Java 8 – Convert List to Map
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?
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’.
you should also include the scenario where duplicate key values should be added in linkedList.
This not working:
Map result1 = list.stream().collect(
Collectors.toMap(Hosting::getId, Hosting));
Map<Integer, Hosting> result2 = list.stream().collect(
Collectors.toMap(Hosting::getId, x -> x));
Collectors.toMap(h -> h.getId(), h -> h)
Function.identity() is the best
Hosting::this –> try this
dfdh
no matter we comment mkyong never response 😛
If u have to create a map<Integer , List> this code won’t work e.g.
list.add(new Hosting(1, “liquidweb.com”, 80000));
list.add(new Hosting(1, “liquidweb1.com”, 90000));
and u want map to have 1={“liquidweb.com” ,”liquidweb1.com” }
Any solution to get Map of Key, value(as list)? I need it. as explained by Prashant
Multimap multimap = ArrayListMultimap.create();
list.forEach(v -> multimap.put(v.getId(), v.getId()));
Multimap multimap = create();
entry.getValue().forEach(v -> multimap.put(v.id(), v.getid()));
Thank you for this blog, helped me a lot to understand in detail the concept.
Thank you, I came for the first need/case, I ended finding all the things I needed
Thanks, the best examples found so far.
Huge help, thank you sir.
Its really nice example … could You please add also an example when we want to create a multi key map from list ? I thing it could be a really helpfull.
Hi Mkyoung, it should be like (newValue, oldValue). I mean second parameter represent old value not first but in your code you referring first parameter as oldValue.
Your code: //Because here it will Old value
Map result1 = list.stream().collect(
Collectors.toMap(Hosting::getName, Hosting::getWebsites,
(oldValue, newValue) -> newvalue //Because here it will Old value
)
);
But it should be for newValue
Map result1 = list.stream().collect(
Collectors.toMap(Hosting::getName, Hosting::getWebsites,
(newValue, oldValue) -> newvalue
)
);
Hi Mkyong,
My reuirement is to convert List<Map to List
Hi Mkyong,
My reuirement is to convert List<Map to List
Please share example