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Arrays of Cards

The reason I chose Cards as the objects for this chapter is that there is an obvious use for an array of cards---a deck. Here is some code that creates a new deck of 52 cards:

    Card[] deck = new Card [52];

Here is the state diagram for this object:

The important thing to see here is that the array contains only references to objects; it does not contain any Card objects. The values of the array elements are initialized to null. You can access the elements of the array in the usual way:

    if (deck[3] == null) {
      System.out.println ("No cards yet!");

But if you try to access the instance variables of the non-existent Cards, you will get a NullPointerException.

    deck[2].rank;             // NullPointerException

Nevertheless, that is the correct syntax for accessing the rank of the "twoeth" card in the deck (really the third---we started at zero, remember?). This is another example of composition, the combination of the syntax for accessing an element of an array and an instance variable of an object.

The easiest way to populate the deck with Card objects is to write a nested loop:

    int index = 0;
    for (int suit = 0; suit <= 3; suit++) {
      for (int rank = 1; rank <= 13; rank++) {
        deck[index] = new Card (suit, rank);

The outer loop enumerates the suits, from 0 to 3. For each suit, the inner loop enumerates the ranks, from 1 to 13. Since the outer loop iterates 4 times, and the inner loop iterates 13 times, the total number of times the body is executed is 52 (13 times 4).

I used the variable index to keep track of where in the deck the next card should go. The following state diagram shows what the deck looks like after the first two cards have been allocated:

As an exercise, encapsulate this deck-building code in a method called buildDeck that takes no parameters and that returns a fully-populated array of Cards.

Last Update: 2011-01-24