Java 8 Convert Map to List
By：Roy.LiuLast updated：2019-08-17
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
References
From：一号门
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