How to handle ConcurrentModificationException in Android

问题

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 any one suggest me how to delete items from ArrayList without getting this error.

回答1:

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

回答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);

回答3:

You could create a defensive copy of the list like so:

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

回答4:

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.