Java 8 – How to Sort a Map entries by its Key – 6 ways

In this article, we will discuss different ways to sort a Map entries (key-value pairs) by its Key

Before proceeding with this sorting examples understand below items,

Different approaches of Sorting a Map by its Key :

  • TreeMap class
  • ArrayList class and Collections.sort() method
  • TreeSet class
  • Java 8Lambda function
  • Java 8 – Stream sorted() method
  • Java 8Map.Entry.comparingByKey() comparator

1. Using TreeMap class

  • In this approach, we are going to use inter-conversion constructor between HashMap & TreeMap for natural order sorting i.e.; alphabetical order of Keys which is in String type
  • For reverse order sorting, create another TreeMap object passing Collections.reverseOrder() comparator as constructor-argument and put all HashMap entries to TreeMap using putAll() method

SortMapByKeysUsingTreeMap.java

package net.bench.resources.map.keys.sorting.ways;

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

public class SortMapByKeysUsingTreeMap {

	public static void main(String[] args) {

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


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


		// 1.2 print - before sorting - random order
		System.out.println("Before Sorting - Random order :- \n");


		// 1.3 print Map entries to console
		countryPopulation.forEach((key, value) -> System.out.println(
				"Key : " + key  + "\t\t"  + "Value : "  + value
				));



		// 2. Sorting according to alphabetical order of Keys
		System.out.println("\n\nSorted according to "
				+ "alphabetical order of Keys :- \n");


		// 2.1 sorting by Map keys in ascending order
		Map<String, Integer> sortedMapAsc = new TreeMap<>(countryPopulation);


		// 2.2 print Map entries to console
		sortedMapAsc.forEach((key, value) -> System.out.println(
				"Key : " + key  + "\t\t"  + "Value : "  + value
				));



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


		// 3.1 sorting by Map keys in descending order
		Map<String, Integer> sortedMapDesc = new TreeMap<>(
				Collections.reverseOrder());


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


		// 3.2 print Map entries to console
		sortedMapDesc.forEach((key, value) -> System.out.println(
				"Key : " + key  + "\t\t"  + "Value : "  + value
				));
	}
}

Output:

Before Sorting - Random order :- 

Key : Pakistan		Value : 220892331
Key : America		Value : 332429717
Key : Brazil		Value : 213728559
Key : Indian		Value : 382357386
Key : Russia		Value : 146748590


Sorted according to alphabetical order of Keys :- 

Key : America		Value : 332429717
Key : Brazil		Value : 213728559
Key : Indian		Value : 382357386
Key : Pakistan		Value : 220892331
Key : Russia		Value : 146748590


Sorted according to reverse alphabetical order of Keys :- 

Key : Russia		Value : 146748590
Key : Pakistan		Value : 220892331
Key : Indian		Value : 382357386
Key : Brazil		Value : 213728559
Key : America		Value : 332429717

2. Using ArrayList class & Collections.sort() method

  • In this approach, first we will get key set from HashMap entries and pass this key-set as constructor-argument while creating ArrayList object and use Collections.sort() method for natural order sorting i.e.; alphabetical order of Keys which is in String type
  • For reverse order sorting, use same ArrayList and while sorting using Collections.sort() method and pass Collections.reverseOrder() comparator as 2nd argument

SortMapByKeysUsingArrayList.java

package net.bench.resources.map.keys.sorting.ways;

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class SortMapByKeysUsingArrayList {

	public static void main(String[] args) {

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


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


		// 1.2 print - before sorting - random order
		System.out.println("Before Sorting - Random order :- \n");


		// 1.3 print Map entries to console
		countryPopulation.forEach((key, value) -> System.out.println(
				"Key : " + key  + "\t\t"  + "Value : "  + value
				));



		// 2. Sorting according to alphabetical order of Keys
		System.out.println("\n\nSorted according to "
				+ "alphabetical order of Keys :- \n");


		// 2.1 sorting by Map keys in ascending order
		List<String> mapKeys = new ArrayList<>(countryPopulation.keySet());


		// 2.2 Map keys - Ascending order sorting ArrayList
		Collections.sort(mapKeys);


		// 2.3 print Map entries for ascending order
		mapKeys.forEach(key -> System.out.println(
				"Key : " + key  + "\t\t" + 
						"Value : " + countryPopulation.get(key)
				));



