How to handle ConcurrentModificationException in Android

Question

Im trying to delete item from a ArrayList. Some times it pops an exception, java.util.ConcurrentModificationException.

First I tried to remove them by array_list_name.remove(i), but it failed and some people were asked to use Iterator instead. So my current code is as follows:

for (Iterator<Collectable> iter = array_list_name.iterator(); iter.hasNext();) {
   Collectable s = iter.next();
   if (s.equals(array_list_name.get(id))){
       iter.remove();
       return true;
   }
}

And I call array_list_name inside onDraw() function in view. My view is a SurfaceView. Can anyone suggest me how to delete items from ArrayList without getting this error?

Solution 1

Try using java.util.concurrent.CopyOnWriteArrayList instead of ArrayList

Solution 2

Seems from the comments that your ArrayList<Collectable> is accessed from the onDraw() method in one thread, by the UI, concurrently with you removing items from it in another thread.

So, why not just wrap both accessors in a

synchronized(array_list_name)
{
    // UI access code or item removal code
}

Note that this might make your UI laggy if removing items takes a long time. If this is the case, consider making a list of all item indexes to be removed, and remove them in a tight synchronized loop after iterating over the whole list.

Update

It seems to me your whole code snippet could be simplified to just:

synchronized(array_list_name)
    return array_list_name.remove(id);

Solution 3

Have you ever thought to use Vector List ? If you need a thread-safe implementation, you should use Vector instead of ArrayList. Vector list's usage is same with ArrayList. Just change its type with Vector.

Unsafe usage

ArrayList<FutureTask> futureTasks;

Change with

Vector<FutureTask> futureTasks;

That's all.

Comments

[Xion]

Yes, onDraw is called in the UI thread so it might be invoked at the same time as item removal in the other thread.

[Kai]

So that is the problem, see the API: The behavior of an iterator is unspecified if the underlying collection is modified while the iteration is in progress...

[Kai]

But this removes the element in the copy. The asker might want the element removed in array_list_name. If you remove it by its index you can't be sure you get the right element, because the List might have changed.

[xarlymg89]

This is by far the best answer on this thread. Using a class as Vector that was intented precisely for concurrent usage is the right solution. I've already used in my own project, where I faced a similar problem and was getting ConcurrentModificationExceptions. Thanks a lot!