design_patterns

Design Patterns

Creational: The process of object creation

Structural: The composition of classes or objects

Behavioral: Characterize the ways in which classes or objects interact and distribute responsability

Class Scope: Deal with relationships between classes and subclasses set at compile-time

Object Scope: Object relationships at run-time

	Creational	Structural	Behavioral
Class	Factory Method	Adapter(class)	Interpreter
...			Template Method
Object	Abstract Factory	Adapter(object)	Chain of Responsibility
...	Builder	Bridge	Command
...	Prototype	Composite	Iterator
...	Singleton	Decorator	Mediator
...		Facade	Memento
...		Flyweight	Observer
...		Proxy	State
...			Strategy
...			Visitor

Design Pattern Catalog

Creational Patterns

Creational design patterns abstract the instantiation process. There purpose is to make the design more flexible - not necessarily smaller.

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 representations

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 creating new objects by copying this prototype

Singleton

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

Structural Patterns

Structural patterns are concerned with how classes and objects are composed to form larger structures. Mostly focused on inheritance to compose interfaces or implementations.

Adapter

Convert the interface of a class into another interface that clients expect. Adapter lets classes work together that couldnt otherwise because of incompatible interfaces

Bridge

Decouple an abstraction from its implementation so that the two can vary independently

Composite

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

Decorator

Attach additional responsabilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extended 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 number of fine-grained objects efficiently

Proxy

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

Behavioral Patterns

Behavioral patterns are concerned with algorithms and the assignment of responsibilities between objects. They are focused with control flow and communication.

Chain of Responsibility

Avoid coupling the sender of a request to its receiver by giving more than one object a change 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 represenation for its grammer, 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 independently

Memento

Without violating encapsulation, capture and externalize an objects internal state so that the object can be restored to this state later

Observer

Define a one-to-many dependency between objects so that when one object changes 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 interchangable. 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 subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithms 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 element on which it operates

Principals of Object Oriented Design

1. Program to an interface, not an implementation

• Class inheritance, in short, is borrowing another objects implementation
• Interface inheritance (or subtyping) when one object can be used in place of another (because the interfaces are the same)

2. Favor object composition over class inheritance

• Inheritance requires an intimate understanding of the other classes internals - white box
• Composition uses each object as a black-box for composing new functionality
  • Components must have well defined interfaces to work
• This leads to better encapsulation

Designing for Change

The key to maximizing reuse lies in anticipating new requirements and changes to existing ones, and designing systems so they can evolve accordingly.