Java Streams - map() vs flatMap()

Introduction

In this block, we will learn about map and flatMap methods in Java streams API. Both are very useful intermediate methods of Java 8, to perform transformation operations on a list.

• map() method used for transformation (e.g. converting lowercase strings to uppercase strings)

• flatMap() used for transformation and flattening ( e.g converting a stream of lowercase strings to uppercase strings).

• flatmap() -> map() + flattening.

Map() method

• map() method is used for data transformation

• map() takes Stream<T> as input and return Stream <R>

• Stream<R> map(Stream<T> input){}

• Its mapper function produces a single value for each input value hence it is called One-to-One mapping

•         <R> Stream<R> map(Function<? super T,? extends R> mapper);
  

flatMap() method

• flatMap() takes Stream<Stream<T>> as input and return stream<R>

• Stream<R> map(Stream<Stream<T>> input){}

•       <R> Stream <R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper)
  

• Its mapper function produces multiple values for each input value. Hence it is called One-To-Many mapping.

Data Transformation and Flattening

Data Transformation

Data transformation means doing some operations on a list of data.

Ex:- Stream.of("a","b","c","d") -> [A,B,C,D] ( Transform data from lowercase to uppercase)

Flattening

Flattening means converting stream of stream into single stream.

Ex:- [[1,2],[3,4],[5,6],[7,8]] - [1,2,3,4,5,6,7,8]

Real-world example

Suppose there is a customer list containing the email id for each customer and a list of phone numbers for each customer.

We can use the map method to print the list of emails as there is one-to-one mapping whereas, in the case of phone numbers, there is a list so it will need flatMap.

Database.java

public static List<Customer> getAll(){
        return Stream.of(
                new Customer(101, "john", "john@gmail.com", Arrays.asList("397937955", "21654725")),
                new Customer(102, "smith", "smith@gmail.com", Arrays.asList("89563865", "2487238947")),
                new Customer(103, "peter", "peter@gmail.com", Arrays.asList("38946328654", "3286487236")),
                new Customer(104, "kely", "kely@gmail.com", Arrays.asList("389246829364", "948609467"))
        ).collect(Collectors.toList());
    }

MapvsFlatmap.java

public class MapvsFlatMap {

    public static void main(String[] args) {

        List<Customer> customers=EkartDatabase.getAll();

        //List<Customer> convert List<String> -> Data transformation
        //mapping: customer -> customer.getEmail()
        //customer -> customer.getEmail() one to one mapping.
        List<String> emails= customers.stream().map(customer -> customer.getEmail())
        .collect(Collectors.toList());
        System.out.println(emails);
        //o/p:- [john@gmail.com, smith@gmail.com, peter@gmail.com, kely@gmail.com]

        //customer->customer.getPhoneNumber() ->> one to many mapping
        List<String> phones=customers.stream()
                .flatMap(customer -> customer.getPhoneNumbers().stream())
                .collect(Collectors.toList());
        System.out.println(phones);
        //o/p:- [397937955, 21654725, 89563865, 2487238947, 38946328654,
        // 3286487236, 389246829364, 948609467]
    }
}

Differences

Summary

In this post, we have learned about map vs flatMap in Java 8.

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