Using a while loop to solve "Applying a Discount Code"

private String normalizeDiscountCode(String discountCode) {
    int index = 0; // create variable to hold the loop count / index
    while(index < discountCode.length()) { // loop until the index is less than the discountCode length
      char code = discountCode.charAt(index); // use the index to pull out the appropriate char
      if(!Character.isLetter(code) || code != '$' ){
          throw new IllegalArgumentException("Invalid discount");
      }
      index++; // increase the index so that during the next loop we pull out the next character, and to make sure the loop eventually ends.
    }
    discountCode = discountCode.toUpperCase();
    return discountCode;
}