[英]How to pass bounded wildcard type argument in Kotlin?
The class used (in Java, third party API, not changeable): 使用的类(在Java中,第三方API,不可更改):
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: 现在我想从Kotlin那里打电话:
item.queue(BookmarkablePageLink("link", bookmark.page))
bookmark.page
is in Java, and it is: public Class<? extends WebPage> getPage()
bookmark.page
是Java,它是: public Class<? extends WebPage> getPage()
public Class<? extends WebPage> getPage()
None of these work: 这些都不起作用:
item.queue(BookmarkablePageLink("link", bookmark.page))
Error: Not enough information to infer parameter T in
constructor Bookmarkable PageLink<T : Any!, C : Page!>(...)
错误:没有足够的信息来推断
constructor Bookmarkable PageLink<T : Any!, C : Page!>(...)
参数Tconstructor 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 : 这将是用Javaish发言的“假设正确”方式(只是意图,但它不是真正的代码),但Kotlin不支持这一点 :
item.queue(BookmarkablePageLink<Any, ? extends WebPage>("link", bookmark.page))
My best workaround is this, which is ugly, but works: 我最好的解决方法就是这个,这很难看,但有效:
item.queue(BookmarkablePageLink<Any, WebPage>("link", bookmark.page as Class<WebPage>))
Surprisingly in Java this was simply: 令人惊讶的是,这简直就是:
item.queue(new BookmarkablePageLink<>("link", bookmark.getPage() ));
So far as I understand, BookmarkablePageLink(...)
should be approximately equivalent to new BookmarkablePageLink<>
in Java, so this is the option which "should" work. 据我所知,
BookmarkablePageLink(...)
应该与Java中的new BookmarkablePageLink<>
大致相同,因此这是“应该”工作的选项。 All others you tried shouldn't, each for different reasons. 你试过的所有其他人都不应该出于不同的原因。
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. 具有自己的类型参数的构造函数非常罕见(在看到这个问题之前我认为它们是非法的),因此在Kotlin编译器中可能会忽略它们。 A possible workaround is to make it a function instead:
一种可能的解决方法是使其成为一种功能:
fun <T, C : Page> makeBookmarkablePageLink(id: String, clazz: Class<C>): BookmarkablePageLink<T> =
BookmarkablePageLink<T, C>(id, clazz)
and then 接着
item.queue(makeBookmarkablePageLink("link", bookmark.page))
I'll also note that I'm pretty sure 我还要注意,我很确定
the "correct" way to do this in Java-speak
在Java中说“正确”的方法
is actually wrong; 实际上是错的; and in fact you can't write down the type parameters in Java explicitly, because the second type parameter is a captured wildcard .
实际上,您无法明确地在Java中记下类型参数,因为第二个类型参数是捕获的通配符 。
I am creating a answer from all the best comments because those already seem very valuable. 我正在从所有最好的评论中找到答案,因为那些看起来非常有价值。
Workaround from the question is already a good start: 问题的解决方法已经是一个良好的开端:
BookmarkablePageLink<Any, WebPage>("link", bookmark.page as Class<WebPage>)
Also fair is @AlexeyRomanov's intermediate variable (or a similar intermediate function): 同样公平的是@AlexeyRomanov的中间变量(或类似的中间函数):
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 : 对于通过Google发现此问题的所有人来说,也很有价值,可能是Kotlin与Java处理类型差异的简短摘要,如Kotlin的文档中所述 :
in
and out
keywords (which you can't use, because your declaration is in Java) in
和out
关键字(您不能使用,因为您的声明是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. 另外,call-site的Java构造函数只允许从类中指定类型参数,而在Kotlin中,构造函数调用有两个类型参数:一个来自类,另一个来自构造函数。 So in Java, we have to say
所以在Java中,我们不得不说
new BookmarkablePageLink<T>("something", Page.class)
and in Kotlin 在Kotlin
BookmarkablePageLink<T, Page>("something", Page::class.java)
despite both calling the same constructor with the same arguments. 尽管两个都使用相同的参数调用相同的构造函数。
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. 考虑到Kotlin为变体类型选择了一种与Java完全相反的方法,我仍然很高兴,我们只需要在极少数情况下需要解决方法。 ;-)
;-)
请试试
item.queue(BookmarkablePageLink<Any, WebPage>("link", bookmark.page))
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