1.获取过去一周的calendar对象

public static Calendar getLastWeekCalendar(){
        Calendar calendar = Calendar.getInstance();
        calendar.setFirstDayOfWeek(Calendar.MONDAY);
        // 坑，设置为一周的第一天后，必须重新set时间后才会生效
        calendar.setTime(new Date());
        calendar.add(Calendar.DATE,-7);
        // 设置当前时间为当前周的星期一
        calendar.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
        return calendar;
    }

2.特殊设置，遇到跨年的时候，一定要把中间间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，且求出来的周数是第一周，说明该日期实质上是这一年的第53周，也是下一年的第一周
        if (mouth >= 11 && week <= 1) {
            week += 52;
        }
        return week;
    }