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

References

mkyong

Founder of Mkyong.com, love Java and open source stuffs. Follow him on Twitter, or befriend him on Facebook or Google Plus.

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private static final Comparator<Map.Entry<String, String>> valueComparator = new Comparator<Map.Entry<String, String>>() { @Override public int compare(Map.Entry<String, String> e1, Map.Entry<String, String> e2) { //comparing by values, if it's equals, compare by keys // in other case, entries with equals values will be removed if(e1.getValue().equals(e2.getValue())) return e1.getKey().compareTo(e2.getKey()); return (e1.getValue()).compareTo(e2.getValue()); } }; private static Map<String, String> sortByComparator( Map<String, String> unsortMap) { // sorted set based on comparator SortedSet<Map.Entry<String, String>> set = new TreeSet<>( valueComparator); set.addAll(unsortMap.entrySet()); // LinkedHashMap make sure order in which keys were inserted Map<String, String> sortedMap = new LinkedHashMap<>(); for (Iterator<Map.Entry<String, String>> it = set.iterator(); it .hasNext();) { Map.Entry<String, String> entry = it.next(); sortedMap.put(entry.getKey(), entry.getValue()); } return sortedMap; }

public static <K, V extends Comparable> Map sortByValues(final Map map) { Comparator valueComparator = new Comparator<K>() { public int compare(K k1, K k2) { int compare = ((String) map.get(k2)).compareTo((String) map.get(k1)); if (compare == 0) { return 1; } else { return compare; } } }; Map sortedByValues = new TreeMap<K, V>(valueComparator); sortedByValues.putAll(map); return sortedByValues; }

Soumyasanta Bhowmik
Hello mkyong,
I have used below and I am getting key as sorted way.
Map<String, List> objNormalTemplateHashMap=new TreeMap<String, List>();
Please let me know, is this correct way to do?
Thanks , Soumyasanta Bhowmik.
Java
Sorting TreeMap with key will not work when the key is more than 10. The result will not be in a soreted manner.
http://www.river2c.com Free Job Posting and CV or Resume download Job Portal
It is very useful information in my one of functional part in my Project
andresrolo
this is perfect.
Thanks
from colombia
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http://randomtechieblog.blogspot.com Mahendra
Very good example but it will fail if you try using null as value.
Alex
Why not to use SortedMap? It exists in the Java API since 1.2 version. There is no need to re-invent anything!
- Daniel
  Because SortedMap orders by Keys, this implementation sorts by the values.
http://www.echidna-band.com Potney
Thanks for the very nice piece of code :)
Balasubramaniyan Murugesan
Very nice website thanks for ur work..
Pingback: ???Java??Map????? | Wenming's Blog
Marian
Nice man,
Mine, very quick attempt without much thinking, after having a few beers:) :
sortedValues = new TreeSet(unsortMap.values()).asList();
HashMap sortedMap = new HashMap
for (Object value : sortedValues) { //sorted iteration
for ( Set entry : unsortMap.entrySet() {
if (entry.getValue().equals(value) {
sortedMap.put(entry.getKey(), entry.getValue());
}
}
}
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Anonymous
There’s no guarantee the posted implementation will work. Or work on every platform.
The contract for a Map does not guarantee the ordering of its Map.Entry members.
So, just because you sort the values and then put them back in the map in that order, there’s no contract guarantee that the collection returned from values() or an iteration of the entrySet() will be in that order.
The replies that use TreeMap with a special comparator might work, but they defy the contract of TreeMap, which says the entries are sorted by KEY; wrap that TreeMap in a new class that makes an explicit contract to order its entries by value and hide all the implementation details inside.
- http://www.mkyong.com mkyong
  Thanks for your sharing, So, do you have alternative way to sort a Map by ts value?
  - Anonymous
    why does one need to do this, I wonder… then I may have a solution…
    LinkedHashMap is a concrete class? I’ve never needed to do what is being described, so… ignorance is bliss… I stick to using Map as an interface and do not write code that requires a specific implementation.
    - Nan Shi
      A use case of a sorted map is that: 1. when u put pairs into a map, u want to remove or update on duplicated keys, i.e. keep keys uniq. 2. you need a sorted list, say then u can ask for the top 5 keys that have the bigger values. This is actually a very valuable and useful case.
  - cane
    I think, it possible to Sort a Map by values using Set instead of List.
    private static final Comparator<Map.Entry<String, String>> valueComparator = new Comparator<Map.Entry<String, String>>() { @Override public int compare(Map.Entry<String, String> e1, Map.Entry<String, String> e2) { //comparing by values, if it's equals, compare by keys // in other case, entries with equals values will be removed if(e1.getValue().equals(e2.getValue())) return e1.getKey().compareTo(e2.getKey()); return (e1.getValue()).compareTo(e2.getValue()); } }; private static Map<String, String> sortByComparator( Map<String, String> unsortMap) { // sorted set based on comparator SortedSet<Map.Entry<String, String>> set = new TreeSet<>( valueComparator); set.addAll(unsortMap.entrySet()); // LinkedHashMap make sure order in which keys were inserted Map<String, String> sortedMap = new LinkedHashMap<>(); for (Iterator<Map.Entry<String, String>> it = set.iterator(); it .hasNext();) { Map.Entry<String, String> entry = it.next(); sortedMap.put(entry.getKey(), entry.getValue()); } return sortedMap; }
- TymTheEnchanter
  Isn’t that what the LinkedHashMap does? Provide a predictable iteration order based on input order?
Chris
Take it to the next level and make it like TreeMap but on the values so whenever you insert it goes into the sorted by value spot.

http://www.japplis.com Anthony Goubard

Here is a shorter version I’ve written that doesn’t use a list

http://www.mkyong.com mkyong
Thanks for this non-list sorting example, but this put the map’s value sorted in descending order, can it be ascending as well?
- http://www.japplis.com Anthony Goubard
  Yes inverse k1 and k2 in the comparator
Harikrishnan
Still more shorter :) try this..
TreeMap sortedByValues = new TreeMap(unsortedMap);
sortedMap – map will be sorted by key.

Zemian Deng
Few comments:
1) The example is probably better named ValueSortedMap, to not confusing with existing java.util.SortedMap, which is by keys.
2) Your example mixed with Java generics and then not using it all the way through. Very annoying. You should correctly typing the List and Map.Entry for example will save you from casting.
3) Probably good to mention a great use case for this is where you use a Map model to display a HTML form (spring form) option list, and want to sort the listing values.
4) If you are not going to have duplicated values in the map, consider using the commons-collections library, which provided TreeBidiMap just for this. You just call inverseBidiMap() to get a reversed map with values sorted as keys! But the library is not fitted with 1.5 generics though, minor thing consider you get all the implementation for free. :)
- http://www.mkyong.com mkyong
  Really appreciated sharing your invaluable experience.
  - Nan Shi
    mkyong, you can probably use Eclipse instead of some geeky code editors, so you will see all those warnings and get it fixed before publishing your snippets.

How to sort a Map in Java

1. Sort a Map by Keys

2. Sort a Map by Values

References

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1. Sort a Map by Keys

2. Sort a Map by Values

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