为什么我可以在 Java 中抛出 null?

问题描述

运行时:

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);
        }
    }
}

回复是:

true  
false

这对我来说相当惊人.我原以为这会导致编译时错误.

Which was fairly stunning for me. I would have thought this would net a compile-time error.

为什么我可以在 Java 中抛出 null，为什么它会向上转换为 NullPointerException?

(实际上，我不知道它是否是向上转换"，因为我抛出的是 null)

除了一个非常愚蠢的面试问题(请不要在面试中问这个)我看不出有任何理由要throw null.也许你想被解雇，但那是......我的意思是，为什么会有人throw null?

Aside from a really really stupid interview question (please nobody ask this in an interview) I cannot see any reason to throw null. Maybe you want to be fired, but that's... I mean, why else would anyone throw null?

有趣的事实 IntelliJ IDEA 12 告诉我，我的台词，e instanceof NullPointerException，将始终为 false.这根本不是真的.

Fun fact IntelliJ IDEA 12 tells me that my line, e instanceof NullPointerException, will always be false. Which isn't true at all.

推荐答案

看起来不是将 null 视为 NullPointerException，而是尝试的行为throw null 本身会抛出一个NullPointerException.

It looks like it's not that null is treated as a NullPointerException, but that the act of attempting to throw null itself throws a NullPointerException.

换句话说，throw 检查其参数是否为非空，如果为空，则抛出 NullPointerException.

In other words, throw checks that its argument is nonnull, and if it is null, it throws a NullPointerException.

JLS 14.18 指定此行为:

JLS 14.18 specifies this behavior:

如果表达式的计算正常完成，产生一个空值，则创建并抛出一个 NullPointerException 类的实例 V' 而不是 null.然后 throw 语句突然完成，原因是值为 V' 的 throw.

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