从ArrayList中抓取随机数

Question

我有一个存储52个卡对象的ArrayList，如下所示：

 public class Pack

{
   private ArrayList<Card> cards;
   private Card RandomCard;


   public static void main(String[] args) {
    ArrayList<Card> cards = new ArrayList<Card>();

    cards.add(new Card('C','A'));
    cards.add(new Card('C','2'));
    cards.add(new Card('C','3'));

等。

我也有这个方法生成一个随机数来从我的ArrayList中抓取一个随机对象。

public Card getRandomCard()

{
   int number = (int) (Math.random() * 52.0);
   return RandomCard;
}

这编译但是当我测试它时，我得到'null'。我必须包括这种方法！ 有什么建议？

Answer 1

尝试

return cards.get(number);

而不是

return RandomCard;

最后一个语句返回RandomCard var，它没有在任何地方分配。

Answer 2

试试这个：

public Card getRandomCard() {
    return cards.get((int) (Math.random() * cards.size()));
}

替代使用随机：

public Card getRandomCard() {
    Random gen = new Random(); // Make this an instance variable instead.
    return cards.get(gen.nextInt(cards.size()));
}

---

修改

您的原始代码尝试在main方法（第9行）中初始化CardsList of Cards，这是与PackList中的ArrayList完全不同的ArrayList。这意味着Pack中的ArrayList（第4行）保持为null，这解释了下面描述的NullPointerException。用以下代码替换您的代码：

public class Pack {

    private ArrayList<Card> cards;
    private Random random;

    public Pack() {
        random = new Random();
        cards = new ArrayList<Card>();

        cards.add(new Card('C','A'));
        cards.add(new Card('C','2'));
        cards.add(new Card('C','3'));
        // And so on..
    }

    public Card getRandomCard() {
        return cards.get(random.nextInt(cards.size()));
    }

    public static void main(String[] args) {
        Pack pack = new Pack();
        Card randomCard1 = pack.getRandomCard(); // Here is your random card.
        Card randomCard2 = pack.getRandomCard(); // Here is another random card.
    }
}