Mapping a Nested Optional?

Question

I'm kind of running into a tedious issue with the Java 8 "Optional" container. I cannot map an Optional to "bubble up" another optional.

Let's say I have a RussianNestingDoll class

  public class NestedOptionalTest {

    public static void main(String[] args) { 

        RussianNestingDoll doll  = RussianNestingDoll.createInstance(RussianNestingDoll.createInstance(RussianNestingDoll.createInstance()));

        Optional<Optional<RussianNestingDoll>> thirdDollContents = doll.getInnerDoll().map(d -> d.getInnerDoll());

        if (thirdDollContents.isPresent() && thirdDollContents.get().isPresent()) { 
            System.out.println(thirdDollContents.get().get());
        }
        else { 
            System.out.println("empty");
        }
    }

    private static final class RussianNestingDoll { 
        private final Optional<RussianNestingDoll> innerDoll;

        public Optional<RussianNestingDoll> getInnerDoll() { 
            return innerDoll;
        }

        private RussianNestingDoll(Optional<RussianNestingDoll> innerDoll) { 
            this.innerDoll = innerDoll;
        }
        public static RussianNestingDoll createInstance() { 
            return new RussianNestingDoll(Optional.empty());
        }
        public static RussianNestingDoll createInstance(RussianNestingDoll innerDoll) { 
            return new RussianNestingDoll(Optional.of(innerDoll));
        }
    }
}

It would be nice to not have to use nested optionals, and instead just have the optional "bubble up". That way I can call "isPresent()" and "get()" just once, rather than calling them both twice. Is there a way I can accomplish this?

Answer 1

I'm not really sure what you want, but you can rewrite your code like this:

RussianNestingDoll doll  = RussianNestingDoll.get(RussianNestingDoll.get(RussianNestingDoll.get()));

String content = doll.getInnerDoll()
                 .flatMap(d -> d.getInnerDoll())
                 .map(d -> d.get().toString())
                 .orElse("empty");
System.out.println(content);

In case you want to use the doll afterwards:

Optional<RussianNestingDoll> thirdDoll = doll.getInnerDoll()
                 .flatMap(d -> d.getInnerDoll());

if (thirdDoll.isPresent()) {
  System.out.println(thirdDoll.get());
}
else {                 
  System.out.println("empty");
}

Answer 2

Do you want to flatMap?

thirdDollContents
    .flatMap(Function.identity()) // un-nest, get back an Optional<RussianNestingDoll>
    .get() // or isPresent()

The flatMap will return an empty Optional if thirdDollContents is empty.