What functionality is implemented by the flatMap method in the Optional class?

The flatMap method in the Optional class in Java allows you to apply a function that returns another Optional to the value inside the original Optional, and "unwrap" the nested Optional into a single level.

If you have an Optional<T> and a function T -> Optional<U>, then flatMap will apply this function and return an Optional<U>, avoiding nested Optional<Optional<U>>.

Example:

Optional<String> optional = Optional.of("123");

Optional<Integer> result = optional.flatMap(s -> {
    try {
        return Optional.of(Integer.parseInt(s));
    } catch (NumberFormatException e) {
        return Optional.empty();
    }
});

System.out.println(result); // Optional[123]

If map were used instead, the result would be Optional<Optional<Integer>>, which is inconvenient for further processing.