Java – How to Sort TreeMap by it Keys in descending order ?

In this article, we will discuss how to sort TreeMap entries by its Keys in descending order

1. Collections.reverseOrder()

  • Returns a comparator that imposes the reverse of the natural ordering on a collection of objects that implement the Comparable interface
  • The natural ordering is the ordering imposed by the objects’ own compareTo method
  • Arrays.sort(a, Collections.reverseOrder()); –> sorts the array in reverse-lexicographic (alphabetical) order
  • The returned comparator is serializable
  • Method signature :- public static <T> Comparator<T> reverseOrder()

1.1 Sorting TreeMap by its Keys in Descending order

  • A TreeMap contains 7 entries with Keys being in String type
  • TreeMap stores entries in ascending-order of keys
  • Desceding-order sorting of Map Keys :- create another TreeMap object passing comparator (i.e.; Collections.reverseOrder()) as constrcutor-argument
  • Put original TreeMap entries into newly created TreeMap for descending-order sorting of keys using putAll() method

SortTreeMapByKeysInDescendingOrder.java

package net.bench.resources.treemap.keys.sorting;

import java.util.Collections;
import java.util.Map;
import java.util.TreeMap;

public class SortTreeMapByKeysInDescendingOrder {

	public static void main(String[] args) {

		// 1. create TreeMap object of type <String, Integer>
		Map<String, Integer> countryPopulation = new TreeMap<>(); 


		// 1.1 adding key-value pairs to TreeMap object
		countryPopulation.put("Indian", 382357386);
		countryPopulation.put("Chinese", 409900000);
		countryPopulation.put("America", 332429717);
		countryPopulation.put("Russia", 146748590);
		countryPopulation.put("Brazil", 213728559);
		countryPopulation.put("Mexico", 127792286);
		countryPopulation.put("Pakistan", 220892331);


		// 1.2 TreeMap - ascending-order
		System.out.println("Original TreeMap in Ascending-order of Keys :- \n");


		// 1.3 print Map entries using forEach() method
		for(Map.Entry<String, Integer> entry : countryPopulation.entrySet()) {

			System.out.println("Key : " + entry.getKey() + "\t\t" 
					+ "Value : "  + entry.getValue());
		}



		// 3. Sorting according to reverse alphabetical order of Keys
		System.out.println("\n\nSorted TreeMap in "
				+ "reverse alphabetical order of Keys :- \n");


		// 3.1 create new TreeMap passing comparator
		Map<String, Integer> sortedMapDesc = new TreeMap<>(
				Collections.reverseOrder());


		// 3.2 add unsorted map to TreeMap for reverse-order sorting
		sortedMapDesc.putAll(countryPopulation);


		// 3.3 print Map entries using forEach() method
		for(Map.Entry<String, Integer> entry : sortedMapDesc.entrySet()) {

			System.out.println("Key : " + entry.getKey() + "\t\t" 
					+ "Value : "  + entry.getValue());
		}
	}
}

Output:

Original TreeMap in Ascending-order of Keys :- 

Key : America		Value : 332429717
Key : Brazil		Value : 213728559
Key : Chinese		Value : 409900000
Key : Indian		Value : 382357386
Key : Mexico		Value : 127792286
Key : Pakistan		Value : 220892331
Key : Russia		Value : 146748590


Sorted TreeMap in reverse alphabetical order of Keys :- 

Key : Russia		Value : 146748590
Key : Pakistan		Value : 220892331
Key : Mexico		Value : 127792286
Key : Indian		Value : 382357386
Key : Chinese		Value : 409900000
Key : Brazil		Value : 213728559
Key : America		Value : 332429717

Important points to remember about Map :

  • HashMap stores entries (Key-Value pairs) in random-order of Keys
  • LinkedHashMap stores entries (Key-Value pairs) as per insertion-order of Keys
  • TreeMap stores entries (Key-Value pairs) in sorted-order of Keys

References:

Java – How to sort TreeSet in descending order using Comparator