Question

This is my hashmap :

Map<String,String> unsortMap = new HashMap<String,String>() this map contains values like follows

unsortMap.put("18/06/2012", "18/06/2012");
unsortMap.put("19/06/2012", "19/06/2012");
unsortMap.put("20/06/2012", "20/06/2012");
unsortMap.put("26/06/2012", "26/06/2012");
unsortMap.put("27/06/2012", "27/06/2012");
unsortMap.put("04/07/2012", "04/07/2012");
unsortMap.put("13/07/2012", "13/07/2012");
unsortMap.put("29/06/2012", "29/06/2012");

the code that i use to sort this map is as follows :

package samples;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;

public class SortMapExample{

   public static void main(String[] args) {

    System.out.println("Unsort Map......");
    Map<String,String> unsortMap = new HashMap<String,String>();

    unsortMap.put("18/06/2012", "18/06/2012");
    unsortMap.put("19/06/2012", "19/06/2012");
    unsortMap.put("20/06/2012", "20/06/2012");
    unsortMap.put("26/06/2012", "26/06/2012");
    unsortMap.put("27/06/2012", "27/06/2012");
    unsortMap.put("04/07/2012", "04/07/2012");
    unsortMap.put("13/07/2012", "13/07/2012");
    unsortMap.put("29/06/2012", "29/06/2012");

    Iterator iterator=unsortMap.entrySet().iterator();

        for (Map.Entry entry : unsortMap.entrySet()) {
            System.out.println("Key : " + entry.getKey() 
                + " Value : " + entry.getValue());
        }

        System.out.println("Sorted Map......");
        Map<String,String> sortedMap =  sortByComparator(unsortMap);

        for (Map.Entry entry : sortedMap.entrySet()) {
            System.out.println("Key : " + entry.getKey() 
                + " Value : " + entry.getValue());
        }
   }

   private static Map sortByComparator(Map unsortMap) {

        List list = new LinkedList(unsortMap.entrySet());

        //sort list based on comparator
        Collections.sort(list, new Comparator() {
             public int compare(Object o1, Object o2) {
               return ((Comparable) ((Map.Entry) (o1)).getValue())
               .compareTo(((Map.Entry) (o2)).getValue());
             }
    });

        //put sorted list into map again
    Map sortedMap = new LinkedHashMap();
    for (Iterator it = list.iterator(); it.hasNext();) {
         Map.Entry entry = (Map.Entry)it.next();
         sortedMap.put(entry.getKey(), entry.getValue());
    }
    return sortedMap;
   }    
}

The result i get is this:

Sorted Map......
Key : 04/07/2012 Value : 04/07/2012
Key : 13/07/2012 Value : 13/07/2012
Key : 18/06/2012 Value : 18/06/2012
Key : 19/06/2012 Value : 19/06/2012
Key : 20/06/2012 Value : 20/06/2012
Key : 26/06/2012 Value : 26/06/2012
Key : 27/06/2012 Value : 27/06/2012
Key : 29/06/2012 Value : 29/06/2012

I want the result to be like the following : (the map should be sorted based on the month also)

18/06/2012
19/06/2012
20/06/2012
26/06/2012
27/06/2012
29/06/2012
04/07/2012
13/07/2012

Pleas help me how to about this?

Answer 1

Convert your data to a SortedMap, specifically a TreeMap instance.

Use the constructor that takes a Comparator and within that, do your comparison logic.