为什么Java方法返回null？

Question

I am trying to debug why my Java method returns null. Here are the details.

We are making a simple card "game" and Im having trouble with external method call and creating a new object.. It is a deck of cards ..So 1 class for the cards, 1 class for deck and 1 for the Game

This is my class the code goes into

 public class Game
 {
  private InputReader reader;
  private Deck deck;
  private ArrayList<Card> listCard;

/**
 * Constructor for objects of class Game
 */
public Game()
{
    deck = new Deck();
    reader = new InputReader()
    listCard = new ArrayList<Card>();
}

/**
 *
 */
public void dealCard()
{
   listCard.add(deck.takeCard());
}

}// End of Game class

and this is the deck class which I will grabbing methods from

 import java.util.ArrayList;
 /**
  * Write a description of class Deck here.
  * 
  * @author  
  * @version 2012.05.31
  */
 public class Deck
{
private ArrayList<Card> redblue;

/**
 * Main constructor for objects of class Deck
 */
public Deck()
{
    redblue = new ArrayList<Card>();
}


   public Card takeCard()
{
      **return redblue.remove(0);**  /// this is the Index.pointer.exception
}



}
}// End of class

So my problem is im trying to pull the first card off the deck and add it to my "hand" ..So im trying to call dealCard() which calls takeCard()..The takeCard() works fine but when I try and call if through dealCard() it returns null and errors out because i cant add null to arrayList.. I think my problem might be external method calling not step up with the right variables, I dunno

Thanks in advance

***Edited. Removed irrelevant methods and Class..

Answer 1

Look at your constructor for Deck:

public Deck()
{
    redblue = new ArrayList<Card>();
}

After that constructor has run, how many cards are there in the redBlue ArrayList?

What do you think might happen if you try to remove a card from that ArrayList?

Answer 2

I think you need to post some more code from your main(game class) showing us the usage of your deck and card classes. Only thing that took my attention now is this:

public Card takeCard()
{
      **return redblue.remove(0);**  /// this is the Index.pointer.exception
}

According to this an exception can be thrown if:

IndexOutOfBoundsException - if index out of range (index < 0 || index >= size()).

Are you sure your deck is filled with cards? You could add a check to your method. If your list of cards is empty, 0 = it's size.

public void dealCard()
{
   Card card = deal.takeCard();

   if (card != null)
      listCard.add(deck.takeCard());
}

public Card takeCard()
{
    if ( !this.redblue.isEmpty() )
        return redblue.remove(0);

    return null;        
} 

Answer 3

Somehow the first element of your

redblue

is null. Nulls are allowed in an ArrayList.

My guess is that there is more code where you fill in the Deck of cards and the first time around you are adding null to the ArrayList

Check the ArrayList specification here.

Answer 4

It seems to me like redblue doesn't actually have anything in it! when you intialise it like so:

redblue = new ArrayList<Card>();

You're just creating an empty container for Cards, not actually putting cards in it. What you may want to do is create a function to generate the cards for redblue.. something like..

public ArrayList<Card> createInitialDeck(){ 
  //Create an empty ArrayList
  //Do some code here to create new cards..
       //you might want to consider nested for loops if you're creating 13 cards of 4 different suits for example
       //while inside those for loop add each new card object to your array list

   //return arrayList;
}

then instead of redblue = new ArrayList<Card>(); you'd do redblue = createInitialDeck(); .