Convert Map to a List in Java 8 and above

This post will discuss how to convert map to a list in Java.

1. Converting `Map<K,V>` to `List<Map.Entry<K,V>>`

We know that Map.entrySet() returns a set view of the mappings contained in this map. In Java 8, we can easily get a list of key-value pairs by converting it to stream, as shown below:

import java.util.HashMap;

import java.util.List;

import java.util.Map;

import java.util.stream.Collectors;

class Main

{

// Generic method to convert `Map<K, V>` to a list of `<Map.Entry<K, V>`

public static<K, V> List<Map.Entry<K, V>> convertToList(Map<K, V> map)

{

return map.entrySet()

.stream()

.collect(Collectors.toList());

}

public static void main(String[] args)

{

Map<String, Integer> asciiMap = new HashMap<>();

asciiMap.put("A", 65);

asciiMap.put("B", 66);

asciiMap.put("C", 67);

// list to store mappings

List<Map.Entry<String, Integer>> entries = convertToList(asciiMap);

System.out.println(entries);

}

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Output:

[A=65, B=66, C=67]

Here’s an even shorter version of the above code that makes use of the ArrayList constructor to create a list of Map.Entry<K,V> objects containing both key and value:

// Generic method to convert `Map<K, V>` to a list of `<Map.Entry<K, V>`

public static<K, V> List<Map.Entry<K, V>> convertToList(Map<K, V> map) {

return new ArrayList<>(map.entrySet());

}

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2. Converting `Map<K,V>` to `List<K>`

We can get a list of keys of the map using Map.entrySet() in Java 8 and above:

import java.util.ArrayList;

import java.util.HashMap;

import java.util.List;

import java.util.Map;

class Main

{

// Generic method to convert `Map<K, V>` to a `List<K>`

public static<K, V> List<K> convertToList(Map<K, V> map)

{

// create an empty list to store keys

List<K> key = new ArrayList<>();

map.entrySet().stream().forEach(entry -> {

key.add(entry.getKey());

});

return key;

}

public static void main(String[] args)

{

Map<String, Integer> asciiMap = new HashMap<>();

asciiMap.put("A", 65);

asciiMap.put("B", 66);

asciiMap.put("C", 67);

// list to store map keys

List<String> keys = convertToList(asciiMap);

System.out.println(keys);

}

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Output:

[A, B, C]

Here’s a better way to do it in Java 8 and above using Map.keySet():

// Generic method to convert `Map<K, V>` to a `List<K>`

public static<K, V> List<K> convertToList(Map<K, V> map)

{

return map.keySet().stream()

.collect(Collectors.toList());

}

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3. Converting `Map<K,V>` to `List<V>`

We can get a list of values of the map using Map.entrySet() in Java 8 and above:

import java.util.ArrayList;

import java.util.HashMap;

import java.util.List;

import java.util.Map;

class Main

{

// Generic method to convert `Map<K, V>` to a `List<V>`

public static<K, V> List<V> convertToList(Map<K, V> map)

{

// create an empty list to store values

List<V> values = new ArrayList<>();

map.entrySet().stream().forEach(entry -> {

values.add(entry.getValue());

});

return values;

}

public static void main(String[] args)

{

Map<String, Integer> asciiMap = new HashMap<>();

asciiMap.put("A", 65);

asciiMap.put("B", 66);

asciiMap.put("C", 67);

// list to store map values

List<Integer> values = convertToList(asciiMap);

System.out.println(values);

}

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Output:

[65, 66, 67]

Here’s a better way to do it in Java 8 and above using Map.values():

// Generic method to convert `Map<K, V>` to a `List<V>`

public static<K, V> List<V> convertToList(Map<K, V> map)

{

return map.values().stream()

.collect(Collectors.toList());

}

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That’s all about converting Map to a List in Java.

Convert Map to a List in Java 8 and above

1. Converting Map<K,V> to List<Map.Entry<K,V>>

2. Converting Map<K,V> to List<K>

3. Converting Map<K,V> to List<V>

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1. Converting `Map<K,V>` to `List<Map.Entry<K,V>>`

2. Converting `Map<K,V>` to `List<K>`

3. Converting `Map<K,V>` to `List<V>`