How to do instanceOf of a variable of a class (not a class type) in scala

Question

I want to use isInstanceOf to decide the type of a variable, however isInstanceOf[T] requires T and T has to be a class decided in compile time. I hope I can make it a variable.

See the example code:

class A{ ... }
class B{ ... }

val class_map = Map( classOf[A] -> 1, classOf[B] -> 2 )
val a = new A()
class_map.keys foreach { i =>
   if (a.isInstanceOf[ i ])  // how to make this statement work?
     println(class_map[i])
}

isInstanceOf is usually a code smell, and this looks like an XY Problem, can you describe the broader problem you want yo solve. There may be better alternatives. — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented Sep 28, 2019 at 16:53

Kolmar · Accepted Answer · 2019-09-28 20:03:06Z

2

You can use isInstance method of the Class object:

val matchingValues = class_map.collect {
  case (clazz, value) if clazz.isInstance(a) => value
}

matchingValues foreach println

answered Sep 28, 2019 at 20:03

Kolmar

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Comments

DeusEx · Accepted Answer · 2019-10-08 19:45:00Z

0

If I were you, I'd take a different approach, along the lines of:

sealed trait Arg
case class A{ ... } extends Arg
case class B{ ... } extends Arg

val class_map: Arg => Int = {
  case A => 1
  case B => 2
}

val a = new A()
println(class_map(a)) // will print 1

edited Oct 8, 2019 at 19:45

answered Sep 29, 2019 at 13:23

DeusEx

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3 Comments

worldterminator Over a year ago

I don't think you can write case A => 1, if A is a class.

DeusEx Over a year ago

@worldterminator On the contrary, that's what pattern matching is for. But also, why don't you try it yourself?

DeusEx Over a year ago

@worldterminator apologies, I didn't realise those were not case classes :facepalm: . Once you make them so, it compiles and works (slightly adjusting the match block's syntax).

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