How to dynamically construct subclass with parameters based on string

Question

Code I have:

abstract class Animal {
   def init(animalType: String, jsonBlob: String)
}

class Dog (name: String, colour: String) {
   def init(animalType: String, jsonBlob: String) : Unit = {
        name = jsonBlob.name
        colour = jsonBlob.colour
    }
}

class Cat (address: String) {
   def init(animalType: String, jsonBlob: String) : Unit = {
        address = jsonBlob.address
    }
}

What I want: to instantiate a Cat or Dog dynamically.

I have made an attempt with code that looks like this:

case class AnimalDefinition(animalType: String, jsonBlob: String)
val animalDefinitions : Array[AnimalDefinition] = // 
val animals : Array[Animal] = animalDefinitions.map(new Animal(_.animalType, _.jsonBlob))

It should dynamically instantiate the correct class using the animalType parameter. I think traditionally I would do this using a case statement (if animalType == "Cat", return new Cat(..)). But I believe there's an automatic way to do this with reflections.

The code doesn't compile. I've tried reading the Scala reflection docs but they don't have examples that show dynamic instantiation of subclasses with additional parameters

Why do you want to include reflection at all? What is wrong with just a simple factory function that instantes the correct type based on input? — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented Oct 23, 2019 at 1:03
@LuisMiguelMejíaSuárez so you are talking about something like this: alvinalexander.com/scala/… ? — user
– user, Commented Oct 23, 2019 at 22:29
Yeah, basically that. I would make the method return an Option[Animal] or an Either[String, Animal] to handle invalid entries. — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented Oct 23, 2019 at 23:14

Dmytro Mitin · Accepted Answer · 2019-10-25 11:45:56Z

1

You can replace

if animalType == "Cat", return new Cat("cat name") ...

with

import scala.reflect.runtime.universe._
val mirror = runtimeMirror(getClass.getClassLoader)
val classSymbol = mirror.staticClass(animalType)
val typ = classSymbol.info
val constructorSymbol = typ.decl(termNames.CONSTRUCTOR).asMethod
val classMirror = mirror.reflectClass(classSymbol)
val constructorMirror = classMirror.reflectConstructor(constructorSymbol)
constructorMirror("cat name").asInstanceOf[Animal]

answered Oct 25, 2019 at 11:45

Dmytro Mitin

53.1k3 gold badges34 silver badges76 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

user Over a year ago

Thank you. At this point, I'm not sure if my desire to use reflection was appropriate. Why would one choose to use the second code block instead of the first line?

Dmytro Mitin Over a year ago

@user Well, if you have let's say 100 cases like if animalType == "Cat", return new Cat("cat name") then reflection can be better than boilerplate code. Surely for 2 cases there's not much sense.

user Over a year ago

Gotcha. I have about 10 cases at this point, but I might have to come back to this in the future. Thanks

Collectives™ on Stack Overflow

How to dynamically construct subclass with parameters based on string

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related