Design Patterns added February 21, 2011

Learning a programming language is not the same thing as learning how to build software. It is good to have coding skills, but they are of limited use apart from a knowledge of how to turn software requirements and dreams into functioning systems. Software architecture drives software development, and good software engineering practices enable systematic approaches to development to be maintained for maximized effectiveness.

Sometimes it is impractical to reuse code, but the conceptual building blocks for software systems remain remarkably consistent across a broad range of applications. This book contains a number of patterns designed to make software development more consistent and specific. A consistent pattern language for software engineering provides increased potential for productivity.

These patterns (as summarized in the front cover of the book) are described:

Creational Patterns

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 presentations.

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.

Prototype Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.

Singleton Ensure a class has only one instance, and provide a global point of access to it.

Structural Patterns

Adapter Conver 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 Composite objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of ojbects uniformly.

Decorator Attach additional responsibilities to an object dynamically. 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.

Behavioral Patterns

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 dependently.

Memento Without violating encapsulation, capture and externalize an object's internal state so that teh object can be restored to this state later.

Observer Define a one-to-many dependency between objects so that when one object changes a state, all its dependents are notified and updated automatically.

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 its 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.

While Design Patterns can be tedious at times, it remains one of the most important software engineering texts and is filled with valuable reference material.