How to create instance with type parameter through scala reflection?

Question

Here is my code

package a1

trait Algorithm[T] {
  def someMethod(a: Int): T
}

package b2

import a1.Algorithm

class Word2Vec[T] extends Algorithm[T] {
  def someMethod(a: Int): T = a.asInstanceOf[T]
}

package c3

import a1.Algorithm

object Main {
  def main(args:Array[String]) = {
    val word2vec:Algorithm[Float] = Class.forName("b2.Word2Vec").newInstance().asInstanceOf[Algorithm[Float]]
    val a = word2vec.someMethod(123)
    println(a)
  }
}

and I got this:

Exception in thread "main" java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.Float
    at scala.runtime.BoxesRunTime.unboxToFloat(BoxesRunTime.java:107)
    at c3.Main$.main(Main.scala:8)
    at c3.Main.main(Main.scala)

by the way,How could I get a type when I get a string name. I have an "int" and I want to pass type Int as the type parameter for generic

First reflection is a bad idea, is slow and unsafe, why do you need it? Why not just instantiating the class on code. PS: Lately there had been a couple of similar questions, are they related in anyway? Same project? - Second, asInstanceOf is a big code smell especially in this case it seems it will never work. - Finally, how did you expect it to work if you passed an Int and asked for a Float? — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented Nov 27, 2019 at 11:38
@LuisMiguelMejíaSuárez "Finally, how did you expect it to work if you passed an Int and asked for a Float?" Int is passed to def someMethod(a: Int)... and Float is T in Word2Vec[T]. There's no problem in casting scala.Int to scala.Float: 123.asInstanceOf[Float]. It's java.lang.Integer that can't be casted to java.lang.Float. — Dmytro Mitin
– Dmytro Mitin, Commented Nov 27, 2019 at 19:14

Dmytro Mitin · Accepted Answer · 2019-11-27 20:12:31Z

The following code with Int doesn't throw exception

val word2vec = Class.forName("b2.Word2Vec").newInstance().asInstanceOf[Algorithm[Int]]
val a = word2vec.someMethod(123)
println(a) // 123

or with _

val word2vec = Class.forName("b2.Word2Vec").newInstance().asInstanceOf[Algorithm[_]]
val a = word2vec.someMethod(123)
println(a) // 123

Actually Class.forName + newInstance is not Scala reflection, it's Java reflection used in Scala. Scala reflection is different.

by the way,How could I get a type when I get a string name. I have an "int" and I want to pass type Int as the type parameter for generic

I'm not sure this can be done with Java reflection. With Scala reflection

import scala.reflect.runtime.universe._
val mirror = runtimeMirror(this.getClass.getClassLoader)
val classSymbol = mirror.staticClass("b2.Word2Vec") // class Word2Vec
// if string is "int"
internal.typeRef(NoPrefix, classSymbol, List(typeOf[Int])) // Word2Vec[Int]
// if string is "float"
internal.typeRef(NoPrefix, classSymbol, List(typeOf[Float])) // Word2Vec[Float]

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