Bug in ArrayList iterator

Remi Forax forax at univ-mlv.fr
Thu Jan 8 08:11:47 UTC 2015

On 01/08/2015 03:24 AM, David Holmes wrote:
> On 7/01/2015 7:45 PM, Remi Forax wrote:
>> A simple Java question, what this code does ?
>>    ArrayList<String> list = new ArrayList<>();
>>    list.add("foo");
>>    list.add("bar");
>>    for(String s: list) {
>>      list.remove(s);
>>    }
>> :(
>> Rémi
>> tip: the bug lies in ArrayList.Itr.hasNext() (and
>> AbstractList.Itr.hasNext()).
> This is not a bug. The only supported way to remove from a collection 
> you are iterating over is to use an Iterator's remove method.
> http://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html#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."
> and as already pointed out the forEach docs make it clear you can't 
> use forEach in such a context as the Iterator is not exposed to you.
> So don't do that.

Hi David,
while I agree that fail-fast behavior should be done on the best effort 
i think like Paul said that the size should be stored in a field of the 
iterator because
i don't see the point to try to support the fact that the size of the 
can change during the iteration.

so in my opinion, it's not a bug but the current implementation of 
ArrayList.Itr should be changed
or maybe there is a case where supporting a size changed is valid ?

> David


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