Java 8 Map vs flatmap method

Map vs FlatMap:

Java 8 stream API provides map() and flatMap() methods. Both these methods are intermediate methods and returns another stream as part of the output.

map() method use for transformation
flatMap() used for transformation & flattering
flatMap()-> map()+flatring

map():

map() takes Stream<T> as input and return Stream<R>
Stream<R> map (String<T> input)
it's mapper function produces single value for each input value. hence it also called as one-to-one mapping.

flatMap():

flatMap() takes Stream<Stream<T>> as input and return Stream<R>
Stream<R> map (String<T> input)
it's mapper function produces multiple value for each input value. hence it also called as one-to-many mapping.

map()	flatMap()
It processes stream of values.	It processes stream of stream of values.
It does only mapping.	It performs mapping as well as flattening.
It’s mapper function produces single value for each input value.	It’s mapper function produces multiple values for each input value.
It is a One-To-One mapping.	It is a One-To-Many mapping.
Data Transformation : From Stream to Stream	Data Transformation : From Stream<Stream to Stream
Use this method when the mapper function is producing a single value for each input value.	Use this method when the mapper function is producing multiple values for each input value.

Let's See the below example for better understanding

Employee.java

import java.util.List;

public class Employee {

    private int id;
    private String name;
    private String email;
    private List<String> mobileNumbers;

    public Customer() {
    }

    public Employee(int id, String name, String email, List<String> mobileNumbers) {
        this.id = id;
        this.name = name;
        this.email = email;
        this.mobileNumbers = mobileNumbers;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public List<String> getMobileNumbers() {
        return mobileNumbers;
    }

    public void setMobileNumbers(List<String> mobileNumbers) {
        this.mobileNumbers = mobileNumbers;
    }
}

Repository.java

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Repository {


    public static List<Employee> getAll() {
        return Stream.of(
                new Employee(001, "Nadeem", "Nadeem@localhost.com", Arrays.asList("737937955", "9812356")),
                new Employee(002, "Shyam", "Shyam@localhost.com", Arrays.asList("65563865", "987654")),
                new Employee(003, "Punneet", "Punneet@localhost.com", Arrays.asList("9412345", "43256")),
                new Employee(004, "Kailash", "Kailash@localhost.com", Arrays.asList("9812345678", "1234456546"))
        ).collect(Collectors.toList());
    }

}

MapFlatMapExample.java

import java.util.List;
import java.util.stream.Collectors;

public class MapFlatMapExample {
    public static void main(String[] args) {

        List<Employee> employee = Repository.getAll();

        List<String> emails = employee.stream()
                .map(employee -> employee.getEmail())
                .collect(Collectors.toList());
        System.out.println(emails);
        List<List<String>> mobileNumbers = customers.
                stream().map(employee -> employee.getMobileNumbers())
                .collect(Collectors.toList());
        System.out.println(mobileNumbers);

        List<String> mobiles = customers.stream()
                .flatMap(employee -> employee.getMobileNumbers().stream())
                .collect(Collectors.toList());
        System.out.println(mobiles);
    }
}

Output:

Nadeem@localhost.com,Shyam@localhost.com,Punneet@localhost.com,Kailash@localhost.com

737937955,9812356,65563865,987654,9412345,43256,9812345678,1234456546

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