CODEWITHSUNDEEP
javascalajava-8option-typeapi-design
pathikrit
pathikrit

Reputation: 33439

When to use Optional<T> vs OptionalT in JDK 8+ (where T is boxed primitive type)?

It seems for performance reasons OptionalDouble does not simply extend Optional<Double>? IMO, that's a perf concern that should not have leaked into SDK design but I digress.

As a library writer, which one should I expose to my users, Optional<T> or OptionalT? What factors would make me decide which one to use when?

EDIT: Based on the comments, elaborating my particular use case. I am writing a bridge to a Scala ML library so people can use it from Java 8+.

The boiled down Scala signature looks like this:

def flattenBy(f: Double => Option[Double]): List[Double]

For those not familiar with Scala, flattenBy is a function that takes in another function from Double to Option[Double] and returns a List of Doubles.

What is the closest signature this maps to in Java 8?

Upvotes: 3

Views: 1008

Answers (2)

Alexey Romanov
Alexey Romanov

Reputation: 170723

It seems for performance reasons OptionalDouble does not simply extend Optional? IMO, that's a perf concern that should not have leaked into SDK design but I digress.

That's not quite what the linked answer says. It explains that OptionalDouble exists at all because of the performance concern. But given that it does it can't extend Optional<Double> because the signatures are incompatible: its get returns double, Optional<Double>'s one returns Double.

If you don't care about types other than double, sure, use OptionalDouble; but consider that List<Double> has a much worse overhead (3 times the memory use of double[] in this example)! There are many libraries providing real primitive collections for Java: e.g. HPPC, Trove, fastutil, Koloboke.

Upvotes: 1

Paul Boddington
Paul Boddington

Reputation: 37645

It looks to me like the closest signature is:

List<Double> flattenBy(DoubleFunction<OptionalDouble> f)

You would only need to use Optional<Double> if it is necessary for the doubles to have an identity as well as a value (It sounds like you don't need this).

Upvotes: 4

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