Kotlin Code Smell 23 - Singletons

// God is the archetypal singleton example
class God {
    // In Kotlin, the companion object is always a singleton
    companion object {
        private var instance: God? = null

        fun getInstance(): God {
            if (instance == null) {
                instance = God()
            }

            return requireNotNull(instance)
        }
    }
}

// Why should we be aware of getInstance when creating an object?
val jewishGod = God.getInstance()

interface Religion {
    // Define behavior
}

class God {
    // There can be as many as you wish
}

class PolytheisticReligion(private val gods: Collection<God>) : Religion
class MonotheisticReligion(private val godAllMighty: God) : Religion

// According to Judaism, there's only one God,
// but this does not hold in other religions.
// Under this context, God is unique. We cannot create or change
// a new one.
val jewishGod = God()
val judaism = MonotheisticReligion(jewishGod)

val jupiter = God()
val saturn = God()
val mythologicalReligion = PolytheisticReligion(listOf(jupiter, saturn))

// Gods are unique (or not) according to context.
// We can create test religions with or without unicity.
// This is less coupled since we break the direct reference to the
// God class.
// The God class has the single responsibility to create gods, not
// to manage them.