And bitwise operation gets negative value

Question

I have this code:

 int code = 0x92011202;
 int a = (code & 0xF0000000) >> 28;
 int b = (code & 0x0F000000) >> 24;
 // ..
 int n = (code & 0x0000000F);

But if most significant bit of code is equal to 1 (from 9 to F) a comes negative value. All other variables works fine.

Why this happen?

That is how `Java` (and many other languages) represent negative numbers, using the most significant bit as a sign bit. — Keppil, Dec 15 '14 at 06:09
See: [What is “2's Complement”?](http://stackoverflow.com/questions/1049722/what-is-2s-complement) — William Price, Dec 15 '14 at 06:11
I suggest reading up on [two's complement](http://en.wikipedia.org/wiki/Two%27s_complement) representation of integers. — Jared, Dec 15 '14 at 06:11
Also see: [Difference between >>> and >>](http://stackoverflow.com/questions/2811319/difference-between-and/2811372#2811372) — William Price, Dec 15 '14 at 06:12

score 5 · Accepted Answer · edited Dec 15 '14 at 06:14

This is explained in The Java Tutorials.

Specifically :

The unsigned right shift operator ">>>" shifts a zero into the leftmost position, while the leftmost position after ">>" depends on sign extension.

Java uses 2s complement variables. The only aspect about 2s complements that you care about is that, if the leftmost bit is a 1, the number is negative. The signed bitshift maintains sign, so if code is negative to begin with, it stays negative after the shift.

To fix your program use >>> instead which is a logical bitshift, ignoring sign

score 0 · Answer 2 · answered Dec 15 '14 at 06:12

The most significant bit of code represents the sign -- 0 means the number is positive and 1 means the number is negative.

If you just print out code you'll find that it's negative.

Because the shift operator takes into account the sign (it's a signed shift), a will get a negative value if code is negative.

score -1 · Answer 3 · answered Dec 15 '14 at 06:23

-1

The max value of "int" is 2^31－1. 0xF0000000 is a negative number. And any number with most significant bit equals to 1 is negative .

answered Dec 15 '14 at 06:23

Teng

1

And bitwise operation gets negative value

3 Answers3