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

Paul Sandoz paul.sandoz at oracle.com
Wed Oct 16 03:03:28 PDT 2013

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.


> Aside: do you still need to re-list every unchecked exception when overriding (i.e. @throws foo {@inheritDoc}) to get them to show up in the subtype docs? I can't tell if this has been forgotten or whether the ConcurrentMap methods truly will never throw such exceptions.
> Cheers,
> David
> -----

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