int a = 6;
    	 int week = 0;
    	 Calendar cal = Calendar.getInstance();
    	 cal.set(Calendar.YEAR, 2018);
    	 cal.set(Calendar.MONTH, a-1);
    	 int lastDay = cal.getMaximum(Calendar.DAY_OF_MONTH);//獲取該月最大一天
    	 cal.set(Calendar.DAY_OF_MONTH, lastDay);
    	 week=cal.get(Calendar.DAY_OF_WEEK)-1==0?7:cal.get(Calendar.DAY_OF_WEEK)-1;//獲得最後一天是星期幾
    	 if (week==7) {
    		 lastDay=lastDay-2;
		}else if(week==6){
			lastDay=lastDay-1;
		}
    	 System.out.println(lastDay);