How to Combine Multiple Lists in Java 8 Using Stream and flatMap

Here we can combine multiple lists in java 8 using the Stream API and flatMap() .

  • First, we create three lists of Integer type data.
  • Next, we create a stream of all lists by using the Stream.of() method.
  • Then, we use flatMap() to convert all list elements into a single stream.
  • After that, we use the collect() method to collect all integer values into one list.
  • Finally, we print the complete data as a single list.
				
					import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class MulipleListAdd {
	public static void main(String[] args) {
		List<Integer> list1 = Arrays.asList(2, 3, 4, 5);
		List<Integer> list2 = Arrays.asList(21, 31, 41, 51);
		List<Integer> list3 = Arrays.asList(20, 30, 40, 50);
		List<Integer> combinedList = Stream.of(list1, list2, list3)
		                             .flatMap(list -> list.stream())
		                             .collect(Collectors.toList());
		System.out.println("All List of data :- " + combinedList);

	}
}

Output :-
All List of data :- [2, 3, 4, 5, 21, 31, 41, 51, 20, 30, 40, 50]

FAQ

What does flatMap() do in Java 8?

flatMap() transforms each element into a stream and then flattens them into a single stream.

 Can we combine lists without Stream API?

Yes, you can use addAll() method, but Stream API provides a cleaner and more functional approach.

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