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

References

  1. Java 8 – Convert List to Map
  2. Java 8 forEach examples

About the Author

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mkyong
Founder of Mkyong.com, love Java and open source stuff. Follow him on Twitter, or befriend him on Facebook or Google Plus. If you like my tutorials, consider make a donation to these charities.

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Sandeep Choudhari
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Sandeep Choudhari

In Java8 Map to List example, the below code snippet needs to change. Instead of map.keySet(), it should be map.values().

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

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

arun singh
Guest
arun singh
public class Test { public static void main(String[] args) { TreeMap hm=new TreeMap(); hm.put(3, “arun singh”); hm.put(5, “vinay singh”); hm.put(1, “bandagi singh”); hm.put(6, “vikram singh”); hm.put(2, “panipat singh”); hm.put(28, “jakarta singh”); ArrayList al=new ArrayList(hm.values()); Collections.sort(al, new myComparator()); System.out.println(“//sort by values n”); for(String obj: al){ for(Map.Entry map2:hm.entrySet()){ if(map2.getValue().equals(obj)){ System.out.println(map2.getKey()+” “+map2.getValue()); } } } } } class myComparator implements Comparator{ @Override public int compare(Object o1, Object o2) { String o3=(String) o1; String o4 =(String) o2; return o3.compareTo(o4); } } OUTPUT= //sort by values 3 arun singh 1 bandagi singh 28 jakarta singh 2 panipat singh 6 vikram singh 5 vinay singh
Caleb Cushing
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Caleb Cushing

why are you recommending what’s probably the most verbose and slowest way possible?, map.keySet() returns a Set fand map.values() return a Collection, though I’m not sure what type of Collection, and that may very. so new ArrayList( map.keySet() ) or new ArrayList( map.values), I mean technically even map.keySet().stream().collect( Collectors.toList() ). The only reason to use an entrySet is if you don’t want to iterate over them twice, and you aren’t showing how to take a map and Partition it in to 2 lists, you can do that, but I’d have to look it up.