Design Pattern

• Design Patterns Outline
  • Creational Patterns
  • Structural Patterns
  • Behavioral Design Patterns
• Factory Method Design Pattern
  • Implementation
  • When to use?
  • Design Principle
  • Simple factory

Design Patterns Outline

《Design Patterns elements of reusable object-oriented software》——A bible for object-oriented design patterns

writtern by Gang of four: Erich Gamma, John Vlissides, Ralph Johnson and Richard Helm. Published Oct 21, 1994

Design Patterns are devided into three categories：creational, structural and beahavioral, and consist of 23 different design pattern.

Creational Patterns

Deals with object creation. Complexity and coupling associated with new object creation and address it.

• Abstract Factory
• Factory
• Builder
• Singleton
• Object Pool
• Prototype

Structural Patterns

About class and object composition. Use of inheritance and associations to compose functionality

• Adapter
• Bridge
• Composite
• Decorator
• Facade
• Flyweight
• Private Class Data
• Proxy

Behavioral Design Patterns

• Chain Of Responsibility
• Command
• Interpreter
• Iterator
• Mediator
• Memento
• Null Object
• Observer
• State
• Strategy
• Template Method
• Vistor

Factory Method Design Pattern

Creational Pattern Define an interface for creating an object, but let subclasses decide which object to instantiate. Factory Method lets a class defer instantiation to subclasses.

Implementation

• Provide an interface for creating an object - Here do not consider the word interface as Java interface. It simply means to provide a constract/method to create an object.
• Let the subclass decide what exact object to instantiate - This pattern involves “inheritance”
• Open-closed principle(OCP) - it states a class should be open for extension and close for modifications.

When to use?

• If there is an inheritance hierarchy, where a polymorphic creation is needed.
• When you are modifying a class to accommodate varying requirements(violating open-closed principle)

Design Principle

• Depend on abstractions - Do not depend on concrete class.
• The dependency inversion principle.

Simple factory

• Providing a static creational method. It is static factory, does not cater to factory method design pattern defined by GoF. ex: Calendar.getInstance()

To be continued…