Convert Scala Any to Java Object

Question

I've a problem using Java Reflection from Scala. My Code:

case class MyClass(id: String, value: Double)     

def create(values: Map[String, Any]): MyClass = {
   val constructor = classOf[MyClass].getConstructors.head
   val arguments = classOf[MyClass].getDeclaredFields().map( f => values(f.getName) )
   constructor.newInstance(arguments: _*).asInstanceOf[MyClass]
} 

create(Map("id" -> "CE0D23A", "value" -> 828.32))

My problem is, that I need to pass a Map[String, Any], because one of the values is a Double, but newInstance needs Object, not Any.

I tried the same with scalas universe:

case class MyClass(id: String, value: Double)     

def create(values: Map[String, Any]): MyClass = {
   val m = universe.runtimeMirror(getClass.getClassLoader)
   val myClass = universe.typeOf[MyClass].typeSymbol.asClass
   val cm = m.reflectClass(myClass)
   val ctro = universe.typeOf[MyClass].declaration(universe.nme.CONSTRUCTOR).asMethod
   val ctorm = cm.reflectConstructor(ctro)
   ctorm(values: _*).asInstanceOf[MyClass]
} 

create(Map("id" -> "CE0D23A", "value" -> 828.32))

Problem here is, that I only introduced MyClass for the example. Later it should be a generic function like def create(values: Map[String, Any]): T. But then I got the following exception: "No TypeTag available for T"

Is there any way to transform these values?

Thank you

score 38 · Accepted Answer · answered May 27 '14 at 15:07

38

java.lang.Object is equivalent to AnyRef in Scala, not Any. The idea is, Scala Double (roughly equivalent to Java double) is an Any, but not an AnyRef. java.lang.Double is an AnyRef, thus also an Any.

You can simply cast an Any to AnyRef, which will perform the needed conversion to turn a Scala Double into a java.lang.Double:

scala> val x = 3.5
x: Double = 3.5

scala> x.getClass
res0: Class[Double] = double

scala> val y = x.asInstanceOf[AnyRef]
y: AnyRef = 3.5

scala> y.getClass
res1: Class[_ <: AnyRef] = class java.lang.Double

answered May 27 '14 at 15:07

Dan Getz

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I added a second exampled based on scalas universe. Maybe you know a solution for this problem too? Or is there no way to use scalas universe runtime reflection with generics? – Torben May 27 '14 at 15:18
I don't know how to do that, sorry. Maybe you should post it as a new, separate question. – Dan Getz May 27 '14 at 15:21

score 1 · Answer 2 · edited May 23 '17 at 10:31

Ok, I was a bit late but here goes:

The following works:

constructor.newInstance(arguments.asInstanceOf[Array[AnyRef]]: _*).asInstanceOf[MyClass]

See also: Transforming Scala varargs into Java Object... varargs

Advice: I'd be very cautious using reflection. In Scala that is bad style. One way to limit/encapsulate it could be:

case class MyClass(id: String, value: Double)     

object MyClass {

    import scala.util.Try
    def apply(map: Map[String, Any] /* this is asking for trouble */) : Option[MyClass]  =  for {
            id <- maybeT[String](map.get("id"))
            value <- maybeT[Double](map.get("value"))
        } yield MyClass(id, value)

    // a truly global utility?
    @inline def maybeT[T] ( a: Any /*Option[T]*/ ) : Option[T]= Try(a.asInstanceOf[Option[T]]).toOption.flatten //yep ugly as hell

}


MyClass(Map("id" -> "CE0D23A", "value" -> 828.32))

Convert Scala Any to Java Object

2 Answers2