为什么我可以抛出null在Java?

当运行这个:

public class WhatTheShoot {

    public static void main(String args[]){
        try {
            throw null;
        } catch (Exception e){
            System.out.println(e instanceof NullPointerException);
            System.out.println(e instanceof FileNotFoundException);
        }
    }
}

答案是:

true  
false

这对我来说是相当惊人的。我本以为这会导致编译时错误。

为什么我可以抛出null在Java中，为什么它向上转换它到一个NullPointerException?

(实际上，我不知道它是否是一个“上置”，因为我抛出null)

除了一个非常非常愚蠢的面试问题(请不要在面试中问这个问题)，我找不到任何理由抛出null。也许你想被解雇，但那…我的意思是，为什么有人会抛出null?

有趣的事实IntelliJ IDEA 12告诉我，我的行，einstanceof NullPointerException，将永远是假的。这根本不是真的。

当前回答

bharal……它看起来是javac编译器错误。我认为它是在SE 5中引入的。 Null可以赋给任何引用类型。然而，“null类型”本身并不是引用类型。程序编译它，因为null可以简单地转换为Exception。此外，throw在声明之后寻找对象引用，因为null可以作为对象引用，它显示结果。

JLS文档中关于throw的描述如下:

“A throw statement first evaluates the Expression. If the evaluation of the Expression completes abruptly for some reason, then the throw completes abruptly for that reason. If evaluation of the Expression completes normally, producing a non-null value V, then the throw statement completes abruptly, the reason being a throw with value V. If evaluation of the Expression completes normally, producing a null value, then an instance V’ of class NullPointerException is created and thrown instead of null. The throw statement then completes abruptly, the reason being a throw with value V’.”

2013-07-11 12:54:22

其他回答

用这种方式思考可以更明显地解释为什么这样做是有效的:

try {
    Exception foo = null;
    if(false) {
        foo = new FileNotFoundException();
    } // Oops, forgot to set foo for the true case..
    throw foo;
} catch (Exception e){
    System.out.println(e instanceof NullPointerException);
    System.out.println(e instanceof FileNotFoundException);
}

2013-07-10 22:19:14

为什么要向上转换为NullPointerException?

As for JLS 14.18:

A throw statement first evaluates the Expression. If the evaluation of the Expression completes abruptly for some reason, then the throw completes abruptly for that reason. If evaluation of the Expression completes normally, producing a non- null value V, then the throw statement completes abruptly, the reason being a throw with value V. If evaluation of the Expression completes normally, producing a null value, then an instance V’ of class NullPointerException is created and thrown instead of null. The throw statement then completes abruptly, the reason being a throw with value V’.