Factory method pattern

Factory method in UML

Factory Method in LePUS3

The factory method pattern is an object-oriented design pattern. Like other creational patterns, it deals with the problem of creating objects (products) without specifying the exact class of object that will be created. The factory method design pattern handles this problem by defining a separate method for creating the objects, whose subclasses can then override to specify the derived type of product that will be created. More generally, the term factory method is often used to refer to any method whose main purpose is creation of objects.

Definition

The essence of the Factory Pattern is to "Define an interface for creating an object, but let the subclasses decide which class to instantiate. The Factory method lets a class defer instantiation to subclasses."

Common usage

Factory methods are common in toolkits and frameworks where library code needs to create objects of types which may be subclassed by applications using the framework.

Parallel class hierarchies often require objects from one hierarchy to be able to create appropriate objects from another.

Factory methods are used in test-driven development to allow classes to be put under test. If such a class Foo creates another object Dangerous that can't be put under automated unit tests (perhaps it communicates with a production database that isn't always available), then the creation of Dangerous objects is placed in the virtual factory method CreateDangerous in class Foo. For testing, TestFoo (a subclass of Foo) is then created, with the virtual factory method CreateDangerous overridden to create and return FakeDangerous, a fake object. Unit tests then use TestFoo to test the functionality of Foo without incurring the side effects of using a real Dangerous object.

Other benefits and variants

Although the motivation behind the factory method pattern is to allow subclasses to choose which type of object to create, there are other benefits to using factory methods, many of which do not depend on subclassing. Therefore, it is common to define "factory methods" that are not polymorphic to create objects in order to gain these other benefits. Such methods are often static.

Encapsulation

Factory methods encapsulate the creation of objects. This can be useful if the creation process is very complex, for example if it depends on settings in configuration files or on user input.

Consider as an example a program to read image files and make thumbnails out of them. The program supports different image formats, represented by a reader class for each format:

public interface ImageReader {
    public DecodedImage getDecodedImage();
}
 
public class GifReader implements ImageReader { 
    public DecodedImage getDecodedImage() {
        return decodedImage;
    }
}
 
public class JpegReader implements ImageReader {
    // ....
}

Each time the program reads an image it needs to create a reader of the appropriate type based on some information in the file. This logic can be encapsulated in a factory method:

public class ImageReaderFactory {
    public static ImageReader getImageReader(InputStream is) {
        int imageType = figureOutImageType(is);

        switch(imageType) {
            case ImageReaderFactory.GIF:
                return new GifReader(is);
            case ImageReaderFactory.JPEG:
                return new JpegReader(is);
            // etc.
        }
    }
}

The code fragment in the previous example uses a switch statement to associate an imageType with a specific factory object. Alternatively, this association could also be implemented as a mapping. This would allow the switch statement to be replaced with an associative array lookup.

Limitations

There are three limitations associated with the use of the factory method. The first relates to refactoring existing code; the other two relate to inheritance.

• The first limitation is that refactoring an existing class to use factories breaks existing clients. For example, if class Complex were a standard class, it might have numerous clients with code like:

Complex c = new Complex(-1, 0);

Once we realize that two different factories are needed, we change the class (to the code shown earlier). But since the constructor is now private, the existing client code no longer compiles.

• The second limitation is that, since the pattern relies on using a private constructor, the class cannot be extended. Any subclass must invoke the inherited constructor, but this cannot be done if that constructor is private.

• The third limitation is that, if we do extend the class (e.g., by making the constructor protected -- this is risky but possible), the subclass must provide its own re-implementation of all factory methods with exactly the same signatures. For example, if class StrangeComplex extends Complex, then unless StrangeComplex provides its own version of all factory methods, the call StrangeComplex.fromPolar(1, pi) will yield an instance of Complex (the superclass) rather than the expected instance of the subclass.

All three problems could be alleviated by altering the underlying programming language to make factories first-class class members (rather than using the design pattern).

Example

Pizza example in Java

abstract class Pizza {
    public abstract double getPrice();
}

class HamAndMushroomPizza extends Pizza {
    private double price = 8.5;

    public double getPrice() {
        return price;
    }
}

class DeluxePizza extends Pizza {
    private double price = 10.5;

    public double getPrice() {
        return price;
    }
}

class HawaiianPizza extends Pizza {
    private double price = 11.5;

    public double getPrice() {
        return price;
    }
}

class PizzaFactory {
    public enum PizzaType {
        HamMushroom,
        Deluxe,
        Hawaiian
    }

    public static Pizza createPizza(PizzaType pizzaType) {
        switch (pizzaType) {
            case HamMushroom:
                return new HamAndMushroomPizza();
            case Deluxe:
                return new DeluxePizza();
            case Hawaiian:
                return new HawaiianPizza();
        }
        throw new IllegalArgumentException("The pizza type " + pizzaType + " is not recognized.");
    }
}

class PizzaLover {
    /**
     * Create all available pizzas and print their prices
     */
    public static void main (String args[]) {
        for (PizzaFactory.PizzaType pizzaType : PizzaFactory.PizzaType.values()) {
            System.out.println("Price of " + pizzaType + " is " + PizzaFactory.createPizza(pizzaType).getPrice());
        }
    }
}

Uses

• In ADO.NET, IDbCommand.CreateParameter is an example of the use of factory method to connect parallel class hierarchies.
• In Qt, QMainWindow::createPopupMenu is a factory method declared in a framework which can be overridden in application code.
• In Java, several factories are used in the javax.xml.parsers package. e.g. javax.xml.parsers.DocumentBuilderFactory or javax.xml.parsers.SAXParserFactory.

See also

• Design Patterns, the highly-influential book
• Abstract Factory, a pattern often implemented using factory methods
• Builder pattern, another creational pattern
• Template method pattern, which may call factory methods
• Factory object

References

• Fowler, Martin (1999). Refactoring: Improving the Design of Existing Code. Addison-Wesley. ISBN 0-201-48567-2. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
• Gamma, Erich; Helm, Richard; Johnson, Ralph; Vlissides, John (1994). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley. ISBN 0-201-63361-2.{{cite book}}: CS1 maint: multiple names: authors list (link)
• Cohen, Tal (2007). "Better Construction with Factories" (PDF). Journal of Object Technology. Bertrand Meyer. Retrieved 2007-03-12. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

External links

• Factory method Example UML and Code
• Factory method in UML and in LePUS3 (a Design Description Language)
• Description from the Portland Pattern Repository
• Jt J2EE Pattern Oriented Framework