Java OR 运算符变得疯狂

Question

冒着 future 几天感到羞愧的风险...请向我解释以下内容。
我需要用字节对整数进行算术运算。

int a = 0x0100;
byte b = (byte)0xff;
int c = a | b;

我希望 c 为 0x100 | 0xff = 0x1ff = 511。
但它是 0xffffffff = -1为什么？

Answer 1

b 是 -1。当你做 a | b，b 被提升为一个 int，它仍然是 -1。

15.22.1. Integer Bitwise Operators &, ^, and |

When both operands of an operator &, ^, or |
    are of a type that is convertible (§5.1.8) to a primitive integral type,
binary numeric promotion is first performed on the operands (§5.6.2).

因此，a | b 被评估为好像 a | -1.

final int a = 0x0100;
final int b = 0xFF;
final int c = a | b;

我不确定你到底想做什么，但是。

How could I accomplish adding 8 bits to the end of a int value in simple steps?

int appendDummyOnes(final int value, final int size) {
    return (value << size) | (-1 >>> (Integer.SIZE - size));
}

int remarkDummyOnes(final int value, final int size) {
    return value | (-1 >>> (Integer.SIZE - size));
}