Scala如何为地图编写get（name：String，type：Class）

Question

I want to be able to have a map implementation of type [String,Any] in which i can store objects of different classes then i want to be able to do

import app.User;
....
usr = map.getAs("user",User)// with usr being an object of Class/Object User. If user is not of that type it throws an exception.

where user was previously stored in the map.

Is this possible?

I can even be a standalone method eg

 usr = getAs(map,"user",User)

Thanks

Answer 1

You can use ClassTag . I don't have a lot of experience with it, but something like that should work:

def getAs[T: ClassTag](m: Map[String, Any], key: String): T = 
  m(key).asInstanceOf[T]

Usage:

val map = Map[String, Any]("one" -> 1, "hello"-> "world")
getAs[String](map, "hello")
res1: String = world

I let you deal with the exception, you can simply wrap the asInstanceOf in a Try , or whichever strategy you prefer. I also recommend changing the return type to Option[T] .

Answer 2

Thanks vptheron. That definitely works. I took your advice and did it using Option[T] and used matching instead.

def getAsOption[T: ClassTag](m: Map[String, Any], key: String): Option[T] =
m.get(key) match {
    case x : Option[T] => x
    case _ => None
}   

I wanted a mechanism to pass random stuff in the play framework and this looks like a good solution for me though the purists wont like it.

Answer 3

I think using TypeTag will do better job unless you will never cast the values into generic types.

import scala.reflect.runtime.universe._

trait Typed[A] {
  val value: A
  implicit val ttag: TypeTag[A]
}

def cast[A, B: TypeTag](v: Typed[A]): Option[B] = {
  val tA = typeOf[A](v.ttag)
  val tB = typeOf[B]
  def vB = v.value.asInstanceOf[B]
  if (tA <:< tB) Some(vB) else None
}

def getTyped[A, B, C: TypeTag](map: Map[A, Typed[B]], key: A): Option[C] = {
  map get key flatMap cast[B, C]
}