TweetFollow Us on Twitter

Patterns Volume Number: 17 (2001)
Issue Number: 1
Column Tag: Software Engineering

Patterns

By By Paul E. Sevinç, Switzerland

Introduction

Reuse is a key objective of software engineering. A well-known form of reuse is code (i.e., implementation) reuse, as promoted by a procedure library for instance. A domain-specific framework also promotes implementation reuse, and in addition the reuse of analysis (namely of the domain) and design (that led to the architecture of the framework). Capturing and passing on analysis and design experience without the need for code can be achieved by the use of patterns.

In this article, we give definitions of pattern, analysis pattern, design pattern, and architectural pattern. (In a future article on frameworks, we will also discuss the relationship between patterns and frameworks.) We do not, however, recap the history of patterns (see, for instance, Appleton [3]). Nevertheless, note that software-development patterns were inspired by Christopher Alexander who, in the 1970s, developed a pattern language for architecture [2]-the "house-building" discipline, that is, not software or computer architecture!

Pattern

Patterns are schematic, proven solutions to recurring problems. Basically, patterns are characterized by at least a name, a problem description, and a problem solution [15]. A well-known name allows us to concisely refer to a specific pattern. It is certainly easier to refer to "the Adapter pattern" (see the example below) than "the pattern that consists of entities which...". The problem description tells us in what situations the respective pattern is applicable. It includes conditions that must be met before applying the pattern. The problem solution explains how we can solve the problem. It typically does so as abstractly as makes sense for the particular kind of pattern.

"Schematic" refers to the fact that, especially in patterns books, patterns are usually discussed according to some template. (For the sake of brevity, we will not do that in this article.) Different authors use different templates [e.g., 6, 11]. "Recurring" refers to the fact that a problem/solution pair must be observed at least three times to be accepted as a pattern.

Let us look at a simple example, the Adapter pattern. Assume that, in a certain context, we defined the interface of encryption/decryption Java classes to be as shown in Listing 1.

Listing 1

public interface Cipher
{
   void encrypt( int[] plainText, int[] cipherText );
   void decrypt( int[] cipherText, int[] plainText );
}

Now we would like to have a Cipher instance that performs IDEA-encryption and -decryption [14]. For that, we need a concrete class that implements the Cipher interface. We could, of course, develop such a class from scratch. However, a good soul already did most of the hard work by developing a class that realizes IDEA and even made its source code freely available (see Listing 2). We are going to reuse this class.

Listing 2

public class IDEA
{
   // fields
   ...
   
   public IDEA( int[] secretKey )
   {
      ...
   }
   
   public void cipher( boolean flag, int[] source, int[] drain )
   {
      ...
   }
   
   // private methods
   ...
}

One problem remains, though: the type IDEA is not a subtype of Cipher, but Java is strongly typed. And even if Java was not strongly typed, we would still have a problem since the interfaces did not match in the first place.-Enter the Adapter pattern. Instead of forgetting about the IDEA class altogether or of copying and modifying its source code (a highly error-prone approach), the Adapter pattern suggests to develop a class, a subtype of Cipher, that simply forwards requests to IDEA (see Listing 3).

Listing 3

public final class IDEACipher implements Cipher
{
   private final static int keyLength = 16;
   
   private final IDEA adaptee;
   
   public IDEACipher( int[] key ) throws InvalidKeyException
   {
      if ( key.length != keyLength ) {
         throw new InvalidKeyException( "..." );
      }
      
      for ( int i = 0; i < keyLength; ++i ) {
         if ( key[ i ] < 0 || key[ i ] > 255 ) {
            throw new InvalidKeyException( "..." );
         }
      }
      
      adaptee = new IDEA( key );
   }
   
   public void encrypt( int[] plainText, int[] cipherText )
   {
      adaptee.cipher( true, plainText, cipherText );
   }
   
   public void decrypt( int[] cipherText, int[] plainText )
   {
      adaptee.cipher( false, cipherText, plainText );
   }
}

Note that, since IDEA was developed first, we could also have defined IDEACipher as shown in Listing 4.

