public class TestWeekNumAndDate {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		//初始化，第一週至少四天
		WeekFields wfs= WeekFields.of(DayOfWeek.MONDAY, 4);
		//一年最後一天日期的LocalDate，如果該天獲得的週數為1或52，那麼該年就只有52周，否則就是53周
		//獲取指定時間所在年的週數
		int num= LocalDate.of(2019, 12, 31).get(wfs.weekOfWeekBasedYear());
		num = num == 1 ? 52 : num;
		System.out.println("第" + num + "周，週一日期:" + getDateByYearAndWeekNumAndDayOfWeek(2019, num, DayOfWeek.MONDAY));
		System.out.println("第" + num + "周，週二日期:" + getDateByYearAndWeekNumAndDayOfWeek(2019, num, DayOfWeek.TUESDAY));
		System.out.println("第" + num + "周，週三日期:" + getDateByYearAndWeekNumAndDayOfWeek(2019, num, DayOfWeek.WEDNESDAY));
		System.out.println("第" + num + "周，週四日期:" + getDateByYearAndWeekNumAndDayOfWeek(2019, num, DayOfWeek.THURSDAY));
		System.out.println("第" + num + "周，週五日期:" + getDateByYearAndWeekNumAndDayOfWeek(2019, num, DayOfWeek.FRIDAY));
		System.out.println("第" + num + "周，週六日期:" + getDateByYearAndWeekNumAndDayOfWeek(2019, num, DayOfWeek.SATURDAY));
		System.out.println("第" + num + "周，週日日期:" + getDateByYearAndWeekNumAndDayOfWeek(2019, num, DayOfWeek.SUNDAY));
		//該格式週日為一週第一天，週六為一週最後一天
		System.out.println(LocalDate.parse("2019-52-1", DateTimeFormatter.ofPattern("YYYY-ww-e", Locale.CHINA)));
	}
	
	private static LocalDate getDateByYearAndWeekNumAndDayOfWeek(Integer year, Integer num, DayOfWeek dayOfWeek) {
		//週數小於10在前面補個0
        String numStr = num < 10 ? "0" + String.valueOf(num) : String.valueOf(num);
        //2019-W01-01獲取第一週的週一日期，2019-W02-07獲取第二週的週日日期
        String weekDate = String.format("%s-W%s-%s", year, numStr, dayOfWeek.getValue());
        return LocalDate.parse(weekDate, DateTimeFormatter.ISO_WEEK_DATE);
    }

}