ArrayList是空的，但为什么呢？

Question

I'm trying to add Cards to ArrayList deck, but it doesn't seem to work(most of the code is an example on oracle.com ). I'm probably doing something really stupid, but I can't seem to find it.. this is the code:

public class Card {

    public enum Rank 
    {
        DEUCE, THREE, FOUR, FIVE, SIX, SEVEN, EIGHT, NINE,
        TEN, JACK, QUEEN, KING, ACE
    }

    public enum Suit
    {
        HEARTS, DIAMONDS, SPADES, CLUBS
    }

    private final Rank rank;
    private final Suit suit;
    private static final List<Card> deck = new ArrayList<Card>();

    public Card(Rank rank, Suit suit)
    {
        this.suit = suit;
        this.rank = rank;
    }
    // initializes deck
    public void initDeck()
    {
        for (Suit suit : Suit.values())
        {
            for (Rank rank : Rank.values())
            {
                deck.add(new Card(rank, suit));
            }
        }
    }
    // returns a copy of the deck    
    public static ArrayList<Card> newDeck()
    {
        return new ArrayList<Card>(deck);
    }

    public Rank getRank()
    {
        return rank;
    }

    public Suit getSuit()
    {
        return suit;
    }

    public String toString()
    {
        return rank +" of "+ suit;
    }

    public static void main(String[] args)
    {
        System.out.println(deck.toString());
    }

}

Answer 1

Your problem is that you never actually invoke initDeck, so the deck remains empty, as it was when the static initializer ran:

private static final List<Card> deck = new ArrayList<Card>();

Other problems:

1. initDeck() is an instance method, but deck is a static reference.
2. initDeck() is a method on Card.
3. deck is a static member of Card , but a card does not own or define a deck.
4. newDeck is nonsensical in the face of a static final deck.
5. Cards should not be delivering decks anyway.

In short, your design is messed up - you need to think more and harder about the entities and their inter-relationships.

Answer 2

it is very suspicious that your deck is private static final and your init method public and non-static . It can be initialized several times and contain more cards then you would expect. You may want to do the initialization in static block instead.

I would replace the code as follows:

1. make the deck public static final and unmodifiable (via Collections)
2. copy the body of your init method in static initializer like this:

 static { for (Suit suit : Suit.values()) { for (Rank rank : Rank.values()) { deck.add(new Card(rank, suit)); } } } 

1. delete initDeck completely

Answer 3

It's simple, in your main method, you're just toString() an empty ArrayList . You haven't called initDeck() method in your main method at all.

When you ran your program private static final List<Card> deck = new ArrayList<Card>(); , deck was assigned an empty ArrayList .

Answer 4

It's because initDeck() isn't called anywhere. Try to replace public void initDeck() with static .

Answer 5

The keyword final in you List<card> definition is for object that will have an starting value that cannot be changed.

Check it here