RFR: JDK-8205461 Create Collector which merges results of two other collectors

[Tagir Valeev]

Hello, Brian!

> Of the three phases, teeing is the most important and least obvious, so
> I think something that includes that in the name is going to be
> helpful.  Perhaps "teeingAndThen" is more evocative and not totally
> unwieldy.

Ok, sounds acceptable to me. Renamed pairing to teeingAndThen.

> > By the way looking into CollectorsTest.java I found some minor things to
> > cleanup:
> > 1. `.map(mapper::apply)` and `.flatMap(mapper::apply)` can be replaced with
> > simple `.map(mapper)` and `.flatMap(mapper)` respectively
> Does IntelliJ have an inspection for eliminating such locutions?

Sure, that's how I found them. Well, I took the liberty to fix these two things.

> > 2. In many methods redundant `throws ReflectiveOperationException` is
> > declared while exception is never thrown
> For test code where a significant fraction of test cases are going to
> throw something, we often do this, since its easier to just uniformly
> tag such methods rather than thinking about which test methods actually
> throw the exception and which don't.  So I think this is harmless
> (though cleaning it up is harmless too.)

I'm not thinking about this, because my IDE thinks for me :-) Ok, I'll
leave them as is for now.

> You may want to optimize the EnumSet mechanics for the case where
> neither collector has interesting characteristics.

Added a special case when reported characteristics for either of
collectors are empty or IDENTITY_FINISH only.
I think this should be a common case.

The updated webrev is posted here (along with Peter suggestion to
rename finisher to merger):
http://cr.openjdk.java.net/~tvaleev/webrev/8205461/r3/
Also copyright year is updated

With best regards,
Tagir Valeev