#Creational Patterns

Singleton

• Concepts
  • Only one instance created
  • Guarantees control of a resource
  • Lazily loaded
  • Examples:
    • Runtime
    • Logger
    • Spring Beans
    • Graphics Managers

Class is responsible for creating itself and managing its lifecycle Static in nature Needs to be thread safe Private instance Private constructor No parameters required for construction

Pitfalls

• Often overused
  • Don't expose an interface and have private constructors, as well as private member variables they can be difficult to unit test.
• Difficult to unit test
• If not careful, not thread-safe
• Sometimes confused for Factory
  • If it needs an argument in that method, it is a factory.

Builder

• Concepts
  • Handles complex constructors
  • Large number of parameters
  • Force immutability once created
  • Works with legacy code
  • Examples:
    • StringBuilder
    • DocumentBuilder
    • Locale.Builder

Telescoping Constructors - Creation of multiple constructors wiht each parameter variation

• Flexibility over telescoping constructors
• Static inner class
• Calls appropriate constructor
• Negates need for exposed setters
• Can take advantage of generics

Prototype

• Implemented around a clone

• Avoids calling complex constructors

• Difficult to implement in legacy code

• Concepts

  • Avoids costly construction
  • Avoids subclassing
  • Typically does not use "new"
  • Often utilizes an Interface
  • Usually implemented in a Registry
  • Example:
    • java.lang.Object#clone()

• Pitfalls

  • Somewhat not clear when to use
    • Application creates a lot of objects
  • Used with other patterns
    • Registry
  • Shallow VS Deep Copy

Factory

• Returns various instances

  • Multiple constructors

• Interface driven

• Adaptable to environment more easily

• Concepts

  • Does not expose instantiation logic
  • Defer to subclass
  • Common interface
  • Specified by architecture, implemented by user
  • Examples:
    • Calendar
    • ResourceBundle
    • NumberFormat

• Responsible for lifecycle

• Common Interface

• Concrete Classes

• Parameterized create method

• Pitfalls

  • Complexity
  • Creation in subclass
  • Refactoring (Need to design at beginning and plan accordingly)

Abstract Factory

• Concepts

  • Factory of Factories
  • Factory of related objects
  • Common Interface
  • Defer to Subclasses
  • Examples:
    • DocumentBuilder
    • Frameworks

• Pitfalls

  • Complexity
  • Runtime switch
  • Pattern within a pattern
  • Problem specific
  • Starts as a Factory