Few Java 8 examples to show you how to convert a List of objects into a Map, and how to handle the duplicated keys.
HostingSite.java
package com.favtuts.java8;
public class HostingSite {
private int Id;
private String name;
private long websites;
public HostingSite(int id, String name, long websites) {
Id = id;
this.name = name;
this.websites = websites;
}
//getters, setters and toString()
@Override
public String toString() {
return "Hosting{" +
" Id='" + getId() + "'" +
", name='" + getName() + "'" +
", websites='" + getWebsites() + "'" +
"}";
}
public int getId() {
return this.Id;
}
public void setId(int Id) {
this.Id = Id;
}
public String getName() {
return this.name;
}
public void setName(String name) {
this.name = name;
}
public long getWebsites() {
return this.websites;
}
public void setWebsites(long websites) {
this.websites = websites;
}
}
1. List to Map – Collectors.toMap()
Create a list of the HostingSite objects, and uses Collectors.toMap to convert it into a Map.
TestListMap.java
package com.favtuts.java8;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class Java8TestListMap {
public static void main(String[] args) {
List<HostingSite> list = new ArrayList<>();
list.add(new HostingSite(1, "liquidweb.com", 80000));
list.add(new HostingSite(2, "linode.com", 90000));
list.add(new HostingSite(3, "digitalocean.com", 120000));
list.add(new HostingSite(4, "aws.amazon.com", 200000));
list.add(new HostingSite(5, "tuts.heomi.net", 1));
// key = id, value - websites
Map<Integer, String> result1 = list.stream().collect(
Collectors.toMap(HostingSite::getId, HostingSite::getName));
System.out.println("Result 1 : " + result1);
// key = name, value - websites
Map<String, Long> result2 = list.stream().collect(
Collectors.toMap(HostingSite::getName, HostingSite::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=tuts.heomi.net}
Result 2 : {tuts.heomi.net=1, liquidweb.com=80000, digitalocean.com=120000, linode.com=90000, aws.amazon.com=200000}
Result 3 : {1=liquidweb.com, 2=linode.com, 3=digitalocean.com, 4=aws.amazon.com, 5=tuts.heomi.net}2. List to Map – Duplicated Key!
2.1 Run below code, and duplicated key errors will be thrown!
TestDuplicatedKey.java
package com.favtuts.java8;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class Java8TestListMap {
public static void main(String[] args) {
testListToMapDuplicatedKey();
}
private static void testListToMapDuplicatedKey() {
List<HostingSite> list = new ArrayList<>();
list.add(new HostingSite(1, "liquidweb.com", 80000));
list.add(new HostingSite(2, "linode.com", 90000));
list.add(new HostingSite(3, "digitalocean.com", 120000));
list.add(new HostingSite(4, "aws.amazon.com", 200000));
list.add(new HostingSite(5, "tuts.heomi.net", 1));
list.add(new HostingSite(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(HostingSite::getName, HostingSite::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(HostingSite::getName, HostingSite::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(HostingSite::getName, HostingSite::getWebsites,
(oldValue, newValue) -> newvalue
)
);Output
Result 1 : {..., aws.amazon.com=200000, linode.com=100000}3. List to Map – Sort & Collect
TestSortCollect.java
package com.favtuts.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 Java8TestListMap {
public static void main(String[] args) {
sortListToMap();
}
private static void sortListToMap() {
List<HostingSite> list = new ArrayList<>();
list.add(new HostingSite(1, "liquidweb.com", 80000));
list.add(new HostingSite(2, "linode.com", 90000));
list.add(new HostingSite(3, "digitalocean.com", 120000));
list.add(new HostingSite(4, "aws.amazon.com", 200000));
list.add(new HostingSite(5, "tuts.heomi.net", 1));
list.add(new HostingSite(6, "linode.com", 100000));
//example 1
Map result1 = list.stream()
.sorted(Comparator.comparingLong(HostingSite::getWebsites).reversed())
.collect(
Collectors.toMap(
HostingSite::getName, HostingSite::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, tuts.heomi.net=1}P.S In above example, the stream is sorted before collect, so the “linode.com=100000” became the ‘oldValue’.
Download Source Code
$ git clone https://github.com/favtuts/java-core-tutorials-examples
$ cd java-basic/java8