公牛和牛游戏程序Java。运行程序时空指针异常错误

Question

我正在为游戏公牛和牛制作代码（http://en.wikipedia.org/wiki/Bulls_and_cows） 我的代码编译了，但是当我尝试在终端中运行它时，我得到了一个空指针异常错误，用于howManyBylls， playOneTurn和playGame。我找不到这个错误来自哪里。此外，该版本的游戏与其他公牛和牛的游戏不同，用户可以输入四位数的重复数字。我认为在大多数其他代码中，他们应该输入具有唯一数字的四位数字。

public int howManyBulls(String guess) 
    { 
    input = guess; 
    bulls = 0; 
    for (int i = 0; i<4; i++) 
    { 
     int k = Integer.parseInt(pattern.substring(i, i+1)); 
     int l = Integer.parseInt(input.substring(i, i+1)); 
     if (k == l) // checking for same value at same location 
     { 


      bulls++; // add one to bull if there is a match 
     } 
    } 
    return bulls; 
    } 


public class Game{ 
    private int turns; 
    private Oracle computer; 
    private Scanner input; 
    public String userInput; 
    public int bulls; 
    public int cows; 

    public Game(){ 
    // creates new data type Oracle 
    computer = new Oracle(); 
    turns = 0; 
    input = new Scanner(System.in); 
    } 

    public void playGame(){ 
    // your code for the Game playGame method goes here 
    System.out.print("Please enter a 4 digit number: "); 
    userInput=input.next(); 
    playOneTurn(); 
    } 

    // plays a turn 
    public void playOneTurn(){ 
    turns++; 

    // passes userInput into methods of howManyBulls() and howManyCows() 
    bulls = computer.howManyBulls(userInput); 
    cows = computer.howManyCows(userInput); 

    System.out.println(bulls+ " bulls"); 
    System.out.println(cows+ " cows"); 

    // checks if game is over 
    if (bulls < 4) // if bulls less than four then continue playing game 
     // if bulls = 4 then the number is correct 
    { 
     playGame(); 
    } else { // done with game and print out number of turns 
     System.out.print("It took" + turns + " turns to guess " 
       + "the correct number"); 
    } 
    } 


    public void setPattern(String solution){ 
     computer.setPattern(solution); 
    } 

    public String getPattern(){ 
     return computer.getPattern(); 
    } 
} 

Answer 1

它出现在howManyBulls（）尚未初始化或已被设置为null的变量“pattern”中。它没有在您发布的代码中定义，并且是唯一尚未初始化的变量。空指针异常通常是通过调用空对象上的方法引起的，如果变量尚未初始化，则可能发生这种情况。