java日期比较，日期计算

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都是常用的日期之间的比较方法，供以后参考。

热身：获取当前时间

SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");//设置日期格式
String nowDate = df.format(new Date());// new Date()为获取当前系统时间

注：楼主一直以为date类型不如string类型随心所欲，所以下面的比较都是string类型的date进行比较。如果你真的真的就是那么倔强，ok，下面是date转string的方法：

SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");//设置日期格式
String date = df.format(Date类型的时间);

1.两个string类型的日期比较大小

  public static int compare_date(String DATE1, String DATE2) {
          DateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
          try {
              Date dt1 = df.parse(DATE1);
              Date dt2 = df.parse(DATE2);
              if (dt1.getTime() > dt2.getTime()) {
                  System.out.println("dt1 在dt2前");
                  return 1;
              } else if (dt1.getTime() < dt2.getTime()) {
                  System.out.println("dt1在dt2后");
                  return -1;
              } else {
                  return 0;
              }
          } catch (Exception exception) {
              exception.printStackTrace();
          }
          return 0;
      }

2.返回两个string类型日期之间相差的天数

  public static int daysBetween(String smdate,String bdate){
          SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd");
          Calendar cal = Calendar.getInstance();
          long time1 = 0;
          long time2 = 0;

          try{
               cal.setTime(sdf.parse(smdate));
               time1 = cal.getTimeInMillis();
               cal.setTime(sdf.parse(bdate));
               time2 = cal.getTimeInMillis();
          }catch(Exception e){
              e.printStackTrace();
          }
          long between_days=(time2-time1)/(1000*3600*24);  

         return Integer.parseInt(String.valueOf(between_days));
      }  

3.返回两个string类型日期相差的小时数

  public static int daysBetween2(String startTime, String endTime) {
           SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH");
           Calendar cal = Calendar.getInstance();
           long time1 = 0;
           long time2 = 0;

           try{
                cal.setTime(sdf.parse(startTime));
                time1 = cal.getTimeInMillis();
                cal.setTime(sdf.parse(endTime));
                time2 = cal.getTimeInMillis();
           }catch(Exception e){
               e.printStackTrace();
           }
           long between_days=(time2-time1)/(1000*3600);  

          return Integer.parseInt(String.valueOf(between_days));
       }

4.计算两段日期的重合日期

 /**
      * 计算两段日期的重合日期
      * @param str1 开始日期1
      * @param str2 结束日期1
      * @param str3 开始日期2
      * @param str4 结束日期2
      * @return
      * @throws Exception
      */
     public static Map<String,Object> comparisonRQ(String str1, String str2, String str3,
             String str4) throws Exception {
         String mesg = "";
         DateFormat df = new SimpleDateFormat("yyyy-MM-dd");
         String startdate = "";
         String enddate = "";
         try {
             Date dt1 = df.parse(str1);
             Date dt2 = df.parse(str2);
             Date dt3 = df.parse(str3);
             Date dt4 = df.parse(str4);
             if (dt1.getTime()<=dt3.getTime()&&dt3.getTime()<=dt2.getTime()&&dt2.getTime()<=dt4.getTime()) {
                 mesg = "f";//重合
                 startdate = str3;
                 enddate = str2;
             }
             if (dt1.getTime()>=dt3.getTime()&&dt3.getTime()<=dt2.getTime()&&dt2.getTime()<=dt4.getTime()) {
                 mesg = "f";//重合
                 startdate = str1;
                 enddate = str2;
             }

             if (dt3.getTime()<=dt1.getTime()&&dt1.getTime()<=dt4.getTime()&&dt4.getTime()<=dt2.getTime()) {
                 mesg = "f";//重合
                 startdate = str1;
                 enddate = str4;
             }
             if (dt3.getTime()>=dt1.getTime()&&dt1.getTime()<=dt4.getTime()&&dt4.getTime()<=dt2.getTime()) {
                 mesg = "f";//重合
                 startdate = str3;
                 enddate = str4;
             }

             System.out.println(startdate+"----"+enddate);

         }catch (ParseException e) {
             e.printStackTrace();
             throw new ParseException(e.getMessage(), 0);
         }catch(Exception e){
             e.printStackTrace();
             throw new Exception(e);
         }
         Map<String,Object> map = new HashMap<String,Object>();
         map.put("startdate", startdate);
         map.put("enddate", enddate);
         return map;
     }