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.naresh.map;

import java.util.HashMap;

import java.util.Map;

import java.util.TreeMap;

public class SortMapOnKeyExample {

      public static void main(String[] mnb) {

            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.naresh.map;

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[] mnb) {

            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);

      }

      @SuppressWarnings("unchecked")

      private static Map sortByComparator(Map unsortMap) {

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

            // sort list based on comparator

            Collections.sort(list, new Comparator() {

                  @SuppressWarnings({ "rawtypes", "unchecked" })

                  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