java - 无法在java中读取文件

Question

我试图让用户输入一个整数，基于整数值，我正在调用 mix 函数来读取文件内容，如下面的代码所示。我得到了，这个错误：

Project2.java:43: variable urlScan might not have been initialized
                        while (urlScan.hasNext())
                               ^
Project2.java:34: unreported exception java.io.FileNotFoundException; must be caught or declared to be thrown
                fileScan = new Scanner (new File("input.txt"));
                       ^

任何想法，我可能在这里做错了什么？

import java.util.Scanner;
import java.io.*;

public class Project2
{
    public static void main(String[] args) 
    {

                System.out.println("Select an item from below: \n");
                System.out.println("(1) Mix");
                System.out.println("(2) Solve");
                System.out.println("(3) Quit");

                int input;
                Scanner scan= new Scanner(System.in);
                input = scan.nextInt();

                System.out.println(input);  
                if(input==1) {
                    mix();
                }
                else{
                    System.out.println("this is exit");
                    } 
        }




        public static void mix()
          {
                String url;
                Scanner fileScan, urlScan;
                fileScan = new Scanner (new File("input.txt"));
                // Read and process each line of the file
                while (fileScan.hasNext())
                    {
                        url = fileScan.nextLine();
                        System.out.println ("URL: " + url);
                        //urlScan = new Scanner (url);
                        //urlScan.useDelimiter("/");
                        //  Print each part of the url
                        while (urlScan.hasNext())
                        System.out.println ("   " + urlScan.next());
                        System.out.println();
                     }
            }
     }

Answer 1

这些错误非常具有表现力。

• 初始化你的urlScan 局部变量（局部变量没有默认值）
• 环绕fileScan = new Scanner (new File("input.txt"));四周try/catch。或声明您的方法可能会在您的方法签名中抛出 FileNotFoundException。（new File(str) 可能会抛出FileNotFoundException已检查的异常，编译器将强制您处理它）。

Answer 2

首先，urlScan没有初始化。

其次，你应该fileScan = new Scanner (new File("input.txt"));用 try/catch for包围FileNotFoundException。

Answer 3

局部变量必须在使用前初始化，所以取消注释这一行：

urlScan = new Scanner (url);