Elements and Categorization of Design Patterns

Key Insight

Design patterns are documented using a consistent, comprehensive format that extends beyond graphical notations to capture critical design decisions, alternatives, and trade-offs. Key sections for each pattern include its Pattern Name and Classification, a brief Intent statement, any 'Also Known As' names, a 'Motivation' scenario illustrating the problem, 'Applicability' conditions, a 'Structure' using OMT notation and interaction diagrams, 'Participants' and their responsibilities, 'Collaborations,' 'Consequences,' 'Implementation' details, 'Sample Code' in C++ or Smalltalk, 'Known Uses,' and 'Related Patterns.' This uniform structure is designed to make patterns easier to learn, compare, and apply effectively.

The catalog of 23 design patterns is organized using two primary classification criteria: purpose and scope. The 'purpose' criterion categorizes patterns into Creational, Structural, and Behavioral. Creational patterns concentrate on the processes of object creation; Structural patterns deal with the composition of classes or objects; and Behavioral patterns characterize how classes or objects interact and distribute responsibilities. This classification helps users quickly navigate the catalog and identify patterns relevant to a specific design goal or problem type.

The 'scope' criterion further distinguishes patterns by whether they primarily apply to classes or objects. Class patterns define relationships between classes and their subclasses, which are established through inheritance and are thus static, fixed at compile-time. Object patterns, conversely, address relationships between objects that can be changed dynamically at run-time, typically through composition. While most patterns involve some degree of inheritance, only those that explicitly focus on static class relationships are designated as 'class patterns.' This distinction highlights the trade-offs between static, inheritance-based reuse and dynamic, composition-based flexibility.