java 我将如何修复以下代码的 NullPointerException？
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How would I go about fixing a NullPointerException of the following code?
提问by Confiqure
I have this code and when I run the script, I pass in valid parameters, but I keep on getting a NPE. Help?
我有这个代码，当我运行脚本时，我传递了有效的参数，但我继续获得 NPE。帮助？
Code:
代码：
private static Date getNearestDate(List<Date> dates, Date currentDate) {
long minDiff = -1, currentTime = currentDate.getTime();
Date minDate = null;
if (!dates.isEmpty() && currentDate != null) {
for (Date date : dates) {
long diff = Math.abs(currentTime - date.getTime());
if ((minDiff == -1) || (diff < minDiff)) {
minDiff = diff;
minDate = date;
}
}
}
return minDate;
}
I get the NullPointerException from line 2 of the code above and I use the following code to pass in thisDate as the currentDate variable.
我从上面代码的第 2 行得到 NullPointerException，并使用以下代码将 thisDate 作为 currentDate 变量传递。
Date thisDate = null;
try {
thisDate = (new SimpleDateFormat("MM/dd/yyyy")).parse(Calendar.getInstance().getTime().toString());
} catch (Exception e) {}
回答by Asaph
Since you've indicated that the NullPointerExceptionis thrown on line 2, we can deduce that you're passing in nullfor the currentDateargument. currentDate.getTime()is the only part of line 2 that can cause a NullPointerException.
既然你已经表明，NullPointerException在第2行抛出，我们可以推断出你在传递null的currentDate参数。currentDate.getTime()是第 2 行中唯一可能导致NullPointerException.
Update:
更新：
I just wrote the following Test.javacode to really understand what your problem is:
我只是写了下面的Test.java代码来真正理解你的问题是什么：
import java.util.*;
import java.text.*;
class Test {
public static void main(String[] args) {
Date thisDate = null;
try {
thisDate = (new SimpleDateFormat("MM/dd/yyyy")).parse(Calendar.getInstance().getTime().toString());
} catch (Exception e) {
e.printStackTrace();
}
}
}
When I run it, I get:
当我运行它时，我得到：
java.text.ParseException: Unparseable date: "Sat Aug 20 12:42:30 PDT 2011"
at java.text.DateFormat.parse(DateFormat.java:337)
at Test.main(Test.java:8)
So the problem is that your SimpleDateFormat.parse()expects the month/day/year format, but the Dateclass's toString()method is giving you something different.
所以问题是你SimpleDateFormat.parse()期望的是月/日/年格式，但是Date类的toString()方法给了你一些不同的东西。
It seems as though all you really want is the current date. Why bother formatting it? Just trim it down to this and be done with it:
似乎您真正想要的只是当前日期。为什么要格式化它？只需将其修剪为这个并完成它：
Date thisDate = new Date(); // this gives you the current date
回答by rsp
What your line:
你的线路：
thisDate = (new SimpleDateFormat("MM/dd/yyyy")).parse(Calendar.getInstance().getTime().toString());
does is
是
- create a new Calender object (initialised with the curren time),
- get a Date() object from it,
- convert that to string which will use a default format like "EEE MMM d HH:mm:ss z yyyy"
- and try to parse the result as a date with the format "MM/dd/yyyy"
- (which will fail to parse by design, resulting in a
null)
- 创建一个新的日历对象（用当前时间初始化），
- 从中获取一个 Date() 对象，
- 将其转换为将使用默认格式的字符串，例如“EEE MMM d HH:mm:ss z yyyy”
- 并尝试将结果解析为格式为“MM/dd/yyyy”的日期
- （这将无法按设计解析，导致
null）
If you fix all problems in this line, the end result will be a Dateobject containing the current time.
如果您修复了这一行中的所有问题，最终结果将是一个Date包含当前时间的对象。
A much easier way to obtain such a Dateis:
获得这样一个更简单的方法Date是：
thisDate = new Date();
