I can make a guess at what you are asking for.

  String abcd = "ab cd";
  byte[] bytes = abcd.getBytes("US-ASCII");

If you have "exotic" characters in your string
maybe you would rather use

  char[] chars = abcd.toCharArray();

Integer has a toHexString() static method, and it's not difficult to
iterate through a string or array, converting each value as you come
to it. Ther are a couple of minor problems which I'll leave you to
solve for yourself - (i) the integer cast from a char, byte, or short
will be sign extended; and (ii) toHexString doesn't pad with 0s.

Pete Barrett