How to sort dates in Map in java with other value

Question

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How to sort a Map<Key, Value> on the values in Java?

I have a map in java with Map<String, String>.Map contaions <Id, Date>. i want to sort these map according to date. how can i do that? This is the map after print it

-------------------------------------
PTRRRRRR     08-20-2011
-------------------------------------
TESTDESC     08-01-2012
-------------------------------------
TESTMYO      12-29-2011
-------------------------------------
TESTTEST     08-01-2012
-------------------------------------
TESTPROL     08-01-2012
-------------------------------------
TESTPRO      08-01-2012
-------------------------------------
PNAMELQ      08-02-2012
-------------------------------------
TESTMYO      12-29-2011
------------------------------------

`TESTPRO` appears more than once, but keys have to be unique?? — Peter Lawrey, Aug 30 '12 at 08:13
Are they actually `Date` objects or are they strings? Date strings in that format don't sort the way you want without effort. — verdesmarald, Aug 30 '12 at 08:15
Check this.http://stackoverflow.com/questions/8298290/how-to-sort-hashmap-based-on-date — Sajith, Aug 30 '12 at 08:15
@Sajith It seems the dates are the values, not the keys, so your link would not apply. — assylias, Aug 30 '12 at 08:16
If you want to sort by value then this would be helpful: http://stackoverflow.com/questions/109383/how-to-sort-a-mapkey-value-on-the-values-in-java — Lorand Bendig, Aug 30 '12 at 08:16
http://stackoverflow.com/questions/109383/how-to-sort-a-mapkey-value-on-the-values-in-java — munyengm, Aug 30 '12 at 08:17

Peter Lawrey · Answer 1 · 2012-08-30T08:25:44.697

4

You can use a custom Comparator like this.

String data = "PTRRRRRR     08-20-2011\n" +
        "TESTDESC     08-01-2012\n" +
        "TESTMYO      12-29-2011\n" +
        "TESTTEST     08-01-2012\n" +
        "TESTPROL     08-01-2012\n" +
        "TESTPRO      08-01-2012\n" +
        "TESTPRO      08-01-2012\n" +
        "PNAMELQ      08-02-2012\n" +
        "TESTMYO      12-29-2011\n";
Map<String, String> map = new LinkedHashMap<String, String>();
for (String line : data.split("\n")) {
    String[] keyValue = line.split(" +");
    map.put(keyValue[0], keyValue[1]);
}
Map.Entry<String, String>[] entries = map.entrySet().toArray(new Map.Entry[map.size()]);
Arrays.sort(entries, new Comparator<Map.Entry<String, String>>() {
    SimpleDateFormat date = new SimpleDateFormat("MM-dd-yyyy");

    @Override
    public int compare(Map.Entry<String, String> o1, Map.Entry<String, String> o2) {
        try {
            return date.parse(o1.getValue()).compareTo(date.parse(o2.getValue());
        } catch (ParseException e) {
            throw new AssertionError(e);
        }
    }
});
for (Map.Entry entry : entries) {
    System.out.println(entry);
}

prints

PTRRRRRR=08-20-2011
TESTMYO=12-29-2011
TESTDESC=08-01-2012
TESTTEST=08-01-2012
TESTPROL=08-01-2012
TESTPRO=08-01-2012
PNAMELQ=08-02-2012

edited Aug 30 '12 at 08:25

answered Aug 30 '12 at 08:18

Peter Lawrey

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using Collections#sort on Map#entrySet would be faster than copying data to an array and using Arrays#sort. – Marc-Christian Schulze Aug 30 '12 at 08:20
Good answer. Really makes one appreciate the beauty of `map.OrderBy(x => DateTime.Parse(x.Value))` though! – verdesmarald Aug 30 '12 at 08:22
We'll have to wait for Java 8 for that. But it will come... – Marc-Christian Schulze Aug 30 '12 at 08:23
@coding.mof It would be faster, except you can't do that. ;) Collections.sort requires a List. – Peter Lawrey Aug 30 '12 at 08:24
Good point. ;) Never realized that. – Marc-Christian Schulze Aug 30 '12 at 08:27

score 1 · Accepted Answer · answered Aug 30 '12 at 08:29

First check http://docs.oracle.com/javase/6/docs/api/java/util/Map.html and notice, that there are differnt Map Implementations:

The order of a map is defined as the order in which the iterators on the map's collection views return their elements. Some map implementations, like the TreeMap class, make specific guarantees as to their order; others, like the HashMap class, do not.

In http://docs.oracle.com/javase/6/docs/api/java/util/TreeMap.html you'll find this definition:

The map is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time, depending on which constructor is used.

Since you want to compare by value (Map<String, Date>), you need to implement a Comparator that compares by value (this is untested):

class ValueComparator implements Comparator<String> {

    Map<String, Date> map;
    public ValueComparator(Map<String, Date> map) {
        this.map = map;
    }

    public int compare(String a, String b) {
        return map.get(a).compareTo(map.get(b));
    }
}

(You may want to build the ValueComparator more generic.)

Finally construct the map with

Map<String, Date> sorted = new TreeMap<String, Date>(new ValueComparator(unsortedMap));
sorted.putAll(unsortedMap);

How to sort dates in Map in java with other value

2 Answers2