Uh, you did mention you need entry with value zero. I knew I miss something. :)
Cheers, Henry > On Dec 4, 2015, at 12:53 PM, ShinyaYoshida <bitterf...@gmail.com> wrote: > > Thank you for your comment. > > 2015-12-05 3:28 GMT+09:00 Henry Jen <henry....@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 <bitterf...@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/ > > bugs: https://bugs.openjdk.java.net/browse/JDK-8144675 > > > > Could you consider this patch and proposal? > > > > Regards, > > shinyafox(Shinya Yoshida) > >
