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.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Vitamin-R 2.45 - Personal productivity t...
Vitamin-R creates the optimal conditions for your brain to work at its best by structuring your work into short bursts of distraction-free, highly focused activity alternating with opportunities for... Read more
BetterTouchTool 2.08 - Customize Multi-T...
BetterTouchTool adds many new, fully customizable gestures to the Magic Mouse, Multi-Touch MacBook trackpad, and Magic Trackpad. These gestures are customizable: Magic Mouse: Pinch in / out (zoom... Read more
NeoOffice 2015.12 - Mac-tailored, OpenOf...
NeoOffice is a complete office suite for OS X. With NeoOffice, users can view, edit, and save OpenOffice documents, PDF files, and most Microsoft Word, Excel, and PowerPoint documents. NeoOffice 3.x... Read more
Together 3.7.5 - Store and organize all...
Together helps you organize your Mac, giving you the ability to store, edit and preview your files in a single clean, uncluttered interface. Features Smart storage. With simple drag-and-drop... Read more
Cocktail 10.3 - General maintenance and...
Cocktail is a general purpose utility for macOS that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
Stylizer 6.17.228.1062 - Real-time CSS e...
Stylizer helps you style websites in a fraction of the time. Features Real-time CSS - Immediate feedback saves countless hours. Say goodbye to the reload button! In Stylizer, your changes are... Read more
Simon 4.2.2 - $99.00
Simon monitors websites and alerts you of crashes and changes. Select pages to monitor, choose your alert options, and customize your settings. Simon does the rest. Keep a watchful eye on your... Read more
iFFmpeg 6.2.6 - Convert multimedia files...
iFFmpeg is a comprehensive media tool to convert movie, audio and media files between formats. The FFmpeg command line instructions can be very hard to master/understand, so iFFmpeg does all the hard... Read more
Dropbox 20.4.19 - Cloud backup and synch...
Dropbox is an application that creates a special Finder folder that automatically syncs online and between your computers. It allows you to both backup files and keep them up-to-date between systems... Read more
DiskCatalogMaker 6.6 - Catalog your disk...
DiskCatalogMaker is a simple disk management tool which catalogs disks. Simple, light-weight, and fast Finder-like intuitive look and feel Super-fast search algorithm Can compress catalog data for... Read more

Tavern Guardians Guide: How to survive e...
Tavern Guardians is a match-three game that brings some much-needed innovation to what is a tired genre in mobile gaming. Players match objects to attack enemies while healing themselves, forcing players to rethink their approach to the typical... | Read more »
Inkle's next big project is Heaven...
Inkle's latest project could be its most exciting yet, following the widespread success of their past two games, Sorcery! and 80 Days. Their latest title, Heaven's Vault, which was announced at GDC this week, steps away from the gamebook genre,... | Read more »
Last week on Pocket Gamer
If you’re wondering what’s going on in the wider world of portable gaming, our sister site PocketGamer has you covered. Each week we like to check in on the PG team and see what they’ve been preoccupied with. From the latest on the Nintendo Switch... | Read more »
Mudd Masher arrives this week
Atooi Games, the minds behind Totes the Goat and Mutant Mudds, have a new game in the works -- Mudd Masher. The game, a hybrid of the independent studio's first two titles, is expected to launch this week on March 2. [Read more] | Read more »
The best sales on the App Store this wee...
The App Store has quite an exciting lineup of discount games this week that range across a variety of genres. It's a great opportunity to catch up on some of the premium games you may have been holding off on -- and some you can even grab for free... | Read more »
The best new games we played this week
Ah, here we are again at the close of another busy week. Don't rest too easy, though. We had a lot of great new releases in mobile games this week, and now you're going to have to spend all weekend playing them. That shouldn't be too much of a... | Read more »
Rollercoaster Tycoon Touch Guide: How to...
| Read more »
Rabbids Crazy Rush Guide: How to unlock...
The Rabbids are back in a new endless running adventure, Rabbids Crazy Rush. It's more ridiculous cartoon craziness as you help the little furballs gather enough fuel (soda) to get to the moon. Sure, it's a silly idea, but everyone has dreams --... | Read more »
Tavern Guardians (Games)
Tavern Guardians 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Tavern Guardians is a Hack-and-Slash action game played in the style of a match-three. You can experience high pace action... | Read more »
Slay your way to glory in idle RPG Endle...
It’s a golden age for idle games on the mobile market, and those addictive little clickers have a new best friend. South Korean developer Ekkorr released Endless Frontier last year, and players have been idling away the hours in the company of its... | Read more »

