[英]Why can't I implicit convert Scala's Function1 to java.util.function.Function?
I'm trying to create a implicit conversion of Scala's Function1 to java.util.function.Function. 我正在尝试创建Scala的Function1到java.util.function.Function的隐式转换。
Here's my code: 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
}
The compiler message is hard to understand: 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
It's just Scala type inference failing, though I don't see why: it seems to be looking for an R
which extends AnyRef
. 这只是Scala的类型推断失败了,虽然我不明白为什么它似乎是在寻找一个
R
延伸AnyRef
。 It works if you use such a type, eg val javaFunc: function.Function[String,String] = (s:String) => s
. 如果使用这样的类型,它可以工作 ,例如
val javaFunc: function.Function[String,String] = (s:String) => s
。
However, it isn't getting an upper bound anywhere: using map[Int]
explicitly works as well. 但是,它没有在任何地方获得上限:使用
map[Int]
明确地起作用 。
Use the following implicit conversion: 使用以下隐式转换:
implicit def javaFuncToScalaFunc[T, R](func1: function.Function[T, R]): Function[T,R] = {
new Function[T, R] {
override def apply(t: T): R = func1.apply(t)
}
}
In your code, you have an implicit converstion from Scala function to Java function, but instead you should have an implicit conversion from Java function to Scala function. 在您的代码中,您有一个从Scala函数到Java函数的隐式转换,但您应该从Java函数到Scala函数进行隐式转换。
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