Java Programming

Thanks to anyone who can help me here - I'm very confused.

I have a class WebAppUtility that looks like this:

private static Set<HttpSession> openSessions = Collections
			.synchronizedSet(new HashSet<HttpSession>());
	
	public static void addSession(HttpSession session) {
		synchronized(openSessions){
			openSessions.add(session);
		}
	}
	
	public static boolean removeSession(HttpSession session) {
		synchronized(openSessions){
			return openSessions.remove(session);
		}
	}
	public static Set<String> getLoggedInUsers() throws Exception {
		Set<String> set = new HashSet<String>();
		synchronized (openSessions) {
			for(Iterator i = openSessions.iterator(); i.hasNext();){
				HttpSession session = (HttpSession)i.next();
                              //grab attribute from session here and stick in "set"
			}
		}
		return set;
	}

Somehow when I have many users logging in and out I'm getting a ConcurrentModificationException under the getLoggedInUsers method:

java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(HashMap.java:787)
at java.util.HashMap$KeyIterator.next(HashMap.java:823)
at com.studentuniverse.utility.WebAppUtility.getLoggedInUsers(WebAppUtility.java:68)

Line 68 is the one that looks like:

HttpSession session = (HttpSession)i.next();

Am I misusing the synchronizedSet in some way? Please guide me to proper threaded Set usage! Thank you very much.

_kris