java计算两个日期相隔的天数

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;


public class  {
 public static void main(String[] args) throws ParseException {

     SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd");
       Date date1 = format.parse("1999-09-09");
     Date date2 = format.parse("2011-11-11");

        System.out.println(differentDays(date1, date2));//4446
      System.out.println(differentDaysByMillisecond(date1, date2));//4446
     System.out.println(differentDaysByCalendar(date1, date2));//4446
    }

   /**
    * date2比date1多的天数
   */
   public static int differentDays(Date date1, Date date2) {
        Calendar cal1 = Calendar.getInstance();
      cal1.setTime(date1);

        Calendar cal2 = Calendar.getInstance();
      cal2.setTime(date2);
     int day1 = cal1.get(Calendar.DAY_OF_YEAR);// 这一天是一年中的第几天
       int day2 = cal2.get(Calendar.DAY_OF_YEAR);

     int year1 = cal1.get(Calendar.YEAR);
        int year2 = cal2.get(Calendar.YEAR);
        if (year1 != year2) {
           int timeDistance = 0;
           for (int i = year1; i < year2; i++) {
               if (i % 4 == 0 && i % 100 != 0 || i % 400 == 0) // 闰年
              {
                    timeDistance += 366;
             } else // 不是闰年
             {
                    timeDistance += 365;
             }
            }
            return timeDistance + (day2 - day1);
        } else {
            System.out.println("判断day2 - day1 : " + (day2 - day1));
          return day2 - day1;
     }
    }

   /**
    * 通过时间秒毫秒数判断两个时间的间隔
     */
   public static int differentDaysByMillisecond(Date date1, Date date2) {
       int days = (int) ((date2.getTime() - date1.getTime()) / (1000 * 3600 * 24));
       return days;
    }

   public static int differentDaysByCalendar(Date date1, Date date2) throws ParseException {
       Calendar calendar = Calendar.getInstance();
      calendar.setTime(date1);
     int cnt = 0;
        while (calendar.getTime().compareTo(date2) != 0) {
          calendar.add(Calendar.DATE, 1);//calendar.add(*,-1)即是减去。calendar没有减法
            cnt++;
       }
        return cnt;
 }
}