我对这个程序有问题

Question

我遇到了这个程序的问题。我认为问题出在我的回复声明上。我可以让程序运行起来，直到确定胜利者然后崩溃。我只是一个初学者，任何帮助将不胜感激。

这里是我的RPS类

/* 
* To change this license header, choose License Headers in Project Properties. 
* To change this template file, choose Tools | Templates 
* and open the template in the editor. 
*/ 
package game; 

import java.util.Random; 
import java.util.Scanner; 

/** 
* 
* @author Owner 
*/ 
public class rPs { 
    Scanner in = new Scanner(System.in); 
    Random random = new Random(); 
    String userChoice = ""; 
    String cpuChoice; 
    String pickWinner; 
    String winner; 
    String tie; 
    String user; 
    String computer; 
    int compRand = 0; 

    //Get user choice 
    public String getUserChoice(){ 
    System.out.println("Rock, Paper, or Scissors?:"); 
    userChoice = in.next(); 

    //Get user choice 
    while(!"rock".equals(userChoice)&& !"paper".equals(userChoice) && 
     !userChoice.equals("scissors")) 
     { 
      System.out.println("Sorry, " + userChoice + " is not a valid entry"); 
      System.out.println("Rock, Paper, or Scissors?:"); 
      userChoice = in.next(); 
     }//end while for r,p,s check 
     return userChoice; 
    }//End Get user 

    //Get the cpu Choice!!!! 
    public String getCPUChoice(){ 


     compRand = random.nextInt(3); 

     if (compRand == 1) { 
      cpuChoice = "Rock"; 
     } 
     if (compRand == 2){ 
      cpuChoice = "Paper"; 
     } 
     else if (compRand == 3){ 
      cpuChoice = "Scissors"; 
     } 
     return cpuChoice; 
    }//End Get cpu choice 

    //Pick a winner by passing 2 strings together 
    public String pickWinner(String userChoice,String cpuChoice){ 

     //need a loop for ties 
     if (userChoice.equalsIgnoreCase(cpuChoice)) { 
       return winner = tie; 

     }//end tie if... 

     if (userChoice.equalsIgnoreCase("Rock")&& cpuChoice.equalsIgnoreCase 
       ("scissors")) 
     { 
      System.out.println("Computer chooses Scissors. You win!"); 
       return winner = user; 
     }//end if 1  
      if (userChoice.equalsIgnoreCase("Rock")&&cpuChoice.equalsIgnoreCase("Paper")){ 

       return winner = computer;  
     }//end if 2 


      if (userChoice.equalsIgnoreCase("Paper") && cpuChoice.equals 
       ("Scissors")) { 
       System.out.println("Computer chooses Scissors. You lose."); 
       return winner = computer; 
     }//end if 3 
      if (userChoice.equalsIgnoreCase("Paper") &&cpuChoice.equalsIgnoreCase 
       ("Rock")){ 
       System.out.println("Computer chooses Rock. You win!"); 
       return winner = user; 
     }//edn if 4 


      if(userChoice.equalsIgnoreCase("Scissors")&& 
       cpuChoice.equalsIgnoreCase("Paper")) { 
       System.out.println("Computer chooses Scissors. You win!"); 
       return winner = user; 
     }//end if 5 
      if (userChoice.equalsIgnoreCase("Scissors")&& 
       cpuChoice.equalsIgnoreCase("Rock")){ 
       System.out.println("Computer chooses Rock. You lose!"); 
       return winner = user; 
     }//end if 6 

     return winner; 


    }//END pick winner 

}//End rPs Class 

********************************** ************************************ 8 这是由讲师创建的主要课程。

package game; 

import java.util.*; 

public class Game 
{ 

    public static void main (String[] args) 
    { 
     Scanner in = new Scanner (System.in); 
     rPs rps = new rPs(); //***Your class 

     int numGames = 0; 
     String userChoice = ""; 
     String cpuChoice = ""; 
     String winner = ""; 
     int userWins = 0; 
     int cpuWins = 0; 


     System.out.println("Welcome to Rock, Paper, Scissors!\n"); 

     //Get odd number of games 
     System.out.println("How many rounds would you like to play?"); 
     numGames = in.nextInt(); 

     while (numGames % 2 == 0) //Even number 
     { 
      System.out.println("Sorry, number of games must be odd. Please try again:"); 
      numGames = in.nextInt(); 
     } 

     //Flush the buffer 
     in.nextLine(); 

     //Play the game for the number of rounds the user entered 
     for (int i = 1; i <= numGames; i++) 
     { 
      //Get the user and computer choices 
      userChoice = rps.getUserChoice(); //***Your method 
      cpuChoice = rps.getCPUChoice(); //***Your method 

      System.out.println("Computer chooses " + cpuChoice); 

      //Pick winner 
      winner = rps.pickWinner(userChoice, cpuChoice); //***Your method 

      if (winner.equalsIgnoreCase("Tie")) 
      { 
       System.out.println("It's a tie! Play again."); 
       numGames++; 
      } 
      else 
      { 
       if (winner.equalsIgnoreCase("User")) 
       { 
        userWins++; 
       } 
       else if (winner.equalsIgnoreCase("Computer")) 
       { 
        cpuWins++; 
       } 
       else 
       { 
        System.out.println("Error in picking winner"); 
       } 

       System.out.println(winner + " wins!"); 
      } 

     } //end for 

     //Print results 
     System.out.println("\nUser wins: " + userWins); 
     System.out.println("Computer wins: " + cpuWins); 

     if (userWins > cpuWins) 
     { 
      System.out.println("\nThe user won!"); 
     } 
     if (cpuWins > userWins) 
     { 
      System.out.println("The computer won!"); 
     } 

     //Close game 
     System.out.println("\nThank you for playing!"); 

    } //end main 

} //end class 

Answer 1

根据你的main方法需要，你pickWinner方法返回值应该是“用户”，“计算机”或“铁”。

变化，如return winner = user;

到

return "User"; 

任何声明目前您的收益报表分配持有空引用另一个变量拿着一个空引用变量，因此它们等同于return null;。

因此，您的主要方法中的语句如winner.equalsIgnoreCase("Tie")会导致NullPointerException，因为winner为空。