C Factory Pattern

[Dot Liljenquist]

FactoryMethod is a creational design pattern that provides an interface for creating objects in a superclass, but allows subclasses to alter the type of objects that will be created.

Great news, right? But how about the code? At present, most of your code is coupled to the Truck class. Adding Ships into the app would require making changes to the entire codebase. Moreover, if later you decide to add another type of transportation to the app, you will probably need to make all of these changes again.

At first glance, this change may look pointless: we just moved the constructor call from one part of the program to another. However, consider this: now you can override the factory method in a subclass and change the class of products being created by the method.

For example, both Truck and Ship classes should implement the Transport interface, which declares a method called deliver. Each class implements this method differently: trucks deliver cargo by land, ships deliver cargo by sea. The factory method in the RoadLogistics class returns truck objects, whereas the factory method in the SeaLogistics class returns ships.

Note, despite its name, product creation is not the primary responsibility of the creator. Usually, the creator class already has some core business logic related to products. The factory method helps to decouple this logic from the concrete product classes. Here is an analogy: a large software development company can have a training department for programmers. However, the primary function of the company as a whole is still writing code, not producing programmers.

The base Dialog class uses different UI elements to render its window. Under various operating systems, these elements may look a little bit different, but they should still behave consistently. A button in Windows is still a button in Linux.

For this pattern to work, the base Dialog class must work with abstract buttons: a base class or an interface that all concrete buttons follow. This way the code within Dialog remains functional, whichever type of buttons it works with.

Inheritance is probably the easiest way to extend the default behavior of a library or framework. But how would the framework recognize that your subclass should be used instead of a standard component?

Now, create a set of creator subclasses for each type of product listed in the factory method. Override the factory method in the subclasses and extract the appropriate bits of construction code from the base method.

In object oriented programming, the factory method pattern is a design pattern that uses factory methods to deal with the problem of creating objects without having to specify their exact class. Rather than by calling a constructor, this is done by calling a factory method to create an object. Factory methods can either be specified in an interface and implemented by child classes, or implemented in a base class and optionally overridden by derived classes. It is one of the 23 classic design patterns described in the book Design Patterns (often referred to as the "Gang of Four" or simply "GoF") and is sub-categorized as a creational pattern.[1]

Creating an object often requires complex processes not appropriate to include within a composing object. The object's creation may lead to a significant duplication of code, may require information not accessible to the composing object, may not provide a sufficient level of abstraction, or may otherwise not be part of the composing object's concerns. The factory method design pattern handles these problems by defining a separate method for creating the objects, which subclasses can then override to specify the derived type of product that will be created.

The factory method pattern relies on inheritance, as object creation is delegated to subclasses that implement the factory method to create objects.[2]As shown in the C# example below, the factory method pattern can also rely on an Interface - in this case IPerson - to be implemented.

In the above UML class diagram, the Creator class that requires a Product object does not instantiate the Product1 class directly.Instead, the Creator refers to a separate factoryMethod() to create a product object,which makes the Creator independent of which concrete class is instantiated.Subclasses of Creator can redefine which class to instantiate. In this example, the Creator1 subclass implements the abstract factoryMethod() by instantiating the Product1 class.

Room is the base class for a final product (MagicRoom or OrdinaryRoom). MazeGame declares the abstract factory method to produce such a base product. MagicRoom and OrdinaryRoom are subclasses of the base product implementing the final product. MagicMazeGame and OrdinaryMazeGame are subclasses of MazeGame implementing the factory method producing the final products. Thus factory methods decouple callers (MazeGame) from the implementation of the concrete classes. This makes the "new" Operator redundant, allows adherence to the Open/closed principle and makes the final product more flexible in the event of change.

In the above code you can see the creation of one interface called IPerson and two implementations called Villager and CityPerson. Based on the type passed into the PersonFactory object, we are returning the original concrete object as the interface IPerson.

You can see we have used MakeProduct in concreteFactory. As a result, you can easily call MakeProduct() from it to get the IProduct. You might also write your custom logic after getting the object in the concrete Factory Method. The GetObject is made abstract in the Factory interface.

In the above snippet, the MazeGame constructor is a template method that makes some common logic. It refers to the makeRoom factory method that encapsulates the creation of rooms such that other rooms can be used in a subclass. To implement the other game mode that has magic rooms, it suffices to override the makeRoom method:

Another example in PHP follows, this time using interface implementations as opposed to subclassing (however, the same can be achieved through subclassing). It is important to note that the factory method can also be defined as public and called directly by the client code (in contrast with the Java example above).

The Factory Design Pattern is probably the most used design pattern in modern programming languages like Java and C#. It comes in different variants and implementations. If you are searching for it, most likely, you'll find references about the GoF patterns: Factory Method and Abstract Factory.

For example a graphical application works with shapes. In our implementation the drawing framework is the client and the shapes are the products. All the shapes are derived from an abstract shape class (or interface). The Shape class defines the draw and move operations which must be implemented by the concrete shapes. Let's assume a command is selected from the menu to create a new Circle. The framework receives the shape type as a string parameter, it asks the factory to create a new shape sending the parameter received from menu. The factory creates a new circle and returns it to the framework, casted to an abstract shape. Then the framework uses the object as casted to the abstract class without being aware of the concrete object type.

The advantage is obvious: New shapes can be added without changing a single line of code in the framework(the client code that uses the shapes from the factory). As it is shown in the next sections, there are certain factory implementations that allow adding new products without even modifying the factory class.

This implementation is the most simple and intuitive (Let's call it noob implementation). The problem here is that once we add a new concrete product call we should modify the Factory class. It is not very flexible and it violates open close principle. Of course we can subclass the factory class, but let's not forget that the factory class is usually used as a singleton. Subclassing it means replacing all the factory class references everywhere through the code.

If you can use reflection, for example in Java or .NET languages, you can register new product classes to the factory without even changing the factory itself. For creating objects inside the factory class without knowing the object type we keep a map between the productID and the class type of the product. In this case when a new product is added to the application it has to be registered to the factory. This operation doesn't require any change in the factory class code.

We have to make sure that the concrete product classes are loaded before they are required by the factory for registration(if they are not loaded they will not be registered in the factory and createProduct will return null). To ensure it we are going to use the Class.forName method right in the static section of the main class. This section is executed right after the main class is loaded. Class.forName is supposed to return a Class instance of the indicated class. If the class is not loaded by the compiler yet, it will be loaded when the Class.forName is invoked. Consequently the static block in each class will be executed when each class is loaded:

This reflection implementation has its own drawbacks. The main one is performance. When the reflection is used the performance on code involving reflection can decrease even to 10% of the poerfomance of a non reflection code. Another issue is that not all the programming languages provide reflection mechanism.

As we saw in the previous paragraph the factory object uses internally a HashMap to keep the mapping between parameters (in our case Strings) and concrete products class. The registration is made from outside of the factory and because the objects are created using reflection the factory is not aware of the objects types.

We don't want to use reflection but in the same time we want to have a reduced coupling between the factory and concrete products. Since the factory should be unaware of products we have to move the creation of objects outside of the factory to an object aware of the concrete products classes. That would be the concrete class itself.

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