flatMapToInt(), flatMapToLong(), and flatMapToDouble()

The flatMapToInt(), flatMapToLong(), flatMapToDouble() intermediate operations produce a numeric stream of a corresponding type:

List<Integer> list = List.of(1, 2, 3);

list.stream().flatMapToInt(i -> IntStream.rangeClosed(1, i))
                        .forEach(System.out::print);    //prints: 112123
list.stream().flatMapToLong(i -> LongStream.rangeClosed(1, i))
                        .forEach(System.out::print);    //prints: 112123
list.stream().flatMapToDouble(DoubleStream::of)
        .forEach(d -> System.out.print(d + " "));  //prints: 1.0 2.0 3.0

As you can see, in the preceding code, we have used int values in the original stream. But it can be a stream of any type:

List<String> str = List.of("one", "two", "three");
str.stream().flatMapToInt(s -> IntStream.rangeClosed(1, s.length()))
                      .forEach(System.out::print);  //prints: 12312312345