How to sort hashmap by key in Java using custom Comparator

This tutorial shows you how to sort hashmap by key in java using java.util.TreeMap. Beside it also shows how to sort a hashmap in Ascending or Descending order by key or value using a custom Comparator and Java 8 Lambda expression.

Sorted HashMap By Key Example
You sort a java.util.HashMap by putting all the entries in a java.util.TreeMap. It will be sorted automatically. Let’s look at the blow code snippet.

package com.javabycode.sort;

import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.Random;
import java.util.TreeMap;

public class MySortByKey {
	
	 public static void main(String... args){
	        Map<Integer, String> random = new HashMap<Integer, String>();
	        for (int i = 0; i < 10; i++) {
	            random.put((int)(new Random().nextInt(100)), String.valueOf((int) (new Random().nextInt(100))));
	        }

	        System.out.println("-------------- Map before sorting --------------");
	        System.out.println("Random Map: " + Arrays.toString(random.entrySet().toArray()));
	        
	        TreeMap<Integer, String> sorted = new TreeMap<Integer, String>(random);
	        
	        System.out.println("-------------- Map after sorting --------------");
	        System.out.println("Sorted Map: " + Arrays.toString(sorted.entrySet().toArray()));
	    }
}

package com.javabycode.sort;

import java.util.Arrays;

import java.util.HashMap;

import java.util.Map;

import java.util.Random;

import java.util.TreeMap;

public class MySortByKey {

public static void main(String... args){

Map<Integer, String> random = new HashMap<Integer, String>();

for (int i = 0; i < 10; i++) {

random.put((int)(new Random().nextInt(100)), String.valueOf((int) (new Random().nextInt(100))));

}

System.out.println("-------------- Map before sorting --------------");

System.out.println("Random Map: " + Arrays.toString(random.entrySet().toArray()));

TreeMap<Integer, String> sorted = new TreeMap<Integer, String>(random);

System.out.println("-------------- Map after sorting --------------");

System.out.println("Sorted Map: " + Arrays.toString(sorted.entrySet().toArray()));

}

Output

-------------- Map before sorting --------------
Random Map: [5=94, 9=9, 90=94, 91=93, 78=73, 30=1, 47=32, 63=73, 15=10]
-------------- Map after sorting --------------
Sorted Map: [5=94, 9=9, 15=10, 30=1, 47=32, 63=73, 78=73, 90=94, 91=93]

-------------- Map before sorting --------------

Random Map: [5=94, 9=9, 90=94, 91=93, 78=73, 30=1, 47=32, 63=73, 15=10]

-------------- Map after sorting --------------

Sorted Map: [5=94, 9=9, 15=10, 30=1, 47=32, 63=73, 78=73, 90=94, 91=93]

Sorted HashMap By Key Reverse using custom comparator
You can sort a java.util.HashMap in descending order by using a custom Comparator.

package com.javabycode.sort;

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

public class MySortByKeyReverse {

	   public static void main(String... args){
		   
	        Map<Integer, String> random = new HashMap<Integer, String>();
	        for (int i = 0; i < 10; i++) {
	            random.put((int)(new Random().nextInt(100)), String.valueOf((int) (new Random().nextInt(100))));
	        }

	        System.out.println("-------------- Map before sorting --------------");
	        System.out.println("Initial Map: " + Arrays.toString(random.entrySet().toArray()));

	        TreeMap<Integer, String> sorted = new TreeMap<Integer, String>(new Comparator<Integer>() {
	            @Override
	            public int compare(Integer o1, Integer o2) {
	                return o2.compareTo(o1);
	            }
	        });
	        sorted.putAll(random);
	        
	        System.out.println("-------------- Map after sorting --------------");
	        System.out.println("Sorted Map: " + Arrays.toString(sorted.entrySet().toArray()));
	    }	
}

package com.javabycode.sort;

import java.util.Arrays;

import java.util.Comparator;

import java.util.HashMap;

import java.util.Map;

import java.util.TreeMap;

public class MySortByKeyReverse {

public static void main(String... args){

Map<Integer, String> random = new HashMap<Integer, String>();

for (int i = 0; i < 10; i++) {

random.put((int)(new Random().nextInt(100)), String.valueOf((int) (new Random().nextInt(100))));

}

System.out.println("-------------- Map before sorting --------------");

System.out.println("Initial Map: " + Arrays.toString(random.entrySet().toArray()));

TreeMap<Integer, String> sorted = new TreeMap<Integer, String>(new Comparator<Integer>() {

@Override

public int compare(Integer o1, Integer o2) {

return o2.compareTo(o1);

}

});

sorted.putAll(random);

System.out.println("-------------- Map after sorting --------------");

System.out.println("Sorted Map: " + Arrays.toString(sorted.entrySet().toArray()));

}

Output

-------------- Map before sorting --------------
Initial Map: [64=9, 65=71, 17=31, 83=61, 5=25, 7=87, 40=69, 26=90, 44=95, 29=11]
-------------- Map after sorting --------------
Sorted Map: [83=61, 65=71, 64=9, 44=95, 40=69, 29=11, 26=90, 17=31, 7=87, 5=25]

-------------- Map before sorting --------------

Initial Map: [64=9, 65=71, 17=31, 83=61, 5=25, 7=87, 40=69, 26=90, 44=95, 29=11]

-------------- Map after sorting --------------

Sorted Map: [83=61, 65=71, 64=9, 44=95, 40=69, 29=11, 26=90, 17=31, 7=87, 5=25]

Sorted HashMap By Value using custom comparator
You can sort a java.util.HashMap by value by using a custom Comparator. Let’s create a custom Comparator with name ValueComparator such as:

package com.javabycode.sort;
import java.util.Comparator;
import java.util.Map;

public class ValueComparator implements Comparator<Integer>{

	 private Map<Integer, String> map;

	    public ValueComparator(Map<Integer, String> map) {
	        this.map = map;
	    }