Price Scanner via MacPrices.net

Back in stock: Apple refurbished 13-inch Reti...
Apple has Certified Refurbished 2015 13″ Retina MacBook Pros available for up to $360 off original MSRP, starting at $1099. An Apple one-year warranty is included with each model, and shipping is... Read more
“Sealed” Apple Notebooks Demand More Battery...
In most aspects, my two MacBook Airs (one a mid-2013 revision and the other the current early-2015 refresh spec.) are the best computers I’ve owned over the past 25 years. The only major... Read more
Intro to Real Life Now Available On The iBook...
Just in time for Lent, Washington based Duccats has released Intro to Real Life, a new iBook with an accessible photo-based introduction to the Christian worldview. Intro to Real Life fills a... Read more
13-inch 2.5GHz MacBook Pro (Apple refurbished...
Apple has Certified Refurbished 13″ 2.5GHz MacBook Pros (MD101LL/A) available for $829, or $270 off original MSRP. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.5GHz MacBook... Read more
Save $150-$180 with Apple refurbished 13-inch...
Apple has Certified Refurbished 2016 13″ MacBook Airs available starting at $849. An Apple one-year warranty is included with each MacBook, and shipping is free: - 13″ 1.6GHz/8GB/128GB MacBook Air: $... Read more
Apple’s Education discount saves up to $300 o...
Purchase a new Mac or iPad using Apple’s Education Store 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
SanDisk Introduces 256GB Storage Expansion Fo...
Western Digital Corporation at Mobile World Congress in Barcelona, Spain on Tuesday announced that it has increased the capacity of its line of iOS mobile flash drives with the introduction of its... Read more
FINSiX DART World’s Smallest and Lightest Lap...
Up to 4x smaller and lighter than today’s typical 65W laptop chargers, and with a built-in Smallest and Lightest Laptop Charger, DART allows you to charge all your devices from a single outlet.... Read more
New iPads May Not Arrive Until May or June –...
Digitimes’ Siu Han and Steve Shen say their sources in the iPad upstream supply chain are anticipating disappointing calendar first-quarter 2017 sales. The same sources at upstream suppliers had... Read more
Olay Unveils Skin Advisor App To Help Women B...
Olay celebrated its Mobile World Congress debut with the global launch of Olay Skin Advisor, a new app designed to help women better understand their skin and find the products best-suited to their... Read more

Jobs Board

*Apple* Retail - Multiple Positions - Apple,...
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* Solutions Consultant - Apple (United...
# Apple Solutions Consultant Job Number: 55676865 Los Angeles, California, United States Posted: Feb. 22, 2017 Weekly Hours: 40.00 **Job Summary** As an Apple Read more
Programmer/Editor *Apple* Music Dance - App...
# Programmer/Editor Apple Music Dance Job Number: 55565967 Culver City, California, United States Posted: Feb. 23, 2017 Weekly Hours: **Job Summary** Apple Music Read more
Digital Marketing Specialist - *Apple* iClo...
# Digital Marketing Specialist - Apple iCloud Job Number: 54729233 Culver City, California, United States Posted: Feb. 22, 2017 Weekly Hours: 40.00 **Job Summary** Read more
Marketing Specialist, iTunes & *Apple*...
# Marketing Specialist, iTunes & Apple Music Job Number: 55704205 Culver City, California, United States Posted: Feb. 23, 2017 Weekly Hours: 40.00 **Job Summary** Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.