java - 在 Java 中读取文件时 FileNotFoundException 捕获错误

Question

我正在尝试编写一个简单的程序，从文本文件中读取整数，然后将总和输出到输出文件。我得到的唯一错误是在第 38 行的 catch block 中“ Unresolved 编译问题:无法解析文件”。请注意，"file"是我的输入文件对象的名称。如果我注释掉这个异常 block ，程序运行正常。任何建议将不胜感激!

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

public class ReadWriteTextFileExample 
{
    public static void main(String[] args) 
    {
        int num, sum = 0;

        try
        {
            //Create a File object from input1.txt
            File file = new File("input1.txt");
            Scanner input = new Scanner(file);

            while(input.hasNext())
            {
            //read one integer from input1.txt
                num = input.nextInt();
                sum += num;
            }
            input.close();  

        //create a text file object which you will write the output to
        File output1 = new File("output1.txt");
        //check whether the file's name already exists in the current directory
        if(output1.exists())
        {
            System.out.println("File already exists");
            System.exit(0);
        }
        PrintWriter pw = new PrintWriter(output1);
        pw.println("The sum is " + sum);
        pw.close();
    }
    catch(FileNotFoundException exception)
    {
        System.out.println("The file " + file.getPath() + " was not found.");
    }
    catch(IOException exception)
    {
        System.out.println(exception);
    }
}//end main method
}//end ReadWriteTextFileExample

Answer 1

file 变量在 try block 中声明。它超出了 catch block 的范围。 (虽然在这种情况下不会发生，但想象一下如果在执行甚至到达变量声明之前抛出异常。基本上，您不能访问在中声明的 catch block 中的变量相应的 try block 。)

您应该在 try block 之前声明它:

File file = new File("input1.txt");
try
{
    ...
}
catch(FileNotFoundException exception)
{
    System.out.println("The file " + file.getPath() + " was not found.");
}
catch(IOException exception)
{
    System.out.println(exception);
}