They are well known and documented solutions to

• Recurring challenges in an OOP environment

• No need to reinvent the wheel and waste your time
• Systematic reuse of design ideas or best pratices yields lower cost and higher quality

• Language Neutral
• Dynamic (always new design patterns are coming)
• Incomplete (You need able to customize it accoording to use)

1. Creational Patterns
2. Structural Patterns
3. Behavioral Patterns

• Useful when needed to create objects for specific purpose in a systematic way
• Benefits: Flexibility
• Mechanisms used: Polymorphism, Interface
• Eg. you need to restrict number of objects instantiated (You can use singleton design pattern)
• Eg: Different subtypes of objects from the same class can be created at runtime
• Other creational patetrns
  • Singleton
  • Factory
  • Abstract Factory
  • Builder
  • Prototype

• Contains Various ways to assemble classes and thier instances
• Establish useful realationships between software components in certain configurtions
• To accomplish both functional an non functional goals.
• Different goals yield diffrerent structures.
• Mechanisms used: Inheritance, Interface
• Examples:
  • Decorator
  • Proxy
  • Adaptor
  • Composite
  • Bridge

• Common solutions to programming problems occuring when trying to effectively regulate communications among objects i.e. best pratcies on how to make different objects interact with each other.
• Focus: Define the protocols in between the objects to work together to achieve the common goal.
• Mechanisms used: Methods and their signatures, Interface
• Examples:
  • Observer
  • Visitor
  • Iterator
  • Strategy
  • Chain of Responsibility

• classes involved to from a design pattern.
• These classes play different roles to achieve the goal

• They include non functional requirements: Usability, modifiability, reliabaility, performance
• Imapct on the entire software
• Addressed by only the architectural solutions

• Various factors or trade offs to conider when adopting a design pattern
• Manifested in quality attributes
• Unintended consequences if not considered like:

• Side effects: worse performance
• Decision makers:
  • choosing the design pattern despite the consequences
  • vital step since design patterns can cause more problems instead of solving prblems when miused.

• Should capture the gist of the pattern
• Become part of vocabulary during design process
• Needs to be meaningful and memorable

• Provides scenario and insights when patterns may be used

• Describes a design challenge the pattern is trying to address

• Specifies the pattern itself in terms of:
  • Structure: specifies relationship among elements used in the pattern
  • Behavior: specifies interactions between all those elements

• Lists other patterns:
  • used together with the pattern being dwescribed or
    • Similar but different patterns
    • Important to describe the subtle differenes bwteen the patterns