Question

好的家伙所以我试图通过if ... else子句从3个文件中的1个读入

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条件中创建单独的新Scanner。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();
       }

Answer 2

而不是

 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");
        }

使用

Scanner myScanner;
int Mazenumber = 0;
do {
    System.out.println("Choose 1, 2 or 3");
    Mazenumber = myScannerinput.nextInt();
    if(Mazenumber >=1 && Mazenumber <=3) {
         String fileName = "maze"+Mazenumber+".txt";
        myScanner = new Scanner(new File(fileName));
    } else {
           System.out.println("You did not choose one of the 3 mazes");      
     }

} while(Mazenumber>3 && Mazenumber <1);

Main.main（Main.java:33）中线程“main”java.lang.NullPointerException中的异常

异常是因为，您已在Scanner myScanner阻止内声明了if-else。

Answer 3

您在if块中定义并初始化myScanner，因此一旦离开它就会超出范围

文件输入java，扫描器使用if .. else语句。提供错误

3 个答案: