Confusion I do not get the difference betwee - java help on CodeGym

I understand that try-with resources automatically closes the streams but what if you wantt to use try with catch block. I'm clueless as why this codes is wrong and the validator is even more confusing.

package com.codegym.task.task18.task1824;

/*
Files and exceptions

*/

import java.io.*;

public class Solution {
    public static void main(String[] args) throws IOException {
        //step 1: The program should read file names from the console.
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        String fileName = reader.readLine();

        while(true) {
            FileInputStream fis = null;
            try {
                fis = new FileInputStream(fileName);//if fileName doesn't exist is will automatically throw a FileNotFoundException
            } catch (FileNotFoundException ex) {
                System.out.println(fileName);
                fis.close();
                reader.close();
                break;
            }

        }




        /*while(true){
            String fileName = reader.readLine();
            try (FileInputStream fis = new FileInputStream(fileName)){

            } catch (FileNotFoundException ex){
                System.out.println(fileName);
                break;
            } catch (IOException e) {
                e.printStackTrace();
            }
        }*/


    }
}

while(true) { String fileName = reader.readLine(); try { FileInputStream fis = new FileInputStream(fileName);//if fileName doesn't exist is will automatically throw a FileNotFoundException fis.close(); } catch (FileNotFoundException ex) { System.out.println(fileName); break; } } reader.close();

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Guadalupe Gagnon

Level 37 , Tampa, United States

8 January 2021, 18:01

You don't need try-with-resources in this task. The problem with the code is that: 1) it should read file names from the console until an invalid file is entered. This code only reads one file name. 2) it needs to close the streams. In the above code the only stream that would close is the one that was an invalid file when the exception was caught. You shouldn't close that stream as it is still going to be null because the creation of the object threw a FileNotFoundException and never was initialized. Trying to use any method on a null object would throw a NullPointerException

Liliane Top Backend Developer at Procura

9 January 2021, 09:37

The try-with resources passed the test. I'm trying to refactor it to a try-catch but then I fail the test. That is where I am confused. 1) you're right the reader.readLine() should be within the while loop 2) So the other mistake is that the files that are valid should be closed and the one with an invalid filename is never opened? Thanks again. SO nice that you're helping me.

9 January 2021, 09:48

I solved it and passed the test but I do not understand why this is correct. It doesn't make sense to open a stream and close it straightaway. What am I missing? Ans I was not able to close it outside the try-block as it is a local variable.

Nouser

Level 36 , Germany

9 January 2021, 13:46

solution

Here you just want to test if a file really exists. So they try to open a stream and if that throws a fnfe, the file of course won't exist and the code continues in the catch block. If the file exists, all is good and the stream can be closed. What if we declare the InputStream outside the try catch block and we try to close it outside, too? In case the file doesn't exist, we won't have a fis object (would be null). And when we try to close null it'll throw a NullPointerException. To avoid that a check for null is required before closing fis. When the fis is declared inside the try block and it throws no exception, means it's continuing to run inside the try block, then you know you have a valid input stram object and you can close it (now without the need to check for null). To sum it up. The input stream is just needed inside the catch block so this is also the best place to declare it. The one and only thing you need from that stream is to know whether it throws a FileNotFoundException or not.

9 January 2021, 14:08

Now it makes sense. Thanks Nouser!!

Confusion I do not get the difference between try-catch and try-with-resources