Differences between Java 8 Map() Vs flatMap() :

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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.

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//map() function produces one output for one input value, whereas flatMap() function produces an arbitrary no of values as output (ie zero or more than zero) for each input value

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ages.map(d => d + " age Range") //one output

//example ["<10 age Range", "10-19 age Range", "20-29 age Range", "30-39 age Range", "40-49 age Range", "50-59 age Range", "60-69 age Range", "70-79 age Range", "≥80 age Range"]

​

ages.flatMap(age => state.map(d => ({state:d.name, age, population: d[age]})))

/*

 0: Object {state: "AL", age: "<10", population: 598478}

  1: Object {state: "AK", age: "<10", population: 106741}

  2: Object {state: "AZ", age: "<10", population: 892083}

  3: Object {state: "AR", age: "<10", population: 392177}

  4: Object {state: "CA", age: "<10", population: 5038433}

  ....

  flatMap creates a single array instead of array of arrays when using map 

  */ 

// Java program using flatMap() function

import java.io.*;

import java.util.*;

import java.util.ArrayList;

import java.util.List;

import java.util.stream.Collectors;

class GFG {

    public static void main(String[] args)

    {

        // making the arraylist object of List of Integer

        List<List<Integer> > number = new ArrayList<>();

        // adding the elements to number arraylist

        number.add(Arrays.asList(1, 2));

        number.add(Arrays.asList(3, 4));

        number.add(Arrays.asList(5, 6));

        number.add(Arrays.asList(7, 8));

        System.out.println("List of list-" + number);

        // using flatmap() to flatten this list

        List<Integer> flatList

            = number.stream()

                  .flatMap(list -> list.stream())

                  .collect(Collectors.toList());

        // printing the list

        System.out.println("List generate by flatMap-"

                           + flatList);

    }

}

// Java program using map() function

import java.io.*;

import java.util.*;

import java.util.ArrayList;

import java.util.List;

import java.util.stream.Collectors;

class GFG {

    public static void main(String[] args)

    {

        // making the array list object

        ArrayList<String> fruit = new ArrayList<>();

        fruit.add("Apple");

        fruit.add("mango");

        fruit.add("pineapple");

        fruit.add("kiwi");

        System.out.println("List of fruit-" + fruit);

        // lets use map() to convert list of fruit

        List list = fruit.stream()

                        .map(s -> s.length())

                        .collect(Collectors.toList());

        System.out.println("List generated by map-" + list);

    }

}