code: public static String getFirstDayOfMonth(Date date){
          Calendar   ca   =   Calendar.getInstance();
          ca.setTime(date);
          ca.set(Calendar.DAY_OF_MONTH,   1);
          Date   firstDate   =   ca.getTime();
          return dateToString(firstDate);
      }
docs: 获取月份的第一天
  @param date
  @return