如何按日历以字符串类型对地图排序？

Question

I have this structure:

Map<String, String> map = new HashMap<String, String>();

and data looks like this:

("10/11/2015", "A")
("10/12/2015", "B")
("10/13/2015", "C")
("10/14/2015", "D")
("10/15/2015", "E")

I want to sort the map by date (key), so my attempt was to convert the string to Calendar type and then move them into TreeMap like below:

DateFormat formatter = new SimpleDateFormat("MM/dd/yyyy");
Calendar calendar = Calendar.getInstance();

TreeMap<Calendar, String> tree = new TreeMap<Calendar, String>();

for(Map.Entry<String, String> entry : map.entrySet()){
    String key = entry.getKey();
    String value =  entry.getValue();

    calendar.setTime(formatter.parse(key));
    tree.put(calendar, value);
}

But when I print the key, the result looks like below:

10/11/2015
10/15/2015
10/14/2015
10/12/2015
10/13/2015

What should I do to sort the map by date (date in String type) correctly?

Answer 1

Based on your example, you should end up with a TreeMap with a single Calendar key, because you're using the same instance of Calendar .

Instead, if you used a new instance of Calendar for each entry, for example...

    DateFormat formatter = new SimpleDateFormat("MM/dd/yyyy");

    Map<String, String> map = new HashMap<>();
    map.put("10/11/2015", "A");
    map.put("10/12/2015", "B");
    map.put("10/13/2015", "C");
    map.put("10/14/2015", "D");
    map.put("10/15/2015", "E");

    TreeMap<Calendar, String> tree = new TreeMap<Calendar, String>();

    for (Map.Entry<String, String> entry : map.entrySet()) {
        System.out.println(entry);
        String key = entry.getKey();
        String value = entry.getValue();

        Calendar calendar = Calendar.getInstance();
        calendar.setTime(formatter.parse(key));
        tree.put(calendar, value);
    }

    for (Calendar key : tree.keySet()) {
        System.out.println(key.getTime());
    }

It would print something more like...

Sun Oct 11 00:00:00 EST 2015
Mon Oct 12 00:00:00 EST 2015
Tue Oct 13 00:00:00 EST 2015
Wed Oct 14 00:00:00 EST 2015
Thu Oct 15 00:00:00 EST 2015

If you're using Java 8(+), then you really should be using the new Date/Time API, for example...

Map<LocalDate, String> tree = new TreeMap<>();

for (Map.Entry<String, String> entry : map.entrySet()) {
    System.out.println(entry);
    String key = entry.getKey();
    String value = entry.getValue();

    LocalDate ld = LocalDate.parse(key, DateTimeFormatter.ofPattern("MM/dd/yyyy"));
    tree.put(ld, value);
}

for (LocalDate key : tree.keySet()) {
    System.out.println(key);
}