Applying a Discount Code 2-2.

public class Order {
  private String itemName;
  private int priceInCents;
  private String discountCode;

  private String normalizeDiscountCode(String discountCode) { // THIS IS WHERE THE CHALLENGE IS!
    for (char element : discountCode.split("")) {
      if (! Character.isLetter(element)) {
        if (element != '$') {
          throw new IllegalArgumentException("Invalid discount code.");
        }
      }
    }
    return discountCode.toUpperCase();
  }

  public Order(String itemName, int priceInCents) {
    this.itemName = itemName;
    this.priceInCents = priceInCents;
  }

  public String getItemName() {
    return itemName;
  }

  public int getPriceInCents() {
    return priceInCents;
  }

  public String getDiscountCode() {
    return discountCode;
  }

  public void applyDiscountCode(String discountCode) {
    this.discountCode = normalizeDiscountCode(discountCode);
  }
}