java - 线程 "main"java.lang.NullPointerException : program compiles fine, 中出现异常，但运行时显示错误

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 3 年前。

public void traverseRowLR(String [] list, String [] key, String pool)
   { 
      pool = "left to right";
      ArrayList<Location> obj = new ArrayList<Location>();
      for(int i = 0; i<key.length; i++)
      {
         for(int a=0;a<list.length;a++)
            if(list[a].indexOf(key[i])>-1)
               finalList.add(new Location(key[i],a+1,list[a].indexOf(key[i]),pool));        
      }

   }

上面就是方法了。错误出现在第二个 for 循环的行上。

这里提到了错误的另一个地方:

 public MazeAH(String [] key, String [][] wordBank)
   {
      maze = wordBank;
      answer = key; 
   }

 //other methods 

   public ArrayList<Location> solve()
   {  
      mergeRow(maze, rowWords);
      mergeCol(maze, colWords);   
      traverseRowLR(rowWords, answer, horizontal);
      traverseRowRL(rowWords, answer, horizontal);
      traverseRowTB(colWords, answer, vertical);
      traverseRowBT(colWords, answer, vertical);

      return finalList;
   }      

错误所在行:

traverseRowLR(rowWords, answer, horizontal);

最后，这是驱动程序，抱歉，如果我发布了太多代码，我不确定需要什么:该程序的目标是将行和列合并到字符串中，然后在“关键”词中搜索单词银行。我最初使用非 void 方法完成了这个项目，但我的教授希望我使用 void 方法重做它。

import java.util.*;

public class SolveWordSearchAH
{

   public static void main(String [] args)
   {

      String [][] letters = {{"e","p","l","i","r","q","k","o","s","a"},
                        {"u","k","i","e","s","y","x","c","n","f"},
                        {"c","p","o","z","g","f","q","a","y","r"},
                        {"p","e","m","c","z","a","n","t","e","i"},
                        {"g","u","g","q","t","a","b","r","h","e"},
                        {"g","i","q","v","b","e","d","b","s","s"},
                        {"c","o","f","f","e","e","i","l","r","q"},
                        {"h","r","e","t","a","w","b","d","e","l"},
                        {"y","q","o","k","w","v","p","x","h","z"},
                        {"c","d","n","t","f","q","k","b","p","w"}};

      String [] key = {"coffee", "water", "taco", "hershey", "fries"};
      //can have another key array if needed-depends on your implementation

      MazeAH m=new MazeAH(key, letters);
      ArrayList<Location> answer=m.solve();
      System.out.println(answer);

   }
}

我的变量在这里声明:

private ArrayList<Location> finalList = new ArrayList<Location>();
   private String [][] maze;
   private String [] answer;
   private String vertical="";
   private String horizontal = ""; 
   private String [] rowWords;
   private String [] colWords;

这是编译日志中的错误:

Exception in thread "main" java.lang.NullPointerException
    at MazeAH.mergeRow(MazeAH.java:117)
    at MazeAH.solve(MazeAH.java:26)
    at SolveWordSearchAH.main(SolveWordSearchAH.java:24)

这里合并行:

 public void mergeRow(String[][] letters, String [] words)
   {
      String []temp = new String [letters.length];
      for(int i=0; i<letters.length; i++)
      {
         temp[i]="";
         for(int a=0; a<letters[0].length; a++)
         {
            temp[i]+=letters[i][a];
         }
      }
      for(int s = 0; s<temp.length; s++)
      {
      temp[s] = words[s];
      }

   }

这是更新后的跟踪:

Exception in thread "main" java.lang.NullPointerException
    at MazeAH.mergeRow(MazeAH.java:117)
    at MazeAH.solve(MazeAH.java:26)
    at SolveWordSearchAH.main(SolveWordSearchAH.java:24)

Answer 1

您声明 rowWords 和 colWords 但从未向它们分配任何内容。这意味着它们将保留默认值 null，并且任何访问数组元素的尝试都将失败。

我不知道您需要在 rowWords 和 colWords 中存储多少个元素，但如果 100 个就足够了，您可以使用以下方法初始化它们:

 public MazeAH(String [] key, String [][] wordBank)
   {
      maze = wordBank;
      answer = key; 
      rowWords = new String[100];
      colWords = new String[100];
   }