Calendarish.java

package calendarish;

import java.time.DayOfWeek;
import java.time.Duration;
import java.time.LocalDate;
import java.time.Month;
import java.time.ZoneId;
import java.time.temporal.ChronoField;
import java.time.temporal.WeekFields;
import java.util.Calendar;
import java.util.GregorianCalendar;
import java.util.List;
import java.util.stream.Stream;

public class Calendarish {

    interface Strategy {
        int getWeek(final LocalDate date);
    }

    private List<Integer> weeksBetween(
            final LocalDate startDate,
            final LocalDate endDate,
            final Strategy strategy) {

        final long durationInDays = Duration.between(
                startDate.atStartOfDay(),
                endDate.atStartOfDay()).toDays();

        return Stream.iterate(startDate, date -> date.plusDays(1))
                .limit(durationInDays)
                .map(strategy::getWeek)
                .distinct()
                .toList();
    }

    public static void main(String[] args) {
        final var startDate = LocalDate.of(2022, Month.FEBRUARY, 1);
        final var endDate = LocalDate.of(2022, Month.FEBRUARY, 13);

        final Strategy theGoodOldWay = date -> GregorianCalendar
                .from(date.atStartOfDay(ZoneId.systemDefault()))
                .get(Calendar.WEEK_OF_YEAR);

        final Strategy theISOWay = date -> date.get(WeekFields.ISO.weekOfYear());

        final Strategy theAlignedWay = date ->
                date.get(ChronoField.ALIGNED_WEEK_OF_YEAR);

        final var broadcastCalendar = WeekFields.of(DayOfWeek.MONDAY, 1);
        final Strategy theBroadcastWay = date ->
                date.get(broadcastCalendar.weekOfYear());

        final var calendarish = new Calendarish();
        System.out.println("The good old way: "
                + calendarish.weeksBetween(startDate, endDate, theGoodOldWay));
        System.out.println("The ISO way: "
                + calendarish.weeksBetween(startDate, endDate, theISOWay));
        System.out.println("The aligned way: "
                + calendarish.weeksBetween(startDate, endDate, theAlignedWay));
        System.out.println("The broadcast way: "
                + calendarish.weeksBetween(startDate, endDate, theBroadcastWay));
    }
}