Kotlin Code Smell 16 - Instance Type Checking For Polymorphism

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

Problems

• Coupling: Objects are tightly coupled due to instance type checking.

• Metamodel interference: The use of instance type checking interferes with the metamodel.

• IFs: Excessive use of if statements.

Solutions

• Avoid using 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, favor immutability, and you will never need if statements.

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 objects with accidental decisions and violates bijection since no such control exists in the real world. It is a code smell that indicates our models are not good enough.

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Credits

• Code Smell 23 - Instance Type Checking by @Maxi Contieri