java - 引用 toString 时的 NPE

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 9 年前。

Error: Exception in thread "main" java.lang.NullPointerException
at Deck.toString(Deck.java:83)
at DeckDriver.main(DeckDriver.java:25)

我完全不明白为什么会收到此错误消息。 d.toString应该显示描述 Deck d 的 52 行代码。

甲板等级

import java.util.Random;
public class Deck
{
    private Card[] deck;
    private int nextCard;
    Face face;
    Suit suit;


    /**
     * Default Constructor        
     *
     * <hr>
     * Date created: Feb 17, 2014 
     *
     * 
     */
    public Deck()
    {
        nextCard = 0;
        deck = new Card[52];
        int iCount;
        for(iCount=0; iCount<52; iCount++)
        {
            Card c = new Card(iCount);
        }
    }


    /**
     * Copy Constructor        
     *
     * <hr>
     * Date created: Feb 17, 2014 
     *
     * 
     * @param existingDeck
     */
    public Deck(Deck existingDeck)
    {
        int i;
        for(i=0;i<52;i++)
        {
            this.deck[i] = existingDeck.deck[i];
        }
    }

    /**
     * toString         
     *
     * <hr>
     * Date created: Feb 17, 2014 
     *
     * <hr>
     * @return
     * @see java.lang.Object#toString()
     */
    public String toString()
    {
        int iCount = 0;
        String description = "";
        for(iCount=0; iCount<52;iCount++)
        {
            description += deck[iCount].toString();
        }
        return description;
    }

    /**
     * Shuffles the deck       
     *
     * <hr>
     * Date created: Feb 17, 2014
     *
     * <hr>
     */
    public void shuffle()
    {
        Random r = new Random();
        nextCard = 0;
        int i;
        for(i=0;i<52;i++)
        {
            int x = r.nextInt(52);
            Card c = new Card();
            c=deck[x];
            deck[x]=deck[i];
            deck[i]=c;
        }
    }

    /**
     * Deals individual card.        
     *
     * <hr>
     * Date created: Feb 17, 2014
     *
     * <hr>
     * @return
     */
    public Card dealACard()
    {
        Card c;
        c=deck[nextCard];
        nextCard++;

        return c;
    }

    public String dealAHand(int handSize)
    {
        int i;
        String hand="";
        for(i=0;i==handSize;i++)
        {
            hand+="" + dealACard();
        }
        return hand;
    }
}

DeckDriver 类

public class DeckDriver
{
    public static void main(String[]args)
    {
        Deck d = new Deck();
        System.out.print(d.toString()); //(this is the DeckDriver.main(DeckDriver.java:25))
    }
}

Answer 1

默认构造函数中没有添加任何对象到数组中。您应该像在其他构造函数中一样初始化数组的每个字段。