Converting an unsigned byte array to an integer

November 29, 2004

I found myself today needing to deal with unsigned integers, and shorts in java. In Java there is no unsigned keyword like in C, or other languages. All primitives are signed (meaning they can hold negative values).

So I came up with two functions for converting the unsigned byte arrays into numbers I can use (thanks to Matt Finn, here at ActivSoftware for some pointers). I am posting these to help others, and to see if anyone knows of a more efficient or easier way to do this:

/**
 * Converts a 4 byte array of unsigned bytes to an long
 * @param b an array of 4 unsigned bytes
 * @return a long representing the unsigned int
 */
public static final long unsignedIntToLong(byte[] b) 
{
    long l = 0;
    l |= b[0] & 0xFF;
    l <<= 8;
    l |= b[1] & 0xFF;
    l <<= 8;
    l |= b[2] & 0xFF;
    l <<= 8;
    l |= b[3] & 0xFF;
    return l;
}
    
/**
 * Converts a two byte array to an integer
 * @param b a byte array of length 2
 * @return an int representing the unsigned short
 */
public static final int unsignedShortToInt(byte[] b) 
{
    int i = 0;
    i |= b[0] & 0xFF;
    i <<= 8;
    i |= b[1] & 0xFF;
    return i;
}

You will notice that the functions actually return a long for unsinged integers, and an integer for shorts, this is because we can't store a large unsigned int in a java int. You may also be interested to know that byte's, short's, and int's are all actually stored using 4 bytes by the jvm (so they can perform 32 bit operations).

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Comments

On 11/30/2004 at 12:46:47 AM EST Keith Lea wrote:

You can use my BinaryTools class freely, at http://cvs.sourceforge.net/viewcvs.py/joustim/joscar/src/net/kano/joscar/BinaryTools.java?view=markup

It provides methods for dealing with binary data, including two methods very similar to the two you posted. It uses my ByteBlock class, but you could easily change it (using IDEA's structural search & replace for example) to use byte arrays.

On 11/30/2004 at 3:21:04 PM EST john wrote:

if you want it to run at least 8 times slower you can wrap the byte array in a java.nio.ByteBuffer and use its getInt method

On 12/01/2004 at 4:43:35 AM EST Anton wrote:

You could try doing it like this. It is basically the same, but much faster. For the unsignedShortToInt, it is about 1/3 faster, where unsignedIntToLong is five times faster. <pre> public static final long unsignedIntToLong(byte[] b) { return (b[0] << 24) | (b[1] << 16) | (b[2] << 8) | b[3]; }

public static final int unsignedShortToInt(byte[] b) { return (b[0] << 8) | b[1]; } </pre>

On 12/01/2004 at 12:42:45 PM EST eu wrote:

Shortest version would be (masking to 0xff is necessary):

return ((b[0] & 0xFF) << 24) | ((b[1] & 0xFF) << 16) | ((b[2] & 0xFF) << 8) | b[3] & 0xFF;

You can probably can do some bytecode tweaking to make it bit shorter but not necessary faster.

On 12/03/2004 at 4:21:29 AM EST Rasesh wrote:

Good work dude !!! Thanks I needed it.

On 03/15/2005 at 1:08:20 PM EST dave wrote:

This code converts directly from an int to a long.

int i = ...; long l = i < 0 ? 0x0000000100000000L + i : i;

On 06/07/2005 at 11:32:14 AM EDT Andre wrote:

on my pc (J2SE V1.4.2_04), I have to cast to a long before each operation:

return ((long)(b[0] & 0xFF) << 24) | ((long)(b[1] & 0xFF) << 16) | ((long)(b[2] & 0xFF) << 8) | (long)b[3] & 0xFF;

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Exactly what I was looking for, thanks!

On 07/08/2005 at 4:00:53 PM EDT Jason wrote:

Thanks guys, just what I was looking for!

On 08/08/2005 at 11:22:29 AM EDT Lee Namba wrote:

A probably slower but more flexible way is:

int i = new BigInteger(1,bytes).intValue();

which takes any length array. Especially handy for doing IP filtering for both IPv4 and IPv6

On 11/18/2005 at 8:19:14 AM EST Fernando wrote:

Very useful, thank you

On 11/27/2005 at 8:40:27 AM EST Nusli wrote:

thanx...

i wasnt '&' it with FF. my code worked because i never tested with values which would give me a -ve int. but looking at this i realized that it had a bug and it sometimes might not work without the '& 0xFF'

On 12/30/2005 at 8:48:29 PM EST Martin wrote:

Just another alternative for converting directly from an int (treated as unsigned) to a long:

int i = ...; long res = (i & 0x7FFFFFFF) + ((long)(i>>>31)<<31);

Hope someone (other than me!) finds that helpful

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to byte array - BigInteger.valueOf(num).toByteArray()

from byte array - new BigInteger(array).intValue()

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ByteBuffer keyBuffer = ByteBuffer.wrap(rowByteArray);

try { int nextInt = keyBuffer.getInt(); // do operations on the integer here } catch(BufferUnderflowException e) { // handle the last 3 bytes }

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EXAMPLE:

byte[] MyByteArray; String Str = new String(MyByteArray); int Value = Integer.parseInt(Str);

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