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);
}
} |
[GEEKS, For, GEEKS]
Example 2: To create an immutable 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);
}
} |
[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);
}
} |
{1=Geeks, 2=For, 3=Geeks}
Note:This method is most commonly used for creating immutable collections.