What is a design pattern?

A design pattern is a proven design solution to a common problem faced by software developers. Design patterns became popular with the rise of object oriented analysis and design (OOAD). Design patterns are designed to help developers deliver higher quality, more easily maintained software products in less time and at lower cost.

Design patterns are:

• encapsulated - They embody design knowledge regarding collaboration of classes and objects, distribution of responsibility, and other design issues.
• object-oriented - They incorporate OOAD principles—e.g., low coupling, high cohesion.
• reusable - They are adaptable, flexible, general solutions to classes of problems with broad applicability. Design patterns simplify the task facing the developer.

History

Patterns originated as an architectural concept by Christopher Alexander (1977/79). In 1987, Kent Beck and Ward Cunningham began experimenting with the idea of applying patterns to programming and presented their results at the OOPSLA conference that year. In the following years, Beck, Cunningham and others followed up on this work.

Design patterns gained popularity in computer science after the book Design Patterns: Elements of Reusable Object-Oriented Software was published in 1994 by the so-called "Gang of Four" (Gamma et al.). That same year, the first Pattern Languages of Programming Conference was held and the following year, the Portland Pattern Repository was set up for documentation of design patterns. The scope of the term remains a matter of dispute. Notable books in the design pattern genre include:

• Gamma, Erich; Richard Helm, Ralph Johnson, and John Vlissides (1995). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley. ISBN 0-201-63361-2. 
• Schmidt, Douglas C.; Michael Stal, Hans Rohnert, Frank Buschmann (2000). Pattern-Oriented Software Architecture, Volume 2: Patterns for Concurrent and Networked Objects. John Wiley & Sons. ISBN 0-471-60695-2. 
• Fowler, Martin (2002). Patterns of Enterprise Application Architecture. Addison-Wesley. ISBN 978-0321127426. 
• Hohpe, Gregor; Bobby Woolf (2003). Enterprise Integration Patterns: Designing, Building, and Deploying Messaging Solutions. Addison-Wesley. ISBN 0-321-20068-3. 

Although the practical application of design patterns is a phenomenon, formalization of the concept of a design pattern languished for several years.

How are design patterns classified?

Design patterns were originally grouped into the categories Creational patterns, Structural patterns, and Behavioral patterns, and described using the concepts of delegation, aggregation, and consultation. For further background on object-oriented design, see coupling and cohesion. For further background on object-oriented programming, see inheritance, interface, and polymorphism. Another classification has also introduced the notion of architectural design pattern that may be applied at the architecture level of the software such as the Model-View-Controller pattern.

• Creational design patterns - address the object creation process. Creational design patterns encapsulate knowledge about how, when, and by whom an instance is created.
• Abstract factory Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
• Builder Separate the construction of a complex object from its representation so that the same construction process can create different representations.
• Connection pool Avoid expensive multiple connect/disconnect operations.
• Factory method Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.
• Lazy initialization Tactic of delaying the creation of an object, the calculation of a value, or some other expensive process until the first time it is needed.
• Multiton Ensure a class has only named instances, and provide global point of access to them.
• Object pool Avoid expensive acquisition and release of resources by recycling objects that are no longer in use
• Prototype Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.
• Resource acquisition is initialization Ensure that resources are properly released by tying them to the lifespan of suitable objects.
• Singleton Ensure a class has only one instance, and provide a global point of access to it.
• Behavioral design patterns - characterize the manner of class and object interaction and how responsibilities are distributed among them.
• Blackboard Generalized observer, which allows multiple readers and writers. Communicates information system-wide.
• Chain of responsibility Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.
• Command Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
• Interpreter Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.
• Iterator Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
• Mediator Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.
• Memento Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.
• Null Object designed to act as a default value of an object.
• Observer or Publish/subscribe Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
• Restorer An alternative to the existing Memento pattern.
• Specification Recombinable business logic in a boolean fashion
• State Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
• Strategy Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
• Template method Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure.
• Visitor Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.
• Structural design patterns - address the composition of classes and objects.
• Adapter or Wrapper Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.
• Bridge Decouple an abstraction from its implementation so that the two can vary independently.
• Composite Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
• Decorator Attach additional responsibilities to an object dynamically keeping the same interface. Decorators provide a flexible alternative to subclassing for extending functionality.
• Facade Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.
• Flyweight Use sharing to support large numbers of fine-grained objects efficiently.
• Proxy Provide a surrogate or placeholder for another object to control access to it.

Design patterns vary both in granularity and level of abstraction.

References

• Directory of websites that provide pattern catalogs at hillside.net.
• Ward Cunningham's Portland Pattern Repository.
• Patterns and Anti-Patterns at the Open Directory Project
• PerfectJPattern Open Source Project Design Patterns library that aims to provide full or partial componentized version of all known Patterns in Java.
• Jt J2EE Pattern Oriented Framework
• Printable Design Patterns Quick Reference Cards
• 101 Design Patterns & Tips for Developers
• On Patterns and Pattern Languages by Buschmann, Henney, and Schmidt
• Patterns for Scripted Applications
• Design Patterns Reference at oodesign.com