Abstract Factory in Kotlin

Design patterns / Creational patterns

//abstract factory
interface IFactory {
    fun createA(): IProductA
    fun createB(): IProductB
}

//concrete factory 1
class Factory1: IFactory {
    override fun createA(): IProductA {
        return ProductA1()
    }
    override fun createB(): IProductB {
        return ProductB1()
    }
}

//concrete factory 2
class Factory2: IFactory {
    override fun createA(): IProductA {
        return ProductA2()
    }
    override fun createB(): IProductB {
        return ProductB2()
    }
}

//abstract product A
interface IProductA {
    fun testA()
}

//abstract product B
interface IProductB {
    fun testB()
}

//concrete product A1
class ProductA1: IProductA {
    override fun testA() {
        println("test A1")
    }
}

//concrete product A2
class ProductA2: IProductA {
    override fun testA() {
        println("test A2")
    }
}

//concrete product B1
class ProductB1: IProductB {
    override fun testB() {
        println("test B1")
    }
}

//concrete product B2
class ProductB2: IProductB {
    override fun testB() {
        println("test B2")
    }
}

//client code
private fun testFactory(factory: IFactory) {
    val productA = factory.createA()
    val productB = factory.createB()
    productA.testA()
    productB.testB()
}

testFactory(Factory1())
//printed: test A1
//         test B1
testFactory(Factory2())
//printed: test A2
//         test B2

Provides an interface for creating families of objects whose interfaces are known but concrete classes are not.

The problem. A program draws its windows in two styles, light and dark. Each style needs its own button and its own menu. If the code creates them directly, it can mix a light button with a dark menu by mistake. A third style would also mean a search for every place where a button or a menu is created.

The idea. An abstract factory is an interface with one creation method for each kind of product. Each concrete factory implements it for one family. One factory creates the light button and the light menu; another creates the dark ones. The client receives a factory and creates every product through it. It never names a concrete product, so the products it gets always belong to one family. To change the whole family, you give the client a different factory.

How to read the example. There are two kinds of product, A and B. A concrete factory makes one family: one product of each kind, marked with the same number. The client receives a factory, asks it for one product of each kind and uses both. The client does not know which family it got, but the output shows it. Some languages declare no interfaces at all, but the roles stay the same.

When to use it. Use Abstract Factory when products must work together and there are several families of them, such as visual themes or operating systems. Do not use it when there is only one family. It is also not needed when the products do not depend on each other: a constructor or a Factory Method is enough.

A common mistake. Choosing this pattern while the kinds of product still change often. Each new kind adds a method to the abstract factory and to every concrete factory.