Java 8 –对Map进行排序

Few Java 8 Stream examples to sort a Map

1. Sort by Key

SortByKeyExample.java

package com.mkyong.test;

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

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("j", 50);
        unsortMap.put("m", 2);
        unsortMap.put("f", 9);

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

        Map<String, Integer> result = new LinkedHashMap<>();

        //sort by key, a,b,c..., and put it into the "result" map 
        unsortMap.entrySet().stream()
                .sorted(Map.Entry.<String, Integer>comparingByKey())
                .forEachOrdered(x -> result.put(x.getKey(), x.getValue()));

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

    }

}

Output

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

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

2. Sort by Value

SortByValueExample.java

package com.mkyong.test;

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

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("j", 50);
        unsortMap.put("m", 2);
        unsortMap.put("f", 9);

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

        Map<String, Integer> result = new LinkedHashMap<>();

        //sort by value, and reserve, 10,9,8,7,6...
        unsortMap.entrySet().stream()
                .sorted(Map.Entry.<String, Integer>comparingByValue().reversed())
                .forEachOrdered(x -> result.put(x.getKey(), x.getValue()));

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

    }

}

Output

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

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

3. Generic Example

Create a generic method to sort a Map.

SortByGenericExample.java

package com.mkyong.test;

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

public class SortByGenericExample {

    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("j", 50);
        unsortMap.put("m", 2);
        unsortMap.put("f", 9);

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

        System.out.println("Sort By Key...");
        Map<String, Integer> resultKey = compareByKey(unsortMap);
        System.out.println(resultKey);

        System.out.println("Sort By Value...");
        Map<String, Integer> resultValue = compareByValue(unsortMap);
        System.out.println(resultValue);
    }

    //Reference from java.util.Map source code, try dig inside, many generic examples.
    public static <K, V extends Comparable<? super V>> Map<K, V> compareByValue(Map<K, V> map) {

        Map<K, V> result = new LinkedHashMap<>();

        Stream<Map.Entry<K, V>> mapInStream = map.entrySet().stream();

        mapInStream.sorted(Map.Entry.comparingByValue())
                .forEachOrdered(x -> result.put(x.getKey(), x.getValue()));

        return result;

    }

    public static <K extends Comparable<? super K>, V> Map<K, V> compareByKey(Map<K, V> map) {

        Map<K, V> result = new LinkedHashMap<>();
        Stream<Map.Entry<K, V>> mapInStream = map.entrySet().stream();

        mapInStream.sorted(Map.Entry.comparingByKey())
                .forEachOrdered(x -> result.put(x.getKey(), x.getValue()));

        return result;

    }

}

Output

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

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

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

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. More Fun with Wildcards
3. Collections.sort() signature
4. Difference between and in Java