3.2: Card toString

When you create a new class, the first step is to declare the instance variables and write constructors. A good next step is to write toString, which is useful for debugging and incremental development.

To display Card objects in a way that humans can read easily, we need to map the integer codes onto words. A natural way to do that is with an array of Strings. We can create the array like this:

String[] suits = new String[4];

And then assign values to the elements:

suits[0] = "Clubs";
suits[1] = "Diamonds";
suits[2] = "Hearts";
suits[3] = "Spades";

Or we can create the array and initialize the elements at the same time, as we saw in Section 8.3:

String[] suits = {"Clubs", "Diamonds", "Hearts", "Spades"};

The state diagram in Figure 12.2.1 shows the result. Each element of the array is a reference to a String.

Figure \(\PageIndex{1}\): State diagram of an array of strings.

Now we need an array to decode the ranks:

String[] ranks = {null, "Ace", "2", "3", "4", "5", "6",
           "7", "8", "9", "10", "Jack", "Queen", "King"};

The zeroth element should never be used, because the only valid ranks are 1–13. We set it to null to indicate an unused element.

Using these arrays, we can create a meaningful String using suit and rank as indexes.

String s = ranks[card.rank] + " of " + suits[card.suit];

The expression suits[card.suit] means “use the instance variable suit from the object card as an index into the array suits.”

Now we can wrap all that in a toString method.

public String toString() {
    String[] ranks = {null, "Ace", "2", "3", "4", "5", "6",
               "7", "8", "9", "10", "Jack", "Queen", "King"};
    String[] suits = {"Clubs", "Diamonds", "Hearts", "Spades"};
    String s = ranks[this.rank] + " of " + suits[this.suit];
    return s;
}

When we display a card, println automatically calls toString:

Card card = new Card(11, 1);
System.out.println(card);

The output is Jack of Diamonds.