Differences Between map() And flatMap() in Java 8 ?

In this post, we will clarify Differences Between map() And flatMap() in Java 8 using an example

Below both Methods are available in java.util.stream.Stream Interface from Java 8 onwards

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

These two methods map and flatMap can be applied to a Stream<T> and they both return a Stream<R> only. The map(Function<? super T, ? extends R> mapper) method produces one output value for each input value but flatMap(Function<? super T, ? extends Stream<? extends R>> mapper) method produces an arbitrary number (zero or more) values for each input value.

Let’s try to understand the difference between these two methods using the below example:

package com.infotech.client;

import java.util.Arrays;
import java.util.Collection;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;
import java.util.stream.Stream;

/**
 * Differences Between map() And flatMap() in Java 8
 *An example where we take a map of user names and lists of phones and "flatten" 
 *it down to a list of phones of all the users using flatMap
 *@author KK JavaTutorials
 */
public class MapAndflatMapStreamOperationTest {
	public static void main(String[] args) {
	
		Map<String, List<String>> peopleNameAndPhonesMap = new HashMap<>();
		peopleNameAndPhonesMap.put("KK", Arrays.asList("999-2256-766", "999-4467-788"));
		peopleNameAndPhonesMap.put("John", Arrays.asList("999-5523-877", "999-8766-877"));
		peopleNameAndPhonesMap.put("Sean", Arrays.asList("999-7766-654", "999-8766-754"));
		 
		
		//Use of flatMap Stream to get all phones
		System.out.println("-------Use of flatMap Stream to get all phones--------");
		List<String> phoneList = peopleNameAndPhonesMap.values().stream()
		  .flatMap(Collection::stream).collect(Collectors.toList());
		
		phoneList.forEach(System.out::println);
		
		//Use of map Stream to get all phones
		System.out.println("----------Use of map Stream to get all phones-----------");
		List<Stream<String>> phonesList = peopleNameAndPhonesMap.values().stream()
		  .map(Collection::stream).collect(Collectors.toList());
		for (Stream<String> phoneStream : phonesList) {
			List<String> phones = phoneStream.collect(Collectors.toList());
			System.out.println(phones);
		}
		System.out.println("----------Use of map stream to get all user names-------");
		
		//Use of map stream to get all user names
		Set<String> namesList = peopleNameAndPhonesMap.keySet().stream().map(s->s).collect(Collectors.toSet());
		namesList.forEach(System.out::println);
	}
}

package com.infotech.client;

import java.util.Arrays;

import java.util.Collection;

import java.util.HashMap;

import java.util.List;

import java.util.Map;