为什么 int num = Integer.getInteger(“123”) 会抛出 NullPointerException？
以下代码抛出 NullPointerException：
int num = Integer.getInteger("123");
我的编译器是否在 null 上调用 getInteger，因为它是静态的？这没有任何意义！
发生什么事了？
The following code throws NullPointerException:
int num = Integer.getInteger("123");
Is my compiler invoking getInteger on null since it's static? That doesn't make any sense!
What's happening?
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总体情况
这里有两个问题：
Integer getInteger(String)没有按照您的想法进行操作nullInteger到int的赋值会导致自动拆箱Integer为null，因此抛出NullPointerException解析
(String) "123"为(int) 123，您可以使用例如int Integer.parseInt(String)。参考文献
IntegerAPI 参考static int parseInt(String)static Integer getInteger(String)On
Integer.getInteger这是文档的内容不得不说一下这个方法的作用：
换句话说，此方法与将
String解析为int/Integer值无关，而是与System.getProperty方法。诚然，这可能是相当令人惊讶的。不幸的是，该库有这样的惊喜，但它确实给您上了宝贵的一课：始终查找文档以确认方法的作用。
巧合的是，这个问题的一个变体出现在 谜题者归来：Schlock 和 Awe (TS-5186)，Josh Bloch 和 Neal Gafter 的 2009 年 JavaOne 技术会议演示。这是总结幻灯片：
完整起见，还有这些类似于
Integer.getInteger：Boolean.getBoolean(String)Long.getLong(String)相关问题
关于自动拆箱
当然，另一个问题是
NullPointerException被抛出。为了关注这个问题，我们可以将代码片段简化如下：这里引用了《Effective Java》第 2 版第 49 条：“优先选择原始类型而不是装箱原始类型”：
在某些地方，您别无选择，只能使用装箱基元，例如泛型，但除此之外，您应该认真考虑使用装箱基元的决定是否合理。
相关问题
The Big Picture
There are two issues at play here:
Integer getInteger(String)doesn't do what you think it doesnullin this caseIntegertointcauses auto-unboxingIntegerisnull,NullPointerExceptionis thrownTo parse
(String) "123"to(int) 123, you can use e.g.int Integer.parseInt(String).References
IntegerAPI referencesstatic int parseInt(String)static Integer getInteger(String)On
Integer.getIntegerHere's what the documentation have to say about what this method does:
In other words, this method has nothing to do with parsing a
Stringto anint/Integervalue, but rather, it has to do withSystem.getPropertymethod.Admittedly this can be quite a surprise. It's unfortunate that the library has surprises like this, but it does teach you a valuable lesson: always look up the documentation to confirm what a method does.
Coincindentally, a variation of this problem was featured in Return of the Puzzlers: Schlock and Awe (TS-5186), Josh Bloch and Neal Gafter's 2009 JavaOne Technical Session presentation. Here's the concluding slide:
For completeness, there are also these methods that are analogous to
Integer.getInteger:Boolean.getBoolean(String)Long.getLong(String)Related questions
On autounboxing
The other issue, of course, is how the
NullPointerExceptiongets thrown. To focus on this issue, we can simplify the snippet as follows:Here's a quote from Effective Java 2nd Edition, Item 49: Prefer primitive types to boxed primitives:
There are places where you have no choice but to use boxed primitives, e.g. generics, but otherwise you should seriously consider if a decision to use boxed primitives is justified.
Related questions
来自 http://konigsberg.blogspot.com/2008/ 04/integergetinteger-are-you-kidding-me.html：
你想要这个：
From http://konigsberg.blogspot.com/2008/04/integergetinteger-are-you-kidding-me.html:
You want this:
请检查该方法的文档 getInteger()。
在此方法中，
String参数是一个系统属性，用于确定具有指定名称的系统属性的整数值。正如此处所述，“123”不是任何系统属性的名称。如果您想将此字符串转换为
int，请使用该方法：int num = Integer.parseInt("123")。Please check documentation of the method getInteger().
In this method, the
Stringparameter is a system property that determines the integer value of the system property with the specified name. "123" is not the name of any system property, as discussed here.If you want to convert this String to
int, then use the method asint num = Integer.parseInt("123").