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


		// 3.1 Map keys - Descending order sorting ArrayList
		Collections.sort(mapKeys, Collections.reverseOrder());


		// 3.2 print Map entries for descending order
		mapKeys.forEach(key -> System.out.println(
				"Key : " + key  + "\t\t"  + 
						"Value : " + countryPopulation.get(key)
				));
	}
}

Output:

Before Sorting - Random order :- 

Key : Pakistan		Value : 220892331
Key : America		Value : 332429717
Key : Brazil		Value : 213728559
Key : Indian		Value : 382357386
Key : Russia		Value : 146748590


Sorted according to alphabetical order of Keys :- 

Key : America		Value : 332429717
Key : Brazil		Value : 213728559
Key : Indian		Value : 382357386
Key : Pakistan		Value : 220892331
Key : Russia		Value : 146748590


Sorted according to reverse alphabetical order of Keys :- 

Key : Russia		Value : 146748590
Key : Pakistan		Value : 220892331
Key : Indian		Value : 382357386
Key : Brazil		Value : 213728559
Key : America		Value : 332429717

3. Using TreeSet class

  • In this approach, first we will get key set from HashMap entries and pass this key-set as constructor-argument while creating TreeSet object which will sort keys in natural order i.e.; alphabetical order of Keys which is in String type
  • For reverse order sorting, create another TreeSet object passing Collections.reverseOrder() comparator as constructor-argument and add all Key-set to TreeSet using addAll() method
  • Note: alternatively we can provide our own customized logic for reverse-order sorting by overriding compare() method of Comparator interface which is shown in the below illustration

SortMapByKeysUsingTreeSet.java

package net.bench.resources.map.keys.sorting.ways;

import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;
import java.util.SortedSet;
import java.util.TreeSet;

public class SortMapByKeysUsingTreeSet {

	public static void main(String[] args) {

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


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


		// 1.2 print - before sorting - random order
		System.out.println("Before Sorting - Random order :- \n");


		// 1.3 print Map entries to console
		countryPopulation.forEach((key, value) -> System.out.println(
				"Key : " + key  + "\t\t"  + "Value : "  + value
				));



		// 2. Sorting according to alphabetical order of Keys
		System.out.println("\n\nSorted according to "
				+ "alphabetical order of Keys :- \n");


		// 2.1 TreeSet - Ascending order Sorting by Map keys
		SortedSet<String> mapKeysAsc = new TreeSet<>(
				countryPopulation.keySet());


		// 2.2 print Map entries
		mapKeysAsc.forEach(key -> System.out.println(
				"Key : " + key  + "\t\t"  + 
						"Value : " + countryPopulation.get(key)
				));



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


		// 3.1 TreeSet - Descending order Sorting by Map keys
		SortedSet<String> mapKeysDesc = new TreeSet<>(new Comparator<String>() {

			@Override
			public int compare(String str1, String str2) {
				return str2.compareTo(str1);
			}
		});


		// 3.2 add unsorted keySet to TreeSet for reverse-order sorting
		mapKeysDesc.addAll(countryPopulation.keySet());


		// 3.3 print Map entries
		mapKeysDesc.forEach(key -> System.out.println(
				"Key : " + key  + "\t\t"  + 
						"Value : " + countryPopulation.get(key)
				));
	}
}

Output:

Before Sorting - Random order :- 

Key : Pakistan		Value : 220892331
Key : America		Value : 332429717
Key : Brazil		Value : 213728559
Key : Indian		Value : 382357386
Key : Russia		Value : 146748590


Sorted according to alphabetical order of Keys :- 

Key : America		Value : 332429717
Key : Brazil		Value : 213728559
Key : Indian		Value : 382357386
Key : Pakistan		Value : 220892331
Key : Russia		Value : 146748590


Sorted according to reverse alphabetical order of Keys :- 

Key : Russia		Value : 146748590
Key : Pakistan		Value : 220892331
Key : Indian		Value : 382357386
Key : Brazil		Value : 213728559
Key : America		Value : 332429717

4. Java 8 – Lambda function

  • In this approach, first we will get Map entry set from HashMap and pass this entry-set as constructor-argument while creating LinkedList object and use Collections.sort() method by passing converted LinkedList as 1st argument and below lambda expression for natural order sorting as 2nd argument
    • (map1, map2) -> map1.getKey().compareTo(map2.getKey())
  • For reverse order sorting, just twist the comparison logic in the lambda expression as shown below,
    • (map1, map2) -> map2.getKey().compareTo(map1.getKey())

