TL;DR: Abstract Factory pattern provides an interface for creating families of related or dependent objects without specifying their concrete classes. This pattern is useful when the system needs to be independent of how its products are created, composed, and represented, and when there are multiple families of products that need to be used together.
Also known as
- Kit
Intent
Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
Explanation
Real-world example
To create a kingdom we need objects with a common theme. The elven kingdom needs an elven king, elven castle, and elven army whereas the orcish kingdom needs an orcish king, orcish castle, and orcish army. There is a dependency between the objects in the kingdom.
In plain words
A factory of factories; is a factory that groups the individual but related/dependent factories together without specifying their concrete classes.
Wikipedia says
The abstract factory pattern provides a way to encapsulate a group of individual factories that have a common theme without specifying their concrete classes
Programmatic Example
Translating the kingdom example above. First of all, we have some interfaces and implementations for the objects in the kingdom.
internal interface Castle {
val description: String
}
internal interface King {
val description: String
}
internal interface Army {
val description: String
}
// Elven implementations ->
internal class ElfCastle : Castle {
override val description = "This is the elven castle!"
}
internal class ElfKing : King {
override val description = "This is the elven king!"
}
internal class ElfArmy : Army {
override val description = "This is the elven army!"
}
// Orcish implementations similarly -> ...
Then we have the abstraction and implementations for the kingdom factory.
internal interface KingdomFactory {
fun createCastle(): Castle
fun createKing(): King
fun createArmy(): Army
}
internal class ElfKingdomFactory : KingdomFactory {
override fun createCastle() = ElfCastle()
override fun createKing() = ElfKing()
override fun createArmy() = ElfArmy()
}
internal class OrcKingdomFactory : KingdomFactory {
override fun createCastle() = OrcCastle()
override fun createKing() = OrcKing()
override fun createArmy() = OrcArmy()
}
Now we have the abstract factory that lets us make a family of related objects i.e. elven kingdom factory creates an elven castle, king and army, etc.
val factory = ElfKingdomFactory()
val castle = factory.createCastle()
val king = factory.createKing()
val army = factory.createArmy()
castle.description
king.description
army.description
Program output:
This is the elven castle!
This is the elven king!
This is the elven Army!
Now, we can design a factory for our different kingdom factories. In this example, we created FactoryMaker, responsible for returning an instance of either ElfKingdomFactory or OrcKingdomFactory.
The client can use FactoryMaker to create the desired concrete factory which, in turn, will produce different concrete objects (derived from Army, King, Castle).
In this example, we also used an enum to parameterize which type of kingdom factory the client will ask for.
internal data class Kingdom(
val king: King,
val castle: Castle,
val army: Army,
) {
object FactoryMaker {
enum class KingdomType {
ELF,
ORC
}
fun makeFactory(type: KingdomType): KingdomFactory {
return when (type) {
KingdomType.ELF -> ElfKingdomFactory()
KingdomType.ORC -> OrcKingdomFactory()
}
}
}
}
Now we can use the abstract factory to create the kingdoms
logger.info("elf kingdom")
val elfKingdom = createKingdom(KingdomType.ELF)
logger.info(elfKingdom.army.description)
logger.info(elfKingdom.castle.description)
logger.info(elfKingdom.king.description)
logger.info("orc kingdom")
val orcKingdom = createKingdom(KingdomType.ORC)
logger.info(orcKingdom.army.description)
logger.info(orcKingdom.castle.description)
logger.info(orcKingdom.king.description)
Program output:
elf kingdom
This is the elven castle!
This is the elven king!
This is the elven Army!
orc kingdom
This is the orc castle!
This is the orc king!
This is the orc Army!
Class diagram
Applicability
Use the Abstract Factory pattern when
The system should be independent of how its products are created, composed, and represented
The system should be configured with one of the multiple families of products
The family of related product objects is designed to be used together, and you need to enforce this constraint
You want to provide a class library of products, and you want to reveal just their interfaces, not their implementations
The lifetime of the dependency is conceptually shorter than the lifetime of the consumer.
You need a run-time value to construct a particular dependency
You want to decide which product to call from a family at runtime.
You need to supply one or more parameters only known at run-time before you can resolve a dependency.
When you need consistency among products
You don't want to change existing code when adding new products or families of products to the program.
Example use cases
Selecting to call to the appropriate implementation of FileSystemAcmeService or DatabaseAcmeService or NetworkAcmeService at runtime.
Unit test case writing becomes much easier
UI tools for different OS
Consequences
Dependency injection in Java hides the service class dependencies that can lead to runtime errors that would have been caught at compile time.
While the pattern is great when creating predefined objects, adding new ones might be challenging.
The code becomes more complicated than it should be since many new interfaces and classes are introduced along with the pattern.
Tutorials
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Code Examples
All code examples and tests can be found in the Kotlin Design Patterns repository
Related patterns
Factory Kit
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