Few Java 8 examples to show you how to convert a List of objects into a Map, and how to handle the duplicated keys.

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. List to Map – Collectors.toMap()

Create a list of the Hosting objects, and uses Collectors.toMap to convert it into a Map.

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);

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. List to Map – Duplicated Key!

2.1 Run below code, and duplicated key errors will be thrown!

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);

Output – The error message below is a bit misleading, it should show “linode” instead of the value of the key.

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 To solve the duplicated key issue above, pass in the third mergeFunction argument like this :

	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 Try 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. List to Map – Sort & Collect

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=80000, mkyong.com=1}

P.S In above example, the stream is sorted before collect, so the “linode.com=100000” became the ‘oldValue’.

References

1. Java 8 Collectors JavaDoc
2. Java 8 – How to sort a Map
3. Java 8 Lambda : Comparator example