Calendar設置周一為一周的第一天

1.獲取過去一周的calendar對象

public static Calendar getLastWeekCalendar(){
        Calendar calendar = Calendar.getInstance();
        calendar.setFirstDayOfWeek(Calendar.MONDAY);
        // 坑，設置為一周的第一天后，必須重新set時間后才會生效
        calendar.setTime(new Date());
        calendar.add(Calendar.DATE,-7);
        // 設置當前時間為當前周的星期一
        calendar.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
        return calendar;
    }

2.特殊設置，遇到跨年的時候，一定要把中間間隔的日期算到上一周去

public static Integer getWeekOfYear(Date date) {
        Calendar calendar = Calendar.getInstance();
        calendar.setFirstDayOfWeek(Calendar.MONDAY);
        calendar.setTime(date);
        int week = calendar.get(Calendar.WEEK_OF_YEAR);
        int mouth = calendar.get(Calendar.MONTH);
        // JDK think 2015-12-31 as 2016 1th week
        //如果月份是12月，且求出來的周數是第一周，說明該日期實質上是這一年的第53周，也是下一年的第一周
        if (mouth >= 11 && week <= 1) {
            week += 52;
        }
        return week;
    }