Hello Carla,

Thank you for your help

I tried to run the program it works, but not as expected.
I only changed reverseStr9 into reverseStr11

***************************************************
javac reverseStr11.java

java reverseStr11 piet.txt

result:
===========================================================

.noitseuq ym rewsna nac ohw ydobemos si ereht taht epoh I

Exception in thread "main" java.lang.NullPointerException
       at java.lang.StringBuffer.<init>(StringBuffer.java:129)
       at reverseStr11.main(reverseStr11.java:22)
==========================================================

content file: piet.txt
==========================================================
begin line

Hello this is piet in the hottentottententoonstelling.
I hope that there is somebody who can answer my question.
greetings

end line
==========================================================

I used your modified version.
used java source code:
==========================================================
import java.io.*;
import java.util.*;
//import java.lang.*;

public class reverseStr11{
       public static void main(String args[]) throws Exception{
            File file = new File(args[0]);

                if(!file.exists() || !file.canRead()) {
                System.out.println("Can't read " + file);
                return;
                }

                else
                    try{
                    FileReader fr = new FileReader(file);
                    BufferedReader in = new BufferedReader(fr);
   
                        for ( String line = in.readLine() ; line != null ; line =
in.readLine()) {
       
       
                        StringBuffer strTemp = new StringBuffer(in.readLine());
                       String newString = strTemp.reverse().toString();
                        System.out.println(newString);
       
                        }
                   }

                   catch ( FileNotFoundException e ) {
                       System.out.println( "File Disappeared" );
                   }
                  }
}

==========================================================

I also tested the program with another textfile named piet2.txt and
here are the results:

content piet2.txt
==============================================
begin line
hallo dit is tekst van mij de piet2 DE ED ADE
fdasfhsadhkjfkjsadkfjakjdHF
import java.io.*;
import java.util.*;
import java.lang.*;

class reverseStr9{
    public static void main(String args[]) throws Exception{
        File file = new File(args[0]);
       
        if(!file.exists() || !file.canRead()) {
            System.out.println("Can't read " + file);
            return;
        }
       
        else
            try{       
                FileReader fr = new FileReader(file);
                BufferedReader in = new BufferedReader(fr);
               
               
                StringBuffer strTemp = new StringBuffer(in.readLine());
                String newString = strTemp.reverse().toString();           
                System.out.println(newString);

              }
           
          catch ( FileNotFoundException e ) {
               System.out.println( "File Disappeared" );
           }           
}
}

END LINE
============================================

result after typing in: java reverseStr11 piet2.txt
=========================================
EDA DE ED .......
;*.oi.avaj tropmi

;) "deraeppasiD eliF" (nltnirp.tuo.metsyS
}

Exception in thread "main" java.lang.NullPointerException
       at java.lang.StringBuffer.<init>(StringBuffer.java:129)
       at reverseStr11.main(reverseStr11.java:22)
=========================================
I tried to copy the output result but didn't succeeded so I typed it
in.
But as you can see. The piet2.txt file is more succesfull than the
piet.txt file. My question is why?
Perhaps you got another result.

Thanks.