Remembering Design Patterns

There are 23 design patterns described in the classic GoF book. I have found myself an easier way to remember them:

First, I divide them into three categories:

• Creational Patterns
• Structural Patterns
• Behavior Patterns

Creational Patterns

Their purposes are to parameterize systems by the classes of objects they create.

• Create a group of objects: Abstract Factory
• Create a single object. This can further be categorized by how clients use this pattern.
  • Use by composition:
    • Create by clone: Prototype
    • Create once: Singleton
    • Create complex object: Builder
  • Use by inheritance: Factory Template

Structural Patterns

They are used to compose larger structures from smaller ones.

• One interface (Don’t change or add other interfaces)
  • Group together calls: Composite
  • Intercept calls: Proxy
  • Add state or behavior to calls: Decorator
• Two/Many interfaces
  • Adapt interface A to B: Adapter
  • Two interfaces at both sides of a bridge can evolve independently: Bridge
  • A single interface to hide the complexity of a number of interfaces: Facade
• Structural & Creational pattern: Flyweight, to cache static states as singletons, and passing dynamic states when calling

Behavioral Patterns

They try to encapsulate algorithms or allocate responsibilities.

• encapsulate general algorithm by composition: Strategy
• encapsulate general algorithm by inheritance: Template Method
• encapsulate iteration algorithm: Iterator
• encapsulate state based algorithms: State
• encapsulate snapshot algorithm: Memento
• encapsulate event notification: Observer
• encapsulate nodes (e.g. AST nodes) visit algorithms: Visitor
• encapsulate Abstract Syntax Tree parser algorithm: Interpretor
• decouple interaction between related peer classes: Mediator
• encapsulate how I was called
  • Command, implement Command interface and submit to an Executor, the executor will call me later.
  • Chain of responsibility, implement Handler interface and chain into an existing list of handlers. Upstream Handler will call me, and I have to call downstream handler.

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