Signed bytes and comparaison.

Submitted by galatnm on Thu, 01/08/2009 - 09:25.

In some part of the code, when i run findbugs, there is the following error reported :

"Bad comparison of signed byte : Signed bytes can only have a value in the range -128 to 127. Comparing a signed byte with a value outside that range is vacuous and likely to be incorrect. To convert a signed byte b to an unsigned value in the range 0..255, use 0xff & b".

It occurs for example with the following line :

byte[] data
...
if ((nonfullBitmap == -1) && (data[i] != 0xFF))
...

Is it something we must take care about or not ?

Thanks,

Fabien L.

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Should be false

Submitted by Peter on Thu, 01/08/2009 - 11:00.

Imho we should care about these cases.
The code above evaluates to false as data[i] gets upcasted to int (range -128 to 127) so it'll never be 0xFF (Though I'm not sure about the intended semantics in that case).

Peter is absolutely correct

Submitted by Stephen Crawley on Thu, 01/08/2009 - 12:42.

Any case of this reported by findbugs is deeply suspicious, and probably a real bug. But the only way to tell for sure ... and the only way to be sure of the correction ... is to examine the code carefully.