What data or elements are passed as arguments to the flatMap() function?

In Java, the flatMap() method is used in the Stream API and takes a function as an argument, which returns a new stream of elements for each element of the original stream.

That is, the argument to the flatMap() function is a function that transforms an element of type T into a Stream<R>.

Example:

List<String> words = Arrays.asList("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, for each word, the function returns a stream of letters, and flatMap() combines all these streams into a single flat stream.

Thus, in flatMap(), a function is passed that transforms an element into a stream of elements, and then all these streams are merged into one.