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title category language tag
Factory Method
Creational
en
Extensibility
Gang of Four

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

Taking 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 requested weapons are manufactured:

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}")

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

Factory Method pattern class diagram

Applicability

Use the Factory Method pattern when:

  • Class cannot anticipate the class of objects it must create.
  • 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.

Credits