Map Vs FlatMap

 In Java, the Stream interface has a map() and flatmap() methods and both have intermediate stream operation and return another stream as method output. Both of the functions map() and flatMap are used for transformation and mapping operations. 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.


// 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 Test{
    
    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);
    }
}

Output:

List of fruit-[Apple, mango, pineapple, kiwi]

List generated by map-[5, 5, 9, 4]


// 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 Test{

    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);

    }

}

Output

List of list-[[1, 2], [3, 4], [5, 6], [7, 8]]

List generate by flatMap-[1, 2, 3, 4, 5, 6, 7, 8]




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