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From: Andreas Leitgeb <avl@logic.at>
Newsgroups: comp.lang.java.programmer
Subject: Re: Lambda dereference
Date: Fri, 16 Mar 2018 23:42:34 -0000 (UTC)
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Daniele Futtorovic <da.futt.news@laposte.net> wrote:
> On 2018-03-16 19:26, Daniele Futtorovic wrote:
>> interface Tagged {
>>     boolean tag();
>> }
>> Stream<Tagged> stream = ...;
>> You can do this (exhibit A):
>>    Predicate<Tagged> p = Tagged::tag;
>>    Collection<Tagged> untagged = stream.filter( p.negate()
>> ).collect(Collectors.toSet());
>> But you can't do this (exhibit B):
>>    Collection<Tagged> untagged = stream.filter( (Tagged::tag).negate()
>> ).collect(Collectors.toSet());
>> Why not? It would be pretty neat. Or am I missing something?
>
> This works:
> Collection<Tagged> untagged = stream.filter(
> ((Predicate<Tagged>)Tagged::tag).negate() ).collect(Collectors.toSet());
>
> ... and it makes sense why it wouldn't accept it without the casting.

Huh, why?    Well, Java does have some "expressions" that only work as
initializers (like braced list of elements for initializing arrays), but
that wouldn't look to me like such a case - afterall one could use 
Tagged::tag right there if it weren't for the ".negate()".

What would be the expression type of blank (Tagged::tag)?