Question

我正在使用jgrasp，我不确定为什么我会收到此错误。具体做法是：

Exception in thread "main" java.lang.NullPointerException
at java.io.File.<init>(File.java:277)
at MazeSolver.readMazeFile(MazeSolver.java:87)
at MazeSolver.main(MazeSolver.java:15)

我尝试重写扫描仪部件，但我不知道我在做什么。我会在行号中加上评论。这是我的代码：

public class MazeSolver {

   // The name of the file describing the maze
   static String mazefile;
   static int width;
   static int height;
   public static void main(String[] args) throws FileNotFoundException {
      if (handleArguments(args)) {

         readMazeFile(mazefile);  //line 15
         DrawMaze.draw();

         if (solveMaze())
            System.out.println("Solved!");
         else
            System.out.println("Maze has no solution.");
      }
      else {
         System.out.println("The arguments are invalid.");
      }
   }

   // Handle the input arguments
   static boolean handleArguments(String[] args) {
      if (args.length > 4 || args.length < 1) {
         System.out.println("There are too many or too few command line arguments");
         return false;
      }
      if (args.length == 1) {
         String mazefile = args[0];
         File file = new File(mazefile);
         if (!file.canRead()) {
            return false;
         }
         return true;
      }
      if (args.length == 2) {
         String mazefile = args[0];
         File file = new File(mazefile);
         if (!file.canRead()) {
            return false;
         }
         int cellsize = Integer.parseInt(args[1]);
         if (cellsize < 10) {
            return false;
         }
         return true;
      }
      if (args.length == 3) {
         String mazefile = args[0];
         File file = new File(mazefile);
         if (!file.canRead()) {
            return false;
         }
         int cellsize = Integer.parseInt(args[1]);
         int borderwidth = Integer.parseInt(args[2]);
         if (borderwidth < 5) {
            return false;
         }
         return true;
      }
      if (args.length == 4) {
         String mazefile = args[0];
         File file = new File(mazefile);
         if (!file.canRead()) {
            return false;
         }
         int cellsize = Integer.parseInt(args[1]);
         int borderwidth = Integer.parseInt(args[2]);
         int sleeptime = Integer.parseInt(args[3]);
         if (sleeptime < 0 || sleeptime > 10000) {
            return false;
         }
         return true;
      }   
      return false;
   }

   // Read the file describing the maze.
   static char[][] readMazeFile(String mazefile) throws FileNotFoundException {

      Scanner scanner = new Scanner(new File(mazefile)); //line 87
      height = scanner.nextInt();
      width = scanner.nextInt();
      int arrayHeight = 2  * height + 1;
      int arrayWidth = 2 * width + 1;
      char[][] mazeArrays = new char[arrayHeight][arrayWidth];
      while (scanner.hasNextLine()) {
         String line = scanner.nextLine();
         System.out.println(line);
         for (int row = 0; row < arrayHeight; row++) {
            for (int col = 0; col < arrayWidth; col++) {
               mazeArrays[row][col] = line.charAt(col);
            }
         }

      }
      return mazeArrays;
   }

   // Solve the maze.      
   static boolean solveMaze() {
      return true;
   }
}

Answer 1

问题出在handleArguments()这行（及其他副本）：

String mazefile = args[0];

此语句声明并为局部变量赋值，该阴影（隐藏）同名的字段。< / p>

来自Java语言规范，section 14.4.3：

如果声明为局部变量的名称已经声明为字段名称，那么该外部声明将在局部变量的整个范围内被遮蔽（第6.3.1节）。

但在main()方法中，您传递了字段 mazefile来创建文件：

readMazeFile(mazefile);

但字段mazefile仍然未分配 - 即null。

如果要解决问题，请将args[0]分配到字段：

mazefile = args[0];

线程＆＃34; main＆＃34;中的例外情况java.lang.NullPointerException但我分配给字段

1 个答案: