Multiple threads using iterator.remove() on a single collection returning fail-fast iterator

Question

Oracle says

Note that Iterator.remove is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress.

Could this mean that even if multiple threads are iterating together over the same collection's fail-fast implementation's (Vector,Hashmap,ArrayList,HashSet) object carry out iterator.remove() there would be no ConcurrentModificationException thrown?

No it does not. With the exception of Vector it will likely happen with multiple threads. — Adam Gent
– Adam Gent, Commented Jul 5, 2016 at 10:51
No it does Not.If multiple threads are iterating over the same collection's fail-fast implementation's there would be ConcurrentModificationException, It is not generally permissible for one thread to modify a Collection while another thread is iterating over it. — Mihir
– Mihir, Commented Jul 5, 2016 at 10:56
@Mihir Just a little bit clarity. What if the "modification" is iterator.remove() by one thread when it is traversing and the other thread executes the same while iterating during the iteration of the former thread? — Aakash Verma
– Aakash Verma, Commented Jul 6, 2016 at 5:15

halfer · Accepted Answer · 2018-08-14 11:42:39Z

2

No. This tells you that only safe way to remove elements while iterating (in one thread) is to use iterator.remove. And if collection is accessed (iterated or modified) from other threads - sometime you will get exception, sometime not - in general behavior is not deterministic so you should avoid using it or relying on it.

That being said - only exception to this are Concurrent collections.

edited Aug 14, 2018 at 11:42

halfer

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answered Jul 5, 2016 at 10:59

river

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2 Comments

Aakash Verma Over a year ago

So you mean concurrent collections do not face the problem of having multiple threads executing iterator.remove() all at once (or for that matter anytime when all the iterators are in motion in a loop) , i.e., we don't get ConcurrentModificationException?

river Over a year ago

Exactly, but this is not a solution to a problem, it is just by design. E.g. ConcurrentHashMap will iterate concurrently and you will see some of the edits other threads made (insertions and deletions) - and is says in doc that it will not fail with ConcurrentModificationException. Other collections have other semantics.

sauumum · Accepted Answer · 2016-07-05 10:55:27Z

0

It does not mean multiple thread can remove data using iterator.remove().

If you want to achieve it you need to use Synchronized type of collections. Even in that case you should not try to use the same iterator in two threads. If you have two threads that need to remove entry, then they each should have their own iterators.

answered Jul 5, 2016 at 10:55

sauumum

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1 Comment

Aakash Verma Over a year ago

What if they do have their own and they are iterating during the same time interval? I think you didn't get my question. Let me rephrase it. If two or more threads having their own individual iterators execute iterator.remove() either at the same instant or anytime during which each of those iterators are traversing their collections together, would it cause ConcurrentModificationException?

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