NPE when referencing a toString

Question

Error: Exception in thread "main" java.lang.NullPointerException
at Deck.toString(Deck.java:83)
at DeckDriver.main(DeckDriver.java:25)

I'm completely lost as to why i'm getting this error message. The d.toString is supposed to display 52 lines of code describing the Deck d.

Deck class

import java.util.Random;
public class Deck
{
    private Card[] deck;
    private int nextCard;
    Face face;
    Suit suit;


    /**
     * Default Constructor        
     *
     * <hr>
     * Date created: Feb 17, 2014 
     *
     * 
     */
    public Deck()
    {
        nextCard = 0;
        deck = new Card[52];
        int iCount;
        for(iCount=0; iCount<52; iCount++)
        {
            Card c = new Card(iCount);
        }
    }


    /**
     * Copy Constructor        
     *
     * <hr>
     * Date created: Feb 17, 2014 
     *
     * 
     * @param existingDeck
     */
    public Deck(Deck existingDeck)
    {
        int i;
        for(i=0;i<52;i++)
        {
            this.deck[i] = existingDeck.deck[i];
        }
    }

    /**
     * toString         
     *
     * <hr>
     * Date created: Feb 17, 2014 
     *
     * <hr>
     * @return
     * @see java.lang.Object#toString()
     */
    public String toString()
    {
        int iCount = 0;
        String description = "";
        for(iCount=0; iCount<52;iCount++)
        {
            description += deck[iCount].toString();
        }
        return description;
    }

    /**
     * Shuffles the deck       
     *
     * <hr>
     * Date created: Feb 17, 2014
     *
     * <hr>
     */
    public void shuffle()
    {
        Random r = new Random();
        nextCard = 0;
        int i;
        for(i=0;i<52;i++)
        {
            int x = r.nextInt(52);
            Card c = new Card();
            c=deck[x];
            deck[x]=deck[i];
            deck[i]=c;
        }
    }

    /**
     * Deals individual card.        
     *
     * <hr>
     * Date created: Feb 17, 2014
     *
     * <hr>
     * @return
     */
    public Card dealACard()
    {
        Card c;
        c=deck[nextCard];
        nextCard++;

        return c;
    }

    public String dealAHand(int handSize)
    {
        int i;
        String hand="";
        for(i=0;i==handSize;i++)
        {
            hand+="" + dealACard();
        }
        return hand;
    }
}

DeckDriver class

public class DeckDriver
{
    public static void main(String[]args)
    {
        Deck d = new Deck();
        System.out.print(d.toString()); //(this is the DeckDriver.main(DeckDriver.java:25))
    }
}

By your current code, all the elements in your array are `null`... — Luiggi Mendoza, Feb 17 '14 at 20:51

score 1 · Answer 1 · answered Feb 17 '14 at 20:53

1

There is no object added to array in default constructor. You should initialize every field of the array the same way as you do in the other constructor.

answered Feb 17 '14 at 20:53

Grzegorz Żur

47,257
14
109
105

score 1 · Answer 2 · answered Feb 17 '14 at 20:53

These lines

for(iCount=0; iCount<52; iCount++)
{
    Card c = new Card(iCount);
}

don't have much effect. You're not storing the new Card object anywhere, so it will get thrown away by the garbage collection process. They should probably say something line this.

for(iCount=0; iCount<52; iCount++)
{
    deck[iCount] = new Card(iCount);
}

score 0 · Answer 3 · answered Feb 17 '14 at 20:52

Best way to prevent against NPE is to use Apache Commons Lang StringUtils.

In this case, it looks like StringUtils.defaultString(deck[iCount]) would be the way to go. If it's null, defaultString will return "" (an empty string).

http://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/StringUtils.html#defaultString%28java.lang.String%29

If you decide to not use StringUtils, the best practice is to always check for null before doing .toString().

score 0 · Answer 4 · answered Feb 17 '14 at 20:54

You're initializing your array correctly, but not assigning new Cards to it in your constructor:

public Deck()
{
    nextCard = 0;
    deck = new Card[52];
    int iCount;
    for(iCount=0; iCount<52; iCount++)
    {
        Card c = new Card(iCount);
    }
}

You instantiate each card ... then throw it away. This means every element in your deck array is null as that is the default value.

NPE when referencing a toString

4 Answers4