Java Read/Write Fixed Length file sample

public static void TempPreProcessor_Fixed(String input, String output){
 BufferedReader reader = null;
 BufferedWriter writer = null;
 try 
 {
  String inputfilename = input;
  String outputfilename = output;
  
  reader = new BufferedReader(
     new InputStreamReader(
       new FileInputStream(inputfilename)));
  writer = new BufferedWriter(
     new OutputStreamWriter(
       new FileOutputStream(outputfilename)));
  
  String thisLine = null;

  //line 1
  if((thisLine =reader.readLine()) != null) 
   writer.write("AA,"+thisLine+"\n");

  //line 2-4
  for(int i = 0; i<3; i++){
   if((thisLine =reader.readLine()) != null)
    writer.write(thisLine+"\n");
  }
  
  //remaining lines
  while((thisLine=reader.readLine()) != null){   
   //add AB to start of line
   writer.write("AB,"+thisLine+"\n");
   //copy with no change
   if((thisLine=reader.readLine()) != null)
    writer.write(thisLine+"\n");
  } 
 }catch(Exception e){
  throw new ServiceException("TempPreProcessor: Unknown errors: " + e.toString());
 }finally{
  try{
   if(writer != null) writer.flush();
   if(reader != null) reader.close();
   if(writer != null) writer.close();
   reader=null; writer=null;
  }catch(Exception e){
   throw new ServiceException("TempPreProcessor: Unknown errors: " + e.toString());
  }
 }
}