Determine Generic Type Paramater in Scala

Question

Currently, I'm working with Scala language and I don't know how to determine the type of generic type. I have a class as following code.

class A[K, V]() {
  def print(): Unit = {
    //Check the real type here
  }
}

What I want to do is:

If K is Int => Print out It is an Integer
If K is Long => Print out It is a Long
If K is String => Print out It is a String

Why create a generic class if you need specific type detail? — Yuval Itzchakov
– Yuval Itzchakov, Commented Nov 6, 2017 at 7:05

Shaido · Accepted Answer · 2017-11-06 08:38:38Z

1

You could use TypeTag[T]. From the documentation:

A TypeTag[T] encapsulates the runtime type representation of some type T.

Example code:

import scala.reflect.runtime.universe._
class A[K: TypeTag, V]() {
  def print(): Unit = typeOf[K] match {
    case i if i =:= typeOf[Int] => println("It is an Integer")
    case _ => println("Something else")
  }
}

Testing the code:

val a = new A[Int, String]
val b = new A[Double, String]
a.print()
b.print()

Printed result:

It is an Integer
Something else

answered Nov 6, 2017 at 8:38

Shaido

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Comments

ArunavaS · Accepted Answer · 2017-11-06 06:48:46Z

0

When you are writing generic classes, you should only focus on doing the common/generic operations on the provided values. However, if you really want to do some sort of type checking you can do that using asInstanceOf method.

You need modify you print method something like below.

def print(k: K) = {
    if (k.isInstanceOf[Int]) println("Int type")
    else if ...
}

answered Nov 6, 2017 at 6:48

ArunavaS

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5 Comments

jwvh Over a year ago

Pattern matching is the more common/accepted approach. isInstanceOf is discouraged.

tauitdnmd Over a year ago

@ArunavaS Due to some reasons, there is no way to modify my method to have an instance of K as its param. :

ArunavaS Over a year ago

@jwvh, true... pardon my oo mentality :P

ArunavaS Over a year ago

@tauitdnmd, if you cannot have type instances, then, how can you just do the type checking? Maybe, I am missing something here, according to me, what you are asking is you have a type class and you need to check the provided type from the type class.. which isn't possible practically

tauitdnmd Over a year ago

@ArunavaS Yes, I also have my own answer is that I can not do type detail checking without an instance of K. That's why I need to know do we have any solution for this.

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