java.util.ConcurrentModificationException while using iterator

Question

I have the following code to form a url, but some clients have reported exception at the for loop. java.util.ConcurrentModificationException at java.util.ArrayList$ArrayListIterator.next(ArrayList.java:573)

    List<NameValuePair> requestParams = adRequest
            .createRequestParamsList(view.getContext());

    RequestTask currentTask = new RequestTask(this,
            view.getUserAgent(), view.getContext(), keyCode);

    try {
            currentTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,
                baseRequestUrl + Request.REQ_URI, requestParams);

    } catch (Exception e) {

    }

                String debugUrl = "";
                synchronized (this) {
                    debugUrl = baseRequestUrl + Request.REQ_URI + "?";
                    for (Iterator<NameValuePair> iterator = requestParams.iterator(); iterator.hasNext();) {
                        NameValuePair nvp = iterator.next();
                        debugUrl = debugUrl + nvp.getName() + "=" + nvp.getValue() + "&";
                    }
                    debugUrl = debugUrl.substring(0, debugUrl.length() - 1);
                }

requestParam is modified in RequestTask. why would it fail?

Do watch https://www.youtube.com/watch?v=MZOf3pOAM6A. Avoid Iterators — Raghunandan, Aug 06 '15 at 02:04
Im a little confused, as you arent modifying the requestParams object inside the iterator block. — D. Ben Knoble, Aug 06 '15 at 02:05
If an `ArrayList` is modified while you are iterating over it with an `Iterator` you will get a `Concurrent ModificationException`. — Paul Boddington, Aug 06 '15 at 02:24
The duplicate is irrelevant here because you are not modifying the `ArrayList` in the loop itself. — Paul Boddington, Aug 06 '15 at 02:32
@PaulBoddington re-opened the question as the duplicate was irrelevant. — Raghunandan, Aug 06 '15 at 02:54

score 2 · Answer 1 · answered Aug 06 '15 at 03:43

After your comments, I suspect you should synchronize on the requestParams object, not on this.

The async task does not have a reference to the object that is iterating, so they are not locking on the same object, which is virtually the same as not synchronizing at all.

Make sure both classes synchronize on the same reference of the same object!

PS: if both classes do a synchronized(this), they are effectively synchronizing on different object instances.

score 0 · Answer 2 · answered Aug 06 '15 at 03:05

When using an Iterator over an ordered set you must use iterator.remove() instead of <? extends Collection>.remove(). The latter yields a ConcurrentModificationException. If you are modifying requestParams in another thread and you must use the Iterator either have a field private Iterator iter in your class and provide a getIterator() method and perform the necessary null checks.

java.util.ConcurrentModificationException while using iterator

2 Answers2