Design Patterns

Summary

• History of Patterns
• Patterns Catalog
• Factory Method Pattern
  • Intent
  • Applicability
  • Structure
  • Example
• Strategy Pattern
  • Intent
  • Applicability
  • Structure
• Other Common Patterns
• Choosing a Pattern

History of Patterns

• 1977: Christopher Alexander introduces the idea of patterns
  • “Successful solutions to problems”
• 1987: Ward Cunningham and Kent Beck leverage Alexander’s idea in the context of an OO language
• 1987: Erich Gamma’s dissertation on importance of patterns and how to capture them
• 1992: Jim Coplien’s book on Advanced C++ Programming Styles and Idioms
• Gang of Four: Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides
  • “Design Patterns: Elements of Reusable OO Software”

Patterns Catalog

• Fundamental Patterns
  • Delegation
  • Interface
  • Proxy
• Creational Patterns
  • Abstract Factory
  • Factory Method
  • Lazy initialization
  • Singleton
• Structural Patterns
  • Adapter
  • Bridge
  • Decorator
• Behavioral Patterns
  • Chain of Responsibility
  • Iterator
  • Observer
  • State
  • Strategy
  • Visitor
• Concurrency Patterns
  • Active Object
  • Monitor Object
  • Thread Pool

Factory Method Pattern

Intent

Allows for creating objects without specifying their class, by invoking a factory method (i.e., a method whose main goal is to create class instances)

Applicability

• Class can’t anticipate the type of objects it must create
• Class wants sub-classes to specify the type of objects it creates
• Class needs control over the creation of its objects

Structure

Participants:

• Creator
  • Provides interface for factory method
• Concrete Creator
  • Provides actual method for creating actual object
• Product
  • Object created by the factory method

Example

public class ImageReaderFactory {
  public static ImageReader createImageReader(InputStream is) {
    int imageType = getImageType(is);
    
    switch (imageType) {
      case ImageReaderFactory.GIF
        return new GifReader(is);
      case ImageReaderFactory.JPEG
        return new JpegReader(is);
      // ...
    }
  }
}

Strategy Pattern

Intent

Allows for switching between different algorithms for accomplishing a task

Applicability

• Different variants of an algorithm
• Many related classes that differ only in their behavior

Structure

Participants:

• Context
  • Interface to outside world
• Algorithm/Strategy
  • Common interface for the different algorithms
• Concrete Strategies
  • Actual implementation of the algorithm

Other Common Patterns

• Visitor
  • A way of separating an algorithm from an object on which it operates
• Decorator
  • A wrapper that adds functionality to a class
    • stackable
• Iterator
  • Access elements of a collection without knowing the underlying representation
• Observer
  • Notify dependents when object changes
• Proxy
  • Surrogate controls access to another object

Choosing a Pattern

Approach

• Understand your design context
• Examine pattern catalog
• Identify and study related patterns
• Apply suitable pattern