通过列表迭代，同步并使用迭代器时发生ConcurrentModificationException

Question

How is it possible to get a ConcurrentModificationException for this code-block?

synchronized (list) {
        for (Iterator<?> it = list.iterator(); it.hasNext(); ) {
            Object object = it.next();
            // do something to object without touching list
        }
}

Edit: sorry, that was not specific enough: // do something to object does not touch the list

Answer 1

By having another thread modifying the list, for example.

Just because you synchronize this code block doesn't mean all the accesses to the list are blocked. It only makes sure that all the other blocks synchronized on the list can't execute in parallel.

If the list is shared by several threads, all its usages must be synchronized on the same monitor.

Answer 2

If the "do something" directly or indirectly adds or removes something from list (except by using it.remove() or similar), then you'll get this exception. The synchronization won't stop it from happening in this case.