计算两个日期之间的天数:

  public static void main(String[] args) throws ParseException {
        String startTime = "2019-11-30";
        String endTime = "2019-12-12";
        Calendar startCal = Calendar.getInstance();
        Calendar endCal = Calendar.getInstance();
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        Date startDate =  sdf.parse(startTime);
        Date endDate = sdf.parse(endTime);
        startCal.setTime(startDate);
        endCal.setTime(endDate);
        List<String> list = new ArrayList<>();
        //得到两个日期相差的天数   还可通过去除后面的数 求得相差的小时数,分钟数,秒   等等
         int days = ((int)(endCal.getTime().getTime()/1000)-(int)(startCal.getTime().getTime()/1000))/3600/24;   
         for(int i = 0;i<=days;i++){
             list.add(sdf.format(startCal.getTime()));
             startCal.add(Calendar.DAY_OF_MONTH, 1);
         } 
         System.out.println(days);
         System.out.println(list);
    }

/**
* startTime  起始日期
* endTime   截止日期
* dateType  日期类型 HOUR/DAY/MONTH/YEAR    
*/
public Integer dateTimeNums(String startTime,String endTime,String dateType) throws ParseException{
        String fmtStr = null;
        int timeNum = 3600 * 24;
        switch(dateType){
            case "HOUR" : 
                fmtStr = "yyyy-MM-dd HH"; 
                timeNum = 3600;
                break;
            case "DAY" :
                fmtStr = "yyyy-MM-dd";
                timeNum = 3600 * 24 ;
                break;
            case "MONTH" :
                fmtStr = "yyyy-MM";
                timeNum = 3600 * 24 * 30;
                break;
            case "YEAR" :
                fmtStr = "yyyy";
                timeNum = 3600 * 24 * 365 ;
                break;
            default :
                fmtStr = "yyyy-MM-dd";
                break;
        } 
        
        Calendar startCal = Calendar.getInstance();
        Calendar endCal = Calendar.getInstance();
        SimpleDateFormat sdf = new SimpleDateFormat(fmtStr);//此处修改日期格式
        Date startDate =  sdf.parse(startTime);
        Date endDate = sdf.parse(endTime);
        startCal.setTime(startDate);
        endCal.setTime(endDate);
        startCal.compareTo(endCal);
      //得到两个日期相差的天数   
        int nums = ((int)(endCal.getTime().getTime()/1000)-(int)(startCal.getTime().getTime()/1000))/timeNum;//此处修改日期单位   
        return nums;
    }