TL;DR: Trust your collaborators. Don't check who they are. Ask them to do it instead.

Problems

• Coupling

• Metamodel interference

• IFs

Solutions

• Avoid is , as, ::class, ::class.java , etc...

• Don't use Reflection and Meta-programming for Domain Objects.

• Replace IFs with polymorphism.

• Use complete objects, avoid nulls and setters, favor immutability and you will never ifs.

Sample Code

Wrong

class Rabbit {
    fun run() = println("I'm running! 🏃‍")
}

class Seagull {
    fun fly() = println("I'm flying! ✈️")
}

fun move(animal: Any) =
    when(animal) {
        is Rabbit -> animal.run()
        is Seagull -> animal.fly()
        else -> throw IllegalArgumentException("Unknown animal type")
    }


fun main() {
    val bunny = Rabbit()
    val livingstone = Seagull()

    move(bunny)
    move(livingstone)
}

Right

abstract class Animal {
    abstract fun move()
}

class Rabbit : Animal() {
    override fun move() = println("I'm running! 🏃‍")
}

class Seagull : Animal() {
    override fun move() = println("I'm flying! ✈️")
}

fun main() {
    val bunny = Rabbit()
    val livingstone = Seagull()

    bunny.move()
    livingstone.move()
}

Conclusion

Testing for a class type couples the objects with accidental decisions and violates bijection since no such control exists in the real world. It is a smell our models are not good enough.

Credits

• Code Smell 23 - Instance Type Checking by Maxi Contieri