Java 8 –如何对地图排序

电玩女神 2023-02-15 05:07 7阅读 0赞

![分类][format_png]![分类][format_png 1]

Java 8 `Stream`示例，用于通过键或值对`Map`进行排序。

## 1.快速说明 ##

在Java 8中对地图进行排序的步骤。

1.  将地图转换成流
2.  解决
3.  收集并返回一个新的`LinkedHashMap` （保留订单）

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

*PS默认情况下， `Collectors.toMap`将返回一个`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);
    
        }
    
    }

输出量

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);
    
        }
    }

输出量

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}

## 4. Map <对象，对象> ##

Stream无法直接对`Map<Object,Object>`排序。 要解决该问题，请将其转换为`Map<String,String>` ，请查看以下示例。

> **注意**  
> 如果您有更好的想法进行排序，请告诉我。

DisplayApp.java

package com.mkyong;
    
    import java.util.LinkedHashMap;
    import java.util.Map;
    import java.util.Properties;
    import java.util.Set;
    import java.util.stream.Collectors;
    
    public class DisplayApp {
    
        public static void main(String[] args) {
    
            Properties properties = System.getProperties();
            // not easy to sort this
            Set<Map.Entry<Object, Object>> entries = properties.entrySet();
    
            LinkedHashMap<String, String> collect = entries.stream()
                    //Map<String, String>
                    .collect(Collectors.toMap(k -> (String) k.getKey(), e -> (String) e.getValue()))
                    .entrySet()
                    .stream()
                    .sorted(Map.Entry.comparingByKey())
                    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
                            (oldValue, newValue) -> oldValue, LinkedHashMap::new));
    
            collect.forEach((k, v) -> System.out.println(k + ":" + v));
    
        }
    
    }

> **注意**  
> 不使用Java 8？ 尝试使用这种经典方法[在Java中对Map][Java_Map]进行[排序][Java_Map] 。

## 参考文献 ##

1.  [如何用Java对地图排序][Java_Map]
2.  [LinkedHashMap JavaDoc][]
3.  [Collectors.toMap（）JavaDoc][Collectors.toMap_JavaDoc]
4.  [通配符带来更多乐趣][Link 1]
5.  [Collections.sort（）签名][Collections.sort]
6.  [Java中超级T>和扩展T>之间的区别][Java_T_T]

标记： [通用][Link 2] [java8][] [地图][Link 3] [排序][Link 4] [排序][Link 5] [流][Link 6]

> 翻译自: [https://mkyong.com/java8/java-8-how-to-sort-a-map/][https_mkyong.com_java8_java-8-how-to-sort-a-map]

[format_png]: http://imgconvert.csdnimg.cn/aHR0cDovL3d3dy5ta3lvbmcuY29tL3dwLWNvbnRlbnQvdXBsb2Fkcy8yMDE2LzA4L3NvcnRpbmctdG9vbC5qcGc?x-oss-process=image/format,png
[format_png 1]: http://imgconvert.csdnimg.cn/aHR0cHM6Ly93d3cubWt5b25nLmNvbS93cC1jb250ZW50L3VwbG9hZHMvMjAxNi8wOC9zb3J0aW5nLXRvb2wuanBn?x-oss-process=image/format,png
[Java_Map]: 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-
[Link 1]: http://docs.oracle.com/javase/tutorial/extra/generics/morefun.html
[Collections.sort]: http://docs.oracle.com/javase/6/docs/api/java/util/Collections.html#sort%28java.util.List%29
[Java_T_T]: http://stackoverflow.com/questions/4343202/difference-between-super-t-and-extends-t-in-java
[Link 2]: https://mkyong.com/tag/generic/
[java8]: https://mkyong.com/tag/java8/
[Link 3]: https://mkyong.com/tag/map/
[Link 4]: https://mkyong.com/tag/sort/
[Link 5]: https://mkyong.com/tag/sorting/
[Link 6]: https://mkyong.com/tag/stream/
[https_mkyong.com_java8_java-8-how-to-sort-a-map]: https://mkyong.com/java8/java-8-how-to-sort-a-map/