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NNTP-Posting-Date: Thu, 20 Jun 2013 13:22:16 -0500
Date: Thu, 20 Jun 2013 19:22:13 +0100
From: lipska the kat <"nospam at neversurrender dot co dot uk">
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Subject: Re: convert byte array to hex string using BigInteger
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On 20/06/13 14:32, Laura Schmidt wrote:
> On 06/20/2013 03:09 PM, rossum wrote:
>
>> This is a case of RTFM.  The Javadocs for BigInteger(byte[]) tell you
>> that it expects the byte array in two's complement representation, so
>> a leading 1 bit is interpreted as a negative number.  You need a
>> different constructor:
>>
>>    public BigInteger(int signum, byte[] magnitude)
>
> You are right, sorry.
>
> Now I get a hex string without sign. But now the decode method does not
> return the original bytes anymore:
>
>   private byte [] hex_decode (String val)
>   {
>    BigInteger b = new BigInteger (val,16);
>    byte [] t = b.toByteArray();
>
>    return (t);
>   }
>
> There is no other constructor for byte arrays.
> And I don't really understand why it doesn't return the original byte
> array.

I'm not sure what all this twos compliment stuff is about
twos compliment is just a number representation scheme
There should be no need to 'convert' anything

Anyway

The following program works with the following observation

If the most significant byte is positive e.g >= 1 && <= 127 or 0x7F
the conversion works both ways

If the most significant byte is negative or 0 e.g <= 0 && >= -128 or 
0x80 then the conversion works with one proviso

The contract for BigInteger#toByteArray() contains the following text

"The array will contain the minimum number of bytes required to 
represent this BigInteger, including at least one sign bit"

so, if the MSB is negative there will be an additional byte in the MSG 
position after calling toByteArray set to 0 which indicates that the 
following is a positive number, if the MSB is positive there is no need 
for an additional byte as the first bit in the MSB is 0 thereby marking 
the following number as positive ... interesting, never seen this 
before... apart from that it seems to work.

Try changing the byte variable a to a positive number to see the 
difference.


<code compiles='yes'>

import java.math.BigInteger;

public class Converter {
	
	private static void printBytes(byte[] val){
		for(byte b : val){
			System.out.print(b);
			System.out.print(' ');
		}
		System.out.print("\n");
	}
	
    	
	private static String hexEncode (byte[] val){
		BigInteger b = new BigInteger(1, val);
		String t = b.toString(16);
		return t;
		//Just Say No to this :-)
		//return new BigInteger(1, val).toString(16);
	}
	
	private static byte[] hexDecode (String val){
		BigInteger b = new BigInteger (val, 16);
		byte[] t = b.toByteArray();
		return t;
	}

	public static void main(String[] args) {
		byte a = -1;   //0xFF
		byte b = 127;  //0x7F
		byte c = -128; //0x80
		byte d = 64;   //0x40
		byte e = 16;   //0x10
		byte f = 99;   //0x5A
		
		byte[] val = {a, b, c, d, e, f};
		
		printBytes(val);
		
		String encoded = hexEncode(val);
		
		System.out.println(encoded);
		
		byte[] decoded = hexDecode(encoded);
		
		printBytes(decoded);
		
         	encoded = hexEncode(decoded);
		
		System.out.println(encoded);

	}

}

</code>

lipska

-- 
Lipska the KatŠ: Troll hunter, sandbox destroyer
and farscape dreamer of Aeryn Sun