Java 8 将List转换为Map

矫情吗；* 2023-10-10 11:34 80阅读 0赞

package com.zzg.java8
    public class webSite{
        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()
    }

### List到Map - Collectors.toMap（） ###

创建`webSite`对象的List，`Collectors.toMap`并将其转换为Map。

package com.zzg.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<webSite> list = new ArrayList<>();
            list.add(new webSite(1, "liquidweb.com", 80000));
            list.add(new webSite(2, "linode.com", 90000));
            list.add(new webSite(3, "digitalocean.com", 120000));
            list.add(new webSite(4, "aws.amazon.com", 200000));
            list.add(new webSite(5, "mkyong.com", 1));
            // key = id, value - websites
            Map<Integer, String> result1 = list.stream().collect(
                    Collectors.toMap(webSite::getId, webSite::getName));
            System.out.println("Result 1 : " + result1);
            // key = name, value - websites
            Map<String, Long> result2 = list.stream().collect(
                    Collectors.toMap(webSite::getName, webSite::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);
        }
    }

### 列出Map - 重复键 ###

package com.zzg.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<webSite> list = new ArrayList<>();
            list.add(new webSite(1, "liquidweb.com", 80000));
            list.add(new webSite(2, "linode.com", 90000));
            list.add(new webSite(3, "digitalocean.com", 120000));
            list.add(new webSite(4, "aws.amazon.com", 200000));
            list.add(new webSite(5, "mkyong.com", 1));
            list.add(new webSite(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(webSite::getName, webSite::getWebsites));
            System.out.println("Result 1 : " + result1);
        }
    }

输出 - 下面的错误信息有点误导，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)
    	//...

解决上述重复的关键问题，请传入第三个mergeFunction参数，如下所示：

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

尝试newValue

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

### 列出Map- 排序Collect ###

package com.zzg.java8;
    import java.util.*;
    import java.util.stream.Collectors;
    public class TestSortCollect {
        public static void main(String[] args) {
            List<webSite> list = new ArrayList<>();
            list.add(new webSite(1, "liquidweb.com", 80000));
            list.add(new webSite(2, "linode.com", 90000));
            list.add(new webSite(3, "digitalocean.com", 120000));
            list.add(new webSite(4, "aws.amazon.com", 200000));
            list.add(new webSite(5, "mkyong.com", 1));
            list.add(new webSite(6, "linode.com", 100000));
            //example 1
            Map result1 = list.stream()
                    .sorted(Comparator.comparingLong(webSite::getWebsites).reversed())
                    .collect(
                            Collectors.toMap(
                                    webSite::getName, webSite::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);
        }
    }