How to Sort a HashMap by Keys in Java

We saw how sort a HashMap by values.  In this tutorial, you'll see how to order items by keys using the TreeMap class. We know that HashMap does not keep the order of insertion of elements by default. In java, Map or HashMap can both be sorted by keys or values.

1- Sort HashMap using TreeMap

This code sorts HashMap by keys and is based on the use of   the TreeMap.

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

public class Triclés {

 public static void main(String[] args) {

 Map< String,String> hmap = new HashMap< String,String> (); 
 hmap.put("4", "four"); 
 hmap.put("2", "two"); 
 hmap.put("3", "three"); 
 hmap.put("1", "one"); 
 
 Set set = hmap.entrySet(); 
 Iterator iterator = set.iterator(); 
 System.out.println("Before sorting:"); 
 while(iterator.hasNext()) {
 Map.Entry me = (Map.Entry)iterator.next(); 
 System.out.print(me.getKey() + ": "); 
 System.out.println(me.getValue()); 
 }

 Map sortedMap = new TreeMap(hmap); 

 Set set2 = sortedMap.entrySet(); 
 Iterator iterator2 = set2.iterator(); 
 System.out.println("After sorting:"); 
 while(iterator2.hasNext()) {
 Map.Entry me2 = (Map.Entry)iterator2.next(); 
 System.out.print(me2.getKey() + ": "); 
 System.out.println(me2.getValue()); 
 }
 }
}

Output:

Before sorting: 
3: three
2: two
1: one
4: four
After sorting:  
1: one
2: two
3: three
4: four

2- Sort HashMap with Comparator or Comparable

You can also sort a Map by copying the keys into a list that implements the interface java.util.List, and then sort the elements by calling the Collections.sort(). Here you can use both sorting interfaces: Comparator or Comparable  and choose the ascending or descending order you want to display. Once the key list is created, you can copy the keys and values into a new HashMap.

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

public class Triclés {

 public static HashMap< String, String> sortHashMap(Map< String, String> hmap){
 List< Map.Entry< String, String> > list =
 new LinkedList< Map.Entry< String, String> > ( hmap.entrySet() ); 
 Collections.sort( list, new Comparator< Map.Entry< String, String> > (){
 public int compare
 (Map.Entry< String, String> o1, Map.Entry< String, String> o2 )
 {
 //compare two keys
 return (o1.getKey()).compareTo( o2.getKey() ); 
 }
 }); 

 //create a new HashMap from LinkedList
 HashMap< String, String> hmapTriee = new LinkedHashMap< String, String> (); 
 for (Map.Entry< String, String> entry : list)
 {
 hmapTriee.put( entry.getKey(), entry.getValue() ); 
 }
 return hmapTriee; 
 }
 
 public static void main(String[] args) {

 final HashMap< String, String> hmap = new HashMap< String, String> (); 
 
 hmap.put("4", "four"); 
 hmap.put("2", "two"); 
 hmap.put("3", "three"); 
 hmap.put("1", "one"); 
 
 System.out.println("Before sorting: "+ hmap); 

 System.out.println("After sorting: "+sortHashMap(hmap)); 
 }
}

Output:

Before sorting: {3=three, 2=two, 1=one, 4=four}
After sorting: {1=one, 2=two, 3=three, 4=four}