Library method to partition a collection by a predicate

Question

I have a collection of objects that I would like to partition into two collections, one of which passes a predicate and one of which fails a predicate. I was hoping there wo

Answer 1

Note that in case of limited set of known in advance partiotion keys it may be much more efficient just to iterate the collection once more for each partition key skipping all different-key items on each iteration. As this would not allocate many new objects for Garbage Collector.

LocalDate start = LocalDate.now().with(TemporalAdjusters.firstDayOfYear());
LocalDate endExclusive = LocalDate.now().plusYears(1);
List daysCollection = Stream.iterate(start, date -> date.plusDays(1))
        .limit(ChronoUnit.DAYS.between(start, endExclusive))
        .collect(Collectors.toList());
List keys = Arrays.asList(DayOfWeek.values());

for (DayOfWeek key : keys) {
    int count = 0;
    for (LocalDate day : daysCollection) {
        if (key == day.getDayOfWeek()) {
            ++count;
        }
    }
    System.out.println(String.format("%s: %d days in this year", key, count));
}

Another both GC-friendly and encapsulated approach is using Java 8 filtering wrapper streams around the original collection:

List>> partitions = keys.stream().map(
        key -> new AbstractMap.SimpleEntry<>(
                key, daysCollection.stream().filter(
                    day -> key == day.getDayOfWeek())))
        .collect(Collectors.toList());
// partitions could be passed somewhere before being used
partitions.forEach(pair -> System.out.println(
        String.format("%s: %d days in this year", pair.getKey(), pair.getValue().count())));

Both snippets print this:

MONDAY: 57 days in this year
TUESDAY: 57 days in this year
WEDNESDAY: 57 days in this year
THURSDAY: 57 days in this year
FRIDAY: 56 days in this year
SATURDAY: 56 days in this year
SUNDAY: 56 days in this year