How To Sort A Map In Java

Two Java examples to demonstrate how to sort a Map by its keys – TreeMap, and by its values – Comparator.

1. Sort a Map by Keys

Uses TreeMap, and keys are sorted automatically.

SortMapOnKeyExample.java

package  com.mkyong;
 
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.

Map ---> List ---> Sort ---> Map

SortMapOnKeyExample.java

package com.mkyong;
 
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