Collectors collectingAndThen() method in Java with Examples Last Updated : 16 Jun, 2022 Summarize Comments Improve Suggest changes Share Like Article Like Report 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. Comment More infoAdvertise with us Next Article Collectors collectingAndThen() method in Java with Examples S suman_ptnl Follow Improve Article Tags : Java Java - util package Java-Functions Java-Stream-Collectors Java-Collectors +1 More Practice Tags : Java Similar Reads Java Tutorial Java is a high-level, object-oriented programming language used to build web apps, mobile applications, and enterprise software systems. It is known for its Write Once, Run Anywhere capability, which means code written in Java can run on any device that supports the Java Virtual Machine (JVM).Java s 10 min read Java OOP(Object Oriented Programming) Concepts Java Object-Oriented Programming (OOPs) is a fundamental concept in Java that every developer must understand. It allows developers to structure code using classes and objects, making it more modular, reusable, and scalable.The core idea of OOPs is to bind data and the functions that operate on it, 13 min read Java Interview Questions and Answers Java is one of the most popular programming languages in the world, known for its versatility, portability, and wide range of applications. Java is the most used language in top companies such as Uber, Airbnb, Google, Netflix, Instagram, Spotify, Amazon, and many more because of its features and per 15+ min read Arrays in Java Arrays in Java are one of the most fundamental data structures that allow us to store multiple values of the same type in a single variable. They are useful for storing and managing collections of data. Arrays in Java are objects, which makes them work differently from arrays in C/C++ in terms of me 15+ min read Collections in Java Any group of individual objects that are represented as a single unit is known as a Java Collection of Objects. In Java, a separate framework named the "Collection Framework" has been defined in JDK 1.2 which holds all the Java Collection Classes and Interface in it. In Java, the Collection interfac 15+ min read Inheritance in Java Java Inheritance is a fundamental concept in OOP(Object-Oriented Programming). It is the mechanism in Java by which one class is allowed to inherit the features(fields and methods) of another class. In Java, Inheritance means creating new classes based on existing ones. A class that inherits from an 13 min read Java Exception Handling Exception handling in Java allows developers to manage runtime errors effectively by using mechanisms like try-catch block, finally block, throwing Exceptions, Custom Exception handling, etc. An Exception is an unwanted or unexpected event that occurs during the execution of a program, i.e., at runt 10 min read Java Programs - Java Programming Examples In this article, we will learn and prepare for Interviews using Java Programming Examples. From basic Java programs like the Fibonacci series, Prime numbers, Factorial numbers, and Palindrome numbers to advanced Java programs.Java is one of the most popular programming languages today because of its 8 min read Java Interface An Interface in Java programming language is defined as an abstract type used to specify the behaviour of a class. An interface in Java is a blueprint of a behaviour. A Java interface contains static constants and abstract methods. Key Properties of Interface:The interface in Java is a mechanism to 12 min read Introduction to Java Java is a high-level, object-oriented programming language developed by Sun Microsystems in 1995. It is platform-independent, which means we can write code once and run it anywhere using the Java Virtual Machine (JVM). Java is mostly used for building desktop applications, web applications, Android 4 min read Like