Question

I have a static method which I am calling from an Asynctask in doInBackGround()

In the method there is this part of code:

    ArrayList<Message> messagesList = new ArrayList<Message>();
    if (!clearList) {
        messagesList.addAll(messages.getMessagesList());
            for (Message msg : messagesList) {
                if (msg.getId().length() == 0) {
                    messagesList.remove(msg);
                }
        }
    }

It is sometimes throwing ‘Concurrent modification exception’, I have tried declaring the method as ‘synchronized’ but it still didn’t help, and I cannot declare the block synchronized, since it is a static method and there is no ‘this’ reference.

I have also tried to stop a running asynctask if I need to start another one, but it didn’t help as well.

Help appreciated.

Answer 1

This has nothing to do with synchronization. You’re using an iterator to loop over messagesList, but then using remove to modify it during the iteration. You can’t do that, because ArrayList‘s iterators fail when the list if modified during iteration. From the docs:

The iterators returned by this class’s iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove or add methods, the iterator will throw a ConcurrentModificationException.

Your enhanced for loop is just syntactic sugar around using an Iterator, so you can just make that explicit and then use the iterator’s remove method:

Iterator<Message> it = messagesList.iterator();
while (it.hasNext()) {
    if (it.next().getId().length == 0) {
       it.remove();
    }
}

Alternately, you can just use a simple for loop running backward and indexing into the ArrayList (since get(int) is a cheap and constant-time operation on an ArrayList, which isn’t true of all Lists):

int index;
for (index = messagesList.length - 1; index >= 0; --index) {
    if (messagesList.get(index).getId().length == 0) {
       messagesList.remove(index);
    }
}