In Java: Is List.iterator() thread-safe, i.e. does the returned iterator reflect the current state of the list at any time or just the state of the list at the time of its creation?
3 Answers
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.
It depends on what class you use,
not for Collections.synchronizedList(new ArrayList<>()); reason but
for CopyOnWriteArrayList reason.
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