Concurrent Modification Exception : adding to an ArrayList

Question

The problem occurs at

Element element = it.next();

And this code which contains that line, is inside of an OnTouchEvent

for (Iterator<Element> it = mElements.iterator(); it.hasNext();){
    Element element = it.next();

    if(touchX > element.mX  && touchX < element.mX + element.mBitmap.getWidth() && touchY > element.mY   
            && touchY < element.mY + element.mBitmap.getHeight()) {  

        //irrelevant stuff..

        if(element.cFlag){
            mElements.add(new Element("crack",getResources(), (int)touchX,(int)touchY));
            element.cFlag = false;

        }           
    }
}

All of this is inside synchronized(mElements), where mElements is an ArrayList<Element>

When I touch an Element, it may activate cFlag, which will create another Element with different properties, which will fall off the screen and destroy itself in less than a second. It's my way of creating particle effects. We can call this "particle" crack, like the String parameter in the constructor.

This all works fine until I add another main Element. Now I have two Elements on the screen at the same time, and if I touch the newest Element, it works fine, and launches the particles.

However, if I touch and activate cFlag on the older Element, then it gives me the exception.

 07-28 15:36:59.815: ERROR/AndroidRuntime(4026): FATAL EXCEPTION: main
07-28 15:36:59.815: ERROR/AndroidRuntime(4026): java.util.ConcurrentModificationException
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at java.util.ArrayList$ArrayListIterator.next(ArrayList.java:573)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at com.Juggle2.Panel.onTouchEvent(Panel.java:823)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at android.view.View.dispatchTouchEvent(View.java:3766)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:863)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:863)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:1767)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1119)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at android.app.Activity.dispatchTouchEvent(Activity.java:2086)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:1751)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1785)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at android.os.Handler.dispatchMessage(Handler.java:99)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at android.os.Looper.loop(Looper.java:123)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at android.app.ActivityThread.main(ActivityThread.java:4627)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at java.lang.reflect.Method.invokeNative(Native Method)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at java.lang.reflect.Method.invoke(Method.java:521)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:893)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:651)
07-28 15:36:59.815: ERROR/AndroidRuntime(4026):     at dalvik.system.NativeStart.main(Native Method)

How can I make this work?

Solution 1

ConcurrentModificationException occurs when you modify the list (by adding or removing elements) while traversing a list with Iterator.

Try

List<Element> thingsToBeAdd = new ArrayList<Element>();
for(Iterator<Element> it = mElements.iterator(); it.hasNext();) {
    Element element = it.next();
    if(...) {  
        //irrelevant stuff..
        if(element.cFlag){
            // mElements.add(new Element("crack",getResources(), (int)touchX,(int)touchY));
            thingsToBeAdd.add(new Element("crack",getResources(), (int)touchX,(int)touchY));
            element.cFlag = false;
        }           
    }
}
mElements.addAll(thingsToBeAdd );

Also you should consider enhanced for each loop as Jon suggested.

Solution 2

I normally use something like this:

for (Element element : new ArrayList<Element>(mElements)) {
    ...
}

quick, clean and bug-free

another option is to use CopyOnWriteArrayList

Solution 3

You're not allowed to add an entry to a collection while you're iterating over it.

One option is to create a new List<Element> for new entries while you're iterating over mElements, and then add all the new ones to mElement afterwards (mElements.addAll(newElements)). Of course, that means you won't have executed the loop body for those new elements - is that a problem?

At the same time, I'd recommend that you update your code to use the enhanced for loop:

for (Element element : mElements) {
    ...
}

Solution 4

An indexed for loop should also work.

for (int i = 0; i < collection.size(); i++)

Solution 5

Using Iterators also fixes concurrency problems, like this:

Iterator<Object> it = iterator.next().iterator();
while (it.hasNext()) {
    it.remove();
}

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