如何解決java.lang.NullPointerException？

Question

這是我爲蛇和階梯寫的代碼。請幫助我解決例外問題。例外情況在第72和114行。我已經突出顯示了導致異常的評論。感謝提前尋求幫助。

package practical1; 

import java.io.Console; 
import java.util.ArrayList; 
import java.util.Scanner; 

public class snakeandladder1 { 

int totalpos=100; 
int v=0; 
int[] scores; 
int score=0; 
int l=0; 

public int throwdice() 
{ 
    int i=0; 
    { 
     while(i==0) 
     { 
      i=(int)Math.random()*100; 
      i=i%13; 
     } 
     return i; 
    } 
} 

public int ladder(int score) 
{ 
    if(score==15) 
     score=30; 
    else if(score == 45) 
     score=71; 
    else if(score == 25) 
     score=62; 
    else if(score == 81) 
     score=91; 
    else if(score == 9) 
     score=39; 
    scores[l]=score; 
    return score; 
} 

public int snake(int score) 
{ 
    if(score == 29) 
     score=11; 
    else if(score == 81) 
     score=48; 
    else if(score == 92) 
     score=71; 
    else if(score == 30) 
     score=6; 
    else if(score == 58) 
     score=19; 
    scores[l]=score; 
    return score; 
} 

void start() 
{ 
    System.out.println("Enter the number of players:"); 
    Scanner in=new Scanner(System.in); 
    int n=in.nextInt(); 
    in.close(); 
    l=n; 
    System.out.println("Enter Players names in order:"); 
    ArrayList<String> name1=new ArrayList<String>(); 
    for (int i=0;i<n;i++) 
    { 
     Console in1=System.console(); 
     String name2=in1.readLine(); //---- I am getting this null pointer exception here. 
     name1.add(name2); 
    } 
    while(true) 
    { 
     while(l>0) 
     { 
      System.out.println("Click y to roll dice %d"+(l+1)); 
      Console in2=System.console(); 
      String yes=in2.readLine(); 
      if (yes == "y") 
      { 
       v=throwdice(); 
      } 
      score = scores[l]+v; ((//--- the null pointer exception after changing all user input   to scanner)) 
      if(score==totalpos) 
      { 
       System.out.println("User:"+name1.get(l)+" got "+v+".Winner!!!"); 
       break; 
      } 
      else if(score > totalpos) 
      { 
       scores[l]=scores[l]; 
      } 
      int s1; 
      s1=ladder(score); 
      if(s1==score) 
      { 
       snake(score); 
      } 
      System.out.println("Current score of"+name1.get(l)+"is:"+scores[l]); 
      l--; 
     } 
     if(l==0) 
     { 
      l=n; 
     } 
    } 
} 
public static void main(String[] args) throws Exception{ 
    // TODO Auto-generated method stub 
    snakeandladder1 sal=new snakeandladder1(); ((//null pointer exception here as well after the changes)) 
    sal.start(); //------ i am getting this null pointer exception here 
} 

}

Answer 1

Scanner in=new Scanner(System.in); 
int n=in.nextInt(); 
in.close(); 

請未在上面的代碼close()方法將關閉不僅Scanner對象本身而且其強調流（即鍵盤輸入）。這發生在System.in和實現Readable的所有其他來源。

從the docs：

當掃描器被關閉時，它會如果源 實現可關閉接口關閉其輸入源。

我的建議：

• 刪除in.close();
• 使用name2 = in.readLine();