code: public static byte[] decode(String hexa) throws CoderException {
  	if (hexa == null) {
  	    throw new CoderException("can't decode empty String");
  	}
  	if ((hexa.length() % 2) != 0) {
  	    throw new CoderException("invalid hexadecimal String");
  	}
  	int tamArray = hexa.length() / 2;
  	byte[] retorno = new byte[tamArray];
  	for (int i = 0; i < tamArray; i++) {
  	    retorno[i] = hexToByte(hexa.substring(i * 2, i * 2 + 2));
  	}
  	return retorno;
      }
docs: decodes back a String to a byte array
  
  @param hexa
  @return decoded byte array
  @throws CoderException