RFR: 8024688: j.u.Map.merge doesn't work as specified if contains key:null pair

[Paul Sandoz]

On Oct 16, 2013, at 12:28 PM, David Holmes <david.holmes at oracle.com> wrote:

> On 16/10/2013 8:03 PM, Paul Sandoz wrote:
>> 
>> On Oct 16, 2013, at 6:41 AM, David Holmes <david.holmes at oracle.com> wrote:
>> 
>>> Okay you have incited me to throw in my 2c :) I think the CME issue has been raised a number of times in the past (and if the below doesn't agree with what I've said in the past Hey! It's a brand new day! ;-) )
>>> 
>>> But first, Mike the missing spaces are creeping back in "if(xxx)" :)
>>> 
>>> For Map I don't think looking for external concurrent modification and throwing CME is necessary or worthwhile. These are not thread-safe methods. That covers:
>>> - remove, replace
>>> 
>>> and it implies that putIfAbsent should not check for or throw CME.
>>> 
>>> For compute* and merge it is possible that the computation function modifies the Map - unlikely perhaps but possible - so CME here seems more reasonable. (As for forEach, replaceAll etc.)
>>> 
>>> I fully agree with removing the retry loops in these non-concurrent implementations.
>>> 
>>> That said it makes ConcurrentMap somewhat different to Map as it never throws CME even if it was an internal mutation.
>>> 
>> 
>> HashMap.compute*/merge methods do not throw CME either. I suppose those methods could and do so beyond that of only the entry under computation. I think this really points to the fact that, for non-traversal, only concrete implementations are capable of reliably detecting a CME and therefore it's best to leave it up to those implementations should they choose to do so.
> 
> Perhaps HashMap's implementations should throw CME?
> 

Perhaps, seems to be going beyond the call of duty. My inclination is not to bother. It becomes most relevant with forEach since the consumer will have side-effects that might make it easier to unintentionally slip in a modification to the map itself.


> But the possibility of CME has to be allowed for in the spec of the interfaced methods regardless.
> 

Ideally by not say anything :-) otherwise perhaps a variant of the following:

"If a function value passed to an operation of a non-concurrent map modifies the contents of that map then the result of that operation is undefined.  An implementation may throw {@link ConcurrentModificationException} in such cases and if so that behaviour should be documented."

Paul.