[jdom-interest] JDOM 2 list of elements, remove nulls out entries and fails to remove?

[Rolf Lear]

Hi David.

This problem you describe is very concerning, and should not, in 
'supported' usage ever happen....

I am wracking my brains to figure out what it could be, but, the only 
think I can think is that you are accessing the JDOM objects from 
different threads.... JDOM is *not* thread safe.

If this is happening in a single-threaded mechanism, would it be 
possible to put together a failing 'test case', any code that shows the 
problem?

I have to admit that I thought he iteration/remove/add/etc. type code is 
very well tested, and I would be *very* suprised if there is a problem 
like this.... in fact, I know that there are test cases that cover this 
basic condition that you describe... and they never fail....

so, if you can confirm this is all happening in one thread.... and 
preferably if you can put together an example of the problem, I would 
look in to it immediately....

Rolf

On 11/05/2012 8:42 PM, David Wall wrote:
> I believe I've run into a problem that I don't think I had with JDOM 
> 1.1.3 before I upgraded to 2.0.1.
>
> I create a list of elements of a selected element using:
>
> List<?> documentVersionElements = 
> rootElement.getChildren("DocumentVersion", ns);
>
> I see that I have 6 elements as expected in my list. None are null.
>
> I am basically then trying to find a matching related element id in 
> that list using something like:
>
>                 Element found = null;
>                 ListIterator<?> iter = documentVersionElements 
> .listIterator();
>                 while( iter.hasNext() ) {
>                     Element checkElement = (Element)iter.next();
>                     EsfUUID evParentId = new 
> EsfUUID(checkElement.getChildText("documentId", ns));
>                     if ( evParentId.equals(id) ) {
>                         found = checkElement;
>                         break;
>                     }
>                 }
>
> Then, assuming I find it (found != null), I process it as expected.  
> But I then want to remove the found element from the element list so 
> it cannot be found again, so I use this:
>
> documentVersionElements.remove(found);
>
> Most of the time, this seems to work as expected, and the 
> documentVersionElements list is then shorter by 1.  But there are 
> times when a list of 6 elements remains 6 elements after the remove 
> (and remove() returns false), with 4 of them now null (not removed, 
> but actually a null element), so in when I return to the 
> listIterator() above and get my next() element, the element is null.
>
> It's as if my Element objects are not unique in the list in terms of 
> equals/hashCode as my elements should never be nulled out.
>
> I am not sure why an element I find by iterating cannot then be 
> removed. How could it fail to remove? Why would it change other list 
> elements to NULL instead?  Is this not a valid usage pattern?
>
> Thanks,
> David
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