With the following test case, I would check the type of the elements of a Seq[AnyRef]
,
@Test
def testClassTagAndTypeTag(): Unit = {
import scala.reflect.runtime.universe._
def getTypeTag[T: TypeTag](data: T): TypeTag[T] = typeTag[T]
def getClassTag[T: ClassTag](data: T): ClassTag[T] = implicitly[ClassTag[T]]
val data = Seq(List(1), "Hello", new Box(1))
data.foreach(x => {
println(s"TypeTag: ${getTypeTag(x)}, tpe: ${getTypeTag(x).tpe}")
})
data.foreach(x => {
println(s"ClassTag: ${getClassTag(x)}")
})
}
The output of the above code is:
TypeTag: TypeTag[java.lang.Object], tpe: java.lang.Object
TypeTag: TypeTag[java.lang.Object], tpe: java.lang.Object
TypeTag: TypeTag[java.lang.Object], tpe: java.lang.Object
ClassTag: Object
ClassTag: Object
ClassTag: Object
I thought that the type tag should output the real type of Seq's elements, which should be
List[Int]
String
Box[Int]
By the time you have a Seq[AnyRef]
, it's too late. The type of x
in data.foreach(x => ...)
is just AnyRef
, so getTypeTag(x)
and getClassTag(x)
give corresponding results.
Instead, you can store type tag together with value, eg
case class WithTypeTag[A](x: A)(implicit val tag: TypeTag[A])
val data = Seq(WithTypeTag(List(1)), WithTypeTag("Hello"), WithTypeTag(new Box(1)))
data.foreach(x => println(x.tag))
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