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3.2: Card toString

  • Page ID
    15285
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    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.

    State diagram of an array of strings.
    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.


    3.2: Card toString is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Allen B. Downey (Green Tea Press) .

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