List 转 Map (Convert List to Map)

https://www.mkyong.com/java8/java-8-convert-list-to-map/

Java 8 – Convert List to Map

By mkyong | March 19, 2016 | Updated : May 26, 2017 | Viewed : 392,244 | +3,600 pv/w

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

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()
}

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1. List to Map – Collectors.toMap()

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

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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}

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2. List to Map – Duplicated Key!

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

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);

    }
}

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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)
	//...

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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
                )
        );

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Output

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

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Note
(oldValue, newValue) -> oldValue ==> If the key is duplicated, do you prefer oldKey or newKey?

3.3 Try newValue

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

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Output

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

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3. List to Map – Sort & 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);

    }
}

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Output

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

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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

collectors duplicated key java 8 list map sort stream

HOW MUCH DO YOU KNOW ABOUT ROBOTS?

  

How was the first working robot employed in 1961?

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This first humanoid robot was 7 feet tall and could 'speak' 700 words.

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Who sketched 'mechanical knights' as early as 1945?

In which country did Barack Obama play game of soccer against a robot?

Spirit and Opportunity refer to which robots?

The first case of robot homicide occured when:

15

• Searching for gold

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• The first pair of Google cars

• The 'bots that uncovered Bin Laden

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• Drones made to explore Mars

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About the Author

mkyong

Founder of Mkyong.com, love Java and open source stuff. Follow him on Twitter, or befriend him on Facebook or Google Plus. If you like my tutorials, consider make a donation to these charities.