Listing 4

public final class IDEACipher extends IDEA implements Cipher
{
   // see Listing 3
   ...
   
   public void encrypt( int[] plainText, int[] cipherText )
   {
      super.cipher( true, plainText, cipherText );
   }
   
   public void decrypt( int[] cipherText, int[] plainText )
   {
      super.cipher( false, cipherText, plainText );
   }
}


Figure 1. Object Adapter (adapted from [11])

The Adapter variant of Listing 3 is called the Object Adapter (it connects objects at run time through a reference, see Figure 1). The Adapter variant of Listing 4 is called the Class Adapter (it connects classes at compile time through inheritance, see Figure 2). In Java, we clearly prefer the Object Adapter over the Class Adapter. But in C++, where IDEACipher could have inherited privately from IDEA (and thus not be of type IDEA), the Class Adapter is a viable alternative.


Figure 2. Class Adapter (adapted from [11])

In short: Name: Adapter (also know as Wrapper)

Problem Description: Class (Adaptee) whose implementation cannot be reused (by Client) because its type or at least its interface does not meet certain requirements.

Problem Solution: Develop a class (Adapter) whose type or interface does meet above-mentioned requirements and whose implementation mainly consists of forwarding requests to (and returning replies from) Adaptee in a suitable form.

Architecture and Design

According to Stroustrup [23], software development is an iterative and incremental process basically consisting of analysis, design, and implementation, where the software design steps result in the software architecture. This view of architecture being the result of design is not uncommon. And we adopted it in the first paragraph of the introduction, when we said that framework design leads to the framework architecture.

However, the software-engineering community in general and the patterns community in particular denote by architecture another software-development step (analysis, architecture, design, implementation). We adopted the latter view in the last paragraph of the introduction, when we considered architecture to be a discipline, and for the remainder of this article.

So what is architecture? Just as is the case for components [22, CS99], the software-engineering community is struggling with a definition for architecture [4, 7]. Very, very loosely speaking, architecture is coarse-grained design (higher level), and design is fine-grained architecture (lower level).

Analysis Pattern, Architectural Pattern, and Design Pattern

Many kinds of patterns exist. And more often than not, there are different definitions for the "same" kind of pattern. A prime example is the term "design pattern" which sometimes denotes patterns in general and sometimes a particular class of patterns (as it does in this article).

As a basis for further discussion, we quote five definitions from the literature:

Analysis Pattern

Richter [16, p. 344]:
"An analysis pattern is a way of solving a problem in a particular problem domain."

Architectural Pattern

Richter [16, p. 345]:
"An architectural pattern is a problem-independent way of organizing a system or subsystem. It describes a structure by which the different parts of a system are organized or interact."

Buschmann et al. [6, p. 12]:
"An architectural pattern expresses a fundamental structural organization schema for software systems. It provides a set of predefined subsystems, specifies their responsibilities, and includes rules and guidelines for organizing the relationships between them."

Design Pattern

Richter [16, p. 345]:
"A design pattern is a solution to a small problem that is independent of any problem domain. It represents an attractive solution to a design problem that could occur in any kind of application. The same design pattern can be applied in areas as diverse as order processing, factory control, and meeting room scheduling."

Buschmann et al. [6, p. 13]:
"A design pattern provides a scheme for refining the subsystem or components of a software system, or the relationships between them. It describes a commonly-recurring structure of communicating components that solves a general design problem within a particular context [11]." (Note: here, "component" means software entities in general, not components in the more narrow sense of component-oriented programming [22, CS99].)

Examples

Example analysis patterns can be found in Fowler's book [8] and on his homepage [9].

Probably one of the most successful architectural patterns is the Layers [6, p. 31]:

"The Layers architectural pattern helps to structure applications that can be decomposed into groups of subtasks in which each group of subtasks is at a particular level of abstraction."


