Question

I’m trying to instantiate classes from Maps[String, Any], receiving through some json-rpc.
So I end up with following problem:

val mpa:Map[String, Any] = Map("key"->0.0)

implicit def anyToInt(a:Any):Int = a.asInstanceOf[Double].toInt

When key exists all is OK.

val i:Int = mpa.getOrElse("key", 0.0)
i: Int = 0

But when key is missing …:

scala> val i:Int = mpa.getOrElse("val", 0.0)
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.Double
at scala.runtime.BoxesRunTime.unboxToDouble(Unknown Source)
at .anyToInt(<console>:13

Now, if we’re add some verbosity as:

implicit def anyToInt(a:Any):Int = {
  println(a)
  val b = a.asInstanceOf[Double].toInt
  println("converted")
  b
}

We got:

val i:Int = mpa.getOrElse("val", 0.0)
0.0
converted
0
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.Double
     .....

So i came to the conclusion that anyToInt is called twice. And the second time it received Int as Any.

Questions:

1. Why ?!

2. What should I do to avoid this ?

P.S.: Sorry if it’s newbie question. I’m new in scala.

Answer 1

Having hidden direct conversions and especially for Any as it is in your case is a very bad idea and you’ve discovered why. I’d advice you to simply convert your input map into Map[String, Int]. You don’t need implicits here, but if you still want to go with implicits you should do it with wrapper approach:

implicit def anyExtender (x: Any) = new {
  def toInt = x match {
    case x: Double => x.toInt
  }
}

and use it like so:

mpa.getOrElse("val", 0.0).toInt

Since scala 2.10 the following implementation of implicit wrappers is preferred:

implicit class AnyExtender (x: Any) {
  def toInt = x match {
    case x: Double => x.toInt
  }
}

Also since your conversion function does not work on all subtypes of Any it is incorrect to extend Any with such a function, it would be correct to extend Double with such a function but it already has it. So in your scenario the only correct way of using your map would be simply as follows:

map.getOrElse("val", 0.0).asInstanceOf[Double].toInt

Update

Conversion to Map[String, Int] will work for you, if you move the logic you wanted to place in implicits like so:

val resultMapOfStringToIntType = 
  inputMapOfStringToAnyType.mapValues {
    case x: Double => x.toInt
    case x: String => x.toInt
    case x: Timestamp => //...
    // and so on
  }