Java.Java8.What are the flatMap(), flatMapToInt(), flatMapToDouble(), flatMapToLong() methods for in streams?

🔹 flatMap() and its variants

✅ flatMap(Function<T, Stream<R>> mapper)

Used to:

• Transform each element in the stream into a stream of values
• Flatten those inner streams into one continuous stream

Example:

List<String> words = List.of("hello", "world");

List<String> letters = words.stream()
    .flatMap(word -> Arrays.stream(word.split("")))
    .collect(Collectors.toList());

System.out.println(letters); // [h, e, l, l, o, w, o, r, l, d]

Here:

• "hello" → ["h", "e", "l", "l", "o"]
• "world" → ["w", "o", "r", "l", "d"]
• Then flatMap merges all letters into a single stream

✅ flatMapToInt(Function<T, IntStream> mapper)

Used when the result is primitive int values.

Example:

List<String> numbers = List.of("1,2", "3,4");

IntStream intStream = numbers.stream()
    .flatMapToInt(s -> Arrays.stream(s.split(",")).mapToInt(Integer::parseInt));

intStream.forEach(System.out::print); // 1234

✅ flatMapToDouble(Function<T, DoubleStream> mapper)

Same as above, but for double:

List<String> doubles = List.of("1.5,2.5", "3.0");

DoubleStream doubleStream = doubles.stream()
    .flatMapToDouble(s -> Arrays.stream(s.split(",")).mapToDouble(Double::parseDouble));

doubleStream.forEach(System.out::println);

✅ flatMapToLong(Function<T, LongStream> mapper)

Same, for long values:

List<String> longs = List.of("100,200", "300");

LongStream longStream = longs.stream()
    .flatMapToLong(s -> Arrays.stream(s.split(",")).mapToLong(Long::parseLong));

longStream.forEach(System.out::println);

🔸 When to Use flatMap vs map

Use map() when…	Use flatMap() when…
You return one value per element	You return a stream or collection per element
No need to flatten anything	You need to flatten nested structures