How to solve my try-catch exception handling?

Question

I'm having problems with my try-catch exception here. Actually what it does is to prompt the user for the name of a text file say, Robot.txt but if say the file does not exist, I have to make sure that the application reprompts the user for the file name. Hope you guys can understand I'm still a newbie here so please feel free to provide suggestions or advices on my coding etc. Cheers!

Main method class:

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

class TestVector3 {

public static void main(String [] args)
{
    System.out.println("Please enter the name of the text file to read: ");
    Scanner userInput = new Scanner(System.in);
    Vector <KillerRobot> robotDetails = new Vector <KillerRobot>();
    KillerRobot robot;

    Scanner fileInput = null;
    try
    {
        File textFile = new File(userInput.nextLine());
        fileInput = new Scanner(textFile);
    }
    catch (FileNotFoundException e)
    {
        System.out.println("Error - file not found!");
        System.out.println("Re-enter file name :");             //Reprompt user for name of the text file
        fileInput = new Scanner(userInput.nextLine());
    }

    while(fileInput.hasNext())
    {
        robot = new KillerRobot();

        String first = fileInput.next();
        robot.setName(first);

        String second = fileInput.next();
        robot.setMainWeapon(second);

        int third = fileInput.nextInt();
        robot.setNumberOfKills(third);

        robotDetails.add(robot);
    }

    for(KillerRobot i : robotDetails)
    {
        System.out.println(i);
    }

    fileInput.close();
}
}

KillerRobot class file:

class KillerRobot {

private String name;
private String mainWeapon;
private int numberOfKills;

KillerRobot()
{
}

public String getName()
{
    return name;
}

public String getMainWeapon()
{
    return mainWeapon;
}

public int getNumberOfKills()
{
    return numberOfKills;
}

public String toString()
{
    return name + " used a " + mainWeapon + " to destroy " + numberOfKills + " enemies ";
}

public void setName(String a)
{
    name = a;
}

public void setMainWeapon(String b)
{
    mainWeapon = b;
}

public void setNumberOfKills(int c)
{
    numberOfKills = c;
}
}

Answer 1

As you state that you are a beginner, let us first look at the relevant part of your code, to make sure that we talk about the same thing:

Scanner fileInput = null;
try {
    File textFile = new File(userInput.nextLine());
    fileInput = new Scanner(textFile);
}
catch (FileNotFoundException e) {
    System.out.println("Error - file not found!");
    System.out.println("Re-enter file name :"); 
    fileInput = new Scanner(userInput.nextLine());
}

You have an input and you want to check this input for a condition and require a new input until this condition is fulfilled. This problem can be solved using a loop like the following:

Scanner fileInput = null;
do {
    System.out.println("Enter file name :"); 
    try {
      fileInput = new Scanner(new File(userInput.nextLine()));
    } catch (FileNotFoundException e) {
      System.out.println("Error - file not found!");
   }
} while(fileInput == null);

So finally, why does this work? The fileInput variable is set to null and will remain null until the given file is successfully read from standard input because an exception is thrown otherwise what prevents the fileInput variable to be set. This procedure can be repeated endlessly.

On a side note, for performance reasons, it is normally not a good idea to implement control flow that is based on exceptions. It would be better to check for a condition if a file exists via File::exists. However, if you read the file after checking for its existence, it might have been deleted in the meantime which introduces a racing condition.

Answer to your comment: In Java (or almost any programming language), you can inline expressions. This means that instead of calling two methods in two different statements as in

Foo foo = method1();
Bar bar = method2(foo);

you can simply call

Bar bar = method2(method1());

This way, you save yourself some space (what becomes more and more important if your code gets longer) as you do not need the value that you saved in foo at any other place in your code. Similarly, you can inline (which is how this pattern is called) from

File file = new File(userInput.nextLine())
fileInput = new Scanner(file);

into

fileInput = new Scanner(new File(userInput.nextLine()));

as the file variable is only read when creating the Scanner.

Answer 2

Try putting the try-catch in a loop like below:

Scanner fileInput = null;
while (fileInput==null) 
{
    try
    {
        System.out.println("Please enter the file name.");
        File textFile = new File(userInput.nextLine());
        fileInput = new Scanner(textFile);
    }
    catch (FileNotFoundException e)
    {
        System.out.println("Error - file not found!");
    }
}

Next you could think of moving the File creation part into separate method, so that the code was cleaner.

Answer 3

Do not fall for try-catch instead add this as your functionality. Exceptions are naturally for run time error handling not for logic building.

Check if file exists at given location.

File textFile = new File(userInput.nextLine());

// Check if file is present and is not a directory
if(!textFile.exists() || textFile.isDirectory()) { 

System.out.println("Error - file not found!");

//Reprompt user for name of the   text file
System.out.println("Re-enter file name :");             
fileInput = new Scanner(userInput.nextLine());

 }

You can put while loop instead of if loop if you want to continuously prompt user until correct path is entered.

Answer 4

You can call back your main(), like following

 try
    {
        File textFile = new File(userInput.nextLine());
        fileInput = new Scanner(textFile);
    }
    catch (FileNotFoundException e)
    {
        System.out.println("Error - file not found!");
        main(args); // recursively call main() method 
    }

Now if user first attempt wrong then your code will asked to re enter file name.

How to check isFile exist?

  File file = new File(filePathString); 
  if(file.exists() && !file.isDirectory()){
     System.out.println("file exist");
  }

Answer 5

This really is an XY problem because you assumed the only way to check for a file existence is by catching a FileNotFoundException (hence asking about try-catch exception handling) whereas other means exist to help you avoid a try-catch idiom in an elegant manner.

To check if a file exists at the given path or not you can simply use the File.exists method. Please also see the File.isFile method and/or the File.isDirectory method to verify the nature of the targeted File object.

EDIT : As stated by raphw, this solution is best used in simple scenario since it can incur a race condition in the case of concurrent file deletion happening during the file existence check. See his answer for handling more complex scenario.