空检查抛出NullPointerException [英] Null Check throws NullPointerException

问题描述

有时候我碰巧写了一个代码来检查NullPointerException，如下所示，

Sometimes it happens that I have written a code to check NullPointerException like below,

if(str!=null)
{
    doSomething();
} 

并且空检查本身会引发NUllPointerException。

and the null check itself throws NUllPointerException.

如何解决。

编辑：
实际上，此代码获取的是空指针，

Edited: Actually this code was getting null pointer,

Map params = new HashMap();
if(params != null && params.get("requestType"))
{
    //some null safe code goes here
}

后来我了解到params.get（）抛出了空指针异常。

i understood later that params.get() was throwing null pointer exception.

推荐答案

您似乎在说：

if (str != null) {
    doSomething();
}

抛出 NullPointerException

is throwing a NullPointerException in the comparison.

这是不可能。因此，我希望真正发生的是以下情况之一：

That is impossible. So I expect that what is really happening is one of the following:

• 您误解了堆栈跟踪，而NPE是

• You have misinterpreted the stack trace, and the NPE is not being thrown there.

实际代码与示例有所不同。

The actual code is substantially different to the illustrative example.

您没有执行您认为正在执行的代码。例如，您可能没有重新编译它……或者您正在执行 .class 或 .jar 文件。

You are not executing the code that you think you are executing. For example, you might not have recompiled it ... or you may be executing a stale copy of the .class or .jar file.

您已经设法严重混淆了IDE。 （有时您会从混乱的IDE中获得非常奇怪的行为...）

You have managed to seriously confuse your IDE. (Sometimes you can get very strange behaviour from a confused IDE ...)

^{该代码可能给出的NPE可能不是您所做的其他事情的假象的唯一情况是，如果Java（或IDE）安装损坏，或者您的硬件出现故障。我会把这些解释视作根本难以置信。}

更新

现在您说：

Map params = new HashMap();
if(params != null && params.get("requestType"))
{
    //some null safe code goes here
}

在 params.get 中抛出NPE。我不得不说这是胡说八道。

throws an NPE in params.get. I have to say that this is nonsense.

1. 代码无法编译。 params.get 调用不会返回 boolean 或可以自动转换为 boolean 。

1. The code won't compile. The params.get call doesn't return something that is boolean or that can be automatically converted to a boolean.

如果我们忽略编译错误，则 params.get 参数为 null ，则在线程受限映射上的c $ c>只能抛出NPE。这张地图 是线程受限的，并且先前的检查确保 params 不会 null 。

If we ignore the compilation errors, then params.get on a thread-confined map can only throw an NPE if params is null. This map is thread-confined, and the previous check ensures that params isn't null.

我以前的结论成立。

提示：这可能是线程问题。如果您使用一个线程更新 HashMap 并与另一个线程一起读取，并且未正确同步，则可能会得到间歇性NPE。

Hint: this could be a threading problem. It is possible to get intermittent NPE's if you update a HashMap with one thread and read it with another, and you don't synchronize properly.

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