Using a Random Access File

 try {
   File f = new File("filename");
   RandomAccessFile raf = new RandomAccessFile(f, "rw");
     
   // Read a character.
   char ch = raf.readChar();
   
   // Seek to end of file.
   raf.seek(f.length());
    
   // Append to the end.
   raf.writeChars("aString");
   raf.close();
   } catch (IOException e) {
 }