private static int getDigit(char c) throws StringFormatException {
        switch(c)  {
            case '0': return 0;
            case '1': return 1;
            case '2': return 2;
            case '3': return 3;
            case '4': return 4;
            case '5': return 5;
            case '6': return 6;
            case '7': return 7;
            case '8': return 8;
            case '9': return 9;
            case 'a': return 10;
            case 'b': return 11;
            case 'c': return 12;
            case 'd': return 13;
            case 'e': return 14;
            case 'f': return 15;
            default: throw new StringFormatException("Illegal character: " + c);
        }
    }