Friday, April 29, 2011

Chapter 51: Introduction to Design Patterns

Design Patterns are a very important topic from the SCWCD Exam perspective as well as from being a J2EE Programmer perspective. Any experienced J2EE programmer is expected to know about a few of the J2EE Design Patterns. These patterns are used extensively and are very useful to create enterprise class J2EE Web Applications. In this next few chapters, we are going to look in detail, some of the most commonly used Design Patters (Of Course, only the ones that are part of the SCWCD Exam)

This chapter, is going to be a short and sweet introduction to the J2EE Design Patterns.

So, lets get started!!!

A word of Caution: This chapter assumes you've read the previous chapters (well, obviously) and that you are familiar with object-oriented programming concepts, including the popular UML diagramming notation.

History of Design Patterns

The term “pattern” comes from the architect Christopher Alexander who wrote several books on Construction and Building. Although he was interested in urban planning and building architecture, his notions were clearly useful in many areas. Alexander, a building architect, was the first to “codify” patterns for architecture. Alexander didn't invent the idea of patterns nor was he the first to see them in designs. Of course, they were there before him, but his work on urban planning and building architecture isolated the idea better than anyone before him. Alexander was an architect extraordinaire. He is one of those rare people who take a step back and question why people do things the way they do. He gave the result of his inquiry the name Pattern Language and defined it at length in his seminal book by the same name.

While Alexander was thinking buildings and foundations, it became clear to many that his design patterns were abstract enough to be useful elsewhere. That is when the Gang of Four (GoF as we will refer to them henceforth), as Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides are now known, applied his patterns to software in their classic Design Patterns: Elements of Reusable Object-Oriented Software (1995, Addison-Wesley). It took a while, but the GoF have started a ground swell. There are now dozens of books, and numerous websites in the internet, about design patterns.

What is a Design Pattern?

Design patterns are often defined as “a solution to a problem in a context.” This falls short of the abstract definition that experts prefer.

Sun defines them in the following way: “A design pattern describes a proven solution to a recurring design problem, placing particular emphasis on the context and forces surrounding the problem, and the consequences and impact of the solution.”

Design Patterns are everywhere and are used (knowingly or unknowingly) by almost all J2EE Developers. Don’t worry my friend, you will be using them too and by the time you are done reading the next few chapters, you will be a Design Pattern Expert.

Previous Chapter: Self Test - Chapters 47 to 50

Next Chapter: Chapter 52 - Design Pattern Elements

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