Java 8 – Convert List to Map

package com.sheting.java8;

public class Hosting {
    private int Id;
    private String name;
    private long websites;

    public Hosting() {
    }

    public Hosting(int id, String name, long websites) {
        Id = id;
        this.name = name;
        this.websites = websites;
    }

    public int getId() {
        return Id;
    }

    public void setId(int id) {
        Id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public long getWebsites() {
        return websites;
    }

    public void setWebsites(long websites) {
        this.websites = websites;
    }

    @Override
    public String toString() {
        return "Hosting{" + "Id=" + Id + ", name='" + name + '\'' + ", websites=" + websites + '}';
    }
}

Example 1 List to Map – Collectors.toMap()

package com.sheting.java8;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class ConvertListToMapExample1 {
    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);
    }
}

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}

Example 2 List to Map – Duplicated Key!

package com.sheting.java8;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class ConvertListToMapExample2 {
    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!?
        // 重复key会抛出异常
        /*
         * Map<String, Long> result = list.stream().collect(
         * Collectors.toMap(Hosting::getName, Hosting::getWebsites));
         */

        // 第三个参数，当key相同，value取上一次key的value
        Map<String, Long> result1 = list.stream().collect(Collectors.toMap(Hosting::getName, Hosting::getWebsites, (oldValue, newValue) -> oldValue));

        System.out.println("Result 1 : " + result1);

    }
}

Output:

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

Example 3 List to Map – Sort & Collect

package com.sheting.java8;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class ConvertListToMapExample3 {
    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=80000, mkyong.com=1}