Java 8 Map sort

Java 8 Map 排序

Java 8的Stream例子Map，按鍵或按值排序。

快速說明

在Java 8中排序地圖的步驟

1. 將map轉換爲流
2. 排序
3. Collect and return 新的LinkedHashMap

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

默認情況下Collectors.toMap會返回一個HashMap

2.按鍵排序

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

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

排序...
{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.按值排序

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

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

排序...
{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}