code: public static String getPreOrNextDay(Date date,int num) {
          Calendar   ca   =   Calendar.getInstance();
          ca.setTime(date);
          ca.add(Calendar.DAY_OF_MONTH,  num);//此处要用add方法
          Date   firstDate   =   ca.getTime();
          return dateToString(firstDate);
      }
docs: 获取前后七天
  @param num   7：后七天     -7：前七天
  @return