Ranked Cources, Use of PriorityQueue

Assuming getDirectFriendsForUser returns a list of direct friends in social network, assuming getAttendedCourcesForUser returns a list of courses an user is attending. Try to suggest courses according to its polularity.

I use HashMap to collect statistics of course popularity, and then sorting courses according to their polularity with priority queue.

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public List<String> getRankedCourses(String user) { List<String> rankedCourses = new LinkedList<String>(); // find all first level and second level friends Set<String> friends = new HashSet<String>(); // 1. direct friends List<String> directFriends = getDirectFriendsForUser(user); friends.addAll(directFriends); // 2. second level friends for (String friend : directFriends) { if (!user.equals(friend)) {// not need to process user again friends.addAll(getDirectFriendsForUser(friend)); } } friends.remove(user); // course statistics Map<String, Integer> courseStats = new HashMap<String, Integer>(); // for each of friend, collect statistics of their courses for (String friend : friends) { for (String course : getAttendedCoursesForUser(friend)) { if (courseStats.get(course) == null) { courseStats.put(course, 1); } else { courseStats.put(course, courseStats.get(course) + 1); } } } // use priority queue to sort the map.entry in order to return a list of // cources according to their popularity PriorityQueue<Map.Entry<String, Integer>> pq = new PriorityQueue<Map.Entry<String, Integer>>( courseStats.size(), new Comparator<Map.Entry<String, Integer>>() { public int compare(Map.Entry<String, Integer> e1, Map.Entry<String, Integer> e2) { return e2.getValue() - e1.getValue(); } }); // adding entries to pq for (Map.Entry<String, Integer> e : courseStats.entrySet()) { pq.add(e); } // prepare result list while (!pq.isEmpty()) { rankedCourses.add(pq.poll().getKey());// key from map.entry is the // course name } // excluding user's own cources rankedCourses.removeAll(getAttendedCoursesForUser(user)); return rankedCourses; }