Question

I’m working on a solitaire counting program. I already have the main program working but when I try to implement my own class I get an NullPointerException error on line 19 (whenever it reaches c.getRank).

Note that I first created my main program while importing a class called CardDeck that has all the functions I need for it to work but now I’m supposed to create my own class that does the exact same thing. (Note that I don’t have access to the imported CardDeck class).

Here is the main code:

import se.lth.cs.ptdc.cardGames.Card;

public class Patiens {
public static void main(String[] args) {
    double good = 0;
    double bad = 0;
    double result = 0;

    for (int a = 0; a < 1000000; a++) {
        CardDeck deck = new CardDeck();
        deck.shuffle();
        double fail = 0;
        while (deck.moreCards()) {

            for (int i = 1; i <= 3 && deck.moreCards(); i++) {

                Card c = deck.getCard();

                if (i == 1 && c.getRank() == 1) {
                    fail++;
                }

                if (i == 2 && c.getRank() == 2) {
                    fail++;
                }

                if (i == 3 && c.getRank() == 3) {
                    fail++;
                }
            }
        }
        if (fail >= 1) {
            bad++;      
        }
        else{
            good++;
        }
    }
    System.out.println("Good: " + good + " Bad: " + bad);
    result = good / bad;
    System.out.println("Result= " + result);
}

}

What it does is count the probability that my deck will finish successfully:

It’s counting 1-2-3, 1-2-3 while at the same time drawing a card. Now IF the card happens to be an ACE when it counts “1” the current deck will fail. Same goes for a card of rank 2 while the program counts “2” etc. The probability that it will finish without failing once is 0.8% .

Here is the CardDeck class I’m creating:

import se.lth.cs.ptdc.cardGames.Card;

import java.util.Random;

public class CardDeck {
    private Card[] cards;
    private int current;
    private static Random rand = new Random();

    public CardDeck() {
        cards = new Card[52];
        for(int suit = Card.SPADES; suit <= Card.CLUBS; suit++) {
            for (int i = 0; i < 13; i++) {
                cards[i * suit] = new Card(suit, i);
            }
        }
        current = 0;
    }

    public void shuffle() {
        Card k;
        for(int i = 1000; i > 0; i--) {
            int nbr = rand.nextInt(52);
            int nbr2 = rand.nextInt(52);
            k = cards[nbr2];
            cards[nbr2] = cards[nbr];
            cards[nbr] = k;
        }
    }

    /**
     *Checks for more cards
     */
    public boolean moreCards() {
        if(current > 51) {
            return false;
        } else {
            return true;
        }
    }

    /**
     *Draws the card lying on top.
     */
    public Card getCard() {
        return cards[current++];

    }
}

Here is the import se.lth.cs.ptdc.cardGames.Card; If needed, It is the class that creates the cards.

package se.lth.cs.ptdc.cardGames;

public class Card {
    public static final int SPADES = 1;
    public static final int HEARTS = SPADES + 1;
    public static final int DIAMONDS = SPADES + 2;
    public static final int CLUBS = SPADES + 3;

    private int suit;
    private int rank;

    public Card(int suit, int rank) {
        this.suit = suit;
        this.rank = rank;
    }

    public int getSuit() {
        return suit;
    }

    public int getRank() {
        return rank;
    }
}

(Note that I’m not supposed to change the above class)

Answer 1

Your problem is here:

cards[i * suit] = new Card(suit, i);

If you change this to:

cards[i + ((suit - 1) * 13)] = new Card(suit, i);

It will do what you expect.

Two things to consider: firstly, arrays are zero-based, so your first card needs to be at index 0. Secondly, by multiplying by the suit, you will get multiples of that number, e.g.:

• SPADES: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13
• HEARTS: 2, 4, 6, 8, 10 …
• DIAMONDS: 3, 6, 9, 12 …
• CLUBS: 4, 8, 12, 16 …

So some elements will be filled more than once (12 is filled four times), and some (particular prime numbers > 13) elements (e.g. 23) will be null. In general, it’s probably enough to represent the index with another variable, like so:

int cardIndex = 0;
for (int suit = Card.SPADES; suit <= Card.CLUBS; suit++) {
    for (int i = 0; i < 13; i++) {
        cards[cardIndex++] = new Card(suit, i);
    }
}