java - 在java中对日期值进行排序

Question

我有一个 List<object>我在哪里有属性(property) manufacturedDate我必须在哪里做sorting .在List<Object>有一些值为 null对于 manufacturedDate .排序不正确。我不确定是不是因为 null值。 list不按 manufacturedDate 的升序或降序返回值.

下面是我的代码:

    if(sortType.equalsIgnoreCase("manufacturedDate"))
    {
        DateFormat sdf1 = new SimpleDateFormat("dd-MMM-yyyy", Locale.ENGLISH);
        if(manufacturedDateSortAsc.equalsIgnoreCase("Desc"))
        {
            manufacturedDateSortAsc = "Asc";
            Collections.sort(carList, new Comparator<CarDataTransferObject>() 
            {
                public int compare(CarDataTransferObject object1, CarDataTransferObject object2) 
                {
                    int returnVal = 0;
                    try 
                    {
                        returnVal = sdf1.parse(object1.getManufacturedDate()).compareTo(sdf1.parse(object2.getManufacturedDate()));
                    } 
                    catch (Exception e) 
                    {
                        log.error("Error inside sortList method"+e);
                    }
                    return returnVal;                       
                }
            });
        }
        else
        {
            usReleaseDateSortAsc = "Desc";
            Collections.sort(carList, new Comparator<CarDataTransferObject>() 
            {
                public int compare(CarDataTransferObject object1, CarDataTransferObject object2) 
                {
                    int returnVal = 0;
                    try 
                    {
                        returnVal = sdf1.parse(object2.getManufacturedDate()).compareTo(sdf1.parse(object1.getManufacturedDate()));                             
                    } 
                    catch (Exception e) 
                    {
                        log.error("Error inside sortList method"+e);
                    }
                    return returnVal;                       
                }
            });
        }
    }

Answer 1

我很确定这是一个问题，因为 null值(value)观。

看看你的compare()方法:

int returnVal = 0;
try {
 returnVal = sdf1.parse(object2.getManufacturedDate()).compareTo(sdf1.parse(object1.getManufacturedDate()));                             
} catch (Exception e) {
  log.error("Error inside sortList method"+e);
}
return returnVal;      

如果两者之一是 null 会发生什么？你会得到一个异常并返回 0。

然而，null应该总是被认为大于或小于任何非空值。 Comparator 的要求结果必须是可传递的，即如果a < b和 b < c然后 a < c .现在，如果其中之一是 null (比方说 c )然后你的代码会违反这一点，即它可能导致 a < b , b = c和 a = c这显然违反了契约(Contract)。

因此您需要单独检查空值:

if(date1 == null ) {
  if (date2 == null) {
    return 0;
  } else {
    return /*either -1 or 1 depending on where nulls should end up*/
  }
} else {
  if (date2 == null) {
    return /*either 1 or -1 depending on where nulls should end up*/
  } else {
    return date1.compareTo(date2); //this assumes successful parsing, I'll leave those details for you
  }
}