Make sure you set the this.discountCode = normalizeDiscountCode(discountCode)

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

  private String normalizeDiscountCode (String discountCode) {
     this.discountCode = normalizeDiscountCode(discountCode);
     for (char letter : discountCode.toCharArray()) {
        if (! (Character.isLetter(letter) || letter == '$')) {
           throw new IllegalArgumentException ("Invalid discountcode has been used");
          }
       }
        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 = discountCode;
  }
}