Abstract Factory in action

Our strategy game will consist of buildings and units. Let's start with declaring what all buildings share:

interface Building<in UnitType, out ProducedUnit> 
        where UnitType : Enum<*>, ProducedUnit : Unit {
    fun build(type: UnitType) : ProducedUnit
}

All buildings should implement the build() function. Here we see generics in Kotlin for the first time, so let's discuss them a bit.