Big Java / Java Concepts Lab 13



Consider using the following Card class.

public class Card
    public Card()
        name = "";

    public Card(String n)
        name = n;

    public String getName()
        return name;

    public boolean isExpired()
        return false;

    public String format()
        return "Card holder: " + name;

    private String name;

Use this class as a superclass to implement a hierarchy of related classes:

ID number
Calling Card
Card number, PIN
Driver License
Expiration year

Write definitions for each of the subclasses. For each subclass, supply private instance variables. Leave the bodies of the constructors blank for now.

Calling the Superclass Constructor


Implement constructors for each of the three subclasses. Each constructor should call the superclass constructor to set the name. Here is one example:

public IDCard(String n, String id)
    idNumber = id;

Overriding Methods


Supply the implementation of the format method for the three subclasses. The methods should produce a formatted description of the card details. The subclass methods should call the superclass format method to get the formatted name of the cardholder.


Devise another class, Billfold, which contains slots for two cards, card1 and card2, a method void addCard(Card) and a method String formatCards().

The addCard method checks if card1 is null. If so, it sets card1 to the new card. If not, it checks card2. If both cards are set already, the method has no effect.

Of course, formatCards invokes the format method on each non-null card and concatenates the resulting strings.

What is your Billfold class?


Write a class with a test program that adds two objects of different subclasses of Card to a Billfold object. Print the results of the formatCards methods.

What is the code for your test program?

(Alternatively, if you use BlueJ, explain how you set up the Billfold object.)


What is the output of your test program?


Explain why the output of your program demonstrates polymorphism.


The Card superclass defines a method isExpired, which always returns false. This method is not appropriate for the driver license. Supply a method DriverLicense.isExpired() that checks if the driver license is already expired (i.e., the expiration year is less than the current year).

To find out the current year, you can use the get method of the class Calendar. For example, if you create a Calendar as follows:

GregorianCalendar calendar = new GregorianCalendar();

Then, you can obtain the current year using


What is the code for your isExpired method?


The ID card and the phone card don't expire. What should you do to reflect this fact in your implementation?


Add a method getExpiredCardCount, which counts the number of expired cards in the billfold, to the Billfold class.


Write a test class that populates a billfold with a phone card and an expired driver license. Then call the getExpiredCardCount method. Run your test program to verify that your method works correctly.

What is your test program?

The toString method


Define toString methods for the Card class and its three subclasses. The methods should print:

  • the name of the class
  • the values of all instance fields (including inherited fields)

Typical formats are:

Card[name=Edsger W. Dijkstra]

CallingCard[name=Bjarne Stroustrup][number=4156646425,pin=2234]

Give the code for your toString methods.

The equals method


Define equals methods for the Card class and its three subclasses. Cards are the same if the objects belong to the same class, and if the names and other information (such as the expiration year for driver licenses) match.

Give the code for your equals methods.

Protected access


Change the Card class and give protected access to name. (a) Would that change simplify the toString method of the CallingCard class? How?

(b) Is this change advisable?