Design Pattern Notes

Pattern Organization

Classifications

• Creational
• Structural
• Behavioral
• Security
• Concurrency
• Sql
• User Interface
• Relational
• Social
• Distributed

Creational Patterns:

• Abstract Factory
• Builder
• Factory Method
• Prototype
• Singleton
• Lazy Instantiation
• Utility Pattern

Definition: How to instantiate objects at runtime

Factory Patterns Module

Consider using when:

• Unsure of which concrete implementation of an interface I want to return
• Creation should be separated from representation of an object
• Lots of if/else blocks when deciding which concrete class to create
• Switch statements when deciding which concrete class to create

Intent:

• Separate object creation (instantiation) from the decision of which object to create
• Add new classes and functionality without breaking the O in SOLID
  • Factory-produced objects
  • Factories themselves
• Store which object to create outside of the program
  • In a database
  • In a configuration file

Simple Factory

• Encapsulate object creation
• Allows for late-bound decisions regarding instantiation
  • configuration-based
  • other persistent storage
  • input or other dynamic data
• Caller does know what concrete factory it needs

Factory Method

• "Define an interface for creating an object, but let the subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses."
• Adds an interface to the factory itself from Simple Factory
• Defers object creation to multiple factories that share an interface
• Derived classes implement or override the factory method of the base

Advantages:

• Eliminate references to concrete classes
  • Factories
  • Objects created by Factories
• Factories can be inherited to provide even more specialized object creation
• Rules for object initialization is centralized

Disadvantages:

• May need to create a factory just to get a concrete class delivered
• The inheritance hierarchy gets deeper with coupling between concrete factories and created classes

Abstract Factory

• "Provide an interface for creating families of related or dependent objects without specifying their concrete classes"
• Factories create different types of concrete objects (products)
• A Factory now represents a "family" of objects that it can create
• Factories may have more than one factory method

Null Object Module

Null Object Pattern

Intent:

• Rid program logic of null checks (guard clauses) where possible
• Provide a non-functional object in place of a null reference
• Allow methods to be called on Null objects, unlike a null reference

Objections/Criticism:

• Can make bugs/errors appear as normal program execution
• In most cases no compiler error would occur when the Null Object is a reference type; could lead to run time errors instead of compile time errors
• Some overhead involved in checking for Null Object versus checking for null(nil)
• Must be used anywhere in the codebase where null is assigned currently -- otherwise there isn't a strong guarantee that the Null Object(s) will always be returned

Structural Patterns

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Behavioral Patterns

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Security Patterns

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Concurrency Patterns

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Sql Patterns

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User Interface Patterns

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Relational Patterns

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Social Patterns

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Distributed Patterns

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