#Design Patterns

Christopher Alexander says, "Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice".

The design patterns are descriptions of communicating objects and classes that are customized to solve a general design problem in a particular context.

A Pattern is a solution to a problem in a context.

##OO Fundamentals:##

• Abstraction
• Encapsulation
• Polymorphism
• Inheritance

##OO Principles:##

• Encapsulate what varies.
• Program to an interface, not an implementation.
• Favor object composition over class inheritance.
• Strive for loosely coupled designs between objects that interact.
• Classes should be open for extension, but closed for modification.
• Depend upon abstractions. Do not depend upon concrete classes.
• Principle of Least Knowledge - talk only to your immediate friends.(*)
• The Hollywood Principle - Don't call us, we'll call you.
• Single Responsibility - A class should have only one reason to change.

##OO Patterns:##

• Strategy

  Define a family of algorithms, encapsulate each one, and makes them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

• Observer

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

• Decorator

  Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

• 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.

• Abstract Factory

  Provide an interface for creating families of related or dependent objects without specifying their concrete classes.

• Singleton

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

• Command

  Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

• Adapter

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

• 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.

• 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.

• Iterator

  Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

• Composite

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

• State

  Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

• Proxy

  Provide a surrogate or placeholder for another object to control access to it.

###SOLID Design Principles:###

• The Single Responsibility Principle (SRP)

  Every software module should have only one reason to change.

• The Open-Close Principle (OCP)

  Software modules should be closed for modifications but open for extensions.

• The Liskov Substitution Principle (LSP)

  Subclasses should be substitutable for base classes.

• The Interface Segregation Principle (ISP)

  Clients should not be forced to implement interfaces they don’t use.

• The Dependency Inversion Principle (DIP)

  High level modules should not depend upon low level modules. Rather, both should depend upon abstractions.

###Rference:###

1. Head First Design Patterns, O'Reilly
2. Design Patterns: Elements of Reusable Object-Oriented Software, GoF
3. Agile Software Development, Principles, Patterns, and Practices, Robert C. Martin

###Books:###

• Design Patterns: Elements of Reusable Object-Oriented Software

• Head First Design Patterns

###Articles:###

• Object Oriented Design Principles

##Notes:###

*The principle priovides some guidelines:take any object; now from any method in that object, the principle tells us that we should only invoke methods that belong to:

• The object itself
• Objects passed in as a parameter to the method
• Any object the method creates or instantiates
• Any components of the object