Implement joinMap(...)

[mbruggmann]

Description from README: joinMap is a stream collector that applies an asynchronous operation to a stream of values and collects the results into a map. This is similar to applying an asynchronous operation and using joinList, but retains the mapping from the original value.

There is probably a more optimized way of doing this but I thought I'd push it anyway since it was discussed.

[codecov-io]

Codecov Report

Merging #57 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##             master    #57   +/-   ##
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  Coverage       100%   100%           
- Complexity       69     74    +5     
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  Files             6      6           
  Lines           191    202   +11     
  Branches          9     10    +1     
=======================================
+ Hits            191    202   +11

Impacted Files	Coverage Δ	Complexity Δ
...n/java/com/spotify/futures/CompletableFutures.java	100% <100%> (ø)	46 <6> (+5)	:arrow_up:

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[dflemstr]

Could we solve this generically? There are combinators in the standard library like collectingAndThen etc., similarly here we could build a combinator that would only focus on the "wait for all futures" part and send the resulting future completions to a different collector.

It probably makes sense to have this method as well as a special case; it just might be worth thinking about this for the long run.

[mbruggmann]

@dflemstr Not sure I fully grasp how the collector would work. Do you think we should investigate that before we add the joinMap collector to the public API?

[dflemstr]

I think so. Let us call this hypothetical combinator join(). I would rather do join(toMap(...)) and join(toList()) and join(toSet()) etc. instead of treating each use-case individually.

[mattnworb]

@mbruggmann still want this? 😀

[mbruggmann]

@mattnworb Guess I survived a while without it now, but if anyone feels like this is useful and worthwhile I can have a stab at cleaning it up?

[dflemstr]

I'll take a stab at creating what I wrote in my comment from 2018 :)

[mbruggmann]

Now that we have allAsMap (#70) I'll close this. If we want a collector version of it, we can add that as another PR and possibly revisit the API as @dflemstr suggested.