如果 equals(null) 抛出 NullPointerException 是不是一个坏主意? [英] Is it a bad idea if equals(null) throws NullPointerException instead?

问题描述

equals 对于null，如下:

对于任何非空引用值x，x.equals(null)应该return false.

For any non-null reference value x, x.equals(null) should return false.

这很奇怪，因为如果 o1 != null 和 o2 == null，那么我们有:

This is rather peculiar, because if o1 != null and o2 == null, then we have:

o1.equals(o2) // returns false
o2.equals(o1) // throws NullPointerException

o2.equals(o1) 抛出 NullPointerException 的事实是一件好事，因为它提醒我们程序员的错误.然而，如果出于各种原因，我们只是将其切换到 o1.equals(o2)，则该错误将不会被捕获，这将默默地失败"相反.

The fact that o2.equals(o1) throws NullPointerException is a good thing, because it alerts us of programmer error. And yet, that error would not be catched if for various reasons we just switched it around to o1.equals(o2), which would just "silently fail" instead.

所以问题是:

• 为什么 o1.equals(o2) 应该 return false 而不是抛出 NullPointerException 是个好主意?
• 如果我们尽可能地重写合约，以便 anyObject.equals(null) 总是抛出 NullPointerException，这不是一个坏主意吗?

• Why is it a good idea that o1.equals(o2) should return false instead of throwing NullPointerException?
• Would it be a bad idea if wherever possible we rewrite the contract so that anyObject.equals(null) always throw NullPointerException instead?

相比之下，这就是可比合同说:

In contrast, this is what the Comparable contract says:

注意 null 不是任何类的实例，并且 e.compareTo(null) 应该抛出一个 NullPointerException 即使 e.equals(null) 返回 false.

Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false.

如果NullPointerException适用于compareTo，为什么它不适用于equals?

If NullPointerException is appropriate for compareTo, why isn't it for equals?

• 关于 null 的 Comparable 和 Comparator 契约

这些是 Object.equals(Object obj) 文档:

These are the actual words in the Object.equals(Object obj) documentation:

指示某个其他对象是否等于"这个.

Indicates whether some other object is "equal to" this one.

什么是对象?

一个对象是一个类实例或一个数组.

An object is a class instance or an array.

引用值(通常只是references)是指向这些对象的指针，以及一个特殊的null 引用，它不指向任何对象.

The reference values (often just references) are pointers to these objects, and a special null reference, which refers to no object.

我从这个角度的论点非常简单.

My argument from this angle is really simple.

• equals 测试某个其他对象是否等于"这个
• null 引用没有为测试提供其他对象
• 因此，equals(null) 应该抛出 NullPointerException

• equals tests whether some other object is "equal to" this
• null reference gives no other object for the test
• Therefore, equals(null) should throw NullPointerException

推荐答案

对于这种不对称是否不一致的问题，我觉得不是，请参考这个古老的禅宗公案:

To the question of whether this asymmetry is inconsistent, I think not, and I refer you to this ancient Zen kōan:

• 问任何人他是否和下一个人一样好，每个人都会说是.
• 问任何人他是否和任何人一样好，每个人都会说不.
• 不要问任何人它是否和任何人一样好，你永远不会得到答复.

那一刻，编译器达到了启蒙.

At that moment, the compiler reached enlightenment.

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