Regarding Appending files..

Hello Everyone,
Here I ve a code which lists the keys that are not found between two
properties files and writes that to a log file named
KeysNotFound.log...
now each time,when i compile by changing the property file to be
compared with a standard property file,it overwrites the existing log
file and writes the latest compiled version..
so what i want to know is, the last compiled version has to be appended
to the previous compiled..so if i ve compiled using 4 property files,
say, then all the four has to be appended to the same log file in
sequence..please can you people help me?
Thanks in Advance,
Megha.
import java.io.*;
import java.util.*;
import java.lang.*;
class DirFile{
    public static void main(String args[]){
        Properties properties1 = new Properties();
        Properties properties2 = new Properties();
        Properties properties3 = new Properties();
        String dirname = "/Properties Files";
        File f1 = new File(dirname);
            if(f1.isDirectory()){
                System.out.println("Directory of " +dirname);
                String s[] = f1.list();
                for(int i=0;i<s.length;i++){
                    File f = new File(dirname +s[i]);
                    if(f.isDirectory()){
                        System.out.println(s[i] + " is a directory");
                    }
                    else{
                        System.out.println(s[i] + " is a file");
                    }
                }
        }
        else{
            System.out.println(dirname + " is not a directory");
        }
        try{
            properties1.load(new FileInputStream("FileNames.properties"));
        }
        catch(IOException e){
        }
        Enumeration enumeration = properties1.propertyNames();
        String str,str1,str2,str3;
            while(enumeration.hasMoreElements()){
                str =(String)enumeration.nextElement();
                System.out.println(str);
        }
                str1 = properties1.getProperty("ReferenceFile");
                str2 = properties1.getProperty("File1");
                System.out.println("The keyValue for File1 is: " +str1);
                System.out.print("The keyValue for ReferenceFile is: " +str2);
        try{
            properties2.load(new FileInputStream(str1));
            properties3.load(new FileInputStream(str2));
        }
            catch(IOException e){
            }
             try{
                    BufferedWriter buffwriter = new BufferedWriter(new
FileWriter("KeysNotFound.log"));
                    Date date = new Date(System.currentTimeMillis());
                    System.out.println("\n" +date + "\n");
                    Enumeration enumeration1 = properties2.propertyNames();
                    while(enumeration1.hasMoreElements()){
                        str3 =(String)enumeration1.nextElement();
                        String value2 = properties3.getProperty(str3);
                        if(value2 == null){
                            System.out.println(" " + "Key not found :" +str3);
                            buffwriter.write("\n" + "[" +date + "]" +str3 );
                        }
                    }
                    buffwriter.close();
                }
                catch(IOException e){
            }
    }
}