Collectors collectingAndThen() method in Java with Examples

Last Updated : 08 Jan, 2019

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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