Collectors collectingAndThen() method in Java with Examples

The collectingAndThen(Collector downstream, Function finisher) method of class collectors in Java, which adopts Collector so that we can perform an additional finishing transformation.

Syntax :

public static <T, A, R, RR> 
       Collector <T, A, RR> 
       collectingAndThen(Collector <T, A, R> downstream, 
                         Function <R, RR> finisher)
                                                             

Where,
    T : The type of the input elements
    A :Intermediate accumulation type of the downstream collector
    R :Result type of the downstream collector
    RR :Result type of the resulting collector

Parameters:This method accepts two parameters which are listed below

downstream: It is an instance of a collector, i.e we can use any collector can here.

finisher: It is an instance of a function which is to be applied to the final result of the downstream collector.

Returns: Returns a collector which performs the action of the downstream collector, followed by an additional finishing step, with the help of finisher function.

Below are examples to illustrate collectingAndThen() the method.

Example 1: To create an immutable list

// Write Java code here 
// Collectors collectingAndThen() method 
  
import java.util.Collections; 
import java.util.List; 
import java.util.stream.Collectors; 
import java.util.stream.Stream; 
  
public class GFG { 
    public static void main(String[] args) 
    { 
        // Create an Immutable List 
        List<String> lt 
            = Stream 
                  .of("GEEKS", "For", "GEEKS") 
                  .collect(Collectors 
                               .collectingAndThen( 
                                   Collectors.toList(), 
                                   Collections::<String> unmodifiableList)); 
        System.out.println(lt); 
    } 
} 

Output:

[GEEKS, For, GEEKS]

Example 2: To create an immuitable set.

// Write Java code here 
import java.util.Collections; 
import java.util.List; 
import java.util.Set; 
import java.util.stream.Collectors; 
import java.util.stream.Stream; 
  
public class GFG { 
    public static void main(String[] args) 
    { 
        // Create an Immutable Set 
        Set<String> st 
            = Stream 
                  .of("GEEKS", "FOR", "GEEKS") 
                  .collect( 
                      Collectors 
                          .collectingAndThen(Collectors.toSet(), 
                                             Collections::<String> 
                                                 unmodifiableSet)); 
        System.out.println(st); 
    } 
} 

Output:

[GEEKS, FOR]

Example 2: To create an immutable map

import java.util.*; 
  
public class GFG { 
    public static void main(String[] args) 
    { 
        // Create an Immutable Map 
        Map<String, String> mp 
            = Stream 
                  .of(new String[][] { 
                      { "1", "Geeks" }, 
                      { "2", "For" }, 
                      { "3", "Geeks" } }) 
                  .collect( 
                      Collectors 
                          .collectingAndThen( 
                              Collectors.toMap(p -> p[0], p -> p[1]), 
                              Collections::<String, String> 
                                  unmodifiableMap)); 
        System.out.println(mp); 
    } 
} 

Output:

{1=Geeks, 2=For, 3=Geeks}

Note:This method is most commonly used for creating immutable collections.

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