Design Patterns

What they are?

• General, reusable solutions to commonly occurring problems in software design.
• A description or template (not as in the Standard Template Library) for how to solve a problem that can be used in many different situations.
• Ways of organizing code to increase efficiency and provide greater Object Oriented Design.

What they are not?

• They are not a finished design that can be transformed directly into code. Design patterns exist independently of any particular implementation.
• Algorithms are not design patterns because they (algorithms) solve computational problems not design problems.
• They are not trivial.
• They are not represented by any features that are built into a programming language, e.g. templates are not design patterns.

The definitive reference on Design Patterns

How important are Design Patterns?

• Design Patterns are not something you can just know where to go look up. They must be part of your working knowledge. That is they must be in your brain where you can use them when designing software.
• Those who want to become really good software engineers will take heed and proceed to learn them.

A brief history

• 1977-1979 - Originated as an architectural (as in building design) concept by Christopher Alexander.
• 1987 - In 1987 Kent Beck and Ward Cunningham began experimenting with the idea of applying patterns to programming. They presented their results at the Object-Oriented Programming, Systems, Languages, and Applications (OOPSLA) Conference that year.
• 1987-1994 - The ideas gained in popularity over the next several years.
• 1994 - The GoF published the Design Patterns book.
• 1994 - The same year the first Pattern Languages of Programming confrence was held.
• 1995 - The Portland Pattern Repository was set up to document design patterns.

Advantages of Design Patterns

• Can speed up the development process by providing testing, proven development paradigms.
• Effective software design requires considering issues that may not become visible until later in the implementation. Reusing design patterns helps to prevent subtle issues that can cause major problems.
• Improves readability of the code for coders and designers who are familiar with the patterns.
• Provides patterns for solving problems on a level of abstraction above mere code reuse. Instead of "code reuse" you get "experience reuse."
• Provides a common language for designers.

Disadvantages of Design Patterns

• In order to achieve flexibility design patterns usually add additional levels of indirection which in some cases may complicate the resulting designs and hurt application performance.

Design Pattern Categories

• Creational Patterns - Focus on how objects are created and fften involves isolating the details of object creation in such a way that you don’t have to make major changes to your code when you have to create a new type of object. Includes: Abstract Factory, Builder, Factory Method, Prototype, and Singleton.
• Structural Patterns - Focus on how objects are interconnected, and attempts to ensure that changes in the system don't require changes in those connections. These patterns are often dictated by project constraints. Includes: Adapter, Bridge, Composite, Decorator, Façade, Flyweight, and Proxy.
• Behavioral Patterns - Focus on objects that handle particular types of behavior in an application, and encapsulate processes such as interpreting a language, fulfilling a request, moving through a sequence (as an iterator), or implementing an algorithm. Includes: Chain of responsibility, Command, Interpreter, Iterator, Mediator, Memento, Observer, State, Strategy, Template method, and Visitor.

On Learning Design Patterns

• Learn the basic organization/functionality/makeup of each pattern. Design patterns go in your brain. If you understand them you can use them in designing code.
• Look for places in your code where you can apply them.
• In other words, you must learn what every one means and how to apply them. It simply is not possible to look them up to see which you could use.

From the Code Guru