java - 11111111111111111111111111111110 如何计算为 -2？

Question

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Answer 1

11111111111111111111111111111110 是原始位表示。 Java 使用 two's complement用于有符号值表示，其中第一位(如果设置)表示负数。

Conversion from Two's Complement 很好地解释了这一点:

Use the number 0xFFFFFFFF as an example. In binary, that is:

1111 1111 1111 1111 1111 1111 1111 1111

What can we say about this number? It's first (leftmost) bit is 1, which means that this represents a number that is negative. That's just the way that things are in two's complement: a leading 1 means the number is negative, a leading 0 means the number is 0 or positive.

To see what this number is a negative of, we reverse the sign of this number. But how to do that? The class notes say (on 3.17) that to reverse the sign you simply invert the bits (0 goes to 1, and 1 to 0) and add one to the resulting number.

The inversion of that binary number is, obviously:

0000 0000 0000 0000 0000 0000 0000 0000

Then we add one.

0000 0000 0000 0000 0000 0000 0000 0001

So the negative of 0xFFFFFFFF is 0x00000001, more commonly known as 1. So 0xFFFFFFFF is -1.

上面说明了为什么 0xFFFFFFFF 的二进制补码(如反向位加一)中的负数是 1，这意味着它本身 0xFFFFFFFF 表示 -1。以同样的方式，你的 0xFFFFFFFE 的值有 2 的负数，所以 0xFFFFFFFE 本身意味着 -2 的二分之一补充。