Collectors collectingAndThen() method in Java

This post will discuss the collectingAndThen() method provided by the Collectors class in Java, which adapts a collector to perform an additional finishing transformation.

The collectingAndThen(downstream, finisher) method returns a Collector that performs the action of the downstream collector, followed by an additional finishing step with the help of the finisher Function. It is most commonly used for creating immutable Collections.

1. Creating an immutable empty collection

⮚ Create an immutable empty List

We could adapt the Collectors.toList() collector to always produce an immutable empty List, as shown below:

List<String> immutableList = Stream.of(new String[] {})

.collect(Collectors.collectingAndThen(Collectors.toList(),

Collections::<String>unmodifiableList));

⮚ Create an immutable empty Set

We could adapt the Collectors.toSet() collector to always produce an immutable empty Set, as shown below:

Set<String> immutableSet = Stream.of(new String[] {})

.collect(Collectors.collectingAndThen(Collectors.toSet(),

Collections::<String>unmodifiableSet));

⮚ Create an immutable empty Map

We could adapt the Collectors.toMap() collector to always produce an immutable empty Map, as shown below:

Map<String, String> immutableMap = Stream.of(new String[][] {})

.collect(Collectors.collectingAndThen(

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

Collections::<String, String>unmodifiableMap

));

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2. Creating an Immutable collection

⮚ Create an immutable list

We could adapt the toList() collector to always produce an unmodifiable list with:

List<String> list = Stream.of("USA", "UK", "India")

.collect(Collectors.collectingAndThen(Collectors.toList(),

Collections::<String>unmodifiableList));

⮚ Create an immutable Set

We could adapt the toSet() collector to always produce an unmodifiable set with:

Set<String> set = Stream.of("USA", "UK", "India")

.collect(Collectors.collectingAndThen(Collectors.toSet(),

Collections::<String>unmodifiableSet));

⮚ Create an immutable Map

We could adapt the toMap() collector to always produce an unmodifiable map with:

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

{"USA", "Washington DC"}, {"UK", "London"}, {"India", "New Delhi"}

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

Collections::<String, String>unmodifiableMap));

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3. Reversing a list

We can also write a collector that collects elements in the reversed order with the help of the collectingAndThen() method, as shown below:

import java.util.Arrays;

import java.util.Collections;

import java.util.List;

import java.util.stream.Collectors;

// Java program to reverse a list using `Collectors.collectingAndThen()` method

class Main

{

public static void main(String[] args)

{

List<String> countries = Arrays.asList("USA", "UK", "India");

List<String> reverse = countries.stream()

.collect(Collectors.collectingAndThen(Collectors.toList(),

l -> {

Collections.reverse(l);

return l.stream();

}

)).collect(Collectors.toList());

System.out.println(reverse);

}

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Output:

[India, UK, USA]

4. Slice a list

We can also use the collectingAndThen() method to get a slice of a list, as shown below:

import java.util.List;

import java.util.stream.Collectors;

import java.util.stream.IntStream;

import java.util.stream.Stream;

// Java program to get a slice of a list using `Collectors.collectingAndThen()`

class Main

{

public static void main(String[] args)

{

Stream<Integer> stream = IntStream.rangeClosed(0, 10)

.boxed();

// Get a list containing elements from fromIndex to toIndex

int fromIndex = 2;

int toIndex = 5;

List<Integer> slice = stream.collect(Collectors.collectingAndThen(

Collectors.toList(),

list -> list.stream()

.skip(fromIndex)

.limit(toIndex - fromIndex + 1)

)).collect(Collectors.toList());

System.out.println(slice);

}

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Output:

[2, 3, 4, 5]

That’s all about the Collectors class collectingAndThen() method in Java.

Collectors collectingAndThen() method in Java

1. Creating an immutable empty collection

2. Creating an Immutable collection

3. Reversing a list

4. Slice a list

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