Sort Map by Key in Java

Tags: Map TreeMap Comparator

In this Java core tutorial, we show you how to implement the Java method to sort elements of a Map object by its entry key in the Java program.

Implement method to sort a map by key

Firstly, create a new class named MapUtils and implement a method sortMapByKey() which expects a Map as argument and returns a sorted Map.

MapUtils.java 

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

public class MapUtils {

    public static Map<String, String> sortMapByKey(Map<String, String> map) {
        if(map == null) {
            return null;
        }

        Map<String, String> sortedMap = new TreeMap<>(new Comparator<String>() {
            @Override
            public int compare(String key1, String key2) {
                return key1.compareTo(key2);
            }
        });
        sortedMap.putAll(map);

        return sortedMap;
    }
}

Using MapUtils.sortMapByKey() method to sort map

In the following example Java program we use the MapUtils.sortMapByKey() static method above to sort a map.

SortMapByKeyExample.java 

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

public class SortMapByKeyExample {
    public static void main(String[] args) {
        Map<String, String> sampleMap = new HashMap<>();
        sampleMap.put("key2", "test value two");
        sampleMap.put("key5", "test value five");
        sampleMap.put("key4", "test value four");
        sampleMap.put("key3", "test value three");
        sampleMap.put("key1", "test value one");

        System.out.println("Input Map: ");
        for (Map.Entry entry: sampleMap.entrySet()) {
            System.out.println("Key: " + entry.getKey() + ", Value: " + entry.getValue());
        }

        // sort map by key
        Map<String, String> sortedMap = MapUtils.sortMapByKey(sampleMap);

        System.out.println("\nSorted Map: ");
        for (Map.Entry entry: sortedMap.entrySet()) {
            System.out.println("Key: " + entry.getKey() + ", Value: " + entry.getValue());
        }
    }
}
The output is: 
Input Map: 
Key: key1, Value: test value one
Key: key2, Value: test value two
Key: key5, Value: test value five
Key: key3, Value: test value three
Key: key4, Value: test value four

Sorted Map: 
Key: key1, Value: test value one
Key: key2, Value: test value two
Key: key3, Value: test value three
Key: key4, Value: test value four
Key: key5, Value: test value five

Happy Coding 😊

Sort Map by Value in Java