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problem in bitwise and

thejaswini ramesh
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Joined: Oct 19, 2005
Posts: 74
Hi everyone,

I have a problem with bitwise anding.

1 & 0xff gives me 1
but 254 & 0xff gives me -2

What am I missing here, because I need to display as 254 (not -2)after masking with ff?

Thanks in advance,
Thejaswini.
Jesper de Jong
Java Cowboy
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Joined: Aug 16, 2005
Posts: 14194
    
  20

Can you show us the code that gives -2 when you do 254 & 0xff?

Are you putting the result in a byte? Notice that the range of possible values for the data type byte is -128 to 127. The number 254 doesn't fit in a byte. A byte with the bit pattern 11111110 is interpreted as -2 instead of 254.


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thejaswini ramesh
Ranch Hand

Joined: Oct 19, 2005
Posts: 74
Thanks Jesper!

Yes am assigning the result of (254 & 0xff) to a byte. But I need the value as a byte since am writing it to an output stream.
Stan James
(instanceof Sidekick)
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Joined: Jan 29, 2003
Posts: 8791
Java's lack of an unsigned byte bugs me. I used to enjoy using 0..255 in Pascal. I usually do these bitwise operations in ints (taking care to only use the low 8 bits) then convert to byte only if necessary. At least I can see a value like 254 when I want to.

When you see -2 I bet the right bits are still going to the stream, no?


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