Question

import java.lang.*;

public class Card
{
   String face, suit;
   int value;


   public Card (String a, String b)
   {
      face = a;
      suit = b;
      faceValue(face);
   }

   public void faceValue(String abc)
   {
      if (abc.equals("Jack") || abc.equals("Queen") || abc.equals("King"))
      {
         value = 10;
      }
      else if (abc.equals("Ace"))
      {
         value = 11;
      }
      else
      {
          value = Integer.parseInt(abc);
      }
   }

   public String toString()
   {
      return face + " of " + suit + "\nValue of: " + value;
   }

   public int getValue()
   {
      return value;
   }

   public static void main(String [] args)
   {

      Deck deck = new Deck();
      deck.fillDeck();
      System.out.println(deck.toString());
   }
}
class Deck
{
   Card [] cards;
   String [] faces = {"1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "Jack", "Queen", "King",    "Ace"};
   int  counter = 0;
   String deckList;

   public Deck()
   {
      cards = new Card[52];
   }

   public void fillDeck()
   {
      for (int i = 0; i <= 12; i++)
      {
         cards[i] = new Card(faces[counter], "Hearts");
         counter++;
      }

     counter = 0;

      for (int i = 0; i >= 13 && i <= 25; i++)
      {
         cards[i] = new Card(faces[counter], "Diamonds");
         counter++;
      }

     counter = 0;

      for (int i = 0; i >= 26 && i <= 38; i++)
      {
         cards[i] = new Card(faces[counter], "Clubs");
     counter++;
      }

     counter = 0;

      for (int i = 0; i >= 39 && i <= 51; i++)
      {
         cards[i] = new Card(faces[counter], "Spades");
     counter++;
      }

   }

   public String toString()
   {
      for (int i = 0; i <= 51; i++)
      {
         deckList = cards[i].getValue() + " ";
      }
      return deckList;
   }

}

这是我得到的错误，我不确定它是什么。之前的parseInt（）也存在一些问题，也许它与之有关？这个课程旨在用于黑色杰克游戏，但我似乎无法通过此错误，任何帮助表示赞赏。

Exception in thread "main" java.lang.NullPointerException
    at Deck.toString(Card.java:100)
    at Card.main(Card.java:47)

Answer 1

问题是Deck没有初始化，因为For loop不正确。

您的循环就像在filDeck方法中一样：

for (int i = 0; i >= 39 && i <= 51; i++)
      {
         cards[i] = new Card(faces[counter], "Spades");
     counter++;
      }

但是当你初始化i = 0时，这永远不会运行。所以i的第一个值不符合条件i >= 39 && i <= 51，循环就会中断。

我宁愿把它作为循环：

for (int i = 51; i >= 39; i--)
          {
             cards[i] = new Card(faces[counter], "Spades");
         counter++;
          }

Answer 2

每个for循环都以i = 0开头。你需要将它改为条件的下限，否则for循环不会运行。

ex：让我们逐步完成最后一次

for(int i=0; i >=39 && i <=51; i++)

i = 0，＆gt; = 39＆amp;＆amp;我＆lt; = 51？不，不要输入循环

将其更改为：

(int i = 39; i >= 39 && i <= 51; i++)

Card和Deck类的NullPointerException错误

2 个答案: