Kotlin Code Smell 16 - Instance Type Checking for Polymorphism

Kotlin Code Smell 16 - Instance Type Checking for Polymorphism

Ditch the Type Checks: Let Your Code Dance to Polymorphism's Tune

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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

Sample Code

Wrong

class Rabbit {
    fun run() = println("I'm running! ๐Ÿƒโ€")
}

class Seagull {
    fun fly() = println("I'm flying! โœˆ๏ธ")
}

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! ๐Ÿƒโ€")
}

class Seagull : Animal() {
    override fun move() = println("I'm flying! โœˆ๏ธ")
}

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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