Java 按位移位运算符。签名和未签名
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Bitwise shift operators. Signed and unsigned
提问by
I'm practising for the SCJP exam using cram notes from the Internet.
我正在使用互联网上的补习笔记为 SCJP 考试练习。
According to my notes the >>operator is supposed to be signed right shift, with the sign bit being brought in from the left. While the left shift operator <<is supposed to preserve the sign bit.
根据我的笔记，>>运算符应该被右移签名，符号位是从左边引入的。而左移运算符<<应该保留符号位。
Playing around however, I'm able to shift the sign with the <<operator (f.e. Integer.MAX_VALUE << 1evaluates to -2, while I'm never able to shift the sign with the >>operator.
然而，在玩弄时，我可以用<<操作符移动符号（feInteger.MAX_VALUE << 1评估为-2，而我永远无法用>>操作符移动符号。
I must be misunderstanding something here, but what?
我一定在这里误解了一些东西，但是什么？
回答by Roman
">>" is signed because it keeps the sign. It uses the most left digit in binary representation of a number as a filler. For example:
“>>”是有符号的，因为它保留了符号。它使用数字的二进制表示中最左边的数字作为填充符。例如：
| this value is used as a filler
11011011
>> 11101101
01010010
>> 00101001
">>>" is unsigned version of this operator. It always use zero as a filler:
>>>" 是此运算符的未签名版本。它总是使用零作为填充符：
11011011
>>> 01101101
01010010
>>> 00101001
In binary representation the most left digit determines sign of the number. So, if it's '1' then we have negative value and if it's '0' - then our number is positive. That's why using the most left digit as a filler allows to keep sign permanent.
在二进制表示中，最左边的数字决定了数字的符号。所以，如果它是“1”，那么我们就有负值，如果它是“0”——那么我们的数字就是正值。这就是为什么使用最左边的数字作为填充符可以保持符号永久。
回答by Jacob Zimmerman
The idea behind the shifts is that they can act as multiplying and dividing by powers of 2 ( << 1 is equivalent to *= 2, >> 2 is equivalent to /= 4), which is why the signed version of shifting exists. Unsigned shifting doesn't preserve negatives, necessarily, though. The << operator doesn't actually preserve the sign, as you suggest; it simply happens to in your example. Try doing a left shift on 2,147,483,647; it doesn't stay positive. The reason that they don't bother trying to make a 'signed' left shift is because, if the number shifts from positive to negative (or viceversa), then you went outside the bounds of the variable type anyway.
移位背后的想法是它们可以作为乘以和除以 2 的幂（<< 1 相当于 *= 2，>> 2 相当于 /= 4），这就是为什么存在有符号版本的移位。但是，无符号移位不一定会保留负数。正如您所建议的，<< 运算符实际上并没有保留符号；它只是发生在你的例子中。尝试在 2,147,483,647 上左移；它不会保持积极。他们不费心尝试进行“有符号”左移的原因是，如果数字从正数变为负数（或反之亦然），那么无论如何你都超出了变量类型的范围。
