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)

if you use a game engine like for example libgdx then you will not have trouble with collision detection anymore — donfuxx, Mar 07 '14 at 18:53
Are you trying to update the list while iterating, actually i dnt find this @Shubhankar — Kick, Mar 07 '14 at 19:00
Can you please edit the code and specify the line.dont know i cant fount the line — Kick, Mar 07 '14 at 19:03
I am updating my linked list in function update where if two images collide i remove one item from balloon list in function update as if: balloonList.set(bp, null); al.removeView(iv1); — Shubhankar, Mar 07 '14 at 19:07
@Shubhankar dont remove element like this.use Iterator class method remove() to remove the element,thn you will not face this — Kick, Mar 07 '14 at 19:19
You have a lot of indentation levels in your code... You could avoid most of them by inverting the tests and `continue`ing. For instance, the first test can be rewritten as `if (c2 == null) continue; // rest of code here` — fge, Mar 07 '14 at 19:40

Zeeshan · Answer 1 · 2014-03-07T19:36:41.933

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.

I dnt downvoted but my question 'You are not suppose to modify a collection after getting a iterator on it' so how can we update list while iterating ?? — Kick, Mar 07 '14 at 19:29
@Youngistan Oh i missed the part "except through the Iterator's own remove or add methods." I have edited my answer — Zeeshan, Mar 07 '14 at 19:40

score 1 · Accepted Answer · answered Mar 07 '14 at 19:12

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]))

answered Mar 07 '14 at 19:12

PC.

6,870
5
36
71

only one correction you need to specify the array size otherwise arrayindexoutofbound of exception arises. Thanks. – Shubhankar Mar 07 '14 at 19:30
1

you can specify an array of size zero. Javadoc says - "Parameter a: the array into which the elements of this collection are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose." – PC. Mar 07 '14 at 19:33
but it shows an error in my case as specified in Stacktrace it provides me indexoutofboundexception in the line : for (Candy c2 : candylist.toArray(new Candy[0])) – Shubhankar Mar 07 '14 at 19:44

Getting java.util.ConcurentModificationError in collision detecton

2 Answers2