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What's the difference between map() and flatMap() methods in Java 8?

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A quick guide to learn what's the difference between Stream.map() and Stream.flatMap() methods in Java 8.

1. Overview


In this article, You'll learn what is the difference between the map() and flatMap() methods in Java 8.

Looks both methods do the same thing but actually not. Let us see each method by example programs. So that you can understand how map() and flatMap() works.

Before reading the article, it is good to have a better understanding of How to Lambda Express in java 8?

Remember, both of these methods are present in the Stream API and as well as in the Optional API.

What's the difference between map() and flatMap() methods in Java 8?


2. Stream map() Method


map() is an intermediate operation which means it returns Stream.

package com.javaprogramto.java8.streams.map;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class Java8MapExample {
    public static void main(String[] args) {

        System.out.println("Output with simple list");
        List<String> vowels = Arrays.asList("A","E","I","O","U");

        vowels.stream().map( vowel -> vowel.toLowerCase()).forEach(value -> System.out.println(value));

        List<String> haiList = new ArrayList<>();

        haiList.add("hello");
        haiList.add("hai");
        haiList.add("hehe");
        haiList.add("hi");

        System.out.println("Output with nested List of List<String>");
        List<String> welcomeList = new ArrayList<>();
        welcomeList.add("You got it");
        welcomeList.add("Don't mention it");
        welcomeList.add("No worries.");
        welcomeList.add("Not a problem");

        List<List<String>> nestedList = Arrays.asList(haiList, welcomeList);
        nestedList.stream().map( list -> {return list.stream().map(value -> value.toUpperCase());}).forEach(value -> System.out.println(value));
    }
}
Output:
Output with simple list
a
e
i
o
u

Output with nested List of List<String>

java.util.stream.ReferencePipeline$3@b684286
java.util.stream.ReferencePipeline$3@880ec60
When you tried to get the values from List<List<String>> then map() does not work properly and need to lots of efforts to get the string values from nested List<String> object.
It has printed the intermediate Stream output rather than the actual values of the List<String>.

3. Stream flatMap() Method

Let us change from map() to flatMap() in the above program and see the output.
package com.javaprogramto.java8.streams.map;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class Java8FlatMapExample {

    public static void main(String[] args) {

        List<String> haiList = new ArrayList<>();
      haiList.add("hello");
        haiList.add("hai");
        haiList.add("hehe");
        haiList.add("hi");
        System.out.println("Output with nested List of List<String>");
        List<String> welcomeList = new ArrayList<>();

        welcomeList.add("You got it");
        welcomeList.add("Don't mention it");
        welcomeList.add("No worries.");
        welcomeList.add("Not a problem");
        List<List<String>> nestedList = Arrays.asList(haiList, welcomeList);
        nestedList.stream().flatMap( list ->  list.stream()).map(value -> value.toUpperCase()).forEach(value -> System.out.println(value));
    }
}
Output:
Output with nested List of List<String>
HELLO
HAI
HEHE
HI
YOU GOT IT
DON'T MENTION IT
NO WORRIES.
NOT A PROBLEM

4. Java 8 map() vs flatMap()

Both map() and flatMap() methods can be applied to a Stream<T> and Optional<T>. And also both return a Stream<R> or Optional<U>. The difference is that the map operation produces one output value for each input value, whereas the flatMap operation produces an arbitrary number (zero or more) values for each input value.
In flatMap(), Each input is always a collection that can be List or Set or Map.
The map operation takes a Function, which is called for each value in the input stream and produces one result value, which is sent to the output stream.

The flatMap operation takes a function that conceptually wants to consume one value and produce an arbitrary number of values. However, in Java, it's cumbersome for a method to return an arbitrary number of values, since methods can return only zero or one value.
package com.javaprogramto.java8.streams.map;

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

public class MapVsFlatMapExample {

    public static void main(String[] args) {
        List<Stream> together = Stream.of(Arrays.asList(1, 2), Arrays.asList(3, 4)) // Stream of List<Integer>
              .map(List::stream)
                .collect(Collectors.toList());

        System.out.println("Output with map() -> "+together);

        List<Integer> togetherFlatMap = Stream.of(Arrays.asList(1, 2), Arrays.asList(3, 4)) // Stream of List<Integer>
                .flatMap(List::stream)
                .map(integer -> integer + 1)
                .collect(Collectors.toList());

        System.out.println("Output with flatMap() -> "+togetherFlatMap);
    }
}
Output:
Output with map() -> [java.util.stream.ReferencePipeline$Head@6e8dacdf, java.util.stream.ReferencePipeline$Head@7a79be86]
Output with flatMap() -> [2, 3, 4, 5]

5. Conclusion

In this article, You've seen what is the main differnece between the map() and flatMap() methods in Java 8.
As usual, all the examples over github.

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JavaProgramTo.com: What's the difference between map() and flatMap() methods in Java 8?
What's the difference between map() and flatMap() methods in Java 8?
A quick guide to learn what's the difference between Stream.map() and Stream.flatMap() methods in Java 8.
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