TL;DR: Factory Method is a design pattern that defines an interface for creating objects, allowing subclasses to decide which class to instantiate, thus delegating instantiation logic to child classes.
Also known as
- Virtual Constructor
Intent
Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.
Explanation
Real-world example
Blacksmith manufactures weapons. Elves require Elvish weapons and orcs require Orcish weapons. Depending on the customer at hand the right type of blacksmith is summoned.
In plain words
It provides a way to delegate the instantiation logic to child classes.
Wikipedia says
In class-based programming, the factory method pattern is a creational pattern that uses factory methods to deal with the problem of creating objects without having to specify the exact class of the object that will be created. This is done by creating objects by calling a factory method — either specified in an interface and implemented by child classes, or implemented in a base class and optionally overridden by derived classes—rather than by calling a constructor.
Programmatic Example
Take our blacksmith example above. First of all, we have a Blacksmith interface and some implementations for it:
internal interface Blacksmith {
fun manufactureWeapon(weaponType: WeaponType): Weapon
}
internal class ElfBlacksmith : Blacksmith {
override fun manufactureWeapon(weaponType: WeaponType): Weapon =
ELF_ARSENAL.getOrElse(weaponType) {
throw IllegalArgumentException("Weapon type $weaponType is not supported by elf blacksmith.")
}
}
internal class OrcBlacksmith : Blacksmith {
override fun manufactureWeapon(weaponType: WeaponType): Weapon =
ORC_ARSENAL.getOrElse(weaponType) {
throw IllegalArgumentException("Weapon type $weaponType is not supported by the orc blacksmith.")
}
}
When the customers come, the correct type of blacksmith is summoned and the requested weapons are manufactured:
// Orc
var blacksmith: Blacksmith = OrcBlacksmith()
var weapon: Weapon = blacksmith.manufactureWeapon(WeaponType.SPEAR)
logger.info("$blacksmith manufactured ${weapon.weaponType.title}")
weapon = blacksmith.manufactureWeapon(WeaponType.AXE)
logger.info("$blacksmith manufactured ${weapon.weaponType.title}")
// Elf
blacksmith = ElfBlacksmith()
weapon = blacksmith.manufactureWeapon(WeaponType.SPEAR)
logger.info("$blacksmith manufactured ${weapon.weaponType.title}")
weapon = blacksmith.manufactureWeapon(WeaponType.AXE)
logger.info("$blacksmith manufactured ${weapon.weaponType.title}")
Program output:
The orc blacksmith manufactured spear
The orc blacksmith manufactured axe
The elf blacksmith manufactured spear
The elf blacksmith manufactured axe
Class diagram
Applicability
Use the Factory Method pattern when:
The class cannot anticipate the class of objects it must create.
The class wants its subclasses to specify the objects it creates.
Classes delegate responsibility to one of several helper subclasses, and you want to localize the knowledge of which helper subclass is the delegate.
Code Examples
All code examples and tests can be found in the Kotlin Design Patterns repository