[英]Type mismatch generics with classOf Scala
I have a method that should dynamically cast a field member into a certain type depending on a configuration flag format
.我有一种方法可以根据配置标志format
将字段成员动态转换为特定类型。
This flag format
, accepts one of the following type values:此标志format
接受以下类型值之一:
object Types {
val Str = "string"
val IntNum1 = "int"
val IntNum2 = "integer"
val DoubleNum = "double"
val LongNum = "long"
}
One option would be to use reflection.一种选择是使用反射。 Another option would be to use pattern matching (the method I'm trying to do this with)另一种选择是使用模式匹配(我正在尝试使用的方法)
Here's the method for String
types:下面是String
类型的方法:
private def getValClassIfStr(format: String): Class[String] = {
format.toLowerCase match {
case Types.Str => classOf[String]
case _ => null
}
}
And Here's the method for Numerical types, extending from AnyVal
, that fails to compile:这是从AnyVal
扩展的数字类型的方法,它无法编译:
private def getValueClass[T <: AnyVal](format: String): Try[Class[T]] = {
format.toLowerCase match {
case Types.IntNum1 || Types.IntNum2 => Success(classOf[Int])
case Types.DoubleNum => Success(classOf[Double])
case Types.LongNum => Success(classOf[Long])
case _ => Failure(new Exception)
}
}
The second method doesn't compile because of a: Type mismatch: required Try[Class[T]] found Try[Class[Int]]
由于以下原因,第二种方法无法编译: Type mismatch: required Try[Class[T]] found Try[Class[Int]]
Which I don't understand, given T <: AnyVal
.我不明白,给定T <: AnyVal
。
Any idea?任何的想法? and if that's a bad approach, what would a "clean" alternative be?如果这是一个糟糕的方法,那么“干净”的替代方案是什么?
I think the reason why this doesn't work becomes clearer once you replace Class[T]
in the return value with Class[AnyVal]
:我认为一旦你用Class[AnyVal]
替换返回值中的Class[T]
,为什么这不起作用的原因变得更加清晰:
type mismatch;
found : Class[Int](classOf[scala.Int])
required: Class[AnyVal]
Note: Int <: AnyVal, but Java-defined class Class is invariant in type T.
You may wish to investigate a wildcard type such as `_ <: AnyVal`. (SLS 3.2.10)
In other words, any call to getValueClass[T]
would always have to be populated with T = AnyVal
by the compiler, since format
is only evaluated at runtime.换句话说,任何对getValueClass[T]
调用总是必须由编译T = AnyVal
填充,因为format
只在运行时进行评估。 This, however, is not possible since Class[T]
is invariant, so you can't return a Class[Int]
and a Class[Double]
as a Class[AnyVal]
, since for invariant containers there's no subtyping relationship.但是,这是不可能的,因为Class[T]
是不变的,因此您不能将Class[Int]
和Class[Double]
作为Class[AnyVal]
,因为对于不变容器,没有子类型关系。
To demonstrate:展示:
var x: Class[AnyVal] = _
x = classOf[Int]
// Boom
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