Java方法变量传递问题：牌组抽牌程序重复抽牌故障排查

问题描述 #

我实现了一个从牌组随机抽取卡牌并防止重复抽取的类方法，但该方法失效，原因是迭代值无法在方法调用间持续传递。请问是否可以避免方法中的变量被销毁？以下是相关代码：

Deck类代码 #

public class Deck
{
   // Array for the deck of cards
   private String[] DeckOfCards =
   { " 2 of spades ", " 3 of spades ", " 4 of spades ", " 5 of spades ", " 6 of spades ", " 7 of spades         ", " 8 of spades ", " 9 of spades ", " 10 of spades ", " Jack of spades ", " Queen of spades ",
     " King of Spades ", " Ace of spades ", " 2 of clubs ", " 3 of clubs ", " 4 of clubs ", " 5 of        
     clubs ", " 6 of clubs ", " 7 of clubs ", " 8 of clubs ", " 9 of clubs ", " 10 of clubs ", 
     " Jack of clubs ", " Queen of clubs ", " King of Clubs ", " Ace of clubs ", " 2 of diamonds ",
     " 3 of diamonds ", " 4 of diamonds ", " 5 of diamonds ", " 6 of diamonds ", " 7 of diamonds ",
     " 8 of diamonds ", " 9 of diamonds ", " 10 of diamonds ", " Jack of diamonds ", 
     " Queen of     diamonds ", " King of Diamonds ", " Ace of diamonds ", " 2 of hearts ", 
     " 3 of hearts ", " 4 of hearts ", " 5 of hearts ", " 6 of hearts ", " 7 of hearts ",
     " 8 of hearts ", " 9 of hearts ", " 10 of hearts ", " Jack of hearts ", " Queen of hearts ",
     " King of Hearts ", " Ace of hearts " };

   // Declaring other variables
   int[] alreadyDrawnCards = new int[53];
   int drawnCard;
   int i = 0;
   int j = 0;

   /**
    *  method used to draw cards
    */
   protected int draw()
   {       

      drawnCard = (int)(Math.random() * 53);

      while(j < i)
      {
         if(drawnCard == alreadyDrawnCards[j])
         {

            drawnCard = (int)(Math.random() * 53);
            j = -1;         
         }

         j++;
      }

      if(i < 53) 
      {
         alreadyDrawnCards[i] = drawnCard; // add drawn card to drawn cards
         i++;
      }

      System.out.println("The" + DeckOfCards[drawnCard] + "was drawn from the deck");

      return drawnCard;
   }
}

调用代码 #

public class BlackjackGameSimulator
{

   public static void main(String[] args)
   {
      Deck BlackJackDeck = new Deck();
      BlackJackDeck.draw();

   }
}

问题分析与解决 #

核心问题 #

你误解了变量销毁的问题：类成员变量i、alreadyDrawnCards、j都会随着Deck实例的存在而保留，不会被销毁。真正导致方法失效的是**j作为类成员变量，每次调用draw()后没有重置为0**，导致后续抽牌时循环逻辑混乱。

比如第一次调用draw()后，j的值会走到i的位置（比如i=1时j=1），第二次调用时j从1开始，跳过了对alreadyDrawnCards[0]的检查，就可能抽到重复卡牌。

修复步骤 #

1. 将临时循环变量j移到方法内部
   把j的声明放到draw()方法里，每次调用时重新初始化为0，这样每次抽牌的循环都会从头检查已抽卡牌：

   protected int draw()
   {       
       int j = 0; // 移到方法内，每次调用重置
       drawnCard = (int)(Math.random() * 53);
   
       while(j < i)
       {
           if(drawnCard == alreadyDrawnCards[j])
           {
               drawnCard = (int)(Math.random() * 53);
               j = -1;         
           }
           j++;
       }
   
       if(i < 53) 
       {
           alreadyDrawnCards[i] = drawnCard;
           i++;
       }
   
       System.out.println("The" + DeckOfCards[drawnCard] + "was drawn from the deck");
       return drawnCard;
   }
   

2. 优化抽牌逻辑（更高效的方案）
   当前的重复检测方式效率很低，每次抽牌都要遍历已抽数组。更合理的方式是初始化时洗牌，然后按顺序发牌，完全避免重复检测：

   import java.util.Random;
   
   public class Deck
   {
       private String[] deckOfCards =
       { " 2 of spades ", " 3 of spades ", " 4 of spades ", " 5 of spades ", " 6 of spades ", " 7 of spades ", 
         " 8 of spades ", " 9 of spades ", " 10 of spades ", " Jack of spades ", " Queen of spades ",
         " King of Spades ", " Ace of spades ", " 2 of clubs ", " 3 of clubs ", " 4 of clubs ", " 5 of clubs ", 
         " 6 of clubs ", " 7 of clubs ", " 8 of clubs ", " 9 of clubs ", " 10 of clubs ", 
         " Jack of clubs ", " Queen of clubs ", " King of Clubs ", " Ace of clubs ", " 2 of diamonds ",
         " 3 of diamonds ", " 4 of diamonds ", " 5 of diamonds ", " 6 of diamonds ", " 7 of diamonds ",
         " 8 of diamonds ", " 9 of diamonds ", " 10 of diamonds ", " Jack of diamonds ", 
         " Queen of diamonds ", " King of Diamonds ", " Ace of diamonds ", " 2 of hearts ", 
         " 3 of hearts ", " 4 of hearts ", " 5 of hearts ", " 6 of hearts ", " 7 of hearts ",
         " 8 of hearts ", " 9 of hearts ", " 10 of hearts ", " Jack of hearts ", " Queen of hearts ",
         " King of Hearts ", " Ace of hearts " };
   
       private int currentCardIndex = 0;
   
       // 构造方法里洗牌
       public Deck() {
           Random random = new Random();
           //  Fisher-Yates洗牌算法
           for (int k = deckOfCards.length - 1; k > 0; k--) {
               int swapIndex = random.nextInt(k + 1);
               String temp = deckOfCards[k];
               deckOfCards[k] = deckOfCards[swapIndex];
               deckOfCards[swapIndex] = temp;
           }
       }
   
       protected String draw() {
           if (currentCardIndex >= deckOfCards.length) {
               System.out.println("牌组已经抽完了");
               return null;
           }
           String drawnCard = deckOfCards[currentCardIndex++];
           System.out.println("抽到了：" + drawnCard);
           return drawnCard;
       }
   }
   

   这个方案不仅避免了重复问题，代码更简洁，效率也更高。

总结 #

• 类成员变量不会随方法调用销毁，你的问题是错误地将临时循环变量设为成员变量导致状态混乱。
• 把临时变量移到方法内部初始化即可解决原有逻辑问题；推荐使用洗牌法实现抽牌，更符合真实场景且高效。

内容的提问来源于stack exchange，提问作者Puppy_With_Pinecone