RFR 8144675: Add a filtering collector

[Brian Goetz]

This seems reasonable; mapping() already exists for use in this space 
(as a combinator for the downstream collector of groupingBy and 
friends), filtering() is also reasonable in this context.

On 12/4/2015 3:53 PM, ShinyaYoshida wrote:
> Thank you for your comment.
>
> 2015-12-05 3:28 GMT+09:00 Henry Jen <henry.jen at oracle.com 
> <mailto:henry.jen at oracle.com>>:
>
>     My first thought is what’s wrong with
>
>     stream.filter(predicate).collect(...)?
>
>     In your case, that would be,
>
>     amps.stream().filter(e -> e.getSalary() > 2000)
>     .collect(groupingBy(Employee::getDepartment), Collectors.counting())
>
>     That should work just fine?
>
> Unfortunately, not, Stream#filter could not satisfy us in this case.
> Because when I filter before collecting, the entry which the value is 
> 0 will not be appeared in the result map.
> Ex)
> filtering collector(Collectors#filtering): http://ideone.com/83Lzb7
> filtering before collecting(Stream#filter): http://ideone.com/EmTrXE
>
> Regards,
> shinyafox(ShinyaYoshida)
>
>
>     Cheers,
>     Henry
>
>
>
>     > On Dec 3, 2015, at 10:57 PM, ShinyaYoshida <bitterfoxc at gmail.com
>     <mailto:bitterfoxc at gmail.com>> wrote:
>     >
>     > Hi, core-libs-dev and Brian, Paul,
>     > I'd like to propose adding filtering method to Collectors.
>     >
>     > When I consider the operation what is "grouping the number of
>     employees
>     > whose income is over 2000 by the depertment from employees", we
>     have to
>     > write following because there is no way to filter for Collector:
>     > (Note: In this case, we need the entry which the value is 0)
>     >
>     > Map<Department, Long> map = emps.stream()
>     >    .collect(groupingBy(Employee::getDepartment,
>     >        collectingAndThen(toList(),
>     >            es -> es.stream().filter(e -> e.getSalary() >
>     2000).count())));
>     >
>     > When I add filtering like following to Collectors, we can write
>     it easy,
>     > and it would be more efficient.
>     >    public static <T, A, R>
>     >    Collector<T, ?, R> filtering(Predicate<? super T> filter,
>     >                               Collector<? super T, A, R>
>     downstream) {
>     >        BiConsumer<A, ? super T> downstreamAccumulator =
>     > downstream.accumulator();
>     >        return new CollectorImpl<>(downstream.supplier(),
>     >                                   (r, t) -> {
>     >                                       if (filter.test(t)) {
>     >  downstreamAccumulator.accept(r,
>     > t);
>     >                                       }
>     >                                   },
>     >  downstream.combiner(),
>     > downstream.finisher(),
>     >  downstream.characteristics());
>     >    }
>     >
>     > Map<Department, Long> map = emps.stream()
>     >    .collect(groupingBy(Employee::getDepartment,
>     >        filtering(e -> e.getSalary() > 2000, counting())));
>     >
>     > Here is patch:
>     > webrev: http://cr.openjdk.java.net/~shinyafox/8144675/webrev.00/
>     <http://cr.openjdk.java.net/%7Eshinyafox/8144675/webrev.00/>
>     > bugs: https://bugs.openjdk.java.net/browse/JDK-8144675
>     >
>     > Could you consider this patch and proposal?
>     >
>     > Regards,
>     > shinyafox(Shinya Yoshida)
>
>