map and flatMap

Learn map and flatMap on Optional.

Transforming with Optional map

Optional.map(function) applies the function to the value when it is present and returns the result wrapped in a new Optional. When the value is absent it returns an empty Optional; the function is never called.

Example: opt.map(String::length). If opt is Optional.of("Alex") the result is Optional.of(4). For an empty Optional the result stays empty. This is the shortest way to transform safely without explicit null checks.

Nested Optional: flatMap

If the function you pass to map already returns an Optional, the result becomes Optional<Optional<T>>; this is awkward. flatMap flattens this nested structure.

Example: opt.flatMap(this::lookup). Here lookup returns Optional<String>. With map you would get Optional<Optional<String>>; flatMap gives a single Optional<String>. This is ideal when a second lookup may fail too.

Exercise Find string length with map

Create an Optional<String> with Optional.of("Alex"). Use map(String::length) to get the length as an Optional<Integer>. Fall back to 0 with orElse(0). Print the result with System.out.println.

Show example solution

import java.util.Optional;

public class MapOptional {
    public static void main(String[] args) {
        Optional<String> name = Optional.of("Alex");
        int length = name.map(String::length).orElse(0);
        System.out.println(length);
    }
}

Test yourself

What does Optional.map(String::length) return?
- Optional<Integer>
- void
- Integer
- Stream<Integer>
Show answer

Optional<Integer> — map returns the result wrapped in another Optional.

Complete the map call

Optional<Integer> len = opt.____(String::length);

Show answer

Optional<Integer> len = opt.map(String::length);

Match the map vs flatMap difference

map

Returns Optional<R>
flatMap

Flattens nesting
Function<T,R>

map's function
Function<T,Optional<R>>

flatMap's function

Next lesson: orElse and orElseGet

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