my task is to write to a .dat file

I recently posted some code and tested the code with a .txt file. now that I have the code working my task is to change the .txt to a .dat file, now my code does not write to the file. Am I missing something or do something wrong, I need some help and answers to this. Can anyone help me? I have enclosed the code if anyone needs to look at it to figure out why it doesn't work with a .dat file.

import java.util.*;
import java.io.*;
import java.lang.*;

 public class StringtoFile {  

     String file=new String("UserStrings.dat");  

     FileWriter fileStream = null ;  

       {try {  
         fileStream   = new FileWriter(file);  
       		} 
       catch (IOException e) {  
          System.out.println("there was an Error");
       		}
       }  

     public static void main(String[] args) throws IOException  

     		{  
    	 			StringtoFile h = new StringtoFile ();  
    	 			h.openFile();  
    	 			h.addline();  
    	 			h.closefile();  
     		}     

     
     private Scanner scan, X;     

     
     public void openFile()
     		{            
         		X = new Scanner(file);  
     		}  

     public void addline(){    

         String str="initial";  
         System.out.println("Enter a Sentence....."); 
         
         while(!str.equalsIgnoreCase("done")) {  

            // System.out.println("Enter a Sentence.....");  

             Scanner in=new Scanner(System.in);            

                 str = in.nextLine();
                     try {  
                         if(!str.equalsIgnoreCase("done"))  
                        	 fileStream.write(str+"\n");  
                      	} 
                     catch (IOException e) {  
                    	 System.out.println("There was an Error");
                     }  
         		}
         }  

     	public void closefile(){  

         X.close();  
     	}  
 	}