Figure 3. Layers [6]

Well-known examples of the Layers pattern are the ISO Open System Interconnection (OSI) model (layers: application, presentation, session, transport, network, data link, and physical) or the Java platform (host operating system, Java run-time environment, Java application). Buschmann et al. [6] and Szyperski [24] discuss several variants of Layers.

We do not need to discuss this pattern any further as you are most certainly very familiar with this fundamental architecture. And even if you are not impressed by the Layers at all, it is an excellent example: Patterns are about sound solutions to realistic problems, not about the most elaborate solution to an exotic problem.

The Adapter pattern is one example of a design pattern. Another one is the Wrapper Facade [20]:

"The Wrapper Facade design pattern encapsulates the functions and data provided by existing non-object-oriented APIs within more concise, robust, portable, maintainable, and cohesive object-oriented class interfaces."


Figure 4. Wrapper Facade [20]

As shown in Figure 4, the intent of the Wrapper Facade is to keep an object-oriented application purely object-oriented even though it relies on a procedure-oriented library. Again, the pattern is about an easy-to-understand solution to an easy-to-understand problem.-Nevertheless, the study of patterns is worthwhile: quite a few patterns discuss not-so-obvious (albeit highly elegant) solutions to non-trivial problems.

Discussion

First, note how Richter emphasizes the domain-specific nature of analysis patterns and the domain-independent nature of architectural and design patterns. (Riehle and Züllighoven [18] make a similar distinction between conceptual patterns and design patterns.) Do not take these definitions too literally, though. As Fowler remarks [10], an analysis pattern from one domain can prove useful in a completely different domain. And sooner or later, you will stumble over deceptively contradictory patterns categories such as "security architectural patterns" or "GUI design patterns". This can either mean that a set of general-purpose patterns has been collected for a particular domain, or that a pattern is domain-specific (e.g., for GUIs) but still general enough to be applicable in different situations within that domain (e.g., for different widgets).

Next, both architectural patterns and design patterns are language independent. (Language dependent patterns are called idioms [6] or programming patterns [18], by the way.) And architectural patterns are also paradigm independent. The layers in Figure 3 could each consist of a set of procedures, or some could consist of classes and some of functions, etc. Design patterns on the other hand are typically object oriented. The Wrapper Facade is a border case.

Finally, rotate Figure 4 by 90 degrees and you will see-the Layers (layers: Application, WrapperFacade, Functions)! It should come to no surprise that design patterns refine architectural patterns. However, sometimes it is difficult to determine which design pattern to apply for the refinement. As we said in the introduction, patterns help capturing and passing on experience, but they do not make experience obsolete.

To Probe Further

A good starting point for pattern-related information on the Web is the patterns home page of the Hillside group [12]. And many authors of the patterns community provide information on their personal home pages (e.g., Riehle [17] or Schmidt [19]).

A must-read is Gamma et al.'s Design Patterns [11]. The publication of this seminal book triggered the intensive search for patterns that continues unabated today. Code examples are given in C++ and Smalltalk, so you may want to take a look at Lalonde's article [13]. We hope that the authors will take the time to publish a second edition. This would allow them to modify the diagrams to be UML compliant and to extend the "Known Uses" sections with examples from the Java platform.

A should-read is Buschmann et al.'s Pattern-Oriented Software Architecture [6] which pioneered architectural patterns. In newer publications, this book is also referred to as POSA 1, because other books with the same main title have been published or are in preparation.

The patterns community has its own conference and publishes the proceedings in the Pattern Language of Program Design series [1].

One book you may want to take a look at as well is Brown et al.'s AntiPatterns [5], some sections of which are pretty good while others cannot keep up with the rest of the book. Whether antipatterns really are a concept of their own or just patterns whose template also includes a bad problem "solution" encountered in practice is debatable.

Alas, now that patterns entered the mainstream, a few books are on the market that only try to cash in on a buzzworld-enabled title.

Acknowledgments

This article is partly based on a chapter in my M.S. thesis [21] that was proof-read and commented on by Prof. Dr. Rachid Guerraoui, Dr. Jean-Philippe Martin-Flatin, Luc Girardin, and Dani Seelhofer.

