Is List.iterator() thread-safe?

The behaviour of List.iterator() is not defined or consistent with different List implementations.

For ArrayList, LinkedList, you can get a ConcurrentModificationException if the list is modified when you are iterating over it. (This is not guaranteed) The way to avoid this issue is to use a synchronizedList() and lock the list while iterating over it.

For Vector, the collection is synchronized, but the iterator is not thread safe.

For CopyOnWriteArrayList, you get a snapshot of the elements in the list at the time you call iterator(), This iterator is thread safe, and you don't need to use any locking. Note: the contents of the elements can change.

No iterator is thread-safe. If the underlying collection is changed amidst iteration, a ConcurrentModificationException is thrown.

Even iterators of synchronized collections are not thread-safe - you have to synchronize manually.

One exception is the CopyOnWriteArrayList, which holds a snapshot during iteration.