Sort keys which are date entries in a hashmap

Question

I have a hashMap which has the following values as key value(sql date , integer) pairs:

a.put("31-05-2011",67);
a.put("01-06-2011",89);
a.put("10-06-2011",56);
a.put("25-05-2011",34);

when i try to sort the hashMap based on the keys using : Map modified_a=new TreeMap(a); and display the keys it is as follows :

01-06-2011,10-06-2011,25-05-2011, 31-05-2011

but I want the keys to be sorted as

31-05-2011,25-05-2011,01-06-2011 ,10-06-2011

I can see that the values are being sorted based on the first 2 digits( which is the date value) but I need the month value to also be considered and sort based on months first and then for each month sort the corresponding days. Any clues ??

Answer 1

You can use like

Map<Date, Integer> m = new HashMap<Date, Integer>(); 

    DateFormat dateFormat = new SimpleDateFormat("dd-MM-yyyy");

    m.put(new java.sql.Date(dateFormat.parse("31-05-2011").getTime()),67);
    m.put(new java.sql.Date(dateFormat.parse("01-06-2011").getTime()),89);
    m.put(new java.sql.Date(dateFormat.parse("10-06-2011").getTime()),56);
    m.put(new java.sql.Date(dateFormat.parse("25-05-2011").getTime()),34);


    Map<Date, Integer> m1 = new TreeMap(m);
    DateFormat df = new SimpleDateFormat("dd/MM/yyyy");

    for (Map.Entry<Date, Integer> entry : m1.entrySet())
    {
        System.out.println(df.format(entry.getKey()));
    }

Answer 2

The best solution IMO would be to use a different data type for the keys - a data type which actually represents a date, and which sorts in natural date order. Unless otherwise constrained, I'd use Joda Time's LocalDate type, which represents exactly what you want (just a date, not a date/time etc).

If you really want to use string keys but can change the format of them, you could use a yyyy-MM-dd format, which is naturally sortable.

Alternatively, you could pass in a Comparator<String> to the TreeMap constructor, where the comparator is one which parses two strings when it's asked to compare them, and performs the comparison based on the parsed year/month/day values. There isn't a constructor which takes both a custom comparator and an existing map though, so you'd need something like:

Map<String, Integer> modified = new TreeMap<String, Integer>(customComparator);
modified.putAll(a);

This approach will be relatively slow if you have a lot of data (due to the repeated parsing), and slightly fiddly to write - I'd use a more appropriate data type if you possibly can.