Parsing String read from a text file to Integer

Question

I have a simple program where i read from a text file. Text file contains a number 99. I read this number using a method named readFile. After reading it, i then call writeFile method which increments the number read from the text file by 1 and the again saves it in the text file.

Method to read from file

public void readFile() {

    String line = "";

    try(FileReader fr = new FileReader("Room Number.txt"); 
        BufferedReader br = new BufferedReader(fr)) {

       while((line = br.readLine()) != null) {
           roomNumberField.setText(line);
       }
    } catch (IOException ex) {
        ex.printStackTrace();
    }

    writeFile(line);
}

Problem

As you can see in the above method, i am reading the data from file in a String variable line and at the end of the method, i call writeFile method and pass the line variable in to this method. In the writeFile method, before i can increment the line variable by 1, i have to parse it as int but doing so throws NumberFormatException. Now in my case, this exception will only be thrown only if line cannot be parsed as an int. At first i thought it could be because of line variable containing any space, so i called trim method on it which then threw NullPointerException which led me to believe that line is null.

Question is, why is line variable null ? it is initialized in while loop so how can it be null at the end of the method ?

PS. Number is read from the file correctly and if i move writeFile(line); inside while loop of readFile method, it doesn't throws any exception and my program works as it should.

Then you're not parsing where you should parse. Post the code producing the error. And post the complete stack trace of the exception. Don't force us to guess what you're doing, and t guess what and where the error happens. If the file is supposed to contain a single line, why do you even use a while loop? — JB Nizet
– JB Nizet, Commented Jan 1, 2018 at 14:53
You literally loop until line becomes null. What exactly is tripping you up here? — Mad Physicist
– Mad Physicist, Commented Jan 1, 2018 at 14:54
@MadPhysicist exactly... Silly of me to miss such a simple thing — user7500403
– user7500403, Commented Jan 1, 2018 at 14:59

twain249 · Accepted Answer · 2018-01-01 14:56:19Z

3

Couple of things. Your call to writeFile() should be inside the try block since there is no point in running it if the read fails.

Also your error is because

while((line = br.readLine()) != null) {
           roomNumberField.setText(line);
       }

This loop runs until line == null. So after it exits (before calling writeFile()) the value will be null.

Either you want to write every line then writeFile should be in the loop or you need to figure out which line you want and save it.

answered Jan 1, 2018 at 14:56

twain249

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Comments

captainchhala · Accepted Answer · 2018-01-01 15:02:38Z

1

answer is simple when line get equals to null in

while((line = br.readLine()) != null) {

while loop breaks and line has null value at this spot outside the loop. while inside the loop line has always some values because it is inside the loop :).

answered Jan 1, 2018 at 15:02

captainchhala

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