Java 8 Stream examples to sort a Map, by keys or by values.
1. Quick Explanation
Steps to sort a Map in Java 8.
- Convert a Map into a Stream
- Sort it
- 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.favtuts.java8;
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[] args) {
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.favtuts.java8;
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[] args) {
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>
The Stream can’t sort the Map<Object,Object> directly. To solve it, convert it into Map<String,String>, review below example.
Note
If you have better idea to sort it, do let me know.
DisplayApp.java
package com.favtuts.java8;
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[] args) {
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);
}
}
Note
Not using Java 8? Try this classic way to sort a Map in Java.
Download Source Code
$ git clone https://github.com/favtuts/java-core-tutorials-examples
$ cd java-basic/java8