Convert integer into byte array (Java)

what's a fast way to convert an Integer into a Byte Array?

e.g. 0xAABBCCDD => {AA, BB, CC, DD}

Have a look at the ByteBuffer class.

ByteBuffer b = ByteBuffer.allocate(4);
//b.order(ByteOrder.BIG_ENDIAN); // optional, the initial order of a byte buffer is always BIG_ENDIAN.
b.putInt(0xAABBCCDD);

byte[] result = b.array();

Setting the byte order ensures that result[0] == 0xAA, result[1] == 0xBB, result[2] == 0xCC and result[3] == 0xDD.

Or alternatively, you could do it manually:

byte[] toBytes(int i)
{
  byte[] result = new byte[4];

  result[0] = (byte) (i >> 24);
  result[1] = (byte) (i >> 16);
  result[2] = (byte) (i >> 8);
  result[3] = (byte) (i /*>> 0*/);

  return result;
}

The ByteBuffer class was designed for such dirty hands tasks though. In fact the private java.nio.Bits defines these helper methods that are used by ByteBuffer.putInt():

private static byte int3(int x) { return (byte)(x >> 24); }
private static byte int2(int x) { return (byte)(x >> 16); }
private static byte int1(int x) { return (byte)(x >>  8); }
private static byte int0(int x) { return (byte)(x >>  0); }

Using BigInteger:

private byte[] bigIntToByteArray( final int i ) {
    BigInteger bigInt = BigInteger.valueOf(i);      
    return bigInt.toByteArray();
}

Using DataOutputStream:

private byte[] intToByteArray ( final int i ) throws IOException {      
    ByteArrayOutputStream bos = new ByteArrayOutputStream();
    DataOutputStream dos = new DataOutputStream(bos);
    dos.writeInt(i);
    dos.flush();
    return bos.toByteArray();
}

Using ByteBuffer:

public byte[] intToBytes( final int i ) {
    ByteBuffer bb = ByteBuffer.allocate(4); 
    bb.putInt(i); 
    return bb.array();
}