JDK8 新特性-Map对key和value分别排序实现

在Java 8 中使用Stream 例子对一个 Map 进行按照keys或者values排序.

1. 快速入门

  在java 8中按照此步骤对map进行排序.

1. 将 Map 转换为 Stream
2. 对其进行排序
3. Collect and return a new LinkedHashMap (保持顺序)

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 默认情况下, Collectors.toMap 将返回一个 HashMap

2. 按照key排序

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 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 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 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);
 
    }
 
}

输出：

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. 按照value排序​​​​​​​

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 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 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 result2 = new LinkedHashMap<>();
        unsortMap.entrySet().stream()
                .sorted(Map.Entry.comparingByValue().reversed())
                .forEachOrdered(x -> result2.put(x.getKey(), x.getValue()));
 
        System.out.println("Sorted...");
        System.out.println(result);
        System.out.println(result2);
 
    }
}

输出：

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}