通用类型上的模式匹配

Question

I need to know if the generic type is a String in order to perform a different logic on it, like:

def compute[A](field: String, record: GenericRecord): A match {
   case String => record.get(field).asInstanceOf[Utf8].toString
   case _ => record.get(field).asInstanceOf[A]
}

But I don't know how to get the type of the generic.

Answer 1

You can use ClassTag to do matching if you don't need to handle nested types, and TypeTag if you need to do it:

import scala.reflect.{ClassTag, classTag}

def compute[A: ClassTag](field: String, record: GenericRecord): A = {
  if (classTag[A] == classTag[String]) {
    record.get(field).asInstanceOf[Utf8].toString.asInstanceOf[A]
  } else {
    record.get(field).asInstanceOf[A]
  }
}

For TypeTag s, import scala.reflect.runtime.universe.{TypeTag, typeTag} instead, and change ClassTag to TypeTag and classTag to typeTag .

Answer 2

Maybe something like this (probably could be optimized):

import scala.reflect.ClassTag

def m[A](x: Any)(implicit tag: ClassTag[A]): A = {
  tag.toString match {
    case "java.lang.String" =>
      (x.toString + " is a string").asInstanceOf[A]
    case _ =>
      x.asInstanceOf[A]
  }
}

println(m[String]("123")) // 123 is a string
println(m[Int](456)) // 456