How do I get the class object of Class with type parameters in Scala?

Question

I'm new to Scala and rusty with Java and I'm tripping up on some semantics. I'm trying to get the class object of SequenceFileOutputFormat .

Other stack overflow posts say to simple do the following:

classOf[SequenceFileOutputFormat] 

Which yields the error:

class SequenceFileOutputFormat takes type parameters

Ok fine, so it requires the parameter types, so I do the following:

classOf[SequenceFileOutputFormat[String, String]]

Which yields the error:

[error]  found   : Class[org.apache.hadoop.mapreduce.lib.output.SequenceFileOutputFormat[String,String]](classOf[org.apache.hadoop.mapreduce.lib.output.SequenceFileOutputFormat])
[error]  required: Class[_ <: org.apache.hadoop.mapred.OutputFormat[_, _]]

What gives? How do I get the class object of a class that requires type parameters?

Answer 1

With hadoop-mapreduce-client-core "3.0.0", this here:

import org.apache.hadoop.mapreduce.lib.output._

object HadoopQuestion_48818781 {
  def main(args: Array[String]): Unit = {
    val c = classOf[SequenceFileOutputFormat[_, _]]
    println("Class: " + c)
  }
}

prints:

Class: class org.apache.hadoop.mapreduce.lib.output.SequenceFileOutputFormat

Explanation:

Even though classOf[T] is processed at compile time, the Class[T] thing itself is a runtime construct, and it does not know anything about the generic type parameters of T . Therefore, you have to put underscores _ for the runtime-erased type parameters. These underscores are really just syntactic sugar for existential types in Scala, but in this particular case they fulfill a role similar to that of the wildcards in Java.