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Java Java Objects Delivering the MVP Applying a Discount Code

Checking a string has only letters.


Im looking to check a string to see if it contains only letters. I can use a char.isLetter() to do this with a char, but Im looking for something like this for a String. A quick search on the java API site didn't get me anything that looked fruitful. Can you guys steer me in the right direction?

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

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

  private String normalizeDiscountCode(String discountCode) {
    if (discountCode.isLetter() || discountCode == "$") {
      return discountCode.toUpperCase();
    else {
      throw new IllegalArgumentException("Only letters and $ sign's are allowed");

  public String getItemName() {
    return itemName;

  public int getPriceInCents() {
    return priceInCents;

  public String getDiscountCode() {
    return discountCode;

  public void applyDiscountCode(String discountCode) {

    this.discountCode = normalizeDiscountCode(discountCode);
public class Example {

  public static void main(String[] args) {
    // This is here just for example use cases.

    Order order = new Order(
            "Yoda PEZ Dispenser",

    // These are valid.  They are letters and the $ character only
    order.getDiscountCode(); // ABC

    order.getDiscountCode(); // $ALE

    try {
      // This will throw an exception because it contains numbers
    } catch (IllegalArgumentException iae) {
      System.out.println(iae.getMessage());  // Prints "Invalid discount code"
    try {
      // This will throw as well, because it contains a symbol.
    }catch (IllegalArgumentException iae) {
      System.out.println(iae.getMessage());  // Prints "Invalid discount code"


1 Answer

Craig Dennis
Craig Dennis
Treehouse Teacher

Hi Chris!

Your code works and was more or less what I was intending at this point in time. You'll want to loop toCharArray on the discount code and do that check.

You could do this with a regular expression and use the match method. Again extra credit here but I did a workshop on regular expressions in Java that you might want to check out.