CSC/ECE 517 Fall 2012/ch2b 2w60 ns
Factory Method
General Resources
Overview of the Pattern
Factory method pattern is an Object Oriented Programming concept in which objects can be created without specifying the class to which they belong.Factory method pattern implement the concept of using an object to generate other objects.<ref name="web">http://www.oodesign.com/factory-pattern.html
</ref>Factory method is thus a type of creational pattern. An interface is defined for object creation. However the subclasses decide which class they want to instantiate. Factory methods thus abstract object instantiation from the client.<ref name="userpagesfactory">http://userpages.umbc.edu/~tarr/dp/lectures/Factory.pdf
</ref>
Wikipedia<ref name="wikipedia">http://en.wikipedia.org/wiki/Factory_method_pattern</ref> and dofactory<ref name="dofactory">http://www.dofactory.com/Patterns/PatternFactory.aspx#UML</ref> provide good explanations of the Factory Method Pattern. dofactory<ref name="dofactory"/> is much more concise than Wikipedia<ref name="wikipedia"/>. It is a good resource for quickly viewing the definition and participants of the pattern. Wikipedia<ref name="wikipedia"/>, on the other hand, is much more detailed, it goes into the motivation for the pattern and discusses use of the patterns in several languages such as Java and C# at a high level.
javapapers<ref name="javapapers">http://javapapers.com/design-patterns/factory-method-pattern/</ref> is a good page that gives a basic overview of the pattern through the presentation of a concrete example in java language. There are other Java resources and resources for other languages that are similar below.
UML
Understand the UML diagram for the Factory method pattern can help you understand the purpose of the Factory method pattern, the classes involved and how they interact with each other. UML is not programming language specific, so you should be able look at the UML diagram in any of the resources that have them on this page, regardless of what languages you know.
In the UML diagram on the right, Product is the interface for the type of object created by the factory method. The Product interface is implemented by ConcreteProduct. The factory method which returns an object of type Product, is declared in Creator. The factory method is overridden inside ConcreteCreator to return an instance of ConcreteProduct.
Code Example
This example has been taken from <ref name="sourcemakingfact"> http://sourcemaking.com/design_patterns/factory_method/</ref>
public interface ImageReader {
public DecodedImage getDecodedImage();
}
public class GifReader implements ImageReader {
public GifReader( InputStream in ) {
// check that it's a gif, throw exception if it's not, then if it is decode it.
}
public DecodedImage getDecodedImage() {
return decodedImage;
}
}
public class JpegReader implements ImageReader {
//...
}
In the example above the ImageReader interface is defined. In this interface, a method getDecodedImage is defined that should return a decoded image. However this image may have been encoded in any format for example GIFF or JPEG. Therefore the functionality of actually decoding the image is implemented inside two classes GifReader and JpegReader . In this way, the responsibility of creating objects decoded in the appropriate format has been delegated to the subclass implementing the interface.
Summary
For a general overview and UML diagrams of the Factory method design pattern, we recommend:
Ruby
Summary
Java & C#
There are a variety of resources available for learning how to use the Decorator pattern in Java and C#. We group these together as they are syntactically similar languages. One of the most popular references for learning about Design Patterns in Java is the book "Head First Design Patterns"<ref name="head_first"/>; in this book an in-depth explanation of the Decorator Pattern can be found. This is probably more popular because of how in-depth its explanation is of the design pattern, using diagrams and an ongoing coding example. It is also worth noting the the coding example used in this book (and a number of resources dedicated to the Decorator pattern) is a Coffee class where you wish to "decorate" your coffee with different additional ingredients. If you are really interested in learning the ins and outs of the Decorator pattern and do not mind a lengthy read, this is the best reference to use in regard to learning and understanding how to use this pattern.
If you are interested in learning about the Decorator pattern for Java and/or C# but do not have enough time or energy to read the chapter in the "Head First Design Patterns"<ref name="head_first"/> book, there are other resources that can be used to achieve an understanding but are less textual. Most of the resources that are not highly verbose use an over-arching example to explain the pattern. A good and simple resource for learning how to use the Decorator pattern in Java can be found on the site SourceMaking, a site devoted to teaching IT concepts such as Design Patterns and UML to professional developers<ref name="sourcemaking"> http://sourcemaking.com/design_patterns/decorator - 2012?</ref>. This resource includes a number of Java (and C#) examples meant to explain the usage of the pattern as well as textual explanations if needed. An advantage to using this resource is that it uses less text and more examples to show how the Decorator pattern can be used. It is also helpful that this site has multiple examples that can help increase understanding through reinforcement and multiple viewpoints (and languages). A disadvantage to using this site is that all of the examples are general examples using the pattern as opposed to real life situations or scenarios (like the Coffee example). If it is more useful for you to have a more in-depth walk through of a general example, then using the "Decorator Pattern by Example" page may be more helpful <ref name="dpexample"> http://www.agiledeveloper.com/articles/DecoratorByExample.pdf - ? </ref>
Some sites use over-arching real life examples to explain the Decorator pattern and its usage. The most popular of these examples (in any language) is the Coffee class mentioned previously, however there are other examples based on real life scenarios and situations that can relay the same information. One that seems to be useful in conveying the idea behind the Decorator pattern is using the pattern to report sports statistics for a given team. One site uses the example of a developer hired to create software that reports Soccer sports statistics <ref name="soccer">http://blog.image0.com/c-2/design-patterns-series-the-decorator-pattern-with-a-real-world-soccer-example/ - 2012 </ref>. This site uses a mix of textual explanations and examples to explain the pattern. One advantage to using this site is that it is relatively short and includes a real world example. It also provides the code in a way that would make it simple to attempt to run it yourself and see how it works. A disadvantage for this resource is that it does not break down exactly how it got all of its code so it may be difficult to understand how to get going on your own using your own "problem".
Although most examples for this pattern focus on other usages, it is possible to use the Decorator pattern with GUIs (Graphical User Interfaces). A useful example for understanding this usage can be found on OODesign.com <ref name="gui">http://www.oodesign.com/decorator-pattern-gui-example-java-sourcecode.html - ?</ref>. This site uses code and UML diagrams to explain the usage of Decorator when implementing GUIs. There are small textual explanations in between code snippets but the bulk of the page is code. This site is useful because it offers code that can easily be tested and/or modified if needed to gain a better understanding. Another advantage is that there is not much that has to be read. A disadvantage is that if the code given is not similar to what you are trying to achieve it may be difficult to understand whether you need to be using the Decorator pattern for you particular GUI; outside sources may be needed for this understanding.
Summary
For Java and C# resources, we recommend:
Directory
References
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