String pack and unpack

Question

Hi, i�m having trouble trying to write a methode to unpack a string packed with the following methode.
can some one tell me how to write the unpack methode ?
thanks.
public String pack(String s) {
byte[] a = new byte[16];
//If the string is an odd length then pack with an F
if ( s.length() % 2 == 1 ) {  
s = (char) 0xFF + s ;
//Now pack each pair of bytes into the array
for ( int i = 0, j = 0 ; i < s.length() ; i += 2, j++ ) {
a[j] = (byte) ( ( (s.charAt(i) & 0x0F) << 4 ) +
(s.charAt(i+1) & 0x0F) ) ;
return new String(A);
}

Answer 1

You need know the assumptions about the content of the original string. A packing algorithm like the one you describe might be used to pack a numeric string into an array of bytes. In that case, you assume that the original string consisted of nothing but ASCII digits, and you can recover the string with the following method:
public String unpack(String a) {
  byte [] b = a.getBytes();
  StringBuffer buff = new StringBuffer(b.length * 2);
  for (int i = 0; i < b.length; i++) {
    buff.append((char)(0x30 | ((b[i] & 0xf0) >> 4)));
    buff.append((char)(0x30 | (b[i] & 0x0f)));
  // If the first char was a pad char, then remove it
  if (buff.charAt(0) == (char)0x3f)
    return buff.substring(1);
  return buff.toString();
}