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From: Knute Johnson <nospam@rabbitbrush.frazmtn.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: Removing an element while iterating over a List with enhanced for?
Date: Wed, 15 Feb 2017 17:51:26 -0600
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On 2/15/2017 17:24, Eric Sosman wrote:
> On 2/15/2017 5:56 PM, Knute Johnson wrote:
>> An enhanced for loop on a List is no different than using an Iterator
>> right?  So is there any problem removing the current element from the
>> list while using the enhanced for loop?  I couldn't find anything that
>> said it was OK or not OK in the JLS.
>
>     1) During an iteration, you can only remove elements through
> the Iterator's own remove() method.
>
>     2) The enhanced `for' does indeed use an Iterator, but it
> doesn't expose the Iterator instance: You have no way to refer
> to it.
>
>     3) Since you can't get a reference to the Iterator, you can't
> call the Iterator's remove().
>
>     Therefore,
>
>     4) You can't remove anything while executing an enhanced `for'.
>
>>         List<Integer> list = Stream.of(1,5,7,9,13).collect(toList());
>>
>>         for (Integer i : list)
>>             if (i == 9)
>>                 list.remove(i);
>
>     Not permitted.  From the Javadoc for Iterator#remove: "The behavior
> of an iterator is unspecified if the underlying collection is modified
> while the iteration is in progress in any way other than by calling
> this method."
>
>     Just for fun, I tried your example -- and it worked!!!  But that's
> one of the possible outcomes of "unspecified" behavior, and I rather
> suspected that it "worked" because the removed element just happened
> to be the penultimate and the Iterator's hasNext() might have returned
> false.  So I changed `9' to `7' to attack an earlier element, and got
>
> Exception in thread "main" java.util.ConcurrentModificationException
>     at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
>     at java.util.ArrayList$Itr.next(ArrayList.java:851)
>     at esosman.misc.Knute.main(Knute.java:15)
>
>>         while (iter.hasNext())
>>             if (iter.next() == 9)
>>                 iter.remove();
>
>     This is the correct approach.
>

Thanks!
-- 

Knute Johnson