Java 8 Convert List to Map
By:Roy.LiuLast updated:2019-08-17
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()
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);
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!
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);
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}
Note
(oldValue, newValue) -> oldValue ==> If the key is duplicated, do you prefer oldKey or newKey?
(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 );
Output
Result 1 : {..., aws.amazon.com=200000, linode.com=100000}
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);
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
From:一号门
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