局部變量在聲明內部循環時無法解析，但在外部聲明時不能解析NPE

Question

基本上，我正在做的是通過一副紙牌進行耐心測試，以確定耐心用完的概率，主要方法僅打印出此數字，沒有用戶系統輸入。當第一張牌被抽出時，你說「1」，第二張牌你說「2」，第三張牌「3」，然後你從第四張牌重新開始。所以它是1-2-3-1-2-3-1 -...如果你在說1時畫一個王牌，當你說2時畫一個2，當你說3時畫一個3，那麼耐心就用完了。這是通過一個總共100次（可能我將它設置爲1000）52張牌。下面是兩個類卡（創建卡）和Carddeck（創建卡片組）以及主要方法d84的實現。

public class Card { 
public static final int SPADES = 1; 
public static final int HEARTS = SPADES + 1; 
public static final int DIAMONDS = SPADES + 2; 
public static final int CLUBS = SPADES + 3; 
private int rank; // value 
private int suit; // color 

/** Creates a card with color, suit (SPADES, HEARTS, DIAMONDS, CLUBS) 
    and value, rank (1-13) */ 
public Card(int suit, int rank) { 
    this.suit = suit; 
    this.rank = rank; 
} 

/** Gets suit */ 
public int getSuit() { 
    return suit; 
} 

/** Gets rank */ 
public int getRank() { 
    return rank; 
} 

/** Returns a readable representation of the card, e.g. "diamonds jack" */ 
public String toString(){ 
    String suitString = "";  
    switch(suit) { 
     case SPADES: suitString = "spades"; break; 
     case HEARTS: suitString = "hearts"; break; 
     case DIAMONDS: suitString = "diamonds"; break; 
     case CLUBS: suitString = "clubs"; break; 
    } 
    String rankString = ""; 
    switch(rank) { 
     case 1: rankString = "ace"; break; 
     case 11: rankString = "jack"; break; 
     case 12: rankString = "queen"; break; 
     case 13: rankString = "king"; break; 
     default: rankString = String.valueOf(rank); 
    } 
    return suitString + " " + rankString; 
} 
} 

-

import java.util.Random; 
public class CardDeck { 
private Card[] theCards; 
private int cardNbr = -1; 
public CardDeck() { 
    theCards = new Card[52]; 
    for (int suit = Card.SPADES; suit <= Card.CLUBS; suit++) { 
     for (int r = 0; r < 13; r++) { 
      theCards[r*suit] = new Card(suit, r+1); 
     } 
    } 
} 
public void shuffle() { 
    Random rand = new Random(); 
    for (int i = 0; i < 52; i++) { 
     int n = rand.nextInt(52); 
     theCards[i] = theCards[n]; 
    } 
} 
public boolean moreCards() { 
    return cardNbr < 51; 
} 
public Card getCard() { 
    cardNbr++; 
    return theCards[cardNbr]; 
}} 

-

public class d84 { 
public static void main(String[] args) { 
    int patBroke = 0; 
    for (int j = 0; j < 100; j++){ 
     CardDeck deck = new CardDeck(); 
     deck.shuffle(); 
     while (deck.moreCards() == true) { 
      int say = 1; 
      while (say <= 3) { 
       Card drawn = deck.getCard(); 
       if (say == drawn.getRank()) { 
        patBroke++; 
        break; 
       } 
      say++; 
      } 
      if (say == drawn.getRank()) { 
       break; 
      } 
     } 
    } 
    System.out.print("The probability of patience running out is " + (1-patBroke/100)); 
} 
} 

執行時，在線程 「主要」 java.lang.Error的異常：未解決的問題編譯：取用不能在d84.main解決（d84.java:17）。它被宣佈？當該子句被刪除時，編譯器在d84.java:11處指出相同的錯誤。如果Card被繪製在循環外部，那麼執行會在17或11處拋出NullPointerException。

Class Card在演講中給出，所以我不會在那裏更改任何代碼。另一方面，Carddeck是我自己的實現，主要方法d84也是我自己的代碼。

那麼，在這種情況下，我該如何避免'繪製無法解決'和NPE？

預先感謝您。

Answer 1

您的問題與循環本身無關，而是與{}子句中的變量範圍有關。變量不能在其聲明範圍之外被引用。

你的邏輯更改爲：

 int say = 1; 
     Card drawn = null; // Added 
     while (say <= 3) { 
      drawn = deck.getCard(); // Changed 
      if (say == drawn.getRank()) { 
       patBroke++; 
       break; 
      } 
      say++; 
     } 
     if (say == drawn.getRank()) { 
      break; 
     } 

（但也有極有可能其他問題）