Java 8 - Difference between Optional.flatMap and Optional.map

Question

What\'s the difference between these two methods: Optional.flatMap() and Optional.map()?

An example would be appreciated.

Answer 1

What helped me was a look at the source code of the two functions.

Map - wraps the result in an Optional.

public Optional map(Function mapper) {
    Objects.requireNonNull(mapper);
    if (!isPresent())
        return empty();
    else {
        return Optional.ofNullable(mapper.apply(value)); //<--- wraps in an optional
    }
}

flatMap - returns the 'raw' object

public Optional flatMap(Function> mapper) {
    Objects.requireNonNull(mapper);
    if (!isPresent())
        return empty();
    else {
        return Objects.requireNonNull(mapper.apply(value)); //<---  returns 'raw' object
    }
}