Question

I have a list of specific objects(meetings) which I need to group using a field attribute(java.util.Date) and place them in another Object which is a container for grouped meetings type objects belonging to the same month/year.

So I am thinking about some code which receives a meetings list fetched from Database and than group them by month/year in specific MonthMeetings objects, for this I have a code using a Map like:

public static Map<String, MonthMeetings> groupMeetingsByMonth(
        List<Meeting> meetings) {

    // this is by default sorted by keys I'd like to have it sorted by using
    // comparator on MonthMeetings objects, sorted by map values instead of
    // keys like SortedMap default impl.
    SortedMap<String, MonthMeetings> listMeetingsGroupedByMonth = new TreeMap<String, MonthMeetings>();

    String labelMonthYear = "";

    // reference to put in map.
    MonthMeetings monthMeetings = null;

    for (Iterator iterator = meetings.iterator(); iterator.hasNext();) {

        // grab a meeting from list
        Meeting meeting = (Meeting) iterator.next();

        // get meeting date
        Date dMeeting = meeting.getScheduledDate();

        GregorianCalendar gCalendar = new GregorianCalendar();
        gCalendar.setTime(dMeeting);

        // simple method that build label in format MM_yyyy to use as map
        // key.
        labelMonthYear = generatesLabelMonthYear(gCalendar);

        // check if MonthMeetings for specific month/year already exists in
        // Map
        monthMeetings = (MonthMeetings) listMeetingsGroupedByMonth
                .get(labelMonthYear);

        if (monthMeetings == null) {

            // if not create MonthMeetings and add first meeting to it
            monthMeetings = new MonthMeetings();
            monthMeetings.addMeeting(meeting);
            listMeetingsGroupedByMonth.put(labelMonthYear, monthMeetings);

        } else {

            // if does exist just add another meeting to it
            monthMeetings.addMeeting(meeting);

        }
    }

    return listMeetingsGroupedByMonth;
}

This piece of code, I believe, solves the first part of the problem, grouping meetings on its specific MonthMeetings objects. Now I am looking forward to sort this map by MonthMeetings specific month/year values. I am probably going to try implementing comparable on MonthMeetings objects, but than came the question:

How to sort a map based on it’s values, not keys, when these are objects?

Any suggestions on this?

tx in advance.

Answer 1

Create a TreeSet<MonthMeeting> and add all the values like this:

TreeSet<MonthMeeting> sorted = new TreeSet<MonthMeeting>(new MyComparator());
sorted.addAll(monthMeetingMap.values());

Now it’s sorted on values. MyComparator will be something like:

public class MyComparator implements Comparator<MonthMeeting>() {
...
}