Kotlin的逆差异(Contravariance in Kotlin)

 我从来没有真正理解Java中的泛型，所以Kotlin似乎就是这种情况。 考虑以下代码片段（这是一个人为的例子）： 
 
class AnyComparator: Comparator<Any> {
    override fun compare(o1: Any, o2: Any): Int {
        TODO("not implemented")
    }
}


fun <T> test() {
    val x: Comparator<in Double> = AnyComparator() // OK!
    val y: Comparator<in T> = AnyComparator() // Compilation error
}
 
 第二个赋值因错误而失败 
 
Type mismatch. 
Required: kotlin.Comparator<in T>
Found: AnyComparator
 
 现在，如果我理解正确的in修饰符表明T只被泛型类型Comparator （它产生逆变），那么我应该能够为任何Comparator分配类型参数E ，它是T的基类。 基于此，我应该能够将AnyComparator分配给变量x和y ，因为Any类型是Kotlin中每个类的基类。 事实证明我不能，我不明白为什么。 

I never truly understood generics in Java, so it seems to be the case with Kotlin. Consider the following code snippet (it's a contrived example):
 
class AnyComparator: Comparator<Any> {
    override fun compare(o1: Any, o2: Any): Int {
        TODO("not implemented")
    }
}


fun <T> test() {
    val x: Comparator<in Double> = AnyComparator() // OK!
    val y: Comparator<in T> = AnyComparator() // Compilation error
}
 
The second assignment fails with the error 
 
Type mismatch. 
Required: kotlin.Comparator<in T>
Found: AnyComparator
 
Now, if I understand correctly the in modifier indicates the T is only consumed by the generic type Comparator (it makes contravariant), so I should be able to assign any Comparator with type argument E which is the base class of T. Based on this, I should be able to assign AnyComparator to both variables x and y, since the type Any is the base class of every class in Kotlin. It turns out I can't, and I don't understand why.

原文：https://stackoverflow.com/questions/44861928