RFR: 8029055: Map.merge must refuse null values

[Martin Buchholz]

On Tue, Nov 26, 2013 at 5:35 AM, Stephen Colebourne <scolebourne at joda.org>wrote:

> See the new thread for the general Javadoc issues. I'll focus on null here.
>
> Given a map {"Key" -> null} I find the notion that putIfAbsent() or
> computeIfAbsent() ignore the existing mapping just plain wrong. While
> I can rationalise it (just) it is a horrible reworking of the meaning
> of "absent".
>
> Similarly, for "computeIfPresent()", my expectation is that the
> mapping is present, and that the null should be an input to the
> function.
>

This is also very much my expectation for computeIfPresent.  I can see why
the current semantics may have been chosen, because the obvious
implementation choice needs just one call to map.get without a call to
map.containsKey.  But it seems OK to disambiguate using containsKey when
get returns null.  Concurrent maps don't have problems implementing this
either way because they don't allow nulls.