Java 8 –将列表转换为地图

谁借莪１个温暖的怀抱￠ 2023-02-15 05:09 10阅读 0赞

很少有Java 8示例向您展示如何将对象`List`转换为`Map`以及如何处理重复的键。

Hosting.java

package com.mkyong.java8
    
    public class Hosting {
    
        private int Id;
        private String name;
        private long websites;
    
        public Hosting(int id, String name, long websites) {
            Id = id;
            this.name = name;
            this.websites = websites;
        }
    
        //getters, setters and toString()
    }

## 1.列出地图– Collectors.toMap（） ##

创建`Hosting`对象的列表，并使用`Collectors.toMap`将其转换为Map。

TestListMap.java

package com.mkyong.java8
    
    import java.util.ArrayList;
    import java.util.List;
    import java.util.Map;
    import java.util.stream.Collectors;
    
    public class TestListMap {
    
        public static void main(String[] args) {
    
            List<Hosting> list = new ArrayList<>();
            list.add(new Hosting(1, "liquidweb.com", 80000));
            list.add(new Hosting(2, "linode.com", 90000));
            list.add(new Hosting(3, "digitalocean.com", 120000));
            list.add(new Hosting(4, "aws.amazon.com", 200000));
            list.add(new Hosting(5, "mkyong.com", 1));
    
            // key = id, value - websites
            Map<Integer, String> result1 = list.stream().collect(
                    Collectors.toMap(Hosting::getId, Hosting::getName));
    
            System.out.println("Result 1 : " + result1);
    
            // key = name, value - websites
            Map<String, Long> result2 = list.stream().collect(
                    Collectors.toMap(Hosting::getName, Hosting::getWebsites));
    
            System.out.println("Result 2 : " + result2);
    
            // Same with result1, just different syntax
            // key = id, value = name
            Map<Integer, String> result3 = list.stream().collect(
                    Collectors.toMap(x -> x.getId(), x -> x.getName()));
    
            System.out.println("Result 3 : " + result3);
        }
    }

输出量

Result 1 : {1=liquidweb.com, 2=linode.com, 3=digitalocean.com, 4=aws.amazon.com, 5=mkyong.com}
    Result 2 : {liquidweb.com=80000, mkyong.com=1, digitalocean.com=120000, aws.amazon.com=200000, linode.com=90000}
    Result 3 : {1=liquidweb.com, 2=linode.com, 3=digitalocean.com, 4=aws.amazon.com, 5=mkyong.com}

## 2.列表到地图-重复的钥匙！ ##

2.1运行下面的代码，将引发重复的键错误！

TestDuplicatedKey.java

package com.mkyong.java8;
    
    import java.util.ArrayList;
    import java.util.List;
    import java.util.Map;
    import java.util.stream.Collectors;
    
    public class TestDuplicatedKey {
    
        public static void main(String[] args) {
    
            List<Hosting> list = new ArrayList<>();
            list.add(new Hosting(1, "liquidweb.com", 80000));
            list.add(new Hosting(2, "linode.com", 90000));
            list.add(new Hosting(3, "digitalocean.com", 120000));
            list.add(new Hosting(4, "aws.amazon.com", 200000));
            list.add(new Hosting(5, "mkyong.com", 1));
    		
            list.add(new Hosting(6, "linode.com", 100000)); // new line
    
            // key = name, value - websites , but the key 'linode' is duplicated!?
            Map<String, Long> result1 = list.stream().collect(
                    Collectors.toMap(Hosting::getName, Hosting::getWebsites));
    
            System.out.println("Result 1 : " + result1);
    
        }
    }

输出–下面的错误消息有点让人误解，它应该显示“ linode”而不是键的值。

Exception in thread "main" java.lang.IllegalStateException: Duplicate key 90000
    	at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
    	at java.util.HashMap.merge(HashMap.java:1245)
    	//...

2.2要解决上述重复的键问题，请像下面这样传入第三个mergeFunction参数：

Map<String, Long> result1 = list.stream().collect(
                    Collectors.toMap(Hosting::getName, Hosting::getWebsites,
                            (oldValue, newValue) -> oldValue
                    )
            );

输出量

Result 1 : {..., aws.amazon.com=200000, linode.com=90000}

> **注意**  
> `(oldValue, newValue) -> oldValue` ==>如果密钥重复，您是喜欢oldKey还是newKey？

3.3试试newValue

Map<String, Long> result1 = list.stream().collect(
                    Collectors.toMap(Hosting::getName, Hosting::getWebsites,
                            (oldValue, newValue) -> newvalue
                    )
            );

输出量

Result 1 : {..., aws.amazon.com=200000, linode.com=100000}

## 3.列表到地图–排序和收集 ##

TestSortCollect.java

package com.mkyong.java8;
    
    import java.util.*;
    import java.util.stream.Collectors;
    
    public class TestSortCollect {
    
        public static void main(String[] args) {
    
            List<Hosting> list = new ArrayList<>();
            list.add(new Hosting(1, "liquidweb.com", 80000));
            list.add(new Hosting(2, "linode.com", 90000));
            list.add(new Hosting(3, "digitalocean.com", 120000));
            list.add(new Hosting(4, "aws.amazon.com", 200000));
            list.add(new Hosting(5, "mkyong.com", 1));
            list.add(new Hosting(6, "linode.com", 100000));
    
            //example 1
            Map result1 = list.stream()
                    .sorted(Comparator.comparingLong(Hosting::getWebsites).reversed())
                    .collect(
                            Collectors.toMap(
                                    Hosting::getName, Hosting::getWebsites, // key = name, value = websites
                                    (oldValue, newValue) -> oldValue,       // if same key, take the old key
                                    LinkedHashMap::new                      // returns a LinkedHashMap, keep order
                            ));
    
            System.out.println("Result 1 : " + result1);
    
        }
    }

输出量

Result 1 : {aws.amazon.com=200000, digitalocean.com=120000, linode.com=100000, liquidweb.com=80000, mkyong.com=1}

*PS在上面的示例中，流在收集之前进行了排序，因此“ linode.com = 100000”成为了“ oldValue”。*

## 参考文献 ##

1.  [Java 8收集器JavaDoc][Java 8_JavaDoc]
2.  [Java 8 –如何对地图排序][Java 8]
3.  [Java 8 Lambda：比较器示例][Java 8 Lambda]

标签： [收藏家][Link 1] [重复键][Link 2] [Java 8][Java 8 1] [列表][Link 3] [映射][Link 4] [排序][Link 5] [流][Link 6]

> 翻译自: [https://mkyong.com/java8/java-8-convert-list-to-map/][https_mkyong.com_java8_java-8-convert-list-to-map]

[Java 8_JavaDoc]: https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html
[Java 8]: http://www.mkyong.com/java8/java-8-how-to-sort-a-map/
[Java 8 Lambda]: https://www.mkyong.com/java8/java-8-lambda-comparator-example/
[Link 1]: https://mkyong.com/tag/collectors/
[Link 2]: https://mkyong.com/tag/duplicated-key/
[Java 8 1]: https://mkyong.com/tag/java-8/
[Link 3]: https://mkyong.com/tag/list/
[Link 4]: https://mkyong.com/tag/map/
[Link 5]: https://mkyong.com/tag/sort/
[Link 6]: https://mkyong.com/tag/stream/
[https_mkyong.com_java8_java-8-convert-list-to-map]: https://mkyong.com/java8/java-8-convert-list-to-map/