How to get the next n fridays which are the first, second or third friday in a month?-CodePudding

I need to find the next n Fridays but want to exclude 4th and possibly 5th Fridays, i.e I want to get the next n Fridays which are 1st, 2nd or 3rd of each month. I have found a way to stream over the next n Fridays (example next 10 Fridays):

LocalDate now = LocalDate.now();
Stream.iterate(now , localDate -> localDate.with(TemporalAdjusters.next(DayOfWeek.FRIDAY)))
     .skip(1) // skip today 
     .limit(10)
     .forEach(System.out::println);

Output:

But I need to exclude for example 2021-10-22 & 2021-10-29 because they are 4th and 5th Friday of October. Similarly 2021-11-26 & 2021-12-24 because they are not 1st, 2nd or 3rd Fridays.

I have searched SO and found some ways to calculate nth Friday of a month, for example this one

public static LocalDate nthFridayOfMonth(LocalDate localDate, int week) {
    return localDate.with(TemporalAdjusters.dayOfWeekInMonth(week, DayOfWeek.FRIDAY));
}

which returns for a given date the nth Friday.

How to modify this method so that it returns a boolean if a given LocalDate is a first, second or third Friday of a month? Or use this method to implement another method which does what I need?

Ideally I want to use the stream above and use a method, which returns a boolean as a filter:

LocalDate now = LocalDate.now();
Stream.iterate(now , localDate -> localDate.with(TemporalAdjusters.next(DayOfWeek.FRIDAY)))
     .skip(1) // skip today 
     .filter(d -> is1st2ndOr3rdFriday(d))   
     .limit(10)
     .forEach(System.out::println);

to get

Appreciate any help on how to implement the filter method boolean is1st2ndOr3rdFriday(LocalDate d) {}

CodePudding user response：

You can get the ALIGNED_WEEK_OF_MONTH temporal field of the local date, and check if it is ≤ 3.

Stream.iterate(now , localDate -> localDate.with(TemporalAdjusters.next(DayOfWeek.FRIDAY)))
    .skip(1) // skip today
    .filter(x -> x.get(ChronoField.ALIGNED_WEEK_OF_MONTH) <= 3)
    .limit(10)
    .forEach(System.out::println);

CodePudding user response：

Another solution : you can GroupBy month your dates ( to Map<Month, List< LocalDate > > ) , then for every month key if there are more than 3 date , remove it , and return the result :

public static void main(String[] args) {
    // initialize your date here:
    LocalDate now = LocalDate.now();
    // Grouping Dates By Month 
    Map<Month, List<LocalDate>> map = Stream
            .iterate(LocalDate.of(now.getYear(), now.getMonth(), 1),
                    localDate -> localDate.with(TemporalAdjusters.next(DayOfWeek.FRIDAY)))
            .limit(20).collect(Collectors.groupingBy(element -> element.getMonth()));
    // delete the 4 or 5 fridays
    map.forEach((k, v) -> {
        if (v.size() > 3)
            v.removeIf(e -> e.isBefore(now) || v.indexOf(e) > 2);
    });
    // print results sorted
    Stream.of(map.values()).flatMap(e -> e.stream()).flatMap(List::stream).sorted().limit(10)
            .forEach(System.out::println);
}

Output

2021-11-05

2021-11-12

2021-11-19

2021-12-03

2021-12-10

2021-12-17

2022-01-07

2022-01-14

2022-01-21

2022-02-04