Flatten List of Lists in Java 8 | flatMap Explained in 8 Easy Ways (Interview Ready)

March 29, 2026

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Today I have released new video on "Flatten List of Lists in Java 8 | flatMap Explained in 8 Easy Ways (Interview Ready)"

This is frequently asked question in Java Interview.

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Flatten List of Lists in Java 8 | flatMap Explained in 8 Easy Ways (Interview Ready)

package org.practice.basicprgms;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
import java.util.List;
import java.util.stream.Collectors;

public class FlattenFlatMapDemo {

	public static void main(String[] args) {
		List<List<Integer>> lists = Arrays.asList(
							Arrays.asList(1,2,3),
							Arrays.asList(4,5,6),
							Arrays.asList(7,8,9)				
						);
				
		List<Integer> list = new ArrayList<Integer>();
				
		// 1. Using flatMap() (Most Recommended).
		lists.stream()
			.flatMap(List::stream)
			.forEach(System.out::println);
		
		System.out.println("Using flatMap() (Most Recommended). ");
				
		// 2. Using flatMap() and Collect in another list.
		list = lists.stream()
					.flatMap(List::stream)
					.collect(Collectors.toList());
		System.out.println("Using flatMap() and Collect in another list. "+ list);		
		
		// 3. Using forEach() + addAll()
		list.clear();
		lists.stream()
			.forEach(list::addAll);
		System.out.println("Using forEach() + addAll() "+ list);		

		// 4. Using reduce() (Interview Favorite 🔥)
		list.clear();
		list =
		lists.stream()
			.reduce(
				new ArrayList(),
				(acc, ls) -> {
				acc.addAll(ls);
				return acc;										}					
				);
		
		System.out.println("Using reduce() "+ list);	

		// 5. Using flatMap() + toCollection()
		list.clear();
		list = 
		lists.stream()
				.flatMap(Collection::stream)
				.collect(Collectors.toCollection(ArrayList::new));
		
		System.out.println("Using flatMap() + toCollection() "+ list);	
		
		// 6. Using Parallel Stream (Performance Case)
		list.clear();
		list =
			lists.parallelStream()
				.flatMap(List::stream)
				.collect(Collectors.toList());
		System.out.println("Using Parallel Stream (Performance Case) "+ list);	

		// 7. Using Traditional Loop (No Streams)
		list.clear();
		for(List<Integer> ls: lists) {
			for(int i: ls) {
				list.add(i);
			}
		}
		System.out.println("Using Traditional Loop (No Streams) "+ list);	
		
		// 8. Using Iterator
		list.clear();
		Iterator<List<Integer>> iterator = lists.iterator();
		
		while(iterator.hasNext()) {
			list.addAll(iterator.next());
		}
		
		System.out.println(" Using Iterator "+ list);	
		
	}

}

output:
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2
3
4
5
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7
8
9
Using flatMap() (Most Recommended). 
Using flatMap() and Collect in another list. [1, 2, 3, 4, 5, 6, 7, 8, 9]
Using forEach() + addAll() [1, 2, 3, 4, 5, 6, 7, 8, 9]
Using reduce() [1, 2, 3, 4, 5, 6, 7, 8, 9]
Using flatMap() + toCollection() [1, 2, 3, 4, 5, 6, 7, 8, 9]
Using Parallel Stream (Performance Case) [1, 2, 3, 4, 5, 6, 7, 8, 9]
Using Traditional Loop (No Streams) [1, 2, 3, 4, 5, 6, 7, 8, 9]
Using Iterator [1, 2, 3, 4, 5, 6, 7, 8, 9]