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Question

I had a quick questions about prompting and accepting a file name, then making the file-text a scanner object.

I want the program to prompt the user to enter the name of a file, until he gets one which exists, then for the file-text to be used as a scanner object.

This is the code I have so far, it works to the point where I exit the while {} loop, but then when I try and process the scanner item like while (input.hasNextLine()) { it gives me an error saying it can't find the scanner item.

It's probably a silly mistake, but I just cannot seem to get it.

The whole code is below:

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

public class PersonalityTest {

public static void main(String[] args) throws FileNotFoundException {

    boolean isFile = false;

        Scanner sc = new Scanner(System.in);
        System.out.print("Input file name? ");
        String fileName = sc.next();
        File inputFile = new File(fileName);

    while (isFile == false) {    
        if (inputFile.exists()) {
            Scanner input = new Scanner(inputFile);
            isFile = true;
        }            
    }

    while(input.hasNextLine()) {

    }
}

Answer 1

The scope of the input variable is local to the while (isFile == false) block. Declare it outside otherwise it won't be visible.

For the first part "I want the program to prompt the user to enter the name of a file, until he gets one which exists": Move this code:

    Scanner sc = new Scanner(System.in);
    System.out.print("Input file name? ");
    String fileName = sc.next();
    File inputFile = new File(fileName);

into a method and call it inside the while (isFile == false) block before the exists check (the method should return with the file or make the variable visible in the block by some other means).

Answer 2

You can't access input outside the if statement, sice the compiler is not sure, it will pass the test, you can do this:

Scanner sc = new Scanner(System.in);
Scanner input = null;
boolean isFile = false;

while (isFile == false){    

    System.out.print("Input file name? ");
    String fileName = sc.next();
    File inputFile = new File(fileName);

    if (inputFile.exists()){
        input = new Scanner(inputFile);
        isFile = true;
    }            
}

But will throw a NullPointerException if it is null .

I changed the code a little bit, that way, it will not exceed if the file doesn't exist.

Answer 3

The Scanner input is local to your if statement. Your while (input.hasNextLine()) { statement will not work because of that. The Java compiler will treat input as a separate Scanner object and that is where the problems crop up , because to the Java compiler, the input that you are trying to use does not exist.

I would follow MouseEvent's suggested code as it does not run into the problem mentioned above.

Answer 4

The other answers have addressed your immediate question, but I want to point out a couple of other problems with your code:

The way that you are checking to see if the file can be opened is flawed. A better way to write the code is to attempt to open the file ... and retry when there is an exception. For example:

Scanner input = null;
do {
    System.out.print("Input file name? ");
    String fileName = sc.next();
    File inputFile = new File(fileName);
    try {
        input = new Scanner(inputFile);
    } catch (IOException ex) {
        System.out.println("Cannot open: " + ex.getMessage());
    }          
} while (input == null);

Why is this better than calling File.exists() ?

• There are lots of reasons why you might be able to open a file. It might not exist at all. It might be a directory or a special file that can't be opened as a file. The application might not have permission. The file might be on a remote mounted file system and the remote mount might have just died.

• There is a small time gap between the File.exists() call (and any others that you might make) and actually opening the file. In that time gap, it is possible that something to your program could do something to make the file unopenable; eg it could change its permissions or delete it.

The second problem is that your code potentially leaks a file descriptor because the scanner is not closed. In your specific application (as written) this doesn't matter because you are going to exit the application immediately after using the scanner. But if your weren't ... and this code was called lots of times ... you could find that you are unable to open files after a bit.

The correct way to deal with this would be to write your code something like this:

public static void main(String[] args) {
    try (Scanner input = openInput()) {
        while (input.hasNextLine()) {
            // do stuff
        }
    }
}

This uses Java 7's new "try with resource" syntax, that ensures that the resource is closed when the try statement completes. (You can do the same thing in pre-Java 7 using a try / finally, but the code is a bit more cumbersome.)