Getting java.util.ConcurentModificationError in collision detecton

Question

Here I am getting java.util.ConcurentModificationError in the line for(Candy c2 : candylist) {. Can any one tell me whats wrong? I am trying to check the collision detection of several images managed in two linked lists say

• 1st list of balloon
• 2nd list of fire

This code is the collision detection of balloon and fire :

public void collision(final Balloon mb1) {
    ImageView iv11 = mb1.getIv1();
    Rect r1 = new Rect(iv11.getLeft(), iv11.getTop(), iv11.getLeft() + iv11.getWidth(), iv11.getTop()+iv11.getHeight());
    for (Candy c2 : candylist) {
        if (c2 != null) {
            ImageView iv22 = c2.getIv2();
            if (iv22 != null) {
                Rect r2 = new Rect(iv22.getLeft(), iv22.getTop(), iv22.getLeft() + iv22.getWidth(), iv22.getTop()+iv22.getHeight());
                if (r1.intersect(r2)) {
                    runOnUiThread(new Runnable() {
                        public void run() {
                            mb1.getTt1().cancel();                  
                            if (mb1.isBursted() == false) {
                                mb1.setBursted(true);
                                update(mb1);                            
                            }
                        }
                    });
                }
            }
        }
    }
}

public void update(final Balloon mb1) {
    final int bp = balloonList.indexOf(mb1);

    if (bp != -1) {
        if (sound == true) {
            mp = MyUtils.playSound(R.raw.bur, Game.this);
        }

        final ImageView iv1 = mb1.getIv1(); 
        iv1.setImageResource(R.drawable.negate);
        iv1.setEnabled(false);
        Animation a1 = AnimationUtils.loadAnimation(Game.this, R.anim.fade);
        iv1.startAnimation(a1);
        TimerTask tt = mb1.getTt1();
        tt.cancel();
        TimerTask tt1 = new TimerTask() {
            public void run() {
                runOnUiThread(new Runnable() {
                    public void run() {
                        balloonList.set(bp, null);
                        al.removeView(iv1);
                    }
                });
            }
        };

        t1.schedule(tt1, 1200);
        missed++;
        score = score-10;

        if (music == true) {
            if (mp2 != null) {
                MyUtils.releaseSound(mp2);
            }
        }
        showscore();
        showExitDialog(score);
    }
}

Stacktrace :

03-08 00:07:14.326: E/AndroidRuntime(586): FATAL EXCEPTION: Timer-1
03-08 00:07:14.326: E/AndroidRuntime(586): java.util.ConcurrentModificationException
03-08 00:07:14.326: E/AndroidRuntime(586):  at java.util.LinkedList$LinkIterator.next(LinkedList.java:124)
03-08 00:07:14.326: E/AndroidRuntime(586):  at bitcream.candyhive.candy.Game.collision(Game.java:609)
03-08 00:07:14.326: E/AndroidRuntime(586):  at bitcream.candyhive.candy.Game$11.run(Game.java:229)
03-08 00:07:14.326: E/AndroidRuntime(586):  at java.util.Timer$TimerImpl.run(Timer.java:284)

Answer 1

your collection candylist is modified while you are iterating over it.

Asuming that your collection is ThreadSafe, you should create a copy of the collection before iterating over it.

for (Candy c2 : candylist.toArray(new Candy[0]))

Answer 2

Isn't it obvious. You are not suppose to modify a collection after getting a iterator on it in any way except through the Iterator's own remove or add methods.

Although you haven't provided the complete code I suppose at:

for (Candy c2 : candylist)

Somewhere in the body of the forEach loop you are structurally modifying the candylist and hence you are getting the ConcurrentModificationException

From the stacktrace I presume you are using LinkedList for candylist.

As per LinkedList javadoc:

if the list is structurally modified at any time after the iterator is created, in any way except through the Iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException.

So instead of using enhanced for statement which resolves to :

for (I #i = Expression.iterator(); #i.hasNext(); )

I would suggest you to get an Iterator over candylist and modify it using the Iterator's own remove or add methods.