JAVA Calender 获取当前月的第一周

Calendar calendar = Calendar.getInstance();
        calendar.setFirstDayOfWeek(Calendar.MONDAY);
        int weekYear = calendar.get(Calendar.YEAR);
        String weekId=weekYear + "." + calendar.get(Calendar.WEEK_OF_YEAR);
        String[] weekIdInfo = weekId.split("\\.");
        Calendar calendar2 = Calendar.getInstance();
        calendar2.setFirstDayOfWeek(Calendar.MONDAY);
        calendar2.setWeekDate(Integer.valueOf(weekIdInfo[0]), Integer.valueOf(weekIdInfo[1]), Calendar.MONDAY);
        
        StringBuilder sb = new StringBuilder();
        sb.append(calendar2.get(Calendar.YEAR)).append("-");
        sb.append(calendar2.get(Calendar.MONTH) + 1).append("-");//月是从0开始
        sb.append(calendar2.get(Calendar.DAY_OF_MONTH));
        System.out.println(sb.toString());