In processing you can use saveStrings() and loadStrings(). In java you could do something like the following:
Code:

//Writing to a file:
try{
  //setting the second parameter in the FileWriter to true
  //appends the text to what is already there. Otherwise it 
  //will overwrite the file.
  BufferedWriter out = new BufferedWriter(new FileWriter("myfile.txt",true));
  out.write("Hello again, ");
  out.close();
} catch(IOException e) {
  e.printStackTrace();
}

//reading from the file:
try{
  BufferedReader in = new BufferedReader(new FileReader("myfile.txt"));
  String s;
  while((s = in.readLine()) != null){
    println(s);
  }
  in.close();
}catch(IOException e){
  e.printStackTrace();
}
 

Keep in mind that this works fine from processing but when you create an applet the applet will not have the permissions to read and write to the users computer. If you want this functionality you need to sign the applet. Check out toxi's post for how to do that http://processing.org/discourse/yabb/YaBB.cgi?board=Contributions_Beyond;action=display;num=1102415302#4
Hope this helps.