Is iterator of HashMap fail-safe?

Question

I was reading a properties of HashMap. And found an answer on SO having lot of votes which says:

Iterator in the HashMap is fail-safe while the enumerator for the Hashtable is not.

While the JavaDoc says

The iterators returned by all of this class's "collection view methods" are fail-fast:

Actually the number of votes on that answered has confused me. Else I think it is fail-fast only.

Can someone explain?

upog · Answer 1 · 2013-09-09T07:41:32.873

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Iterator of HashMap is fail-fast It will throw an ConcurrentModificationException if you try to modify while iterating over it, while collection which come under "java.util.concurrent" package have fail-safe iterator. you can modify the collection while iterating over it. Mainly used in Multi threaded environment

edited Sep 09 '13 at 07:41

answered Sep 09 '13 at 07:34

upog

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score 1 · Answer 2 · answered Sep 09 '13 at 07:38

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Yes, your understanding is correct.

Fail-fast: Fail asap by making checks aggressively. HashMap's iterator is fail-fast because it throws a ConcurrentModificationException as soon as it detects change in underlying structure.

answered Sep 09 '13 at 07:38

rocketboy

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Stephen C · Answer 3 · 2013-09-09T08:04:59.030

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I think it is fail-fast only.

You are correct. The person who wrote that Answer probably doesn't understand the difference between fail-fast and fail-safe.

Actually the number of votes on that answered has confused me.

Lots of votes doesn't necessarily mean that an SO Answer is correct. I spotted a couple of other problems with that Answer as well. IMO, it does NOT deserve the large number of upvotes that it has received.

Unfortunately, people vote for Answers for a variety of reasons, which can lead to all sorts of anomalies.

edited Sep 09 '13 at 08:04

answered Sep 09 '13 at 07:44

Stephen C

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You have not replied on that question. You meant It certainly deserve this many upvotes ? – Thinker Sep 09 '13 at 07:45
How have I not answered the question? – Stephen C Sep 09 '13 at 08:03

score 0 · Answer 4 · answered Sep 09 '13 at 12:31

A lot of readers react on what they expect to read and overlook small discrepancies. The answer you linked actually explains the meaning of fail-fast so it’s only a single word that does not match there: “fail-safe” vs. “fail-fast”.

The other point is that a lot of people accept answers sounding plausible. I guess, no one has really tested whether Hashtable’s enumerator really behaves differently than HashMap’s iterator. Both attempt to fail-fast as of version 1.6.0_35 I just checked.

score 0 · Answer 5 · answered Feb 12 '15 at 22:30

Well I was confused by this as well, but actually it is very simple:

Say we have:

Hashtable<String, Integer> ht = new Hashtable<String, Integer>();
HashMap<String, Integer> hm = new HashMap<String, Integer>();

Then for hm we can do hm.keySet().iterator() which returns an iterator and is fast-fail; while for ht, we can both do ht.keySet().iterator which returns an iterator and is fast-fail, and do ht.keys() which returns an enumeration object and is NOT fast-fail.

Is iterator of HashMap fail-safe?

5 Answers5