private List<Integer> findSharedUsers(int movie1, int movie2) {
		List<Integer> sharedUsers = new ArrayList<Integer>();
		HashSet<Integer> movie1Raters = usersByMovie.get(movie1);
		HashSet<Integer> movie2Raters = usersByMovie.get(movie2);

		// Return empty list if one user didn't rate any movies
		if (movie1Raters == null || movie2Raters == null) {
			return sharedUsers;
		}

		Iterator<Integer> it = movie1Raters.iterator();
		while (it.hasNext()) {
			Integer value = (Integer) it.next();
			if (movie2Raters.contains(value)) {
				sharedUsers.add(value);
			}
		}
		return sharedUsers;
	}