How to pass bounded wildcard type argument in Kotlin?

Question

The class used (in Java, third party API, not changeable):

public class BookmarkablePageLink<T> extends Link<T> {

    public <C extends Page> BookmarkablePageLink(final String id, final Class<C> pageClass)

And now I want to call this from Kotlin:

item.queue(BookmarkablePageLink("link", bookmark.page))

bookmark.page is in Java, and it is: public Class<? extends WebPage> getPage()

None of these work:

item.queue(BookmarkablePageLink("link", bookmark.page))

Error: Not enough information to infer parameter T in constructor Bookmarkable PageLink<T : Any!, C : Page!>(...)

item.queue(BookmarkablePageLink<>("link", bookmark.page))

item.queue(BookmarkablePageLink<Any>("link", bookmark.page))

item.queue(BookmarkablePageLink<Any, *>("link", bookmark.page))

item.queue(BookmarkablePageLink<Any, WebPage>("link", bookmark.page))

item.queue(BookmarkablePageLink<Any, in WebPage>("link", bookmark.page))

item.queue(BookmarkablePageLink<Any, out WebPage>("link", bookmark.page))

item.queue(BookmarkablePageLink<Any, T : WebPage>("link", bookmark.page))

This would be the "hypothetically correct" way to do this in Javaish-speak (just the intention, but it's not real code), but this isn't supported by Kotlin:

item.queue(BookmarkablePageLink<Any, ? extends WebPage>("link", bookmark.page))

My best workaround is this, which is ugly, but works:

item.queue(BookmarkablePageLink<Any, WebPage>("link", bookmark.page as Class<WebPage>))

Surprisingly in Java this was simply:

item.queue(new BookmarkablePageLink<>("link", bookmark.getPage() ));

Answer 1

So far as I understand, BookmarkablePageLink(...) should be approximately equivalent to new BookmarkablePageLink<> in Java, so this is the option which "should" work. All others you tried shouldn't, each for different reasons.

Constructors which have their own type parameters are very rare (before seeing this question I thought they were illegal), so they may be overlooked somewhere in Kotlin compiler. A possible workaround is to make it a function instead:

fun <T, C : Page> makeBookmarkablePageLink(id: String, clazz: Class<C>): BookmarkablePageLink<T> = 
    BookmarkablePageLink<T, C>(id, clazz)

and then

item.queue(makeBookmarkablePageLink("link", bookmark.page))

I'll also note that I'm pretty sure

the "correct" way to do this in Java-speak

is actually wrong; and in fact you can't write down the type parameters in Java explicitly, because the second type parameter is a captured wildcard.

Answer 2

I am creating a answer from all the best comments because those already seem very valuable.

Workaround from the question is already a good start:

BookmarkablePageLink<Any, WebPage>("link", bookmark.page as Class<WebPage>)

Also fair is @AlexeyRomanov's intermediate variable (or a similar intermediate function):

val link: BookmarkablePageLink<Any> = BookmarkablePageLink("link", bookmark.page)

Also valuable for all who find this question via Google might be a short summary of Kotlin vs Java handling of type-variance as explained in Kotlin's documentation:

• in Java the handling is at call-site using wildcards (which you can't use, because call-site is in Kotlin)
• and in Kotlin the handling is at declaration site using in and out keywords (which you can't use, because your declaration is in Java)

Additionally, Java constructors at call-site only allow to specify type arguments from the class, while in Kotlin the constructor call has two type arguments: one from the class and the other from the constructor. So in Java, we have to say

new BookmarkablePageLink<T>("something", Page.class)

and in Kotlin

BookmarkablePageLink<T, Page>("something", Page::class.java)

despite both calling the same constructor with the same arguments.

Given that Kotlin chose an approach for variant types which is the exact opposite of Java's, I am still happy, that we only need workarounds in so few cases. ;-)

Answer 3

Please try

item.queue(BookmarkablePageLink<Any, WebPage>("link", bookmark.page))