在一些Java代码中,为什么Deck类会扩展Card类？

Question

我正在阅读一些我不熟悉的Java代码,但似乎很奇怪,如果一个类Deck(对于一副牌),它已经有一个数组的实例变量Cards,那么为什么要Deck扩展或继承Card呢？我认为只有当A是B时,A类才会继承B类(Cat因为猫是动物而继承动物).

代码如下:

public class Deck <T extends Card> {
    private ArrayList<T> cards;
    private int dealtIndex = 0; // marks first undealt card

    public Deck() {
    }

    public void setDeckOfCards(ArrayList<T> deckOfCards) {
        cards = deckOfCards;
    }

    public void shuffle() {
        for (int i = 0; i < cards.size(); i++) {

    [...]
}

public abstract class Card {
    private boolean available = true;

    /* number or face that's on card - a number 2 through 10, 
     * or 11 for Jack, 12 for Queen, 13 for King, or 1 for Ace 
     */
    protected int faceValue;
    protected Suit suit;

    public Card(int c, Suit s) {
        faceValue = c;
        suit = s;
    }

    public abstract int value();

    public Suit suit() { 
        return suit; 
    }

    [...]

}

Answer 1

 public class Deck <T extends Card> 

甲板不延长卡.

这是一个泛型类型注释,它表示Deck可以是T类型,其中T是Card的子类.

这与Java的集合类相同,其中List of String也不扩展String(但包含String实例).

这允许您编写如下代码:

Deck<PokerCard> pokerDeck = new Deck<PokerCard>();

PokerCard c = pokerDeck.getTopCard();   
// now you know it is a PokerCard, not just any old card

pokerDeck.insert(new MagicTheGatheringCard()); 
// get a compile error for wrong card type