Question

我正在尝试制作一个程序，如果它存在，它将读取文件。如果该文件不存在，那么我希望它循环回来并再次询问用户输入。目前我已经知道它会停止但是我想让它循环回来再问一下，有什么想法吗？

System.out.println("Enter the file you want to check (without file extension): ");
Scanner scanner = new Scanner(System.in);
String input = scanner.nextLine();
String in = (input + ".txt");

File f = new File(in);

if(!f.exists()) {
    System.out.println(in + " doesn't exist! Try again.");
    return;
}


String fileName = input;
String content = readFile(in);
System.out.println(content);

Answer 1

一种方法是使用do，while(false)循环（允许f限定为循环体）：

Scanner scanner = new Scanner(System.in);
do {
    System.out.println("Enter the file you want to check (without file extension): ");        
    String input = scanner.nextLine();
    String in = (input + ".txt");

    File f = new File(in);
    if(!f.exists()){
        System.out.println(in + " doesn't exist! Try again.");
        continue; /*go round again*/
    }
} while (false/*i.e. exit the loop if we get here*/);

如果您希望循环后f可用，则在File f;行之前声明do {，并在循环体中写f = new File(in);。

Answer 2

只是为您提供更经典的while循环来执行此操作。

请注意@Bathsheba的回答非常简洁，你绝对应该接受他的回答。

System.out.println("Enter the file you want to check (without file extension): ");
Scanner scanner = new Scanner(System.in);
String input = scanner.nextLine();
String in = (input + ".txt");
File file = new File(input);

while(!file.exists()){

    System.out.println(in + " doesn't exist! Try again.");
    input = scanner.nextLine();
    in = (input + ".txt");
    file = new File(input);

}

Answer 3

我不喜欢do - while和while(false)甚至while(true)和break。

如果没有太多工作，我会做类似的事情：

Scanner scanner = new Scanner(System.in);
File f = getFile();
while (!f.exist()) {
    f = getFile();
}

private File getFile() {
    System.out.println("Enter the file you want to check (without file extension): ");        
    String input = scanner.nextLine();
    String in = (input + ".txt");
    File f = new File(in);
    if (!f.exists()) {
        System.out.println(in + " doesn't exist! Try again.");
    }
    return f;
}

这避免了丑陋的do-while循环，并且很好地封装了获取逻辑。您理论上也可以使其递归（尽管允许堆栈溢出）或让它返回Pair File, in，它会将输入提升到外部循环，从而允许循环访问{{ 1}}变量。

如果文件不存在，请重试

3 个答案: