Design pattern Quick Guide

Observer Pattern :  Behavioral pattern

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

Real Life example:

• RSS feeds.  When I want to get updates from a particular feed, I add it to my feed reader. Any time that the RSS feed has an update, it will appear in my reader automatically.  This is the Observer pattern in action, a publisher/subscriber relationship with one source having many subscribers 
• The whole concept of listeners is based on this pattern.

Adapter Pattern : Structural pattern


Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.

Real Life Example:

• The best example for the adapter pattern is based around AC power adapters.

Downside:
Adapter pattern is just a fix for a badly designed system, which didn't consider all possibilties. While this is a fair point, it is an important part of a pluggable architecture.  It can also add a level of complexity to your code, making debugging more difficult.

The Facade pattern  : Structural pattern

Provide a unified interface to a set of interfaces in a subsystem. Façade defines a higher-level interface that makes the subsystem easier to use.

Real Life Example:

• Concept behind facade is to simplify an interface, service oriented architectures make use of the facade pattern. For example, in web services, one web service might provide access to a number of smaller services that have been hidden from the caller by the facade.

Downside:
 By introducing the Facade into your code, you will be hardwiring subsystems into the Facade. This is fine if the subsystem never changes, but if it does, your Facade could be broken. Therefore, developers working on the subsystem should be made aware of any Facade around their code.

Factory method pattern : Creational pattern

Define an interface for creating an object, but let the subclasses decide which class to instantiate. The Factory method lets a class defer instantiation to subclasses

Real Life Example:

• The idea behind the Factory Method pattern is that it allows for the case where a client doesn't know what concrete classes it will be required to create at run-time, but just wants to get a class that will do the job. The FactoryMethod builds on the concept of a simple Factory, but lets the subclasses decide which implementation of the concrete class to use.  You'll see factories used in logging frameworks, and in a lot of scenarios where the client doesn't need to know about the concrete implementations. It's a good approach to encapsulation.

Abstract Factory Pattern : Creational pattern
Provides an interface for creating families of related or dependent objects without specifying their concrete classes.

Strategy Pattern : Behavioral pattern
Defines a set of encapsulated algorithms that can be swapped to carry out a specific behaviour

Real Life Example:

• Compression algorithm

Visitor Pattern : Behavioral pattern
Allows for one or more operation to be applied to a set of objects at runtime, decoupling the operations from the object structure.