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

[Tomasz Linkowski]

Hello,

I'm responding to the thread at [1] (I'm not sure whether my message will
get appended to it) and the CSR at [2].

I stumbled here from StackOverflow where - indepedently of all the others -
I also came up with this type of "dual" *Collector* [3]. I named it
"collectingBoth" there but I'd like to suggest a slightly different name
here.

I believe that Zheka Kozlov made a great observation about similiarity to
"collectingAndThen" [4]. And it seems Brian Goetz favors the "AndThen"
suffix [5]. On the other hand, this new Collector differs from
"collectingAndThen" in that:
1) it hides the accumulation type parameters (A1, A2), and
2) it needs a few lower bounds on some of its type parameters.

All in all, I'd like to propose to name the method "collectingBothAndThen"
(or alternatively: "biCollectingAndThen".

As an aside, I'd like to suggest changing the name of the parameter from
"merger" to "biFinisher":
1) Peter Levart pointed out in [6] that "finisher" is already taken by the
Collector API (and it's true), but it didn't prevent "collectingAndThen"
from taking a "finisher" and then
calling: downstream.finisher().andThen(finisher)
2) note that in all other contexts "merger" represents a BinaryOperator
because it merges two values of the *same* type, and here the types of
merged values are different.

Complete signatures for comparison below:

*<T,A,R,RR> Collector<T,A,RR> collectingAndThen(*
*    Collector<T, A, R> downstream,*
*    Function<R, RR> finisher)*

*<T,R1,R2,R> Collector<T,?,R> collectingBothAndThen(*
*    Collector<? super T, ?, R1> downstream1,*
*    Collector<? super T, ?, R2> downstream2,*
*    BiFunction<? super R1, ? super R2, R> biFinisher)*


As to the other potential names, I must say:
- I'm not in favor of "teeingAndThen" nor all the duplex-related names (I
find them unintuitive, especially the "teeing" one),
- I really like "composite" and "compound" proposed by Stuart Marks [7]; it
seems that "compouding" could be both intuitive and adhering to the -ing
suffix convention

[1]
http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-August/054718.html
[2] https://bugs.openjdk.java.net/browse/JDK-8209685
[3] https://stackoverflow.com/a/52211175/2032415
[4]
http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-June/053922.html
[5]
http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-August/054736.html
[6]
http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-August/054724.html
[7]
http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-August/054949.html

-- 
Regards,
Tomasz Linkowski