与0xff进行逐位AND运算很重要？ [英] bitwise-ANDing with 0xff is important?

问题描述

对于0xff，不按位进行AND运算本质上意味着返回相同的值，就此而言，在此代码中？

Doesn't bitwise-ANDing with 0xff essentially mean getting the same value back, for that matter, in this code?

byte[] packet = reader.readPacket();
short sh;
sh = packet[1];
sh &= 0xFF;
System.out.print(sh+" ");

奇怪的是，如果不包括ANDing，我会得到-1，但如果包含了255，可能会有人解释原因是什么？

Weirdly, I get a -1 if that ANDing is not included but a 255 when included Could someone explain the reason?

正如我所看到的，0xff只是1111 1111.不是吗？

As I see it 0xff is just 1111 1111. Isn't it?

推荐答案

是的， 0xff 只是 1111 1111 。但这是尝试显示无符号字节值，即使在Java byte 中已签名。对于签名的字节，值 0xff 为 -1 ，但是 255 在短中。

Yes, 0xff is just 1111 1111. But this is attempting to display the unsigned byte value, even though in Java bytes are signed. The value 0xff is -1 for a signed byte, but it's 255 in a short.

当a读取 byte 0xff 的值，打印该值将产生 -1 。所以它分配给短，它有更大的范围并且可以存储 byte 值，这些值通常会溢出为负值数字作为字节作为正整数，例如144作为字节是 0x90 ，或-112，但它可以正确存储为 144 作为短。

When a byte value of 0xff is read, printing the value would yield -1. So it's assigned to a short which has a bigger range and can store byte values that would normally overflow to be a negative number as a byte as a positive integer, e.g. 144 as a byte is 0x90, or -112, but it can be properly stored as 144 as a short.

所以字节 -1 的值分配给 short 。但这又做了什么？进行原始扩展转换，并对负值进行符号扩展。所以 1111 1111 变成 11111111 11111111 ，仍然 -1 ，但这次是短。

So the byte value of -1 is assigned to a short. But what does that do? A primitive widening conversion takes place, and negative values are sign-extended. So 1111 1111 becomes 11111111 11111111, still -1, but this time as a short.

然后位掩码 0xff （ 00000000 11111111 ）用于再次输出最后8位：

Then the bitmask 0xff (00000000 11111111) is used to get the last 8 bits out again:

  -1: 11111111 1111111
0xFF: 00000000 1111111
======================
 255: 00000000 1111111

这只是一种获得无符号字节的方法 value，将其转换为 short ，然后屏蔽字节中的原始位，将其显示为无符号值。

It's just a way to get the unsigned byte value, by converting it to short and then masking out the original bits from the byte, to display it as an unsigned value.

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