I would like to thank Dr. Dirk Riehle for proof-reading and commenting on the current version.

References

  • [1] Addison-Wesley. Software Patterns Series. Home Page.
    Located at <http://cseng.aw.com/catalog.taf?ctype=series&seriesid=34>.
  • [2] C. Alexander. "The Origins of Pattern Theory: The Future of the Theory, and the Generation of a Living World". IEEE Software, Vol. 16, No. 5, pp. 71-82, September/October 1999.
  • [3] B. Appleton. "Patterns and Software: Essential Concepts and Terminology". Available at <http://www.enteract.com/~bradapp/docs/>.
  • [4] Bredemeyer Consulting. Resources for Software Architects. Home Page. Located at <http://www.bredemeyer.com/>.
  • [5] W.H. Brown, R.C. Malveau, H.W. McCormick III, and T.J. Mowbray. AntiPatterns: Refactoring Software, Architectures, and Projects in Crisis. John Wiley & Sons, New York 1998.
  • [6] F. Buschmann, R. Meunier, H. Rohnert, P. Sommerlad, and M. Stal. Pattern-Oriented Software Architecture: A System of Patterns. John Wiley & Sons, Chicester, 1996.
  • [7] Carnegie Mellon Software Engineering Institute. Architecture Tradeoff Analysis Initiative. Home Page.
    Located at <http://www.sei.cmu.edu/ata/ata_init.html>.
  • [8] M. Fowler. Analysis Patterns: Reusable Object Models. Addison-Wesley, Reading (Massachusetts), 1997.
  • [9] M. Fowler. Analysis Patterns. Home Page.
    Located at <http://www.martinfowler.com/apsupp/index.html>.
  • [10] M. Fowler with K. Scott. UML Distilled: A Brief Guide to the Standard Object Modeling Language. Addison-Wesely, Reading (Massachusetts), 2nd edition 2000.
  • [11] E. Gamma, R. Helm, R. Johnson, and J. Vlissides. Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesely, Reading (Massachusetts), 1995.
  • [12] Hillside Group. Patterns Home Page. Home Page.
    Located at <http://hillside.net/patterns/>.
  • [13] W. Lalonde. "I can read C++ and Java, But I Can't Read Smalltalk". JOOP, Vol. 12, No. 9, pp. 40-45, February 2000.
  • [14] A. Menezes, P. van Oorschot, and S. Vanstone. Handbook of Applied Cryptography. CRC Press, New York, 1997.
  • [15] H. Mössenböck. Objektorientierte Programmierung in Oberon-2. Springer, Heidelberg, 3rd edition 1998.
  • [16] C. Richter. Designing Flexible Object-Oriented Systems with UML. Macmillan Technical Publishing, Indianapolis, 1999.
  • [17] D. Riehle. Dirk's Home Page. Home Page.
    Located at <http://www.riehle.org/>.
  • [18] D. Riehle and H. Züllighoven. ìUnderstanding and Using Patterns in Software Development". Theory and Practice of Object Systems, Vol. 2, Nr. 1, pp. 3-13, 1996.
  • [19] D.C. Schmidt. Douglas C. Schmidt's Welcome Page. Home Page.
    Located at <http://www.cs.wustl.edu/~schmidt/>.
  • [20] D.C. Schmidt, M. Stal, H. Rohnert, and F. Buschmann. Pattern-Oriented Software Architecture: Patterns for Concurrent and Networked Objects. John Wiley & Sons, Chicester, 2000.
  • [21 P.E. Sevinç. Design Patterns for the Management of IP Networks. M.S. Thesis, Swiss Federal Institute of Technology Zurich, February 2000.
  • [22] Software Development Magazine. Beyond Objects. Column by B. Meyer, B. Powel Douglas, and C. Szyperski.
    Available at <http://www.sdmagazine.com/uml/beyondobjects/>.
  • [23] B. Stroustrup. The C++ Programming Language. Addison-Wesley, Reading (Massachusetts), 3rd edition 1997.
  • [24] C. Szyperski. Component Software: Beyond Object-Oriented Programming. Addison-Wesely, Reading (Massachusetts), 1998.

