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javalistcollectionsiteratorthread-safety
ubuntudroid
ubuntudroid

Reputation: 3989

Is List.iterator() thread-safe?

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?

Upvotes: 26

Views: 30674

Answers (3)

Ali Bagheri
Ali Bagheri

Reputation: 3419

It depends on what class you use,

not for Collections.synchronizedList(new ArrayList<>()); reason but

for CopyOnWriteArrayList reason.

best Description here

Upvotes: 0

Bozho
Bozho

Reputation: 597046

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.

Upvotes: 15

Peter Lawrey
Peter Lawrey

Reputation: 533492

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.

Upvotes: 40

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