Thread: nullpointerexception error

I'm writing a java video poker game. When I run the test file, I get a nullpointexception error. It points to this line in my shuffling method

 theDeck[i] = theDeck[j];

I'm still a novice in java...

 import java.util.Random;
public class Deck {
 
private Card[] theDeck; 
private int top; 
// add more instance variables if needed
 
public Deck(){
    top = 0;
    Card[] theDeck = new Card[52];
    for(int s = 1; s <= 4; s++)
    {
        for (int v = 1; v <= 13; v++)
        {
            for (int i = 0; i < theDeck.length; i++)
            {
 
                theDeck[i] = new Card(s,v);
            }
        }
    }   
}
 
public void shuffle()
{
    // shuffle the deck here
    Random generator = new Random();
    int i;
    int j;
    i = generator.nextInt(51) + 1;
    j = generator.nextInt(51) + 1;
    Card temp = theDeck[i];
    for(int k = 1; k <100; k++)
    {
        theDeck[i] = theDeck[j];
        theDeck[j] = temp;
    }
    top = 0;
}

this is the part of the class that calls the shuffle method:

 public void play()
{
    cards.shuffle();
    for( int i =1; i <= 5; i++)
    {
        p.addCard(cards.deal());
        cards.incrementTop();
    }
    for( int j = 1; j < 5; j++)
    {
        p.getHand().get(j).toString();
    }   
    System.out.println("These are your cards:");
    for( Card c : p.getHand())
    {

you have this instance private Card[] theDeck;
that statement is similar to private Card[] theDeck = null; a null pointer.
array in java is an Object, and if not equate to array object or make a new object, it would be null. and when you attempt to read of write in an object that points to null, it will make a NullPointerException.

it throws a NullPointerException in your method shuffle(), since it attempted to write in that array

I fixed this error, and now I get another one in these two classes:

 import java.util.Collections;
public class Player 
{
 
 
	private ArrayList<Card> hand; // the player's cards
	// you will likely need more instance variables
 
	public Player()
	{		
		ArrayList<Card> hand = new ArrayList<Card>();
	}
 
	public void addCard(Card c)
	{
		hand.add(c);
		// add the card c to the player's hand
	}
 
	public void removeCard(Card c)
	{
		hand.remove(c);
		// remove the card c from the player's hand
	}
 
	public ArrayList<Card> getHand()
	{
		return hand;
	}
 
	public void sortHand()
	{
		Collections.sort(hand);
	}	
	//public void printHand()
	//{
 
	// you will likely need more methods here
	//DB such as methods for analysing the hand 7
	//and the sorting method.
}

import java.util.Random;
public class Deck {
 
	private Card[] theDeck; 
	private int top; 
	// add more instance variables if needed
 
	public Deck(){
		theDeck = new Card[52];
		for(int s = 1; s <= 4; s++)
		{
			for (int v = 1; v <= 13; v++)
			{
				for (int i = 0; i < theDeck.length; i++)
				{
 
					theDeck[i] = new Card(s,v);
				}
			}
		}	
	}
 
	public void shuffle()
	{
		// shuffle the deck here
		Random generator = new Random();
		int i;
		int j;
		for(int k = 1; k <100; k++)
		{
			i = generator.nextInt(51) + 1;
			j = generator.nextInt(51) + 1;
			Card temp = theDeck[i];
			theDeck[i] = theDeck[j];
			theDeck[j] = temp;
		}
		top = 0;
	}
 
	public Card deal()
	{
		// deal the top card in the deck
		return theDeck[top];
	}	
 
	public void incrementTop()
	{
		top = top + 1;
	}	
 
 
 
 
 
 
	// add more methods here if needed
	//DB getter methods here?
 
}

this is the error I get:

 $ javac Deck.java
$ java PokerTest
Exception in thread "main" java.lang.NullPointerException
        at Player.addCard(Player.java:17)
        at Game.play(Game.java:71)
        at PokerTest.main(PokerTest.java:9)

I believe that the error is in you main method. in PokerTest.java

Any idea why the addCard() method returns a null?

because it points to a null. instance hand and local variable hand in your constructor are not the same. please check

Hmm I doubt it; I think the error must be in the deck class. All I have in the tester class is this:

 Game g = new Game();
                        g.play();

--- Update ---

You're right! Thanks!

believe me that is the error.

public class Player {
 
      private ArrayList<Card> hand; // points to null
 
      public Player {
            ArrayList<Card> hand = new ArrayList<Card>(); // instance variable still points to null
            // hand here is a local variable
      }
 
      public void addCard(Card card) {
            hand.add(card);
      }
}