convert a hexadecimal string to a byte string

public static byte[] HexStringToByteArray(String s) throws IllegalArgumentException {
    int len = s.length();
    if (len % 2 == 1) {
        throw new IllegalArgumentException("Hex string must have even number of characters");
    }
    byte[] data = new byte[len / 2]; // Allocate 1 byte per 2 hex characters
    for (int i = 0; i < len; i += 2) {
        // Convert each character into a integer (base-16), then bit-shift into place
        data[i / 2] = (byte) ((Character.digit(s.charAt(i), 16) << 4)
                + Character.digit(s.charAt(i+1), 16));
    }
    return data;
}