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.

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