Does Kotlin have any secret feature for automatically overriding the return type of builder methods ...

# announcements

[dumptruckman]

Does Kotlin have any secret feature for automatically overriding the return type of builder methods in subclasses?

[Shawn]

Hmmm, I think I might know what you want but would you mind clarifying

[Shawn]

spoiler: it’s not a “secret” feature, just kind of a hack tbh

[dumptruckman]

TrackableMailMessage mailMessage = (TrackableMailMessage) new TrackableEmailBuilder(this)
                .to(emailAddresses)
                .locale(locale)
                .subject(subject)
                .body(content)
                .buildMailMessage();

[dumptruckman]

TrackableEmailBuilder extends EmailBuilder but buildMailMessage() returns MailMessage instead of TrackableMailMessage because each of the setter methods are defined in EmailBuilder

[dumptruckman]

(obviously this is the usage in java)

[Shawn]

Hmm, is EmailBuilder maybe an abstract class?

[dumptruckman]

it’s a sealed class in this case

[dumptruckman]

it could be abstract but ¯\_(ツ)_/¯

[Shawn]

No, that’s fine, sealed types are basically abstract classes

[Shawn]

or, well, gimme a sec to test something real quick before I commit to that statement lmao

[dumptruckman]

alright

[Shawn]

Okay, so, if you want to have overridable return types in child builder classes, you can abuse generics to do so

[Shawn]

Well, in your case specifically it wouldn’t really be abuse, unless you wanted bootleg self types

[Shawn]

actually yeah in your case I’d just include a <T> in your sealed builder definition and then define abstract fun buildMailMessage(): T

[dumptruckman]

oh right

[dumptruckman]

iirc, it’s the same thing in java

[Shawn]

yep