CopyOnWriteArrayList throwing CurrentModificationException

Question

I'm occasionally getting a ConcurrentModificationException when I iterate over a list. A Google search informs me that it's probably because I'm altering that list in another thread while iterating over it and that to make this problem go away I should use java.util.concurrent.CopyOnWriteArrayList....

... except I already am.

Apparently, I'm doing something really stupid somewhere.

Does anybody have any insight into how one might induce CopyOnWriteArrayList to toss a ConcurrentModificationException? If it matters, I'm using Java 5.

Edit: Since the mutators I'm using may matter, I'm modifying this list in two ways:

• Adding elements to the front. (list.add(0, newElement);)
• Using subList to let older items fall off the back. (list = list.subList(0, MAX_LIST_SIZE);)

Do those raise red flags? If so, why? My understanding was that because these operations make a copy of the thing first, any existing iterators would be pointing at the unmodified original and would thus not care. Do I have a hole in my knowledge?

Edit 2: The precise code that's causing the problem is still a bit murky, but I can at least post the exception I'm seeing:

java.util.ConcurrentModificationException
    at java.util.concurrent.CopyOnWriteArrayList$COWSubList.checkForComodification(Unknown Source)
    at java.util.concurrent.CopyOnWriteArrayList$COWSubList.iterator(Unknown Source)
    at....

... where it points to a for-each loop instantiation in my code.

That COWSubList does seem to imply that my call to subList is the root of my problem; I'd still like to understand why.

Edit 3: *facepalm*

CopyOnWriteArrayList.subList() returns a List, not a CopyOnWriteArrayList. The list it returns is under no implied obligation to provide any of COWAL's protections. Which makes using subList() like this to remove elements a Very Bad Idea.

Don't know for certain if this is my culprit, but it's damned suspicious and needs to be corrected regardless.

Answer 1

CopyOnWriteArrayList.subLists throw ConcurrentModificationExceptions if the containing list changes out from underneath it:

public class ListTest {

  private static List<int[]> intList;

  public static void main (String[] args) {
    CopyOnWriteArrayList<Integer> cowal = new CopyOnWriteArrayList<Integer>();
    cowal.add(1);
    cowal.add(2);
    cowal.add(3);

    List<Integer> sub = cowal.subList(1, 2);
    cowal.add(4);
    sub.get(0); //throws ConcurrentModificationException
  }
}