ConcurrentHashMap/ConcurrentMap/Map.compute

[Doug Lea]

On 12/14/12 11:46, Brian Goetz wrote:

> But, I am not quite following your argument.  Why does it fail if you replace
> "map.get(key) == null" with "!map.containsKey(key)"?  For non-null-accepting
> maps they are equivalent.
>

Consider the unlikely prospect of a null-accepting ConcurrentMap.
And let's pick on computeIfPresent. If I try to trigger function
on containsKey(key), then by the time I get(key) for function arg,
it could have been removed (and rechecking doesn't help).
And if I do the opposite, trigger on (v = get(key)) != null ||
containsKey, then v could be wrong. Either way loses.

-Doug