Java 2D 游戏 NullPointerException

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 7 年前。

我正在尝试用 Java 创建一个 2D 游戏，系统会提示用户输入行宽和列长。创建一个“世界”对象，打印时如下所示(其中 P 是玩家的角色):

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

然后，用户可以输入向上、向下、向左、向右或退出，这将移动 P，如果移动会将角色移出 map ，则不执行任何操作。我的问题是在输入宽度和长度后，出现以下错误:

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

我相信这意味着 Java 将 worldDimensions.length 视为空/null？我以为我在创建世界对象时为其分配了一个值。非常感谢任何指导!

这是我的代码:

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++;
        }
    }
}

Answer 1

您正在构造函数内部隐藏 worldDimensions。本地声明不会为您提供与先前声明相同的字段。

只需删除声明即可(至少对于该错误):

worldDimensions = new char[userHeight][userWidth];