Scala：在伴侣对象中获取伴侣类

Question

I need to define a val in my companion object which is initialized with a method which takes the companion class as parameter.

I want to handle this with traits to not repeat myself. My Problem ist, that X.getClass ist not the same as classOf[X]. The first is the class of the companion object and the second is the class of the companion class, but I need to get the companion class without hardcoding it directly.

Basically I need something like this:

trait Foo {

}

object FooCompanionObject[f <: Foo] {
    val fClazz = classOf[f]
}

// Bar's fClass should be classOf[Bar]
case class Bar extends Foo;

object Bar extends FooCompanionObject[Bar];

The problem is that I cannot get the class of an generic type due to type erasure

Answer 1

There are several problems in your code. First, as you already said, the type will be erased, second object s ( object FooCompanionObject[f <: Foo] ) don't take type parameters and third, object s can not be extended ( object Bar extends FooCompanionObject ). To do what you want, you have to create an abstract base class for your companion objects, that takes a type parameter, which may be constrained to a specific type if you like, and has to be context bound on ClassTag . From the ClassTag you can then get the runtime class by calling runtimeClass on it. The final solution could look like this:

import scala.reflect.ClassTag
import scala.reflect.classTag

trait Foo

abstract class Companion[A <: Foo : ClassTag] {
  val fClazz = classTag[A].runtimeClass
}

class Bar extends Foo
object Bar extends Companion[Bar]


scala> Bar.fClazz
res2: Class[_] = class Bar