scala中的Covariant类型参数需要在java接口中不变 [英] Covariant Typeparameter in scala needs to be invariant in java interface
问题描述
我有一个看起来像这样的特质(一些进一步的信息可以在这个相关的问题,虽然我不认为,这是需要这个问题)
trait提取器[-A ,+ B] {
def extract(d:A):B
//许多其他事物
}
要在现有的java框架中使用它,我希望这个 Extractor
有一个函数返回一个 Comparator [B]
(正在 java.util.Comparator ),甚至可以更好地扩展 Comparator [A]
。现在,由于 Comparator
的类型参数应该是不变的,而 A
是逆变的,而<$ c
所以我得到这样的错误:
阶> import java.util.Comparator
import java.util.Comparator
scala> trait Extractor [-A,+ B]扩展Comparator [A]
< console>:6:error:逆变类型A出现在类型为[-A,+ B]的不变位置中,java中的java.lang.Object .util.Comparator [A]特征提取器
特征提取器[-A,+ B]扩展比较器[A]
^
scala>特质提取器[-A,+ B] {
| def comp:比较器[B]
| }
< console>:7:error:协变类型B出现在type =>中的不变位置上。方法comp
的java.util.Comparator [B] def comp:Comparator [B]
^
你是否看到过任何解决方法,或者这只是在scala中使用java泛型而痛苦的一种情况?
scala>在类型边界的帮助下,可以执行以下操作:特质提取器[-A,+ B] {
| def comp:比较器[_<:B]
| }
定义特征提取器
I've got a trait that looks like this (some further information can be found at this related question by myself although I don't think, it's needed for this question)
trait Extractor[-A,+B] {
def extract(d:A):B
//lots of other things
}
To use this in an existing java framework I would like this Extractor
to either have a function that returns a Comparator[B]
(being java.util.Comparator) or even better extend Comparator[A]
. Now that poses a problem because Comparator
s type parameter is ought to be invariant, while A
is contravariant and B
is covariant.
So I get errors like this:
scala> import java.util.Comparator
import java.util.Comparator
scala> trait Extractor[-A,+B] extends Comparator[A]
<console>:6: error: contravariant type A occurs in invariant position in type [-A,+B]java.lang.Object with java.util.Comparator[A] of trait Extractor
trait Extractor[-A,+B] extends Comparator[A]
^
scala> trait Extractor[-A, +B] {
| def comp:Comparator[B]
| }
<console>:7: error: covariant type B occurs in invariant position in type => java.util.Comparator[B] of method comp
def comp:Comparator[B]
^
Do you see any way out of this or is this just one of those cases where "using java generics in scala hurts"?
With help of type-bounds it's possible to do the following:
scala> trait Extractor[-A, +B] {
| def comp:Comparator[_ <: B]
| }
defined trait Extractor
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