java - 文件输入java，扫描器使用if..else语句。提供错误

Question

好的，所以我尝试正常地从 3 个文件中的 1 个读入，而不使用 if ... else 子句

只是 Scanner myScanner = new Scanner(new File("maze1.txt")); 会运行问题，但我希望用户选择他/她想要运行的迷宫

public class Main {
    private static Scanner myScanner;

    public static void main(String[] args) throws FileNotFoundException {

        System.out.println("Please Enter 1,2 or 3 to pick the different maze you would like being solved.");
        Scanner myScannerinput = new Scanner(System.in);



        int Mazenumber = myScannerinput.nextInt();

        if(Mazenumber == 1){
            Scanner myScanner = new Scanner(new File("maze1.txt"));
        }
        else if(Mazenumber == 2){
            Scanner myScanner = new Scanner(new File("maze2.txt"));
        }
        else if(Mazenumber == 3){
            Scanner myScanner = new Scanner(new File("maze3.txt"));
        }
        else{
            System.out.println("You did not choose one of the 3 mazes");
        }



        int numRows = myScanner.nextInt();
        int numCols = myScanner.nextInt();
        myScanner.nextLine();

        int startX = 0;
        int startY = 0;'

请输入 1,2 或 3 以选择您想要解决的不同迷宫。

Exception in thread "main" java.lang.NullPointerException
at Main.main(Main.java:33)

Answer 1

不要在 if 条件内创建单独的新扫描程序。在 if 条件之前创建一个扫描程序，并根据您的偏好分配文件。

        int Mazenumber = myScannerinput.nextInt();
        Scanner myScanner =null;
        if(Mazenumber == 1){
            myScanner = new Scanner(new File("maze1.txt"));
        }
        else if(Mazenumber == 2){
            myScanner = new Scanner(new File("maze2.txt"));
        }
        else if(Mazenumber == 3){
            myScanner = new Scanner(new File("maze3.txt"));
        }
        else{
            System.out.println("You did not choose one of the 3 mazes");
        }
        if(myScanner!=null){
           int numRows = myScanner.nextInt();
           int numCols = myScanner.nextInt();
           myScanner.nextLine();
       }