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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 Kat©: Troll hunter, sandbox destroyer
and farscape dreamer of Aeryn Sun

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Thread

convert byte array to hex string using BigInteger Laura Schmidt <ls@mailinator.com> - 2013-06-20 14:03 +0200
  Re: convert byte array to hex string using BigInteger Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-06-20 09:03 -0400
  Re: convert byte array to hex string using BigInteger rossum <rossum48@coldmail.com> - 2013-06-20 14:09 +0100
    Re: convert byte array to hex string using BigInteger Laura Schmidt <ls@mailinator.com> - 2013-06-20 15:32 +0200
      Re: convert byte array to hex string using BigInteger rossum <rossum48@coldmail.com> - 2013-06-20 15:44 +0100
      Re: convert byte array to hex string using BigInteger lipska the kat <"nospam at neversurrender dot co dot uk"> - 2013-06-20 19:22 +0100
        Re: convert byte array to hex string using BigInteger markspace <markspace@nospam.nospam> - 2013-06-20 11:56 -0700
          Re: convert byte array to hex string using BigInteger lipska the kat <"nospam at neversurrender dot co dot uk"> - 2013-06-20 20:43 +0100
        Re: convert byte array to hex string using BigInteger Lew <lewbloch@gmail.com> - 2013-06-20 13:39 -0700
          Re: convert byte array to hex string using BigInteger lipska the kat <"nospam at neversurrender dot co dot uk"> - 2013-06-21 08:17 +0100
        Re: convert byte array to hex string using BigInteger Arne Vajhøj <arne@vajhoej.dk> - 2013-06-26 22:16 -0400
  Re: convert byte array to hex string using BigInteger Joerg Meier <joergmmeier@arcor.de> - 2013-06-20 22:09 +0200
  Re: convert byte array to hex string using BigInteger Roedy Green <see_website@mindprod.com.invalid> - 2013-06-21 14:51 -0700
    Re: convert byte array to hex string using BigInteger Joerg Meier <joergmmeier@arcor.de> - 2013-06-22 00:26 +0200
      Re: convert byte array to hex string using BigInteger Roedy Green <see_website@mindprod.com.invalid> - 2013-06-23 08:49 -0700
        Re: convert byte array to hex string using BigInteger Joerg Meier <joergmmeier@arcor.de> - 2013-06-23 22:37 +0200
      Re: convert byte array to hex string using BigInteger Roedy Green <see_website@mindprod.com.invalid> - 2013-06-23 12:40 -0700
        Re: convert byte array to hex string using BigInteger Joerg Meier <joergmmeier@arcor.de> - 2013-06-23 22:41 +0200
  Re: convert byte array to hex string using BigInteger Arne Vajhøj <arne@vajhoej.dk> - 2013-06-26 22:12 -0400
    Re: convert byte array to hex string using BigInteger Arne Vajhøj <arne@vajhoej.dk> - 2013-06-26 22:20 -0400
    Re: convert byte array to hex string using BigInteger Laura Schmidt <ls@mailinator.com> - 2013-06-28 07:55 +0200
      Re: convert byte array to hex string using BigInteger Joerg Meier <joergmmeier@arcor.de> - 2013-06-29 00:01 +0200
    Re: convert byte array to hex string using BigInteger Stanimir Stamenkov <s7an10@netscape.net> - 2013-06-29 19:10 +0300
      Re: convert byte array to hex string using BigInteger Arne Vajhøj <arne@vajhoej.dk> - 2013-07-03 22:24 -0400
  Re: convert byte array to hex string using BigInteger aydin.k.abadi@gmail.com - 2013-11-23 16:40 -0800
  Re: convert byte array to hex string using BigInteger aydin.k.abadi@gmail.com - 2013-11-23 16:43 -0800
    Re: convert byte array to hex string using BigInteger Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2013-11-24 10:43 +0000

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