CodeGym/Help with Java Tasks/Problem with third condition

Level 30

London

21.11.2019
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10comments

Problem with third condition

Question about the task

Let's make the code do something useful!

Resolved

It seems that a lot of people are having a problem with this condition; haven't found the answer in other questions though. please help

Task: The program reads in two file names. It copies the first file to the location specified by the second file name.
New task: The program reads in two file names. It copies the first file to the location specified by the second file name.
If the specified file (to be copied) does not exist, then the program should display "File does not exist." and read a file name one more time and only then read the destination file name.

Requirements:

The program must read file names.
The main method must handle exceptions thrown by the getInputStream method. If an exception occurs, you should display "File does not exist.".
The program must copy the contents of the first file to the second one.
The main method should call the getInputStream method.
Don't change the getInputStream method.
The main method should call the getOutputStream method.
Don't change the getOutputStream method.

package com.codegym.task.task09.task0929; import java.io.*; /* Let's make the code do something useful! */ public class Solution { public static void main(String[] args) throws IOException { BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); try { String sourceFileName = reader.readLine(); InputStream fileInputStream = getInputStream(sourceFileName); } catch (FileNotFoundException e){ System.out.println("File does not exist."); } String sourceFileName = reader.readLine(); InputStream fileInputStream = getInputStream(sourceFileName); String destinationFileName = reader.readLine(); OutputStream fileOutputStream = getOutputStream(destinationFileName); while (fileInputStream.available() > 0) { int data = fileInputStream.read(); fileOutputStream.write(data); } fileInputStream.close(); fileOutputStream.close(); } public static InputStream getInputStream(String fileName) throws IOException { return new FileInputStream(fileName); } public static OutputStream getOutputStream(String fileName) throws IOException { return new FileOutputStream(fileName); } }

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Vahan

Level 41 , Tbilisi, Georgia

21 November 2019, 19:05

This program reads the file a second time, when it exists for the first time, while it should read a second time only when it did not exist for the first time and the error was caught, i.e. inside the catch body.

Anthony Chalk

Level 30 , London, United Kingdom

21 November 2019, 20:14

I've done that and now it's returning the following error: Error in com/codegym/task/task09/task0929/Solution.java on line 28 cannot find symbol symbol: variable fileInputStream location: class com.codegym.task.task09.task0929.Solution

Guadalupe Gagnon

Level 37 , Tampa, United States

21 November 2019, 18:45

The problem with this code is that there is no mechanism to check that the first entry is valid or not. Line 21 should only be reached if line 15 reads in an invalid entry. What will happen, if the first entry is valid, line 21 will over write the valid entry and the program will hang up on line 24 (because only 2 entrees are being entered).

Anthony Chalk

Level 30 , London, United Kingdom

21 November 2019, 20:12

So I rewrote it and my code is now:

BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
       try {

           String sourceFileName = reader.readLine();
           InputStream fileInputStream = getInputStream(sourceFileName);
       }
       catch (FileNotFoundException e){
           System.out.println("File does not exist.");
           String sourceFileName = reader.readLine();
           InputStream fileInputStream = getInputStream(sourceFileName);
       }


       String destinationFileName = reader.readLine();
       OutputStream fileOutputStream = getOutputStream(destinationFileName);

       while (fileInputStream.available() > 0) {
           int data = fileInputStream.read();
           fileOutputStream.write(data);
       }

       fileInputStream.close();
       fileOutputStream.close();
   }

but now I'm getting the following error: Error in com/codegym/task/task09/task0929/Solution.java on line 28 cannot find symbol symbol: variable fileInputStream location: class com.codegym.task.task09.task0929.Solution

Guadalupe Gagnon

Level 37 , Tampa, United States

21 November 2019, 20:26

solution

Now, because fileinputStream is contained within inner blocks of code (lines 5 and 10), the outer blocks of code do not have access to it. At line 17 it might as well not exists at all. Change your code to:

InputStream fileInputStream = null;
try {
   String sourceFileName = reader.readLine();
   fileInputStream = getInputStream(sourceFileName);
}  catch (FileNotFoundException e){
   System.out.println("File does not exist.");
   String sourceFileName = reader.readLine();
   fileInputStream = getInputStream(sourceFileName);
}

This will fix your immediate problem. You may want to consider surrounding lines 17-22 in an if block that checks to make sure that the inputStream is not null before those lines are reached.

Anthony Chalk

Level 30 , London, United Kingdom

21 November 2019, 20:32

Thank you, it works :) One question: why? I was under the impression that with a try-catch block at least one of them would be executed, so whatever happens the fileinputStream should have been created and accessible by the rest of the code surely?

Guadalupe Gagnon

Level 37 , Tampa, United States

21 November 2019, 21:12

Yes, but there are still rules that Java itself follows, and that is that variables declared in inner blocks will not be accessible in outer blocks. The same thing would occur if your code looked like this:

Here the fileInputStream object does not exist outside of the if-else structure.

Anthony Chalk

Level 30 , London, United Kingdom

21 November 2019, 21:14

Okay thank you, I think I understand the rules now

Guadalupe Gagnon

Level 37 , Tampa, United States

21 November 2019, 21:22

If you didn't notice, in my first response I moved the variable declaration to outside of the inner blocks and set its value to null. If I did not set the value to null, and used something like this instead: InputStream fileInputStream; (no initialization here) because the outer block(s) still doesn't have access to the inner blocks, the code would have thrown an error saying that at line 17 you were trying to use an uninitialized object. Now of course null isn't going to help you solve the task, but you can change that value in inner blocks or methods as needed and the code in the outer blocks have access to it.

Guadalupe Gagnon

Level 37 , Tampa, United States

21 November 2019, 21:24

And don't worry, you will start to know all this like the back of your hand the more your code breaks because you don't know the rules. That was how I learned all this stuff, by doing it wrong multiple time before I figured out how to avoid these errors.