Java井字棋开发中Visual矩阵未初始化引发NullPointerException如何解决

问题背景 #

开发井字棋游戏时，使用Visual类型二维矩阵存储落子信息，调用drawGame方法渲染界面时抛出NullPointerException，推测与矩阵初始化相关。

错误信息 #

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "com.company.Visual.getSymbol()" because "visual[0][0]" is null
    at com.company.Main.drawGame(Main.java:48)
    at com.company.Main.main(Main.java:21)

相关代码 #

Main.java主类 #

package com.company;

import java.util.Scanner;

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


        //variables
        Visual[][] visual = new Visual[3][3];
        //public static Visual[3][3] visual = {{' ', ' ', ' '}, {' ', ' ', ' '}, {' ', ' ', ' '}};
        char symbol='X';
        boolean game = true;
        String victory="";
        Scanner scan = new Scanner(System.in);
        //int line=0,colunm=0;


        //game
        while(game){
            if(visual!=null)drawGame(visual);

            //drawGame(visual);
            victory = verifyVictory(visual);
            if(victory.equals("")){
                System.out.printf("Player %s won!", victory);
                break;
            }
            try {
                whoPlays(visual, play(scan, symbol), symbol);
                if(symbol=='X'){
                    symbol='O';
                }else{
                    symbol='X';
                }
            } catch (Exception e) {
                System.out.println("Error!");
            }
        }
        System.out.println("End");

    }


    //function to call the Visual class and show the visual of tactoe
    public static void drawGame(Visual[][] visual){
        System.out.printf("    0    1    2\n");
        System.out.printf("0   %c | %c | %C \n", visual[0][0].getSymbol(),visual[0][1].getSymbol(),visual[0][2].getSymbol());
        System.out.printf("   --------------\n");
        System.out.printf("1   %c | %c | %C \n",visual[1][0].getSymbol(),visual[1][1].getSymbol(),visual[1][2].getSymbol());
        System.out.printf("   --------------\n");
        System.out.printf("2   %c | %c | %C \n",visual[2][0].getSymbol(),visual[2][1].getSymbol(),visual[2][2].getSymbol());

    }
    //clean the old drawGame()
    public static void clean(){
        for(int i=0;i<50;i++){
            System.out.println("");
        }
    }

    //coords to play
    public static int[] play(Scanner scan, char symbol){
        int p[] = new int[2];
        System.out.printf("%s %c\n", "Who plays: ", symbol);
        System.out.print("Line: ");
        p[0] = scan.nextInt();
        System.out.print("Colunm: ");
        p[1] = scan.nextInt();
        return p;
    }

    //who is playing
    public static boolean whoPlays(Visual[][] visual, int p[], char symbol){
        if(visual[p[0]][p[1]].getSymbol()==' '){
            visual[p[0]][p[1]].setSymbol(symbol);
            return true;
        }else{
            return false;
        }
    }

    //this is an attempt to inicialize the array visual, since I saw this might be the problem
    public static void start(Visual[][] visual){
        for(int i=0;i<3;i++){
            for(int c=0;c<3;c++){
                visual[i][c] =  new Visual();
            }
        }
    }

    //still doing this, useless by now
    public static String verifyVictory(Visual[][] visual){
        return "";
    }
}

Visual.java辅助类 #

package com.company;

public class Visual {
    private char symbol;

    public Visual(){
        this.symbol=' ';
    }

    public char getSymbol(){
        return this.symbol;
    }
    public void setSymbol(char symbol){
        if(this.symbol==' '){
            this.symbol=symbol;
        }else{
            System.out.println("Error");
        }
    }
}

问题原因 #

Java创建引用类型二维数组时，仅会为数组本身分配内存空间，数组内的所有元素默认值为null，你虽然已经编写了start()初始化方法为每个数组元素实例化Visual对象，但在main方法中声明二维数组后没有调用该初始化方法，导致访问visual[0][0].getSymbol()时触发空指针异常。

修复方案 #

1. 初始化矩阵：在main方法中Visual[][] visual = new Visual[3][3];代码行后，添加调用初始化方法的代码：

start(visual);

2. 补充逻辑优化（可选）：

• 修正胜负判断逻辑：现有代码中if(victory.equals(""))逻辑反向，应改为if(!victory.equals(""))才会在有玩家获胜时触发结束逻辑。
• 优化落子切换逻辑：只有当whoPlays()返回true（落子成功）时，才切换当前玩家符号，避免玩家选中已落子位置时误切换玩家。

内容的提问来源于stack exchange，提问作者Pedro Y