Java 8 – Convert Map to LIST

Java 8 – Convert Map to LIST

Few Java examples to convert a Map to a List

Map<String, String> map = new HashMap<>();

// Convert all Map keys to a List
List<String> result = new ArrayList(map.keySet());

// Convert all Map values to a List
List<String> result2 = new ArrayList(map.values());

// Java 8, Convert all Map keys to a List
List<String> result3 = map.keySet().stream()
.collect(Collectors.toList());

// Java 8, Convert all Map values to a List
List<String> result4 = map.values().stream()
.collect(Collectors.toList());

// Java 8, seem a bit long, but you can enjoy the Stream features like filter and etc.
List<String> result5 = map.values().stream()
.filter(x -> !"apple".equalsIgnoreCase(x))
.collect(Collectors.toList());

// Java 8, split a map into 2 List, it works!
// refer example 3 below

1. Map To List
For a simple Map to List conversion, just uses the below code :

ConvertMapToList.java
package com.mkyong;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class ConvertMapToList {

public static void main(String[] args) {

Map<Integer, String> map = new HashMap<>();
map.put(10, "apple");
map.put(20, "orange");
map.put(30, "banana");
map.put(40, "watermelon");
map.put(50, "dragonfruit");

System.out.println("\n1. Export Map Key to List...");

List<Integer> result = new ArrayList(map.keySet());

result.forEach(System.out::println);

System.out.println("\n2. Export Map Value to List...");

List<String> result2 = new ArrayList(map.values());

result2.forEach(System.out::println);

}

}

Output

1. Export Map Key to List...
50
20
40
10
30

2. Export Map Value to List...
dragonfruit
orange
watermelon
apple
banana

Java 8 – Map To List
For Java 8, you can convert the Map into a stream, process it and returns it back as a List

ConvertMapToList.java
package com.mkyong;

import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class ConvertMapToList {

public static void main(String[] args) {

Map<Integer, String> map = new HashMap<>();
map.put(10, "apple");
map.put(20, "orange");
map.put(30, "banana");
map.put(40, "watermelon");
map.put(50, "dragonfruit");

System.out.println("\n1. Export Map Key to List...");

List<Integer> result = map.keySet().stream()
.collect(Collectors.toList());

result.forEach(System.out::println);

System.out.println("\n2. Export Map Value to List...");

List<String> result2 = map.values().stream()
.collect(Collectors.toList());

result2.forEach(System.out::println);

System.out.println("\n3. Export Map Value to List..., say no to banana");
List<String> result3 = map.keySet().stream()
.filter(x -> !"banana".equalsIgnoreCase(x))
.collect(Collectors.toList());

result3.forEach(System.out::println);

}

}

Output

1. Export Map Key to List...
50
20
40
10
30

2. Export Map Value to List...
dragonfruit
orange
watermelon
apple
banana

3. Export Map Value to List..., say no to banana
dragonfruit
orange
watermelon
apple

3. Java 8 – Convert Map into 2 List
This example is a bit extreme, uses map.entrySet() to convert a Map into 2 List

ConvertMapToList.java
package com.mkyong;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

//https://www.mkyong.com/java8/java-8-how-to-sort-a-map/
public class ConvertMapToList {

public static void main(String[] args) {

Map<Integer, String> map = new HashMap<>();
map.put(10, "apple");
map.put(20, "orange");
map.put(30, "banana");
map.put(40, "watermelon");
map.put(50, "dragonfruit");

// split a map into 2 List
List<Integer> resultSortedKey = new ArrayList<>();
List<String> resultValues = map.entrySet().stream()
//sort a Map by key and stored in resultSortedKey
.sorted(Map.Entry.<Integer, String>comparingByKey().reversed())
.peek(e -> resultSortedKey.add(e.getKey()))
.map(x -> x.getValue())
// filter banana and return it to resultValues
.filter(x -> !"banana".equalsIgnoreCase(x))
.collect(Collectors.toList());

resultSortedKey.forEach(System.out::println);
resultValues.forEach(System.out::println);

}

}

Output

//resultSortedKey
50
40
30
20
10

//resultValues
dragonfruit
watermelon
orange
apple

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

来源：https://www.cnblogs.com/shy1766IT/p/10024987.html