Interpret strings as packed binary data, Python vs Java

Question

I'm struggling a bit with binary in Java and Python to translate a program

In python when I execute the following commands, I've got

>>> print ord(pack('>H', 32809)[0])
128

>>> print ord(pack('>H', 32809)[1])
41

In Java, I expect to have the same result when I execute the following command, but it's not there:

bsh % print ((byte)((32809 & 0xFF00) >> 8)); 
-128

bsh % print ((byte)(32809 & 0x00FF)); 
41

Can somebody explain me why 128 is negative in Java? Many thanks.

Voo · Accepted Answer · 2011-02-13 19:21:02Z

2

Byte in java is a signed data type - and yeah I never understood why they did that either. You'll need to use a short (well since all bit ops are used on ints anyhow, just use a int really) and ignore the higher 8bit (otherwise sign extension would be a problem)

 System.out.println(((short) ((32809 & 0xFF00) >> 8)) & 0xFF);

answered Feb 13, 2011 at 19:21

Voo

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1 Comment

Peter Lawrey Over a year ago

Or just System.out.println((32809 >> 8) & 0xFF);

Hugh Bothwell · Accepted Answer · 2011-02-13 20:45:36Z

1

A signed byte has a range of -128..127, so (127+1) == -128

answered Feb 13, 2011 at 20:45

Hugh Bothwell

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2 Comments

art1go Over a year ago

does it means (byte)128 = -128 ?

Hugh Bothwell Over a year ago

Yes. Incrementing from 127 "wraps around" - the bit pattern for 127 is 0x7F; adding one gives 0x80; interpreted as a signed byte, this is -128. There are not enough bits in a signed byte to properly represent +128.

corsiKa · Accepted Answer · 2011-02-13 19:21:14Z

0

You won't have unsigned types in Java. You could consider casting it to a short or an int instead.

answered Feb 13, 2011 at 19:21

corsiKa

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2 Comments

Peter Lawrey Over a year ago

BTW char is an unsigned type. ;)

corsiKa Over a year ago

That's true, it is. But as far as numbers are concerned, you won't have unsigned types. Yes, I know it is valid to say char c = 100; but seeing how java.lang.Character doesn't extend Number, I don't consider it to be a number.

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