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Collectors collectingAndThen() method in Java with Examples

Last Updated : 16 Jun, 2022
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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 

Java 
// 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 immutable set. 

Java 
// 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 

Java 
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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