java 相当于什么<!--?,?-->在 Kotlin 中？

Question

here is my method

void login(Event<?, ?> event); I am wondering how it should be in Kotlin

Answer 1

In Kotlin wildcard operator is * . It instructs the compiler that it is unknown but once it got known there will be no other object of another type.

Replacement of

Java -> Kotlin

• <?> -> <*>
• Object -> Any?
• <? extends A> <? extends A> -> <out A?>
• <? super A> <? super A> -> <in A?>

Here question mark ( ? ) above means the field can be nullable, in java null and non-null are same but in Kotlin to accept null we need to explicitly mark it nullable.

Answer 2

fun login(event: Event<*, *>?)

Star projection , denoted by the * character. Sometimes you want to say that you know nothing about the type argument, but still want to use it in a safe way. The safe way here is to define such a projection of the GENERIC TYPE .

Read official guideline about Star-projection Kotlin syntax.