Abstract factory pattern
The abstract factory pattern provides a way to encapsulate a group of individual factories that have a common theme without specifying their concrete classes. In normal usage, the client software creates a concrete implementation of the abstract factory and then uses the generic interface of the factory to create the concrete objects that are part of the theme. The client does not know which concrete objects it gets from each of these internal factories, since it uses only the generic interfaces of their products. This pattern separates the details of implementation of a set of objects from their general usage and relies on object composition, as object creation is implemented in methods exposed in the factory interface.
An example of this would be an abstract factory class
DocumentCreator that provides interfaces to create a number of products and createResume). The system would have any number of derived concrete versions of the DocumentCreator class like FancyDocumentCreator or ModernDocumentCreator, each with a different implementation of createLetter and createResume that would create a corresponding object like FancyLetter or ModernResume. Each of these products is derived from a simple abstract class like Letter or Resume of which the client is aware. The client code would get an appropriate instance of the DocumentCreator and call its factory methods. Each of the resulting objects would be created from the same DocumentCreator implementation and would share a common theme. The client would only need to know how to handle the abstract Letter or Resume class, not the specific version that it got from the concrete factory.A factory is the location of a concrete class in the code at which objects are constructed. The intent in employing the pattern is to insulate the creation of objects from their usage and to create families of related objects without having to depend on their concrete classes. This allows for new derived types to be introduced with no change to the code that uses the base class.
Use of this pattern makes it possible to interchange concrete implementations without changing the code that uses them, even at runtime. However, employment of this pattern, as with similar design patterns, may result in unnecessary complexity and extra work in the initial writing of code. Additionally, higher levels of separation and abstraction can result in systems that are more difficult to debug and maintain.
Overview
The Abstract Factorydesign pattern is one of the twenty-three well-known
GoF design patterns
that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.
The Abstract Factory design pattern solves problems like:
- How can an application be independent of how its objects are created?
- How can a class be independent of how the objects it requires are created?
- How can families of related or dependent objects be created?
because it commits the class to particular objects and makes it impossible to change the instantiation later independently from the class.
It stops the class from being reusable if other objects are required,
and it makes the class hard to test because real objects cannot be replaced with mock objects.
The Abstract Factory design pattern describes how to solve such problems:
- Encapsulate object creation in a separate object. That is, define an interface for creating objects, and implement the interface.
- A class delegates object creation to a factory object instead of creating objects directly.
A class can be configured with a factory object, which it uses to create objects, and even more, the factory object can be exchanged at run-time.
Definition
The essence of the Abstract Factory Pattern is to "Provide an interface for creating families of related or dependent objects without specifying their concrete classes.".Usage
The factory determines the actual concrete type of object to be created, and it is here that the object is actually created. However, the factory only returns an abstract pointer to the created concrete object.This insulates client code from object creation by having clients ask a factory object to create an object of the desired abstract type and to return an abstract pointer to the object.
As the factory only returns an abstract pointer, the client code does not know — and is not burdened by — the actual concrete type of the object that was just created. However, the type of a concrete object is known by the abstract factory; for instance, the factory may read it from a configuration file. The client has no need to specify the type, since it has already been specified in the configuration file. In particular, this means:
- The client code has no knowledge whatsoever of the concrete type, not needing to include any header files or class declarations related to it. The client code deals only with the abstract type. Objects of a concrete type are indeed created by the factory, but the client code accesses such objects only through their abstract interface.
- Adding new concrete types is done by modifying the client code to use a different factory, a modification that is typically one line in one file. The different factory then creates objects of a different concrete type, but still returns a pointer of the same abstract type as before — thus insulating the client code from change. This is significantly easier than modifying the client code to instantiate a new type, which would require changing every location in the code where a new object is created. If all factory objects are stored globally in a singleton object, and all client code goes through the singleton to access the proper factory for object creation, then changing factories is as easy as changing the singleton object.
Structure
UML diagram
In the above UML class diagram,the
Client class that requires ProductA and ProductB objects does not instantiate the ProductA1 and ProductB1 classes directly.Instead, the
Client refers to the AbstractFactory interface for creating objects,which makes the
Client independent of how the objects are created.The
Factory1 class implements the AbstractFactory interface by instantiating the ProductA1 and ProductB1 classes.The UML sequence diagram shows the run-time interactions:
The
Client object calls createProductA on the Factory1 object, which creates and returns a ProductA1 object.Thereafter,
the
Client calls createProductB on Factory1, which creates and returns a ProductB1 object.Lepus3 chart
Java example
The full implementation of the Abstract Factory pattern is available at https://java-design-patterns.com/patterns/abstract-factory/. Here is the shortened version.King.java
public interface King
Army.java
public interface Army
ElfKing.java
public class ElfKing implements King
ElfArmy.java
public class ElfArmy implements Army
KingdomFactory.java
public interface KingdomFactory
ElfKingdomFactory.java
public class ElfKingdomFactory implements KingdomFactory
App.java
var factory = new ElfKingdomFactory;
var king = factory.createKing;
var army = factory.createArmy;
king.getDescription;
army.getDescription;
Program output
This is the Elven king!
This is the Elven Army!
Python">Python (programming language)">Python example
from abc import ABC, abstractmethod
from sys import platform
class Button:
@abstractmethod
def paint:
pass
class LinuxButton:
def paint:
return 'Render a button in a Linux style'
class WindowsButton:
def paint:
return 'Render a button in a Windows style'
class MacOSButton:
def paint:
return 'Render a button in a MacOS style'
class GUIFactory:
@abstractmethod
def create_button:
pass
class LinuxFactory:
def create_button:
return LinuxButton
class WindowsFactory:
def create_button:
return WindowsButton
class MacOSFactory:
def create_button:
return MacOSButton
if platform 'linux':
factory = LinuxFactory
elif platform 'darwin':
factory = MacOSFactory
elif platform 'win32':
factory = WindowsFactory
else:
raise NotImplementedError
button = factory.create_button
result = button.paint
Alternative implementation using the classes themselves as factories:
from abc import ABC, abstractmethod
from sys import platform
class Button:
@abstractmethod
def paint:
pass
class LinuxButton:
def paint:
return 'Render a button in a Linux style'
class WindowsButton:
def paint:
return 'Render a button in a Windows style'
class MacOSButton:
def paint:
return 'Render a button in a MacOS style'
if platform "linux":
factory = LinuxButton
elif platform "darwin":
factory = MacOSButton
elif platform "win32":
factory = WindowsButton
else:
raise NotImplementedError
button = factory
result = button.paint