Paul E. Sevinç currently works as a software engineer for Switzerland-based Teamup AG. From January 2001 on, he will work for Trilogy Software, Inc. in Austin (Texas) and Paris (France). You can reach him at paul.sevinc@ubilab.org.

 
AAPL
$111.83
Apple Inc.
+2.42
MSFT
$47.25
Microsoft Corpora
+1.51
GOOG
$506.89
Google Inc.
+2.00

MacTech Search:
Community Search:

Software Updates via MacUpdate

BBEdit 11.0.2 - Powerful text and HTML e...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more
ExpanDrive 4.2.1 - Access cloud storage...
ExpanDrive builds cloud storage in every application, acts just like a USB drive plugged into your Mac. With ExpanDrive, you can securely access any remote file server directly from the Finder or... Read more
Adobe After Effects CC 2014 13.2 - Creat...
After Effects CC 2014 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous After Effects customer). After Effects CS6 is still available... Read more
Command-C 1.1.7 - Clipboard sharing tool...
Command-C is a revolutionary app which makes easy to share your clipboard between iOS and OS X using your local WiFi network, even if the app is not currently opened. Copy anything (text, pictures,... Read more
Tidy Up 4.0.2 - Find duplicate files and...
Tidy Up is a complete duplicate finder and disk-tidiness utility. With Tidy Up you can search for duplicate files and packages by the owner application, content, type, creator, extension, time... Read more
Typinator 6.3 - Speedy and reliable text...
Typinator turbo-charges your typing productivity. Type a little. Typinator does the rest. We've all faced projects that require repetitive typing tasks. With Typinator, you can store commonly used... Read more
GraphicConverter 9.5 - Graphics editor w...
GraphicConverter is an all-purpose image-editing program that can import 200 different graphic-based formats, edit the image, and export it to any of 80 available file formats. The high-end editing... Read more
Toast Titanium 12.0.1 - The ultimate med...
Toast Titanium goes way beyond the very basic burning in the Mac OS and iLife software, and sets the standard for burning CDs, DVDs, and now Blu-ray discs on the Mac. Create superior sounding audio... Read more
QuickBooks 2015 16.0.2.1422 R3 - Financi...
Save 20% on QuickBooks Pro for Mac today through this special discount link QuickBooks Pro 2013 helps you manage your business easily and efficiently. Organize your finances all in one place, track... Read more
Remotix 3.0.6 - Access all your computer...
Remotix is a fast and powerful application to easily access multiple Macs (and PCs) from your own Mac. Features: Complete Apple Screen Sharing support - including Mac OS X login, clipboard... Read more

