Question

我的代码读取用户输入文件名的文件。如果用户键入不存在的文件名，则会捕获异常并打印未找到的文件。我试图做的是如果文件名无效，则循环代码。然而，会发生的是代码继续打印出未找到的文件，它不会停止。那我的代码出了什么问题？

public static Scanner readFile(String filename){
    File input = new File(filename);
    Scanner sc = null;
    do {
        try {
            sc = new Scanner(input);
        }
        catch(FileNotFoundException e) {
            System.out.println("Filename not valid");
        }
    } while (!new File(filename).exists()); 
    return sc;
}

没有一个答案对我有帮助，所以也许我会尝试发布整个代码，看看是否有帮助。

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

public class Report{
    public static void main(String[] args){
        Scanner scanner = new Scanner(System.in);
        String filename = scanner.next();
        Scanner input = readFile(filename);
        CO2Data[] aDataArray  = null;
        aDataArray = readData(filename);
        String highestvalue = highest(aDataArray);
        String lowestvalue = lowest(aDataArray);
        String highest_road = highest_per_person(aDataArray);
        String lowest_road = lowest_per_person(aDataArray);
        try{
        PrintStream output = new PrintStream(new File("Report.txt"));
        output.println("The country with the lowest CO2 emissions is " + lowestvalue);
        output.println("The country with the highest CO2 emissions is " + highestvalue);
        output.println();
        output.println("The country with the lowest per person road emissions is " + lowest_road);
        output.println("The country with the highest per person road emissions is " + highest_road);
        }
        catch(FileNotFoundException e){
            System.out.println("Error printing to file");
            System.exit(-1);
            }
        }

    public static Scanner readFile(String filename){
    Scanner stdin = new Scanner(System.in);
    File input;
    do {
       input = new File(filename);
        try {
            stdin = new Scanner(input);
        }
        catch(FileNotFoundException e) {
            System.out.println("Filename not valid. Please try again:");
            filename = stdin.nextLine();
        }
    } while (!input.exists());
    return stdin;
}

    public static CO2Data[] readData(String filename){
    File input = new File(filename);
        Scanner sc = null;
        try{
            sc = new Scanner(input);
        }
        catch(FileNotFoundException e){
            System.out.println("Filename not valid");
            System.exit(-1);
        }
    String info = sc.nextLine();
    int total = sc.nextInt();
    CO2Data[] arr = new CO2Data[total];
    for(int i=0; i<10;i++){
        arr[i] = new CO2Data();
        }
    for(int i=0; i<10;i++){ 
        arr[i].setCountry(sc.next());
        arr[i].setTotalCO2(sc.nextDouble());
        arr[i].setRoadCO2(sc.nextDouble());
        arr[i].setCO2PerPerson(sc.nextDouble());
        arr[i].setCarsPerPerson(sc.nextInt());
        }
    return arr;
    }

    public static String highest (CO2Data []  arr2){
    Scanner sc = new Scanner(System.in);
    CO2Data highestindex = arr2[0];
    for (int i = 0; i<arr2.length; i++){
        if (arr2[i].getTotalCO2() > highestindex.getTotalCO2()){
            highestindex = arr2[i];
            }
        }
    return highestindex.getCountry();
    }

    public static String lowest (CO2Data [] arr3){
        Scanner sc = new Scanner(System.in);
        CO2Data lowestindex = arr3[0];
            for (int i = 0; i<arr3.length; i++){
                if (arr3[i].getTotalCO2() < lowestindex.getTotalCO2()){
                    lowestindex = arr3[i];
                    }
                }
        return lowestindex.getCountry();
        }

            public static String highest_per_person (CO2Data []  arr2){
    Scanner sc = new Scanner(System.in);
    CO2Data highestindex = arr2[0];
    for (int i = 0; i<arr2.length; i++){
        if (arr2[i].getRoadCO2() > highestindex.getRoadCO2()){
            highestindex = arr2[i];
            }
        }
    return highestindex.getCountry();
    }

    public static String lowest_per_person (CO2Data [] arr3){
        Scanner sc = new Scanner(System.in);
        CO2Data lowestindex = arr3[0];
            for (int i = 0; i<arr3.length; i++){
                if (arr3[i].getRoadCO2() < lowestindex.getRoadCO2()){
                    lowestindex = arr3[i];
                    }
                }
        return lowestindex.getCountry();
        }
    }

Answer 1

当文件名无效时，您应该从用户那里获得一个新的文件名，而不是一遍又一遍地重新检查相同的文件名。 E.g：

Scanner stdin = new Scanner(System.in);
File input = new File(filename);
Scanner sc = null;
do {
    try {
        sc = new Scanner(input);
    }
    catch(FileNotFoundException e) {
        System.out.println("Filename not valid. Please try again:");
        filename = stdin.readLine();
    }
} while (!new File(filename).exists());

Answer 2

readFile(String filename)收到filename参数，但如果此类文件不存在，该方法可以做什么？它肯定无法自己发明正确的文件名。所以问题不在于这种方法。您需要确保使用有效的文件名调用此方法。

public Scanner getScanner() {
    Scanner stdin = new Scanner(System.in);
    while (true) {
        String path = null;
        try {
            System.out.println("Please enter path to input file: ");
            path = stdin.nextLine();
            return getScanner(path);
        } catch (FileNotFoundException e) {
            System.err.println("No such file: " + path);
        }
    }
}

private Scanner getScanner(String path) throws FileNotFoundException {
    return new Scanner(new File(path));
}

如何检查程序错误然后继续循环

2 个答案: