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() methodimport 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 hereimport 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.
