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Java Java Objects Delivering the MVP Applying a Discount Code

Todd Anderson
Todd Anderson
4,260 Points

How do you convert a string to check that it consists only of letters?

In the video his methods are already using char parameters, in this example we are asked to validate a string to make sure it consists of only letters. I have tried several options and all result in incomparable types.

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 != "$"){
      throw new IllegalArgumentException("Invalid discount code");}
    return discountCode.toUpperCase();}

  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

You need to check each letter in the string to see if it's correct or not. Just loop through each character and check it. One way to do it:

private String normalizeDiscountCode(String discountCode){
    for (int i = 0; i < discountCode.length(); i++) {
        char c = discountCode.charAt(i);
        if (!Character.isLetter(c) && c != '$') {
            throw new IllegalArgumentException("Invalid discount code");   
    return discountCode.toUpperCase();