CODEWITHSUNDEEP
javaexceptionjava.util.scanner
m1ddleware
m1ddleware

Reputation: 78

Catching exception and requesting user to re-enter input

I'm creating a program that opens and then reads files which users have specified, currently the code I have looks like:

    System.out.println("Enter the name of the file you want to open: ");
    FileN = scan.nextLine();
    // I want the program to return to this point here if an error has occured.
    try
    {
        scan = new Scanner(new File (FileN));
    }
    catch(Exception e)
    {
        System.out.println("Could not find file" + e);
        System.out.println("Please enter a valid file name: ");

    }

I have specified above where I want the program to return to within the code, I have currently tried creating a loop and then using continue however it wont let me put a try within the loop. Also I've tried to create a new void and it still wont work. Currently the program continues to run even if the user has entered an invalid file name.

I have searched for an answer already and can only find this relating to what I want: Java - Exception handling - How to re-enter invalid input

Also clarifying what I mean by putting a try in a loop; yes, it is possible. However I want to know whether for the continue to work in my program, do I put the try inside the loop or the loop inside the try? I have referred to: Should try...catch go inside or outside a loop?

This is the error I'm currently getting with my latest code

Upvotes: 0

Views: 5777

Answers (3)

codebrane
codebrane

Reputation: 4650

It becomes a little easier if you use Exception in the sense it's meant to be used, to handle something unexpected. A file not existing isn't really an exception as it is to be expected. A file existing but not being able to be opened, or being opened but has zero content even though it has 1MB content is something unexpected, so is an Exception. Taking into account the expected behaviour of a file not existing (as it's being typed in by a user, who may type it incorrectly), you could use something like:

boolean fileExists = false;
File newFile;
while(!fileExists) {
  System.out.println("Enter the name of the file you want to open: ");
  FileN = scan.nextLine();
  newFile = new File(FileN);
  fileExists = newFile.exists();
  if (!fileExists) {
    System.out.println(FileN + " not found...");
  }
}
try {
    Scanner scan;
    scan = new Scanner(newFile);
    ... do stuff with the scanner
}
catch(FileNotFoundException fnfe) {
  System.out.println("sorry but the file doesn't seem to exist");
}

Upvotes: 0

Aaron Digulla
Aaron Digulla

Reputation: 328850

Your problem is that your order of operations is bad. Here is your order:

  • Process a file
  • Give an error when the file doesn't exist
  • Ask for a new file name

I suggest this approach:

  • Ask for a file name
  • Check whether the file exists
  • If it doesn't exist, ask again
  • Process the file

In a nutshell, my approach is:

  • Read some input
  • Validate the input. If it's bad do some error handling
  • Process the input further

Coming back to your problem: Create a loop which asks for file names until File.exists() returns true. Maybe also check File.isFile() (so people can't enter directories).

Create the scanner only after the loop. It will still throw an exception (Java doesn't know that you've already made sure the file exists). But the exception handler code won't need to ask for a file name (so no loop there).

Upvotes: 2

SF23
SF23

Reputation: 184

You can try a while loop:

        boolean again =true;

    while(again){
     FileN = scan.nextLine();

            try
            {
                scan = new Scanner(new File (FileN));
                again=false;
            }
            catch(Exception e)
            {
                System.out.println("Could not find file" + e);
                System.out.println("Please enter a valid file name: ");

            }}

Upvotes: -1

Related Questions