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

Solution 1:

Use map if the function returns the object you need or flatMap if the function returns an Optional. For example:

public static void main(String[] args) {
  Optional<String> s = Optional.of("input");
  System.out.println(s.map(Test::getOutput));
  System.out.println(s.flatMap(Test::getOutputOpt));
}

static String getOutput(String input) {
  return input == null ? null : "output for " + input;
}

static Optional<String> getOutputOpt(String input) {
  return input == null ? Optional.empty() : Optional.of("output for " + input);
}

Both print statements print the same thing.

Solution 2:

They both take a function from the type of the optional to something.

map() applies the function "as is" on the optional you have:

if (optional.isEmpty()) return Optional.empty();
else return Optional.of(f(optional.get()));

What happens if your function is a function from T -> Optional<U>?
Your result is now an Optional<Optional<U>>!

That's what flatMap() is about: if your function already returns an Optional, flatMap() is a bit smarter and doesn't double wrap it, returning Optional<U>.

It's the composition of two functional idioms: map and flatten.