奇数NullPointerException [英] Odd NullPointerException

问题描述

我的NPE中的Stacktrace开头为:

Stacktrace from my NPE starts with:

Caused by: java.lang.NullPointerException
    at pl.yourvision.crm.web.servlets.listExport.ProductListExport.writeCells(ProductListExport.java:141)

此文件中的行号141为:

Line number 141 in this file is:

Double availablePieces = store != null ? store.getAvailablePieces() : 0.0;

其中store不为null且store.getAvailablePieces()为null的地方.我不明白为什么我会在这里例外.

Where store is not null and store.getAvailablePieces() is null. I do not understand why I get exception in here.

有什么想法吗?

推荐答案

我99％确信这是由于条件运算符的行为引起的.我相信您的代码等同于:

I'm 99% sure this is because of the behaviour of the conditional operator. I believe your code is equivalent to:

double tmp = store != null ? store.getAvailablePieces() : 0.0;
Double availablePieces = tmp;

换句话说，它会将 store.getAvailablePieces()的结果装箱到 double ，然后装箱回到 Double .如果 store.getAvailablePieces()返回 null ，则确实会导致 NullPointerException .

In other words, it's unboxing the result of store.getAvailablePieces() to a double, then boxing back to Double. If store.getAvailablePieces() returns null, that will indeed cause a NullPointerException.

解决方法是使第三个操作数也成为 Double :

The fix is to make the third operand Double as well:

Double availablePieces = store != null ? store.getAvailablePieces()
                                       : Double.valueOf(0.0);

现在将没有装箱或拆箱，因此 store.getAvailablePieces()返回 null 很好.然后，您可能想使用 0.0 代替，但这是另一回事.如果要这样做，可以更改为:

Now there won't be any boxing or unboxing, so it's fine for store.getAvailablePieces() to return null. You may want to then use 0.0 instead, but that's a different matter. If you're going to do that, you can change to:

Double tmp = store != null ? store.getAvailablePieces() : null:
double availablePieces = tmp == null ? 0.0 : tmp;

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