How does flatMap behave in multithreaded code?

In multithreaded Java code, the behavior of flatMap depends on how it's being used and the nature of the data being processed. When you apply flatMap on a Stream, it can merge multiple streams into a single stream, which is particularly useful when working with collections of collections or handling optional values.

However, if the flatMap operation involves threads that modify shared mutable data, you may encounter concurrency issues, including race conditions and inconsistent data states. It is crucial to ensure proper synchronization when dealing with shared resources in a multi-threaded context.

Example

    import java.util.Arrays;
    import java.util.List;
    import java.util.concurrent.CompletableFuture;
    import java.util.stream.Collectors;

    public class FlatMapExample {
        public static void main(String[] args) {
            List> listOfLists = Arrays.asList(
                Arrays.asList(1, 2),
                Arrays.asList(3, 4),
                Arrays.asList(5, 6)
            );

            List flatList = listOfLists.parallelStream()
                .flatMap(List::stream) // Use flatMap to flatten the structure
                .collect(Collectors.toList());

            System.out.println(flatList);
        }
    }