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POCopyOnWriteArrayList throwing CurrentModificationException
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copied!<p>I'm occasionally getting a <code>ConcurrentModificationException</code> 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 <code>java.util.concurrent.CopyOnWriteArrayList</code>....</p> <p>... except I already am.</p> <p>Apparently, I'm doing something <i>really</i> stupid somewhere.</p> <p>Does anybody have any insight into how one might induce <code>CopyOnWriteArrayList</code> to toss a <code>ConcurrentModificationException</code>? If it matters, I'm using Java 5.</p> <p><b>Edit:</b> Since the mutators I'm using may matter, I'm modifying this list in two ways:</p> <ul><li>Adding elements to the front. (<code>list.add(0, newElement);</code>)</li> <li>Using subList to let older items fall off the back. (<code>list = list.subList(0, MAX_LIST_SIZE);</code>)</li> </ul> <p>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?</p> <p><b>Edit 2:</b> The precise code that's causing the problem is still a bit murky, but I can at least post the exception I'm seeing: <pre><code> java.util.ConcurrentModificationException at java.util.concurrent.CopyOnWriteArrayList$COWSubList.checkForComodification(Unknown Source) at java.util.concurrent.CopyOnWriteArrayList$COWSubList.iterator(Unknown Source) at.... </pre></code></p> <p>... where it points to a for-each loop instantiation in my code.</p> <p>That <code>COWSubList</code> does seem to imply that my call to <code>subList</code> is the root of my problem; I'd still like to understand why.</p> <p><b>Edit 3:</b> <b>*facepalm*</b></p> <p><code>CopyOnWriteArrayList.subList()</code> returns a <code>List</code>, <em>not</em> a <code>CopyOnWriteArrayList</code>. The list it returns is under no implied obligation to provide any of COWAL's protections. Which makes using <code>subList()</code> like this to remove elements a Very Bad Idea.</p> <p>Don't know for certain if this is my culprit, but it's damned suspicious and needs to be corrected regardless.</p>

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