RFC: Adding ConcurrentModificationException for HashMap.computeIfAbsent(), and JDK-8071667

[Brent Christian]

Hi, Paul

On 3/10/15 8:29 AM, Paul Sandoz wrote:
>
> On the Map.compute* methods.
>
> Perhaps we can reuse similar language to that we added for Matcher:
>
> * The mapping function should not modify this map during computation.
> * This method will, on a best-effort basis, throw a
> * ConcurrentModification if such modification is detected.
>
> It's tempting to place the second sentence on the @throws
> ConcurrentModificationException as i believe will not show up for
> methods on ConcurrentMap, so keeps it somewhat contained
> documentation wise.

The language in Matcher is nice, and we can use it in HashMap.  But the 
default code in Map.compute* won't throw a CME (no iteration).  For the 
compute* methods in the Map interface, I think we should stick to 
recommending against modifying the Map during the computation, and 
encouraging implementing classes to check for CME.

>> In Map/HashMap, there are already two methods that accept lambdas
>> and throw a CME: forEach(BiConsumer) replaceAll(BiFunction)
>>
>> (The default code does not throw a CME ifitems are added).
>
> Right, although the iterator might still be fail-fast.

Ahh - I needed to update my test code (if a Map only has a single entry, 
the iterator doesn't CME when items are added because a second 
"iteration" never happens to detect the changed modCount).  A Map with 
 >= 2 entries is needed for the default code to CME due to items being 
added during forEach()/replaceAll().

Thanks,
-Brent