What is a Pattern?

• Current use comes from the work of the architect Christopher Alexander
• Alexander studied ways to improve the process of designing buildings and urban areas
• “Each pattern is a three-part rule, which expresses a relation
  between a certain context, a problem and a solution.”
• Patterns can be applied to many different areas of human
  endeavor, including software development

 Why Patterns? 

• "Designing object-oriented software is hard and designing
  reusable object-oriented software is even harder." - Erich Gamma
• Experienced designers reuse solutions that have worked in the
  past
• Well-structured object-oriented systems have recurring patterns of
  classes and objects
• Knowledge of the patterns that have worked in the past allows a
  designer to be more productive and the resulting designs to be
  more flexible and reusable 

Types Of Software Patterns

• Riehle and Zullighoven in “Understanding and Using Patterns in
  Software Development” mention three types of software patterns
• Conceptual Pattern
• Pattern whose form is described by means of terms and concepts from the application domain
• Design Pattern
• Pattern whose form is described by means of software design constructs, such as objects, classes, inheritance and aggregation
• Programming Pattern (Programming Idiom)
• Pattern whose form is described by means of programming language
  constructs

Benefits Of Design Patterns

• Capture expertise and make it accessible to non-experts in a
  standard form
• Facilitate communication among developers by providing a
  common language
• Make it easier to reuse successful designs and avoid alternatives
  that diminish reusability
• Facilitate design modifications
• Improve design documentation
• Improve design understandability