Java 8 – 怎样对Map排序

我会带着你远行 2022-06-06 09:20 354阅读 0赞

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keys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); } }

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values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); } }

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}

## 引用： ##

1.  [How to sort a Map in Java][]
2.  [LinkedHashMap JavaDoc][]
3.  [Collectors.toMap() JavaDoc][Collectors.toMap_ JavaDoc]
4.  [More Fun with Wildcards][]
5.  [Collections.sort() signature][Collections.sort_ signature]
6.  [Difference between and in Java][Difference between_and_in Java]

[How to sort a Map in Java]: https://www.mkyong.com/java/how-to-sort-a-map-in-java/
[LinkedHashMap JavaDoc]: https://docs.oracle.com/javase/8/docs/api/java/util/LinkedHashMap.html
[Collectors.toMap_ JavaDoc]: http://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html#toMap-java.util.function.Function-java.util.function.Function-
[More Fun with Wildcards]: http://docs.oracle.com/javase/tutorial/extra/generics/morefun.html
[Collections.sort_ signature]: http://docs.oracle.com/javase/6/docs/api/java/util/Collections.html#sort%28java.util.List%29
[Difference between_and_in Java]: http://stackoverflow.com/questions/4343202/difference-between-super-t-and-extends-t-in-java