将 Null-Object 拆箱为原始类型会导致 NullPointerException，好吗? [英] Unboxing Null-Object to primitive type results in NullPointerException, fine?

问题描述

这个代码段抛出一个 NullPointerException 因为它被拆箱为原始类型并且 Long.longValue() 被调用，对吧?

This snippet throws an NullPointerException due to the fact that its unboxed to a primitive type and Long.longValue() is called, right?

如果您有这样的片段，那就更容易了:

Thats even easy to see if you have a snippet like this:

long value = (Long) null;

但是 NullPointerException 在更复杂的情况下更难得到:

But the NullPointerException is even harder to get in a more complex situation like this:

long propertyValue = (Long) obj.getProperty(propertyModel.getName());

那么，Java 编译器是否有可能从中生成更舒适的异常?我更喜欢 IllegalArgumentException 带有类似 您正在尝试将空对象转换为原始类型的消息，这是不可能的！"

So isn't there any possibility for the Java-Compiler to make a more comfortable Exception out of this? I would prefer an IllegalArgumentException with a message like "You're trying to cast a null-Object into a primitive type, this can't be done!"

这样不是更合适吗?你怎么认为?这在运行时甚至可能吗?我们能确定这个演员表吗?我还没有看过java字节码.也许它可以用于解决方案.

Wouldn't this be more appropriate? What do you think? Is this even possible at runtime? Are we able to determine this cast? I haven't yet looked at the java bytecode. Maybe it could be used in a solution.

这个问题可以回答:我想知道是否有可能实现这种行为！

This question can be answered: I'd like to know if it's possible to achieve this behaviour!

推荐答案

根据Java 语言规范，通过调用Number.longValue()、Number.intValue() 进行拆箱等.没有发生特殊的字节码魔术，这与您手动调用这些方法完全相同.因此，NullPointerException 是拆箱 null 的自然结果(实际上是 JLS 强制要求的).

According to the Java language specification, unboxing happens via calling Number.longValue(), Number.intValue(), etc. There is no special byte code magic happening, it's exactly the same as if you call those methods manually. Thus, the NullPointerException is the natural result of unboxing a null (and in fact mandated by the JLS).

抛出不同的异常需要在每次拆箱转换期间检查 null 两次(一次确定是否抛出特殊异常，一次隐式，当方法实际上是称为).我想语言设计者认为它的用处不足以保证这一点.

Throwing a different exception would require checking for null twice during every unboxing conversion (once to determine whether to throw the special exception, and once implicitly when the method is actually called). I suppose the language designers didn't think it useful enough to warrant that.

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