import java.time.LocalDate;
import java.time.format.DateTimeFormatter;
import java.time.temporal.WeekFields;
import java.util.*;
import java.util.stream.Collectors;

class Event {
    private String name;
    private LocalDate date;

    public Event(String name, LocalDate date) {
        this.name = name;
        this.date = date;
    }

    public LocalDate getDate() { return date; }
    public String getName() { return name; }

    @Override
    public String toString() {
        return name + " on " + date;
    }
}

public class GroupEventsByWeek {
    public static void main(String[] args) {
        List<Event> events = Arrays.asList(
            new Event("Event1", LocalDate.of(2023, 4, 1)),
            new Event("Event2", LocalDate.of(2023, 4, 15)),
            new Event("Event3", LocalDate.of(2023, 12, 25)),
            new Event("Event4", LocalDate.of(2023, 1, 20))
        );

        // 分組函數(shù)，返回年份和周數(shù)組成的標識符（如“2023-W17”）
        Map<String, List<Event>> eventsByWeek = events.stream()
            .collect(Collectors.groupingBy(event -> 
                event.getDate().getYear() + "-W" + getWeekNumber(event.getDate())
            ));

        eventsByWeek.forEach((week, eventList) ->
            System.out.println(week + ": " + eventList.stream().map(Event::toString).collect(Collectors.joining(", ")))
        );
    }

    /**
     * 根據(jù)給定的日期確定該日期屬于一年中的第幾周
     */
    private static int getWeekNumber(LocalDate date) {
        WeekFields weekFields = WeekFields.ISO; // 使用ISO標準周定義
        return date.get(weekFields.weekOfWeekBasedYear());
    }
}