Latest Forum Discussions

See All

Mahjong Journey Review
Mahjong Journey Review By Jennifer Allen on December 18th, 2014 Our Rating: :: STEADY MATCHINGiPad Only App - Designed for the iPad Aimed at the more laid back gamer, Mahjong Journey isn’t for everyone, but those looking for some... | Read more »
Emoji Type - custom keyboard with predic...
Emoji Type - custom keyboard with predictive emojis 0.4.0 Device: iOS iPhone Category: Utilities Price: $.99, Version: 0.4.0 (iTunes) Description: Emoji Type is custom keyboard for iOS 8 that auto suggests emojis as you type. ABOUT... | Read more »
Game of the Year 2014 – 148Apps Staff Pi...
The end of 2014 is almost here, which can only mean one thing. Okay it can mean a lot of things, but in this specific context it means Game of the Year lists! Which is why the 148Apps staff have all picked their favorites from the past year. And why... | Read more »
UponPixels Review
UponPixels Review By Jennifer Allen on December 18th, 2014 Our Rating: :: CREATIVE TYPOGRAPHYUniversal App - Designed for iPhone and iPad Add cool typography and objects to your photos with the easy to use UponPixels.   | Read more »
The Vikings are Coming! CastleStorm’s Ne...
The Vikings are Coming! CastleStorm’s New Update Adds a Survival Mode Posted by Jessica Fisher on December 18th, 2014 [ permalink ] | Read more »
Duet Display (Productivity)
Duet Display 0.3.3 Device: iOS Universal Category: Productivity Price: $9.99, Version: 0.3.3 (iTunes) Description: Duet Display allows you to use your iPad or iPhone as an extra display. Developed by a team of ex-Apple engineers,... | Read more »
Dragon Quest III Review
Dragon Quest III Review By Jennifer Allen on December 18th, 2014 Our Rating: :: DATED BUT NOT WITHOUT MERITUniversal App - Designed for iPhone and iPad A walk down memory lane isn’t foolproof for Dragon Quest III, but it has its... | Read more »
8 KEMCO JRPGs Are Just $0.99 in Celebrat...
8 KEMCO JRPGs Are Just $0.99 in Celebration of the Holidays Posted by Jessica Fisher on December 18th, 2014 [ permalink ] KEMCO has announc | Read more »
Brothers in Arms 3: Sons of War Review
Brothers in Arms 3: Sons of War Review By Jennifer Allen on December 18th, 2014 Our Rating: :: FUN BUT PUSHYUniversal App - Designed for iPhone and iPad Brothers in Arms 3: Sons of War could be great fun, but its plethora of... | Read more »
The Snow is Falling on the French Countr...
The Snow is Falling on the French Countryside of Carcassonne’s Newest Update Posted by Jessica Fisher on December 18th, 2014 [ permalink ] < | Read more »

Price Scanner via MacPrices.net

Save up to $300 on Macs, $30 on iPads with Ap...
Purchase a new Mac or iPad at The Apple Store for Education and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free,... Read more
iOS and Android OS Targeted by Man-in-the-Mid...
Cloud services security provider Akamai Technologies, Inc. has released, through the company’s Prolexic Security Engineering & Research Team (PLXsert), a new cybersecurity threat advisory. The... Read more
KMI MIDI K-Board Great Gift for Amateur &...
The K-Board is a MIDI Nano keyboard for music creation for iPad, Android, And computers; the easiest way to make music with iPads & Android tablets, and Mac, Windows, or Linux computers. Ultra-... Read more
Amazon offers 15-inch 2.2GHz Retina MacBook P...
 Amazon.com has the 15″ 2.2GHz Retina MacBook Pro on sale for $1699 including free shipping. Their price is $300 off MSRP. Stock is limited, so act now if you’re interested. Read more
Holiday sales continue: MacBook Pros for up t...
 B&H Photo has new MacBook Pros on sale for up to $300 off MSRP as part of their Holiday pricing. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $1699... Read more
Google Search App For iOS Gets A Major Makeov...
Google has given iOS users an early Christmas present with a substantial update of it’s not-very-often-upgraded Google Search app. Google Search has been my go-to tool for Web searches since it was... Read more
ShopKeep Apple Pay And Chip Card Reader Avail...
ShopKeep, a cloud-based technology provider to more than 10,000 small business owners to manage retail shops and restaurants with iPads, has released its new Apple Pay and chip card reader. This... Read more
Holiday sale! 27-inch 5K iMac for $2299, save...
 B&H Photo has the 27″ 3.5GHz 5K iMac in stock today and on sale for $2299 including free shipping plus NY sales tax only. Their price is $200 off MSRP, and it’s the lowest price available for... Read more
Holiday Sale! 3.7GHz Quad Core Mac Pro availa...
 B&H Photo has the 3.7GHz Quad Core Mac Pro on sale for $2599 including free shipping plus NY sales tax only. Their price is $400 off MSRP, and it’s the lowest price for this model from any... Read more
iPhone 6 Number 3 Canadian Google Search Of 2...
CTVNews.ca reports that Apple’s iPhone 6 was the third highest-trending Google Canada search topic of 2014, exceeded only by Robin Williams largely after his death by suicide in August, and the FIFA... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.