of, hmm… Particles. So you’ve got 5,000 particles in a frame (a high figure, but not unlikely). They’re stored in an ArrayList. Each entry in the ArrayList takes 4 bytes, so that’s 20kb of RAM, fitting neatly in the L1 data cache on many if not all desktops.

Now, imagine the rather unlikely even that every single particle dies one frame, and you discover this as you iterate through the ArrayList (using an integer index, not an iterator, just for absolute efficiency’s sake). You have to shuffle 4,999 particles down 4 bytes. It’s all in the L1 cache so memory access is effectively free to the CPU; you spend 4,999 cycles moving your particles assuming the loop that copies the data is about as simple and efficient as it can be. Then you have to do it again on the next particle. 4998 cycles. Repeat to solve triangular number, approximately (5000x5000)/2, or in the end 12,500,000 clock cycles, without ever having to touch the L2 or L3 caches most likely, let alone system RAM, until the end of the operation.

That’s 12.5m cycles out of your 3.3 million or so you’ve typically got in a single video frame (2GHz core). Oh dear, you just spent 4 frames ditching a mere 5000 particles. Judder. Imagine if you had, somehow, 10000 particles. That’d be nearly a second wasted doing something utterly trivial and would present itself as a horrible jarring delay in your buttery smooth animation.

But what if, weirdly but possibly, the worst case occurred when even going backwards - every other particle died in one frame? Well, then you’d be compacting a slowly more complex list of particles even if you were scanning backwards. Scanning forwards would be exactly the same speed too.

So you’d be crafty and use the third and final technique which is to make a copy of the original arraylist, and copy the surviving particles into it each frame. This performs consistently no matter how many live or dead particles you have in any particular frame or the pattern of their expiration.

Anyway - for the OP - basically you need to do Just That. Each frame, scan your list of Interactables, and copy each live one into a second ArrayList, and then point your game at that ArrayList. Flip between two ArrayLists, alternating each frame, rather than creating (and expanding!) an ArrayList every frame, or that’ll be slow.

Cas

@theagentd

LinkedList is rarely useful, since it has so much overhead even when it is supposed to be faster. For a very very large list where objects are constantly removed randomly hundreds of times per frame or so, LinkedList might be faster. The problem with LinkedList is that it creates an Entry object which it wraps in each object added to store the next and previous entries. This allocation (and later garbage collection) makes it a lot slower to add and remove stuff than ArrayList in most use cases. ArrayList’s only weakness is removing objects in the beginning or middle of the list, since all following objects have to be shifted to fill the hole created. The longer the list the slower it becomes.

Even when you have lots of stuff to remove, ArrayList can be a good choice. Lists are often used to keep track of game objects. So you say “But wait! The objects will always be added at the end of the list, but as they die they will be removed! I have thousands of objects, so I should use a LinkedList to avoid shifting thousands of objects on every remove()!”. Nope! Most likely the best solution is to use TWO ArrayLists and pingpong your objects between them. By doing that you can avoid all shifting while still getting fast (and random) access to all objects.

ArrayList<GameObject> currentList, nextList;

//Lists created at load time


//Example game loop:

while(true){

    update(); //Updates all objects
    render(); //Draws all objects

}

private void update(){
    for(int i = 0; i < currentList.size(); i++){
        GameObject go = currentList.get(i);
        go.update();
        if(go.isAlive()){
            nextList.add(go);
        }
    }
    
    //Alive objects are now in nextList. Clear currentList.
    currentList.clear();
    
    //Swap current and next list
    ArrayList<GameObject> temp = nextList;
    nextList = currentList;
    currentList = temp;
}

@Jay_PC

Also Doesn’t LinkedList leave null spaces in the List if you remove an object? or was that Hashtable…

Nope, LinkedList does not leave null spaces. Only arrays do that (obviously). Hashtables/HashMaps KIND OF do that, but that’s simply because the key no longer maps to anything, so it returns null instead. Not really the same in my opinion.

related post (ConcurrentModificationProblem with bullets): http://www.java-gaming.org/topics/indexoutofbounds-index-7-size-7-need-help/28638/msg/261156/view.html#msg261156

quotes from: http://www.java-gaming.org/topics/arraylist-vs-linkedlist/27016/msg/240144/view.html#msg240144

Posts require 10% of the contents be written by the poster. So, to remedy this, here’s a short story.

むかしむかし、亀は森の中にすんでいました。彼は遅い亀でした。終わりが。

[65K]

@tdegroot96

Alright, that was really helpful! It works now, but still I have the ConcurrentModificationException when I try to shoot really fast.

Can’t comment without the related code, probably something similiar, or you’re adding and removing at the same time.