RFR 8144675: Add a filtering collector

[Paul Sandoz]

> On 10 Dec 2015, at 15:29, ShinyaYoshida <bitterfoxc at gmail.com> wrote:
> 
> Hi Stuart,
> Thank you for your review and comment.
> The doc comment is wonderful!
> I like it, it must be helpful to users when they understand this method and
> how the filtering collector works!
> 
> I've updated the webrev:
> http://cr.openjdk.java.net/~shinyafox/8144675/webrev.04/
> 

Pushed:

  http://hg.openjdk.java.net/jdk9/dev/jdk/rev/bf153b12450b

I took the liberty of tweaking the parameters and return tags to better align with the notion of predicate:

+     * @param downstream a collector which will accept values that match the
+     * predicate
+     * @return a collector which applies the predicate to the input elements
+     * and provides matching elements to the downstream collector

Thanks,
Paul.