Lazy way on how to generate pretty huge set of distinct (unique) hex values

We sometimes need to generate huge set of quite short distinct identifiers. Another requirement is that the identifiers generated with the chosen method (despite of a huge generated set) are still from the much bigger set and actually represent a small subset of that (this for example could help when you generate the number which you would like is hard to guess. probably to generate the invites for your site).

Here is the lazy way on how to do that using java. Using it I generated 500K distinct hex values (8-digit in a maximum) from 4.3 bil total space in a few seconds. That means the user will have to try to guess the right value for approximately 9K times.

So.. this is the code

import java.io.BufferedOutputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.util.ArrayList;
import java.util.UUID;


public class Generator {
 
 public class DummyStringKeeper {
  
  String string;
  
  public DummyStringKeeper(String string){
   this.string = string;
  }
 }

 public static void main(String args[]) throws IOException{
  BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream("c:/temp/items_expoted.txt"));
  ArrayList<DummyStringKeeper> list = new ArrayList<DummyStringKeeper>();
  for (int i=0; i<500000; i++){
   DummyStringKeeper randomItem = new Generator().new DummyStringKeeper(UUID.randomUUID().toString());
   list.add(randomItem);
   String randomItemString = randomItem.toString().substring(randomItem.toString().indexOf("@")+1) + "\n";
   bos.write(randomItemString.getBytes());
  }
  bos.flush();
  bos.close();
 }
 
}