java编程--03介绍关于日期常用的计算

     /**
      * 获取2个日期之间的天数差
      * d2-d1
      * @return
      * @throws Exception
      * @Description:
      */
     public static int getDiffDays(Date d1,Date d2) throws Exception{
         if(null == d1 || null == d2){
             throw new Exception("日期不允许为空");
         }
         if(d1.compareTo(d2) > 0){
             throw new Exception("开始日期不能大于结束日期");
         }
         int daysDiff = 0;
         Calendar cal = Calendar.getInstance();
         Calendar cal2 = Calendar.getInstance();
         cal.setTime(d1);
         cal2.setTime(d2);

         int date_of_year1 = cal.get(Calendar.DAY_OF_YEAR);
         int date_of_year2 = cal2.get(Calendar.DAY_OF_YEAR);
         int year1 = cal.get(Calendar.YEAR);
         int year2 = cal2.get(Calendar.YEAR);
         if(year1 == year2 ){//同一年
             daysDiff = (date_of_year2 - date_of_year1);
         } else {//非同一年
             for (int i = year1; i < year2; i++) {
                 if(i%4==0&&i%100==0 || i%400==0){//如果是否闰年。能同时被4和100，或者被400整除的年份是闰年
                     daysDiff += 366;
                 } else {//如果不是闰年
                     daysDiff += 365;
                 }
             }
             daysDiff += (date_of_year2 - date_of_year1);
         }
         return daysDiff;
     }

     /**
      * 测试
      * @param args
      * @Description:
      */
     @Test
     public void testGetDiffDays(){
          String dateStr = "2017-11-15 1:21:28";
             String dateStr2 = "2017-11-20 1:21:28";
             SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
             SimpleDateFormat format2 = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
             try
             {
                 Date date2 = format.parse(dateStr2);
                 Date date = format.parse(dateStr);

                 System.out.println("两个日期的差距：" + getDiffDays(date,date2));
             } catch (Exception e) {
                 e.printStackTrace();
             }

     }

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