sorting

Java 8 Stream examples to sort a Map, by keys or by values.

1. Quick Explanation

Steps to sort a Map in Java 8.

  1. Convert a Map into a Stream
  2. Sort it
  3. Collect and return a new LinkedHashMap (keep the order)

Map result = map.entrySet().stream()
	.sorted(Map.Entry.comparingByKey()) 			
	.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
	(oldValue, newValue) -> oldValue, LinkedHashMap::new));

P.S By default, Collectors.toMap will returns a HashMap

2. Sort by Keys

SortByKeyExample.java

package com.mkyong.test;

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

public class SortByKeyExample {

    public static void main(String[] argv) {

        Map<String, Integer> unsortMap = new HashMap<>();
        unsortMap.put("z", 10);
        unsortMap.put("b", 5);
        unsortMap.put("a", 6);
        unsortMap.put("c", 20);
        unsortMap.put("d", 1);
        unsortMap.put("e", 7);
        unsortMap.put("y", 8);
        unsortMap.put("n", 99);
        unsortMap.put("g", 50);
        unsortMap.put("m", 2);
        unsortMap.put("f", 9);

        System.out.println("Original...");
        System.out.println(unsortMap);

        // sort by keys, a,b,c..., and return a new LinkedHashMap
        // toMap() will returns HashMap by default, we need LinkedHashMap to keep the order.
        Map<String, Integer> result = unsortMap.entrySet().stream()
                .sorted(Map.Entry.comparingByKey())
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
                        (oldValue, newValue) -> oldValue, LinkedHashMap::new));


        // Not Recommend, but it works.
        //Alternative way to sort a Map by keys, and put it into the "result" map
        Map<String, Integer> result2 = new LinkedHashMap<>();
        unsortMap.entrySet().stream()
                .sorted(Map.Entry.comparingByKey())
                .forEachOrdered(x -> result2.put(x.getKey(), x.getValue()));

        System.out.println("Sorted...");
        System.out.println(result);
        System.out.println(result2);

    }

}

Output

Original...
{a=6, b=5, c=20, d=1, e=7, f=9, g=50, y=8, z=10, m=2, n=99}

Sorted...
{a=6, b=5, c=20, d=1, e=7, f=9, g=50, m=2, n=99, y=8, z=10}
{a=6, b=5, c=20, d=1, e=7, f=9, g=50, m=2, n=99, y=8, z=10}

3. Sort by Values

SortByValueExample.java

package com.mkyong.test;

package com.mkyong;

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

public class SortByValueExample {

    public static void main(String[] argv) {

        Map<String, Integer> unsortMap = new HashMap<>();
        unsortMap.put("z", 10);
        unsortMap.put("b", 5);
        unsortMap.put("a", 6);
        unsortMap.put("c", 20);
        unsortMap.put("d", 1);
        unsortMap.put("e", 7);
        unsortMap.put("y", 8);
        unsortMap.put("n", 99);
        unsortMap.put("g", 50);
        unsortMap.put("m", 2);
        unsortMap.put("f", 9);

        System.out.println("Original...");
        System.out.println(unsortMap);

        //sort by values, and reserve it, 10,9,8,7,6...
        Map<String, Integer> result = unsortMap.entrySet().stream()
                .sorted(Map.Entry.comparingByValue(Comparator.reverseOrder()))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
                        (oldValue, newValue) -> oldValue, LinkedHashMap::new));

						
        //Alternative way
        Map<String, Integer> result2 = new LinkedHashMap<>();
        unsortMap.entrySet().stream()
                .sorted(Map.Entry.<String, Integer>comparingByValue().reversed())
                .forEachOrdered(x -> result2.put(x.getKey(), x.getValue()));

        System.out.println("Sorted...");
        System.out.println(result);
        System.out.println(result2);

    }
}

Output

Original...
{a=6, b=5, c=20, d=1, e=7, f=9, g=50, y=8, z=10, m=2, n=99}

Sorted...
{n=99, g=50, c=20, z=10, f=9, y=8, e=7, a=6, b=5, m=2, d=1}
{n=99, g=50, c=20, z=10, f=9, y=8, e=7, a=6, b=5, m=2, d=1}
Note
Not using Java 8? Try this classic way to sort a Map in Java.

References

  1. How to sort a Map in Java
  2. LinkedHashMap JavaDoc
  3. Collectors.toMap() JavaDoc
  4. More Fun with Wildcards
  5. Collections.sort() signature
  6. Difference between and in Java