Scala: create instance from generic type with private constructor

Question

I have a class with a generic type parameter and I would like to create an instance of this type using reflection. Moreover, the type has a private constructor. Here is a code sample of the problem:

class X private (x: Int) {
}
class Y private (y: Int) {
}
abstract class Foo[T](z : Int) {
  var bar : T = createInstance[T](z)
  def createInstance[T](z: Int) : M = {
     ???
  }
}

This would allow me to instantiate Foo and access bar:

class xFoo extends Foo[X]{
}

class yFoo extends Foo[Y]{
}

var xfoo = new xFoo(5)
println(xfoo.bar.x)

var yfoo = new yFoo(6)
println(yfoo.bar.y)

This should print out '5' and '6'.

I have tried using implicit in the following way.

def createInstance[T](z: Int)(implicit tag: TypeTag[T]) : T = {
  val clazz = tag.tpe.getClass
  val ctor = class.getDeclaredConstructor(classOf[Int])
  ctor.setAccessible(true)
  return ctor.newInstance(z).asInstanceOf[T]
}

However, then I get the error:

No TypeTag available for T

in line

var bar : T = createInstance[T](z)

Any suggestions? Thanks for your help!

I think that you should be using scala.reflect.ClassTag instead of a TypeTag. — sarveshseri
– sarveshseri, Commented Oct 19, 2016 at 10:50
Do you really want for createInstance to have its own T type parameter, different from Foo's? — Alexey Romanov
– Alexey Romanov, Commented Oct 19, 2016 at 16:25

jvliwanag · Accepted Answer · 2016-10-19 13:04:50Z

1

Assuming that the super class does expose some method/value you're looking for like:

class X private(val x: Int) {}

You can extend it like so:

import scala.reflect._

class Foo[T](z : Int)(implicit ct: ClassTag[T]) {
  var bar : T = createInstance[T](z)
  def createInstance[T](z: Int) : T = {
    val ctor = ct.runtimeClass.getDeclaredConstructor(classOf[Int])
    ctor.setAccessible(true)
    ctor.newInstance(Int.box(4)).asInstanceOf[T]
  }
}

This enables you to call:

class XFoo(n: Int) extends Foo[X](n)
println((new XFoo(5)).bar.x)

edited Oct 19, 2016 at 13:04

answered Oct 19, 2016 at 10:52

jvliwanag

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4 Comments

Michael Kamp Over a year ago

Thanks, but in the line

Michael Kamp Over a year ago

Thanks, but in the line val ctor = classTag[X].runtimeClass.getDeclaredConstructor(classOf[Int]) you are using classTag[X]. Shouldn't that be classTag[T]?

Michael Kamp Over a year ago

Sorry, I think my question was not precise enough. The abstract class Foo should serve as base class for several classes that differ in their type T: xFoo extends Foo[X] yFoo extends Foo[Y]

jvliwanag Over a year ago

Fixed and updated. I'm assuming that the class you're extending does expose the method/field you're after though.

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