Java 8 – Convert List to Map

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()
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!
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) {

// 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}
Copy
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
package com.mkyong.java8;

import java.util.*;
import java.util.stream.Collectors;

public class TestSortCollect {

public static void main(String[] args) {

//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);

}
}

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

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

来源：https://www.cnblogs.com/shy1766IT/p/10021748.html

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convert