5.3: Inheritance

At this point we have a class that represents a collection of cards. Next we’ll use it to define Deck and Hand. Here is the complete definition of Deck:

public class Deck extends CardCollection {

    public Deck(String label) {
        super(label);

        for (int suit = 0; suit <= 3; suit++) {
            for (int rank = 1; rank <= 13; rank++) {
                cards.add(new Card(rank, suit));
            }
        }
    }
}

The first line uses the keyword extends to indicate that Deck extends the class CardCollection. That means a Deck object has the same instance variables and methods as a CardCollection. Another way to say the same thing is that Deck “inherits from” CardCollection. We could also say that CardCollection is a superclass, and Deck is one of its subclasses.

In Java, classes may only extend one superclass. Classes that do not specify a superclass with extends automatically inherit from java.lang.Object. So in this example, Deck extends CardCollection, which in turn extends Object. The Object class provides the default equals and toString methods, among other things.

Constructors are not inherited, but all other public attributes and methods are. The only additional method in Deck, at least for now, is a constructor. So you can create a Deck object like this:

Deck deck = new Deck("Deck");

The first line of the constructor uses something new, super, which is a keyword that refers to the superclass of the current class. When super is used like a method, as in this example, it invokes the constructor of the superclass.

So in this case, super invokes the CardCollection constructor, which initializes the attributes label and cards. When it returns, the Deck constructor resumes and populates the (empty) ArrayList with Card objects.

That’s it for the Deck class. Next we need a way to represent a hand, which is the collection of cards held by a player, and a pile, which is a collection of cards on the table. We could define two classes, one for hands and one for piles, but there is not much difference between them. So we’ll use one class, called Hand, for both hands and piles. Here’s what the definition looks like:

public class Hand extends CardCollection {

    public Hand(String label) {
        super(label);
    }

    public void display() {
        System.out.println(getLabel() + ": ");
        for (int i = 0; i < size(); i++) {
            System.out.println(getCard(i));
        }
        System.out.println();
    }
}

Like Deck, Hand extends CardCollection, so it inherits methods like getLabel, size, and getCard, which are used in display. Hand also provides a constructor, which invokes the constructor of CardCollection (and nothing else).

In summary, a Deck is just like a CardCollection, but it provides a different constructor. And a Hand is just like a CardCollection, but it provides an additional method, display.