Java List and recursion leads to Concurrent Modification Exception

Question

The following function walks recursively through a list and divide it always by half and do something with the sublists. The recursion breaks when the listsize is 2. I know a concurrent modification exception occurs if I change the list when I iterate over it. But I don't use iterations and it still happens:

    private static List<ParticipantSlot> divide(List<ParticipantSlot> list) {
        int n = list.size();

        //do something 

        if (n>2){
            List<ParticipantSlot> l = divide(list.subList(0, n/2-1));
            List<ParticipantSlot> r= divide(list.subList(n/2, n));

            l.addAll(r);
            return l;
        }else{
            return list;
        }
    }

Answer 1

You're using addAll() which will iterate over the collection you provide in the argument. Now subList only returns a view onto the original list, so you're trying to add values onto a view of the original list, and iterate over a different part of the original list at the same time. Bang.

If you created a copy of the sublist each time, it should work - although it'll be pretty inefficient.

Answer 2

You get a concurrent modification exception because sublist is backed by the original list:

The returned list is backed by this list, so non-structural changes in the returned list are reflected in this list, and vice-versa. The returned list supports all of the optional list operations supported by this list.

If you would like to avoid an exception, make a copy of the first sublist before modifying it.

Answer 3

If you are using ArrayList, you may want to change it to a CopyOnWriteArrayList , or ConcurrentLinkedQueue .

If you are on a Multi-thread environment, you will want to put a synchronized around your Array.

Hope it helps.