Collectors Class – toList(), toSet() and toMap() methods

This post will discuss toList(), toSet(), and toMap() methods of Collectors class in Java.

1. Using `Collectors.toList()` method

Collectors.toList() method returns a Collector which collects all the input elements into a new list. It doesn’t offer any guarantee on the type of list returned.

⮚ Get a list from the specified values

1 2	List<Integer> list = Stream.of(1, 2, 3, 4, 5) .collect(Collectors.toList());

⮚ Convert a primitive array to a list

int[] array = { 1, 2, 3, 4, 5 };

List<Integer> list = Stream.of(array) // returns `Stream<int[]>`

.flatMapToInt(Arrays::stream)

.boxed()

.collect(Collectors.toList());

⮚ Convert a non-primitive array to a list

Integer[] arr = { 1, 2, 3, 4, 5 };

List<Integer> list = Stream.of(arr)

.collect(Collectors.toList());

⮚ Convert string to list of Characters

List<Character> chars = "Techie".chars() // IntStream

.mapToObj(e -> (char)e) // Stream<Character>

.collect(Collectors.toList());

Download Run Code

2. Using `Collectors.toSet()` method

Collectors.toSet() method works in the similar way as Collectors.toList() but returns a set which discards the duplicates present in the input.

⮚ Get a set from specified values

1	Set<Integer> set = Stream.of(1, 2, 3, 4, 5).collect(Collectors.toSet());

⮚ Convert a primitive array to a set

int[] array = { 1, 2, 3, 4, 5 };

Set<Integer> set = Stream.of(array) // returns `Stream<int[]>`

.flatMapToInt(Arrays::stream)

.boxed()

.collect(Collectors.toSet());

⮚ Convert a non-primitive array to a set

1 2	Integer[] arr = { 1, 2, 3, 4, 5 }; Set<Integer> set = Stream.of(arr).collect(Collectors.toSet());

Download Run Code

3. Using `Collectors.toMap()` method

Since a stream is a sequence of items, to construct a map out of it, we need to specify how to extract keys/values pairs. The Collectors.toMap() returns a Collector that collects all the input elements into a new map by applying the specified mapping functions to the input elements, which help identify the keys and values.

Map<String, String> map = Stream.of(new String[][] {

{"A", "value1"},

{"B", "value2"},

{"C", "value3"}

}).collect(Collectors.toMap(p -> p[0], p -> p[1]));

If we want a map with different types for key and value, we can do something like :

Map<String, Integer> map = Stream.of(new Object[][] {

{"A", 65},

{"B", 66},

{"C", 67}

}).collect(Collectors.toMap(p -> (String)p[0], p -> (Integer)p[1]));

Another approach to accommodate different types for keys and values is to use the AbstractMap.SimpleEntry<K,V> or AbstractMap.SimpleImmutableEntry<K,V>, which are just implementations of the Map.Entry<K,V> interface.

import java.util.AbstractMap;

import java.util.Map;

import java.util.stream.Collectors;

import java.util.stream.Stream;

class Main

{

public static void main(String[] args)

{

Map<String, String> person =

Stream.of(

new AbstractMap.SimpleEntry<>("Name", "John"),

new AbstractMap.SimpleEntry<>("Age", "26"),

new AbstractMap.SimpleEntry<>("Sex", "Male")

).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

for (String key: person.keySet()) {

System.out.println(key + " is " + person.get(key));

}

Download Run Code

Output:

Sex is Male
Age is 26
Name is John

We can also use an overloaded version of the Collectors.toMap() method to specify the desired map type and how to deal with collisions between multiple values mapping to the same key.

import java.util.AbstractMap;

import java.util.Arrays;

import java.util.List;

import java.util.Map;

import java.util.TreeMap;

import java.util.stream.Collectors;

class Main

{

public static void main(String[] args)

{

List<Map.Entry<String, String>> entries = Arrays.asList(

new AbstractMap.SimpleEntry<>("Name", "John"),

new AbstractMap.SimpleEntry<>("Age", "26"),

new AbstractMap.SimpleEntry<>("Sex", "Male")

);

Map<String, String> treeMap = entries.stream()

.collect(Collectors.toMap(Map.Entry::getKey,

Map.Entry::getValue,

(oldValue, newValue) -> newValue,

TreeMap::new));

for (String key: treeMap.keySet()) {

System.out.println(key + " is " + treeMap.get(key));

}

Download Run Code

Output:

Age is 26
Name is John
Sex is Male

This is equivalent to:

import java.util.*;

class Main

{

public static void main(String[] args)

{

List<Map.Entry<String, String>> entries = Arrays.asList(

new AbstractMap.SimpleEntry<>("Name", "John"),

new AbstractMap.SimpleEntry<>("Age", "26"),

new AbstractMap.SimpleEntry<>("Sex", "Male")

);

Map<String, String> treeMap = new TreeMap<>();

for (Map.Entry<String, String> entry : entries) {

treeMap.put(entry.getKey(), entry.getValue());

}

for (String key: treeMap.keySet()) {

System.out.println(key + " is " + treeMap.get(key));

}

Download Run Code

Output:

Age is 26
Name is John
Sex is Male

That’s all about the Collectors class in Java 8.

Collectors Class – toList(), toSet() and toMap() methods

1. Using Collectors.toList() method

2. Using Collectors.toSet() method

3. Using Collectors.toMap() method

1. Using `Collectors.toList()` method

2. Using `Collectors.toSet()` method

3. Using `Collectors.toMap()` method