FileWriter Question

[Chintan B Shah]

Hi all,

I am trying to understand how write(), flush() and close() work, however I am having some trouble understanding the concept.

Here's the code.

    
package com.scjp6.FileandIOAPI;
import java.io.*;
import static java.lang.System.out;
public class FileAPI {
    
    public static void main(String[] args){
        File f1= new File("chintan.txt");//just creating a File Object..this doesnt actually create a file
        out.println("File exists?"+f1.exists());
     try{
        
           
        //WRITE TO A FILE USING FileWriter Object
        FileWriter fw= new FileWriter(f1);
        FileWriter fw1= new FileWriter(f1,true);
        
        fw.write("Hello Chintan");
        fw1.write("\nAppending fw1");
        
        
        fw.write("\nHello World");
        fw1.write("\nAppending fw1 again");
        
        fw1.flush();
        fw1.close();
        
        fw.flush();
        fw.close();
       
     }
     catch(IOException e){
        e.printStackTrace();
     }
   
    }
    

}

My expected output is:

1. fw1 is flushed first in code so(also, fw1's constructor says append is true, so it should actually append to an empty file first), here's output I am expecting
Appending fw1
Appending fw1 again
Hello Chintan
Hello World

2. This output is true if fw flushes before fw1 (which should not be possible).
Hello Chintan
Hello World
Appending fw1
Appending fw1 again

However, when I open the file to see output it comes to:
Hello Chintan
Hello Worldfw1 again


Any help much appreciated.

Thanks
Chintan.

[Rob Spoor]

The two FileWriters work on the same file, but they don't know about each other. Therefore:
- fw empties the file, then starts at the start, increasing its own position in the file, ignoring everything that fw1 does
- fw1 starts at the end of the current file, but that has just been emptied. It also uses its own position in the file, ignoring everything that fw does.

So fw writes "Hello Chintan\nHello World" and fw1 writes "\nAppending fw1\nAppending fw1 again". Since fw is flushed later, its contents will be leading. So the combined version (I replaced \n with _ so it will also take up only one character):

Hello Chintan_Hello World
_Appending fw1_Appending fw1 again

The result, with fw overwriting everything fw1 did but not removing any of its data: "Hello Chintan\nHello World" followed by the remaining "fw1 again". This is what you are getting:

Hello Chintan
Hello Worldfw1 again

[Chintan B Shah]

Wow! Great explanation! I am now able to understand how code works.

Thanks so much Rob.

Best Regards,
Chintan.

[Chintan B Shah]

Just out of curiosity I commented out the lines that performed "flush()" and the output still remains same.

So, is it really necessary to use flush()?

If yes, then under what circumstances do we actually need to use flush()?

I have poked around close() API and couple of other threads in this forum and it says close() API flushes first and then it closes.

It looks like its behaving this way (at least in the above code ).


Thanks
Chintan.

[Rob Spoor]

You can use flush() to ensure the data is written immediately. You can use this to write intermediate data.