SortMapByKeysUsingJava8Lambda.java

package net.bench.resources.map.keys.sorting.ways;

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

public class SortMapByKeysUsingJava8Lambda {

	public static void main(String[] args) {

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


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


		// 1.2 print - before sorting - random order
		System.out.println("Before Sorting - Random order :- \n");


		// 1.3 print Map entries to console
		countryPopulation.forEach((key, value) -> System.out.println(
				"Key : " + key  + "\t\t"  + "Value : "  + value
				));


		// 1.4 convert Map EntrySet into List
		List<Map.Entry<String, Integer>> list = new LinkedList<>(
				countryPopulation.entrySet());



		// 2. Sorting according to alphabetical order of Keys
		System.out.println("\n\nSorted according to "
				+ "alphabetical order of Keys :- \n");


		// 2.1 Map keys - Ascending order sorting
		Collections.sort(list, 
				(map1, map2) -> map1.getKey().compareTo(map2.getKey())
				);


		// 2.2 put above sorted map into LinkedHashMap by iterating
		Map<String, Integer> tempMapAsc = new LinkedHashMap<>();


		// 2.3 iterate and store in newly created LinkedHashMap
		for (Map.Entry<String, Integer> map : list) {
			tempMapAsc.put(map.getKey(), map.getValue());
		}


		// 2.4 print Map entries to console
		tempMapAsc.forEach((key, value) -> System.out.println(
				"Key : " + key  + "\t\t"  + "Value : "  + value
				));



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


		// 3.1 Map keys - Descending order sorting
		Collections.sort(list, 
				(map1, map2) -> map2.getKey().compareTo(map1.getKey())
				);


		// 3.2 put above reverse sorted map into LinkedHashMap by iterating
		Map<String, Integer> tempMapDesc = new LinkedHashMap<>();


		// 3.3 iterate and store in newly created LinkedHashMap
		for (Map.Entry<String, Integer> map : list) {
			tempMapDesc.put(map.getKey(), map.getValue());
		}


		// 3.4 print Map entries to console
		tempMapDesc.forEach((key, value) -> System.out.println(
				"Key : " + key  + "\t\t"  + "Value : "  + value
				));
	}
}

Output:

Before Sorting - Random order :- 

Key : Pakistan		Value : 220892331
Key : America		Value : 332429717
Key : Brazil		Value : 213728559
Key : Indian		Value : 382357386
Key : Russia		Value : 146748590


Sorted according to alphabetical order of Keys :- 

Key : America		Value : 332429717
Key : Brazil		Value : 213728559
Key : Indian		Value : 382357386
Key : Pakistan		Value : 220892331
Key : Russia		Value : 146748590


Sorted according to reverse alphabetical order of Keys :- 

Key : Russia		Value : 146748590
Key : Pakistan		Value : 220892331
Key : Indian		Value : 382357386
Key : Brazil		Value : 213728559
Key : America		Value : 332429717

5. Java 8 – Stream sorted() method

  • In this approach, we are going to use Stream‘s sorted() method for sorting Map Keys by passing lambda expression as argument to sorted() method
  • Ascending order sorting :- for alphabetical order of keys pass below lambda expression,
    • (map1, map2) -> map1.getKey().compareTo(map2.getKey())
  • Descending order sorting :- for reverse alphabetical order of keys pass below lambda expression,
    • (map1, map2) -> map2.getKey().compareTo(map1.getKey())

SortMapByKeysUsingStreamSortedMethod.java

package net.bench.resources.map.keys.sorting.ways;

import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.stream.Collectors;

public class SortMapByKeysUsingStreamSortedMethod {

	public static void main(String[] args) {

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


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


		// 1.2 print - before sorting - random order
		System.out.println("Before Sorting - Random order :- \n");


		// 1.3 print Map entries to console
		countryPopulation.forEach((key, value) -> System.out.println(
				"Key : " + key  + "\t\t"  + "Value : "  + value
				));



		// 2. Sorting according to alphabetical order of Keys
		System.out.println("\n\nSorted according to "
				+ "alphabetical order of Keys :- \n");


