How to sort a TreeMap by (Date) values?

Question

Possible Duplicate:
TreeMap sort by value
Map that could be iterated in the order of values

I have a TreeMap of type <String, Date>. I want to sort it by the Dates (most recent first), and I can't use them as the keys as I cannot guarantee that they will be unique. Is there any way to do this?

@Nambari, the answer to that question doesn't work when values are non-unique. — Louis Wasserman, Dec 12 '12 at 17:31
@Louis, why shouldn't it work when the values are non-unique? — ignis, Dec 12 '12 at 17:32
@LouisWasserman: Ok. Actually I was looking for your answer, but thought link I have does same job. I am removing it. — kosa, Dec 12 '12 at 17:32
@AleksanderBlomskøld's linked solution also fails _really weirdly_ for duplicate values. Look at the comments. — Louis Wasserman, Dec 12 '12 at 17:41

Jack · Accepted Answer · 2012-12-12T17:39:32.137

5

You can't directly do it, what you can do is to copy the content of the collection into a new one and then sort it, eg:

List<Map.EntrySet<String, Date>> copy = new ArrayList<Map.EntrySet<String, Date>>(treeMap.entrySet());
Collections.sort(copy, new CustomComparator());

class CustomComparator implements Comparator<Map.Entry<String,Date>> {
  public int compareTo(Map.Entry<String,Date> e1, Map.Entry<String,Date> e2) {
    // compare your dates
  }
}

Of course this collection won't be synced with the original one so you must do it again every time the original TreeSet is modified.

edited Dec 12 '12 at 17:39

answered Dec 12 '12 at 17:33

Jack

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Is the comaprsion and sort based on todays date? Like a birth day thing? – Skynet Dec 09 '13 at 07:45

Evgeniy Dorofeev · Answer 2 · 2012-12-12T17:56:05.320

1

Actually you can have duplicate keys in TreeMap if you really want it

    Map<Date, String> map = new TreeMap<Date, String>(new Comparator<Date>() {
        @Override
        public int compare(Date d1, Date d2) {
            return d1.after(d2) ? 1 : -1;
        }
    });
    Date d1 = new Date(-100000000000L);
    Date d2 = new Date(100000000000L);
    map.put(d2, "s1");
    map.put(d1, "s2");
    map.put(d1, "s3");
    System.out.println(map);

output

{Mon Oct 31 16:13:20 EET 1966=s3, Mon Oct 31 16:13:20 EET 1966=s2, Sat Mar 03 11:46:40 EET 1973=s1}

Note that 2 entries have the same key, and output is sorted by Date as you wanted

edited Dec 12 '12 at 17:56

answered Dec 12 '12 at 17:47

Evgeniy Dorofeev

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A `Map` by definition cannot have two values for one key. What am I missing? – Miserable Variable Dec 12 '12 at 17:55
Comparator used in the test allows it. All depends on Comparator annd your desire, you can make TreeMap think that 2 is less than 1 if need be. – Evgeniy Dorofeev Dec 12 '12 at 17:58
Oh, your comparator never returns 0. – Miserable Variable Dec 12 '12 at 18:04
So you can put things into this map and iterate it but can't get anything from it, amirite? – Miserable Variable Dec 12 '12 at 18:33
Of course get() does not make sense. You cant get 2 values with one get – Evgeniy Dorofeev Dec 12 '12 at 18:48

How to sort a TreeMap by (Date) values?

2 Answers2