Java 8 – How to sort a Map

1. Quick Explanation

Map result = map.entrySet().stream()

.sorted(Map.Entry.comparingByKey())

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

(oldValue, newValue) -> oldValue, LinkedHashMap::new));

2. Sort by KEYS

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

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}

4. Map<Object,Object>

package com.mkyong;

import java.util.LinkedHashMap;

import java.util.Map;

import java.util.Properties;

import java.util.Set;

import java.util.stream.Collectors;

public class DisplayApp {

public static void main(String[] args) {

Properties properties = System.getProperties();

// not easy to sort this

Set<Map.Entry<Object, Object>> entries = properties.entrySet();

LinkedHashMap<String, String> collect = entries.stream()

//Map<String, String>

.collect(Collectors.toMap(k -> (String) k.getKey(), e -> (String) e.getValue()))

.entrySet()

.stream()

.sorted(Map.Entry.comparingByKey())

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

(oldValue, newValue) -> oldValue, LinkedHashMap::new));

collect.forEach((k, v) -> System.out.println(k + ":" + v));

}

}

http://www.mkyong.com/java8/java-8-how-to-sort-a-map/