java - 为什么 Double.NaN==Double.NaN 返回 false？

Question

我只是在研究 OCPJP 问题，发现了这个奇怪的代码:

public static void main(String a[]) {
    System.out.println(Double.NaN==Double.NaN);
    System.out.println(Double.NaN!=Double.NaN);
}

当我运行代码时，我得到了:

false
true

当我们比较两个看起来相同的东西时，输出 false 怎么样？ NaN 是什么意思？

Answer 1

NaN 表示“不是数字”。

Java Language Specification (JLS) Third Edition says :

An operation that overflows produces a signed infinity, an operation that underflows produces a denormalized value or a signed zero, and an operation that has no mathematically definite result produces NaN. All numeric operations with NaN as an operand produce NaN as a result. As has already been described, NaN is unordered, so a numeric comparison operation involving one or two NaNs returns false and any != comparison involving NaN returns true, including x!=x when x is NaN.