flatMap untuk Koleksi Bersarang

List<List<Integer>> nested = List.of(List.of(1,2), List.of(3,4), List.of(5));
// With map:
Stream<List<Integer>> s = nested.stream().map(l -> l); // Stream<List<Integer>>
// With flatMap:
Stream<Integer> flat = nested.stream().flatMap(List::stream); // Stream<Integer>
flat.forEach(System.out::println); // 1 2 3 4 5

// map: List<String> -> Stream<Stream<Character>>
Stream<Stream<Character>> nested = List.of("hi","bye").stream()
    .map(s -> s.chars().mapToObj(c -> (char)c));
// flatMap: List<String> -> Stream<Character>
Stream<Character> flat = List.of("hi","bye").stream()
    .flatMap(s -> s.chars().mapToObj(c -> (char)c));

List<String> allItems = orders.stream()
    .flatMap(order -> order.getItems().stream())
    .collect(Collectors.toList());
System.out.println(allItems);

List<Optional<String>> maybes = List.of(Optional.of("a"), Optional.empty(), Optional.of("c"));
List<String> values = maybes.stream()
    .flatMap(Optional::stream)  // Java 9+
    .collect(Collectors.toList());
System.out.println(values); // [a, c]

List<Path> files = List.of(Path.of("a.txt"), Path.of("b.txt"));
List<String> allLines = files.stream()
    .flatMap(p -> {
        try { return Files.lines(p); }
        catch (IOException e) { return Stream.empty(); }
    })
    .collect(Collectors.toList());

List<String> sentences = List.of("hello world", "java streams rock");
Map<String, Long> freq = sentences.stream()
    .flatMap(s -> Arrays.stream(s.split("\\s+")))
    .collect(Collectors.groupingBy(w -> w, Collectors.counting()));
System.out.println(freq);

Map<String, List<String>> catalog = Map.of(
    "fruit", List.of("apple","banana"),
    "veg",   List.of("carrot","pea"));
List<String> all = catalog.values().stream()
    .flatMap(List::stream)
    .sorted()
    .collect(Collectors.toList());
System.out.println(all); // [apple, banana, carrot, pea]

departments.stream()
    .flatMap(dept -> dept.getEmployees() == null
        ? Stream.empty()
        : dept.getEmployees().stream())
    .forEach(System.out::println);

long distinctWords = sentences.stream()
    .flatMap(s -> Arrays.stream(s.toLowerCase().split("[^a-z]+")))
    .filter(w -> !w.isEmpty())
    .distinct()
    .count();
System.out.println("Distinct: " + distinctWords);

int totalQuantity = orders.stream()
    .flatMapToInt(order -> order.getItems().stream().mapToInt(Item::getQuantity))
    .sum();
System.out.println("Total qty: " + totalQuantity);