Kotlin Code Smell 19 - Setters

The first exercise that junior programmers often do involves IDEs, tutorials, and senior developers who teach them an anti-pattern.

Problems

• Mutability

• Information Hiding

• Anemic Models

• Fail Fast

• Integrity

• Duplicated Code

• Concurrent programming execution

Solutions

• Avoid Setters

• Set essential attributes on object construction.

Sample Code

Wrong

// Anemic mutable class
data class PhoneCall(
    var origin: String? = null,
    var destination: String? = null,
    var duration: Long? = null
)

fun main() {
    val janePhoneCall = PhoneCall()
    janePhoneCall.origin = "555-5555"
    janePhoneCall.destination = "444-4444"
    janePhoneCall.duration = 60
}

Mutation brings lots of problems to your code.

fun main() {
    // Since we have a setter, we can create invalid combinations.
    // For example:
    // - We can't exchange the call destination during the call,
    // However, this is not enforced due to the setters' usage.
    val janePhoneCall = PhoneCall()
    janePhoneCall.origin = "555-5555"
    janePhoneCall.destination = "555-5555"
    janePhoneCall.duration = 60
}

// Origin and Destination cannot be the same
// To validate this, we're repeating the same code twice
class PhoneCall(
    origin: String? = null,
    destination: String? = null,
    duration: Long? = null
) {
    var origin: String? = origin
        set(value) {
            if (value == origin)
                throw IllegalArgumentException("Destination cannot be the same as origin")
            field = value
        }

    var destination: String? = destination
        set(value) {
            if (value == destination)
                throw IllegalArgumentException("Destination cannot be the same as origin")
            field = value
        }

    // duration is exposed in seconds as a ripple effect
    // this violates information hiding principle and prevents
    // us from changing its representation
    var duration: Long? = duration
        set(durationInSeconds) {
            field = durationInSeconds
        }
}

// Moreover, multiple threads (or coroutines) can change the same
// object. What is the right state of the object at the end?

Right

class PhoneCall(
    val origin: String,
    val destination: String,
    val duration: Long
) {
    // Single control point.
    // We only create valid phone calls, and they remain valid
    // since they cannot mutate.
    init {
        if (origin == destination)
            throw IllegalArgumentException("Destination cannot be the same as origin")
    }

    // We're explicit about which measure of unit is used
    fun durationInSeconds() = duration
    fun durationInMilliSeconds() = duration * MILLISECONDS_IN_SECOND

    companion object {
        private const val MILLISECONDS_IN_SECOND = 1000
    }
}

Examples

• DTOs

Exceptions

• Setting attributes is safe for non-essential attributes. However, it has all the drawbacks and considerations already mentioned.

Conclusion

Creating incomplete and anemic objects is a very bad practice that violates mutability, the fail fast principle, and real-world bijections.

Credits

• Code Smell 28 - Setters by Maximiliano Contieri