检查 Scala class 实例的运行时类型

Question

If I run the following code, then I get an error:

import scala.reflect.ClassTag

class General {
  
}

class SubGeneral extends General {
  def test() = println("tested")
}


class ProGeneral[T <: General: ClassTag] {

  var array = Array.ofDim[T](3, 3)
  
  def funcForSubGeneral(): Unit =
        if (array(0)(0).isInstanceOf[SubGeneral]) then array(0)(0).test()
    
}

That is because General does not have the function test(). I know that I can fix this with pattern matching. This instead of the above funcForSubGeneral() works:

def funcForSubGeneral(): Unit =
        array(0)(0) match {
         case s: SubGeneral => s.test()
         case _ => println("nope")
        }

But I was wondering if it is possible to get the runtime type of array(0)(0) and check if it is a SubGeneral, and if that is the case then I call test(), which shouldn't cause a problem? That is what I was actually trying by using isIntanceOf. I want to omit pattern matching since I'm just interested in one type.

Answer 1

isInstanceOf doesn't change anything, you would need to do array(0)(0).asInstanceOf[SubGeneral].test() in order to force the casting.

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Note that the casting may fail at runtime, so that is why you need to check with the if before. Thus the end code looks like this:

if (array(0)(0).isInstanceOf[SubGeneral]) then array(0)(0).asInstanceOf[SubGeneral].test()

But, since this is cumbersome and error-prone, we have pattern matching:

array(0)(0) match {
  case subGeneral: SubGeneral => subGeneral.test()
}

However, note that type tests are considered a bad practice; mainly because they are actually class checks and may fail certain circumstances. For example:

List("a", "b", "c") match {
  case numbers: List[Int] => numbers.head + 1
}

Will throw an exception in runtime since, due to type erasure, we lost the [String] part and it matches only List then it tries to read the first element as an Int which is an error.

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Anyways, this is the fourth time in two days you ask a question that shows bad practices and unidiomatic code.

My advice:

1. I would bet you are not following an appropriate resource to learn the language. Rather, it seems you are just trying to mimic another language ( Python ?) with different syntax. - Thus, I encourage you to pick an appropriate book, course, tutorial, etc; that properly introduces the language and its idioms.
2. I would encourage you to join the official Discord server which is more suitable for newcomers than StackOverflow.
3. I would recommend you to explain the meta-problem you are trying to solve in detail (either here in a new question or in the Discord server) , I am pretty sure there are better and more idiomatic ways to solve it.