Sorting a Map In Java

In this Post we will see how to sort Map by its keys and by values.


1. Sort a Map by Keys

Uses TreeMap, and keys are sorted automatically.

SortMapOnKeyExample.java

import java.util.HashMap;
import java.util.Map;
import java.util.TreeMap;

public class SortMapOnKeyExample {



public static void main(String[] args) {

Map<String, String> unsortMap = new HashMap<String, String>();
unsortMap.put("2", "B");
unsortMap.put("1", "A");
unsortMap.put("4", "D");
unsortMap.put("3", "B");
unsortMap.put("7", "C");
unsortMap.put("5", "z");
unsortMap.put("6", "b");
unsortMap.put("8", "a");

System.out.println("Unsort Map......");
printMap(unsortMap);

System.out.println("Sorted Map......");
Map<String, String> treeMap = new TreeMap<String, String>(unsortMap);
printMap(treeMap);

}

public static void printMap(Map<String, String> map) {
for (Map.Entry entry : map.entrySet()) {
System.out.println("Key : " + entry.getKey() + " Value : "
+ entry.getValue());
}
}

}



Output:

Unsort Map......
Key : 3 Value : B
Key : 2 Value : B
Key : 1 Value : A
Key : 7 Value : C
Key : 6 Value : b
Key : 5 Value : z
Key : 4 Value : D
Key : 8 Value : a
Sorted Map......
Key : 1 Value : A
Key : 2 Value : B
Key : 3 Value : B
Key : 4 Value : D
Key : 5 Value : z
Key : 6 Value : b
Key : 7 Value : C
Key : 8 Value : a




2. Sort a Map by Values

The overall idea is, convert the Map into a List, sort the List by Comparator and put the sorted list back to a Map.


SortMapOnKeyExample.java


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 SortMapOnValueExample {

public static void main(String[] args) {

Map<String, String> unsortMap = new HashMap<String, String>();
unsortMap.put("2", "B");
unsortMap.put("1", "A");
unsortMap.put("4", "D");
unsortMap.put("3", "B");
unsortMap.put("7", "C");
unsortMap.put("5", "z");
unsortMap.put("6", "b");
unsortMap.put("8", "a");

System.out.println("Unsort Map......");
printMap(unsortMap);

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

}

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
                //LinkedHashMap make sure order in which keys were inserted
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;
}

public static void printMap(Map<String, String> map){
for (Map.Entry entry : map.entrySet()) {
System.out.println("Key : " + entry.getKey()
                                   + " Value : " + entry.getValue());
}
}
}



Output:

Unsort Map......
Key : 3 Value : B
Key : 2 Value : B
Key : 1 Value : A
Key : 7 Value : C
Key : 6 Value : b
Key : 5 Value : z
Key : 4 Value : D
Key : 8 Value : a
Sorted Map......
Key : 1 Value : A
Key : 3 Value : B
Key : 2 Value : B
Key : 7 Value : C
Key : 4 Value : D
Key : 8 Value : a
Key : 6 Value : b
Key : 5 Value : z