This is a java counter part of map vs flatmap blogpost http://www.brunton-spall.co.uk/post/2011/12/02/map-map-and-flatmap-in-scala/

MapVsFlatMap.java

public class MapVsFlatMap {
    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(1, 2, 3);

        //Stream<X> --map (square)--> Stream<X>
        list.stream()
                .map(it -> it * 2)
                .forEach(it -> System.out.println(it));

        // Stream<X> --map--> Stream<Optional<X>> --filter--> Stream<Optional<X>> --each--> Print
        list.stream()
                .map(MapVsFlatMap::greaterThan2)
                .forEach(optional -> {
                    System.out.println(optional);
                });

        //Stream<X> --map g(x)--> Stream<List<Integer>>
        list.stream()
                .map(MapVsFlatMap::g)
                .forEach(it -> System.out.println(it));

        list.stream()
                .flatMap(it -> g(it).stream())
                .forEach(it -> System.out.println(it));

        list.stream()
                .map(MapVsFlatMap::greaterThan2)
                .filter(MapVsFlatMap::filterEmpty)
                .forEach(it -> System.out.println(it));
    }

    public static List<Integer> g(Integer value) {
        return Arrays.asList(value - 1, value, value + 1);
    }

    public static java.util.Optional<Integer> greaterThan2(Integer val) {
        return val > 2 ? Optional.of(val) : Optional.empty();
    }

    public static <T> boolean filterEmpty(Optional<T> optional) {
        return optional.isPresent();
    }
}