		// 2.1 Stream.sorted - Ascending order of keys 
		Map<String, Integer> tempMapAsc = countryPopulation
				.entrySet()
				.stream()
				.sorted(
						(map1, map2) -> map1.getKey().compareTo(map2.getKey())
						)
				.collect(
						Collectors.toMap(
								Map.Entry::getKey, 
								Map.Entry::getValue, 
								(es1, es2) -> es1, LinkedHashMap::new
								)
						);


		// 2.2 print Map entries
		tempMapAsc.forEach((key, value) -> System.out.println(
				"Key : " + key  + "\t\t"  + "Value : "  + value
				));



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


		// 3.1 Stream.sorted - Descending order of keys 
		Map<String, Integer> tempMapDesc = countryPopulation
				.entrySet()
				.stream()
				.sorted(
						(map1, map2) -> map2.getKey().compareTo(map1.getKey())
						)
				.collect(
						Collectors.toMap(
								Map.Entry::getKey, 
								Map.Entry::getValue, 
								(es1, es2) -> es1, LinkedHashMap::new
								)
						);


		// 3.2 print Map entries
		tempMapDesc.forEach((key, value) -> System.out.println(
				"Key : " + key  + "\t\t"  + "Value : "  + value
				));
	}
}

Output:

Before Sorting - Random order :- 

Key : Pakistan		Value : 220892331
Key : America		Value : 332429717
Key : Brazil		Value : 213728559
Key : Indian		Value : 382357386
Key : Russia		Value : 146748590


Sorted according to alphabetical order of Keys :- 

Key : America		Value : 332429717
Key : Brazil		Value : 213728559
Key : Indian		Value : 382357386
Key : Pakistan		Value : 220892331
Key : Russia		Value : 146748590


Sorted according to reverse alphabetical order of Keys :- 

Key : Russia		Value : 146748590
Key : Pakistan		Value : 220892331
Key : Indian		Value : 382357386
Key : Brazil		Value : 213728559
Key : America		Value : 332429717

6. Java 8 – Map.Entry.comparingByKey() comparator

  • In this approach, we are going to use Stream‘s sorted() method for sorting Map Keys by passing comparator as argument to sorted() method
  • For alphabetical order of keys pass Map.Entry.comparingByKey() comparator
  • For reverse alphabetical order of keys pass Map.Entry.comparingByKey(Comparator.reverseOrder()) comparator

SortMapEntryUsingComparingByKey.java

package net.bench.resources.map.keys.sorting.ways;

import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;

public class SortMapEntryUsingComparingByKey {

	public static void main(String[] args) {

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


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


		// 1.2 print - before sorting - random order
		System.out.println("Before Sorting - Random order :- \n");


		// 1.3 print Map entries to console
		countryPopulation.forEach((key, value) -> System.out.println(
				"Key : " + key  + "\t\t"  + "Value : "  + value
				));



		// 2. Sorting according to alphabetical order of Keys
		System.out.println("\n\nSorted according to "
				+ "alphabetical order of Keys :- \n");


		// 2.1 Ascending Stream sorting using Map.Entry.comparingByKey()
		countryPopulation
		.entrySet()
		.stream()
		.sorted(Map.Entry.comparingByKey())
		.forEach(entry -> 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 according to "
				+ "reverse alphabetical order of Keys :- \n");


		// 3.1 Descending Stream sorting using Map.Entry.comparingByKey()
		countryPopulation
		.entrySet()
		.stream()
		.sorted(Map.Entry.comparingByKey(Comparator.reverseOrder()))
		.forEach(entry -> System.out.println(
				"Key : " + entry.getKey() + "\t\t"  + 
						"Value : " + entry.getValue()
				));
	}
}

Output:

Before Sorting - Random order :- 

Key : Pakistan		Value : 220892331
Key : America		Value : 332429717
Key : Brazil		Value : 213728559
Key : Indian		Value : 382357386
Key : Russia		Value : 146748590


Sorted according to alphabetical order of Keys :- 

Key : America		Value : 332429717
Key : Brazil		Value : 213728559
Key : Indian		Value : 382357386
Key : Pakistan		Value : 220892331
Key : Russia		Value : 146748590


Sorted according to reverse alphabetical order of Keys :- 

Key : Russia		Value : 146748590
Key : Pakistan		Value : 220892331
Key : Indian		Value : 382357386
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:

Happy Coding !!
Happy Learning !!

Java 8 – How to Sort a Map entries by its Value – 6 ways
Java 8 – How to Sort HashMap entries by its Values