RFR 8071600: Add a flat-mapping collector

[Stuart Marks]

Hi Paul,

On 2/3/15 5:48 AM, Paul Sandoz wrote:
> http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8071600-Collector-flatMapping/webrev/
>
> This patch adds a new flat mapping collector to Collectors. This can be useful if one needs to map 0 or more items into a downstream collector.

Mostly pretty good, just a couple minor nits.

Re the comment at TabulatorsTest.java line 513, this isn't a two-level groupBy 
anymore, it's a groupByWithMapping (as the test name indicates).

Given the new tests of closing in FlatMapOpTest.java, is it necessary to have 
testFlatMappingClose() in TabulatorsTest.java?

> A following patch, which i plan to fold into the above patch, performs some renames on the collectors test to be consistent with the current naming:
>
> http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8071600-Collector-flatMapping-test-rename/webrev/

Renaming looks good and makes it easy to correlate the tests, the assertion 
classes, and the collector implementations under test. Unfortunately, the word 
for the act of collecting is "collection" which is confusing since "collection" 
already has another meaning, but oh well.

s'marks