Java 8 Map sort

Java 8 Map 排序

Java 8的Stream例子Map，按键或按值排序。

快速说明

在Java 8中排序地图的步骤

1. 将map转换为流
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));

默认情况下Collectors.toMap会返回一个HashMap

2.按键排序

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}