	    @Override
	    public int compare(Integer a, Integer b) {
	        return map.get(a).compareTo(map.get(b));
	    }

}

package com.javabycode.sort;

import java.util.Comparator;

import java.util.Map;

public class ValueComparator implements Comparator<Integer>{

private Map<Integer, String> map;

public ValueComparator(Map<Integer, String> map) {

this.map = map;

}

@Override

public int compare(Integer a, Integer b) {

return map.get(a).compareTo(map.get(b));

}

And we can use the ValueComparator to sort a HashMap by Value.

package com.javabycode.sort;

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

public class MySortByValue {

    public static void main(String... args){
        Map<Integer, String> random = new HashMap<Integer, String>();
        for (int i = 0; i < 10; i++) {
            random.put((int)(new Random().nextInt(100)), String.valueOf((int) (new Random().nextInt(100))));
        }

        System.out.println("-------------- Map before sorting --------------");        
        System.out.println("Random Map: " + Arrays.toString(random.entrySet().toArray()));

        TreeMap<Integer, String> sorted = new TreeMap<Integer, String>(new ValueComparator(random));               
        sorted.putAll(random);
        
        System.out.println("-------------- Map after sorting --------------");
        System.out.println("Sorted Map: " + Arrays.toString(sorted.entrySet().toArray()));
    }
}

package com.javabycode.sort;

import java.util.Arrays;

import java.util.HashMap;

import java.util.Map;

import java.util.TreeMap;

public class MySortByValue {

public static void main(String... args){

Map<Integer, String> random = new HashMap<Integer, String>();

for (int i = 0; i < 10; i++) {

random.put((int)(new Random().nextInt(100)), String.valueOf((int) (new Random().nextInt(100))));

}

System.out.println("-------------- Map before sorting --------------");

System.out.println("Random Map: " + Arrays.toString(random.entrySet().toArray()));

TreeMap<Integer, String> sorted = new TreeMap<Integer, String>(new ValueComparator(random));

sorted.putAll(random);

System.out.println("-------------- Map after sorting --------------");

System.out.println("Sorted Map: " + Arrays.toString(sorted.entrySet().toArray()));

}

Output

-------------- Map before sorting --------------
Random Map: [96=84, 64=0, 81=13, 98=81, 18=62, 85=86, 56=76, 11=56, 12=66]
-------------- Map after sorting --------------
Sorted Map: [64=0, 81=13, 11=56, 18=62, 12=66, 56=76, 98=81, 96=84, 85=86]

-------------- Map before sorting --------------

Random Map: [96=84, 64=0, 81=13, 98=81, 18=62, 85=86, 56=76, 11=56, 12=66]

-------------- Map after sorting --------------

Sorted Map: [64=0, 81=13, 11=56, 18=62, 12=66, 56=76, 98=81, 96=84, 85=86]

Java 8 Lambda Sorted Map By Value
It’s easy for you to sort java.util.HashMap by value in Java 8. You can just do it because Java 8 has a new streaming API on top of the collections framework.

package com.javabycode.sort;

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

public class MySortByValueLambda {

    public static void main(String... args){
        Map<Integer, String> random = new HashMap<Integer, String>();
        for (int i = 0; i < 10; i++) {
        	random.put((int)(new Random().nextInt(100)), String.valueOf((int) (new Random().nextInt(100))));
        }

        System.out.println("-------------- Map before sorting --------------");        
        System.out.println("Random Map: " + Arrays.toString(random.entrySet().toArray()));

        Map<Integer, String> sortedMap =
                random.entrySet().stream()
                        .sorted(Map.Entry.comparingByValue())
                        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
                                (e1, e2) -> e1, LinkedHashMap::new));
        
        System.out.println("-------------- Map after sorting --------------");        
        System.out.println("Sorted Map: " + Arrays.toString(sortedMap.entrySet().toArray()));
    }
}

package com.javabycode.sort;

import java.util.Arrays;

import java.util.HashMap;

import java.util.LinkedHashMap;

import java.util.Map;

import java.util.Random;

import java.util.stream.Collectors;

public class MySortByValueLambda {

public static void main(String... args){

Map<Integer, String> random = new HashMap<Integer, String>();

for (int i = 0; i < 10; i++) {

random.put((int)(new Random().nextInt(100)), String.valueOf((int) (new Random().nextInt(100))));

}

System.out.println("-------------- Map before sorting --------------");

System.out.println("Random Map: " + Arrays.toString(random.entrySet().toArray()));

Map<Integer, String> sortedMap =

random.entrySet().stream()

.sorted(Map.Entry.comparingByValue())

.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,

(e1, e2) -> e1, LinkedHashMap::new));

System.out.println("-------------- Map after sorting --------------");

System.out.println("Sorted Map: " + Arrays.toString(sortedMap.entrySet().toArray()));

}

Output

-------------- Map before sorting --------------
Random Map: [81=98, 54=65, 88=17, 10=99, 75=58, 45=57, 61=84, 14=26, 78=27]
-------------- Map after sorting --------------
Sorted Map: [88=17, 14=26, 78=27, 45=57, 75=58, 54=65, 61=84, 81=98, 10=99]

-------------- Map before sorting --------------

Random Map: [81=98, 54=65, 88=17, 10=99, 75=58, 45=57, 61=84, 14=26, 78=27]

-------------- Map after sorting --------------

Sorted Map: [88=17, 14=26, 78=27, 45=57, 75=58, 54=65, 61=84, 81=98, 10=99]

That’s it on how to sort hashmap by key in Java. I hope you can use these tips to optimize your source code and increate performace of program.

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How to sort Hashmap by key in Java

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