将Null-Object拆箱到基本类型导致NullPointerException，好吗？ [英] Unboxing Null-Object to primitive type results in NullPointerException, fine?

问题描述

此代码段抛出 NullPointerException ，因为它已取消装入原始类型并且 Long.longValue（）被叫，对吗？

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

如果你有这样的代码片段，很容易看到：

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

long value = (Long) null;

但是 NullPointerException 更难获得在这种更复杂的情况下：

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

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

因此，Java-Compiler没有任何可能使Exception更加舒适？我希望 IllegalArgumentException 带有像这样的消息你试图将null-Object转换为基本类型，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!"

这不是更合适吗？你怎么看？这甚至可以在运行时？我们能够确定这个演员吗？我还没有看过java字节码。也许它可以用在解决方案中。

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（）等。没有特殊字节代码魔术发生，它与手动调用这些方法完全相同。因此， NullPointerException 是取消装箱 null 的自然结果（实际上是由JLS强制执行）。

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).

在每次拆箱转换期间，抛出一个不同的异常需要检查 null 两次确定是否抛出特殊异常，并在实际调用该方法时隐式执行）。我认为语言设计师并不认为它有用得足以保证。

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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