Java 8 将List转换为Map

几个Java 8示例来向您展示如何将一个List对象转换为一个Map，以及如何处理重复的键

Hosting.java

package com.mkyong.java8public 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()}

List到Map - Collectors.toMap（）

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

TestListMap.java

package com.mkyong.java8import 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);
    }}

Output

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.列出Map - 重复键

2.1运行下面的代码，重复的关键错误将被抛出！

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)
	//...

2.2要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(Hosting::getName, Hosting::getWebsites,
                        (oldValue, newValue) -> oldValue                )
        );

Output

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

3.3尝试newValue

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

Output

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

3.列出Map- 排序Collect

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);
    }}

Output

Result 1 : {aws.amazon.com=200000, digitalocean.com=120000, linode.com=100000, liquidweb.com=8000