为什么我不能将Scala的Function1隐式转换为java.util.function.Function? [英] Why can't I implicit convert Scala's Function1 to java.util.function.Function?
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问题描述
我正在尝试创建Scala的Function1到java.util.function.Function的隐式转换。
I'm trying to create a implicit conversion of Scala's Function1 to java.util.function.Function.
这是我的代码:
object Java8ToScala extends App {
implicit def javaFuncToScalaFunc[T, R](func1: Function[T, R]): function.Function[T,R] = {
new function.Function[T, R] {
override def apply(t: T): R = func1.apply(t)
}
}
val javaFunc:function.Function[String,Int] = (s:String) => s.length
println(javaFunc.apply("foo")) // this works
private val strings = new util.ArrayList[String]()
println(strings.stream().map(javaFunc).collect(Collectors.toList())) // this doesn't work
}
编译器消息很难理解:
[error] /xxx/Java8ToScala.scala:74: no type parameters for method map: (x$1: java.util.function.Function[_ >: String, _ <: R])java.util.stream.Stream[R] exist so that it can be applied to arguments (java.util.function.Function[String,Int])
[error] --- because ---
[error] argument expression's type is not compatible with formal parameter type;
[error] found : java.util.function.Function[String,Int]
[error] required: java.util.function.Function[_ >: String, _ <: ?R]
[error] Note: String <: Any, but Java-defined trait Function is invariant in type T.
[error] You may wish to investigate a wildcard type such as `_ <: Any`. (SLS 3.2.10)
[error] .map(javaFunc).collect(Collectors.toList()))
[error] ^
[error] /xxx/Java8ToScala.scala:74: type mismatch;
[error] found : java.util.function.Function[String,Int]
[error] required: java.util.function.Function[_ >: String, _ <: R]
[error] .map(javaFunc).collect(Collectors.toList()))
[error] ^
[error] two errors found
[error] (compile:compileIncremental) Compilation failed
[error] Total time: 7 s, completed Dec 18, 2015 10:51:15 AM
推荐答案
这只是Scala类型推断失败,虽然我不明白为什么:它似乎在寻找一个 R
扩展 AnyRef
。如果您使用这种类型,请工作,例如 val javaFunc:function.Function [String,String] =(s:String)=> s
。
It's just Scala type inference failing, though I don't see why: it seems to be looking for an R
which extends AnyRef
. It works if you use such a type, e.g. val javaFunc: function.Function[String,String] = (s:String) => s
.
然而,它没有获得任何上限:使用 map [Int]
也明确工作。
However, it isn't getting an upper bound anywhere: using map[Int]
explicitly works as well.
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