Java 2D游戏NullPointerException

我正尝试在Java中创建一个用户提示输入行宽和列长度的2D游戏。 A“世界”对象被创建，看起来像这样印刷时（其中P是玩家的角色）：Java 2D游戏NullPointerException

P--- 
---- 
---- 
----

然后，用户可以输入上，下，左，右或退出应围绕移动P和如果移动将角色从地图上移开，则什么也不做。我的问题是在我进入的宽度和长度，我得到以下错误：

java.lang.NullPointerException 
at World.displayWorld(Driver.java:90) 
at Driver.main(Driver.java:28)

我认为，这意味着Java是看到worldDimensions.length为空/空？我认为当世界对象被创建时，我正在为它赋值。任何指导都非常感谢！

这里是我的代码：

import java.util.Scanner; 

public class Driver { 
    public static void main(String args[]) { 

     //Create a scanner object 
     Scanner input = new Scanner(System. in); 

     boolean exit = true; 

     //Prompt user to enter world width and height 
     System.out.print("World width: "); 
     int userWidth = input.nextInt(); 
     System.out.print("World height: "); 
     int userHeight = input.nextInt(); 

     //Create a world object 
     World world1 = new World(userWidth, userHeight); 
     world1.displayWorld(); 

     System.out.println("Possible inputs: up, down, left, right, exit"); 

     while (exit = true) { 
      //display world 
      world1.displayWorld(); 

      System.out.print("ACTION > "); 
      String userAction = input.next(); 

      if (userAction == "up" || userAction == "down" || userAction == "left" || userAction == "right" || userAction == "exit") { 
       switch (userAction) { 
        case "up": 
         world1.moveUp(); 
         break; 
        case "down": 
         world1.moveDown(); 
         break; 
        case "left": 
         world1.moveLeft(); 
         break; 
        case "right": 
         world1.moveRight(); 
         break; 
        case "exit": 
         exit = false; 
         break; 
       } 
      } else { 
       System.out.println("Invalid Input. Possible inputs are: up, down, left, right, or exit."); 
      } 
     } 

    } 
} 

class World { 
    static private char[][] worldDimensions; 
    static private int characterRow; 
    static private int characterColumn; 

    public World(int userWidth, int userHeight) { 
     char[][] worldDimensions = new char[userHeight][userWidth]; 
     int characterRow = 0; 
     int characterColumn = 0; 

     for (int row = 0; row < worldDimensions.length; row++) { 
      for (int column = 0; column < worldDimensions[row].length; column++) { 
       if (characterRow == row && characterColumn == column) { 
        worldDimensions[row][column] = (char) 
        'P'; 
       } else { 
        worldDimensions[row][column] = (char) 
        '-'; 
       } 
      } 
     } 
    } 

    public void displayWorld() { 
     for (int row = 0; row < worldDimensions.length; row++) { 
      for (int column = 0; column < worldDimensions[row].length; column++) { 
       System.out.print(worldDimensions[row][column]); 
      } 
      System.out.println(); 
     } 
    } 

    public void moveUp() { 
     if (characterRow - 1 >= 0) { 
      characterRow--; 
     } 
    } 

    public void moveDown() { 
     if (characterRow + 1 <= worldDimensions[0].length) { 
      characterRow++; 
     } 
    } 

    public void moveLeft() { 
     if (characterColumn - 1 >= 0) { 
      characterColumn--; 
     } 
    } 

    public void moveRight() { 
     if (characterRow + 1 <= worldDimensions.length) { 
      characterRow++; 
     } 
    } 
}

来源

2015-11-04 JimBob117

使用['equals（）方法来比较strings'（http://stackoverflow.com/questions/513832/how-do-i-compare-strings-in- JAVA）。还要检查['什么是空指针异常，以及如何解决它？']（http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception-and-how-do -i-fix-it） – sam

为了将来的参考，它有用于指示空指针在哪一行 - 而不是我们必须找到它。 – Sh4d0wsPlyr

这里至少有三个错误 - 使用'='而不是* *'exit'，使用'=='比较字符串，并将冗余转换为'char' ... – Makoto

你遮蔽worldDimensions你的构造函数里面。本地声明不会像以前声明的那样为您提供相同的字段。

只需删除的声明，你会好起来的（至少是对于那些错误）：

worldDimensions = new char[userHeight][userWidth];

来源

2015-11-04 19:01:58 Makoto

你覆盖类变量worldDimensions。你的世界构造看起来应该像下面

public World(int userWidth, int userHeight) 
{ 
//here is where you were overwriting the global variable, leaving it null 
//and populating the local one instead 
worldDimensions = new char[userHeight][userWidth]; 
int characterRow = 0; 
int characterColumn = 0; 

for (int row = 0; row < worldDimensions.length; row++) 
    { 
    for (int column = 0; column < worldDimensions[row].length; column++) 
     { 
     if (characterRow == row && characterColumn == column) 
      { 
      worldDimensions[row][column] = (char)'P'; 
      } 
     else 
      { 
      worldDimensions[row][column] = (char)'-'; 
      } 
     } 
    } 
}

来源

2015-11-04 19:03:06

Java 2D游戏NullPointerException

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