design-pattern-application

name	design-pattern-application
description	Applies object design patterns judiciously. Use when creation, adaptation, algorithm variation, part-whole structure, event notification, or family-of-products variation creates design pressure.

Design Pattern Application

Overview

Design patterns are named solutions to recurring design forces. Apply them after identifying the force; do not paste patterns into a design because they are familiar.

When to Use

• GRASP reveals coupling, cohesion, creation, or variation pressure.
• The design must adapt external APIs, select algorithms, compose recursive structures, notify dependents, or create related object families.
• You need a concise rationale for a pattern choice.
• Do not use to make a simple design look sophisticated.

Language Adaptation

When the implementation language is known, also use Software Design Language Adaptation and read only its matching language reference. Preserve the design force and consequences, but prefer a native language mechanism over reproducing a canonical class structure from another language.

Workflow

1. State the force. Identify the concrete design problem: variation, creation, notification, adaptation, composition, or dependency direction.
2. Try the simplest GRASP-consistent design. Patterns should solve a pressure that the simple design cannot handle well.
3. Choose a narrow pattern.
   • Adapter: external or incompatible interface must look like a stable domain-facing interface.
   • Factory: creation is complex, variable, or would couple clients to concrete classes.
   • Strategy: interchangeable algorithms or policies vary independently from clients.
   • Composite: clients should treat individual and grouped objects uniformly.
   • Observer: dependents need notification without tight coupling to the subject.
   • Abstract Factory: families of related products must vary together.
4. Define participants in project terms. Name actual classes/interfaces, not generic pattern roles only.
5. Check consequences. Record benefits and costs: indirection, extra types, testability, runtime flexibility, and cognitive load.
6. Integrate with UML. Update interactions and class diagrams to show participants and dependencies.
7. Stop when the force is resolved. Avoid chaining patterns unless each one has a separate pressure.

Decision Template

## Pattern Decision: [Pattern]

Force:
- [Problem creating design pressure]

Participants:
- [Project type] as [pattern role]

Alternatives Considered:
- [Simpler option]: [why insufficient or acceptable]

Consequences:
- Benefits: [coupling/cohesion/variation impact]
- Costs: [indirection/complexity impact]

Red Flags

• The pattern is selected before a force is stated.
• A factory only wraps a constructor with no creation complexity or variation.
• Strategy is used for behavior that will not vary.
• Observer hides important ordering, delivery, or error-handling requirements.
• Pattern names appear in class names where domain names would communicate better.

Verification

• The pattern solves a named design force.
• Project-specific participants are identified.
• Simpler alternatives were considered.
• UML interactions/classes reflect the pattern accurately.
• Added indirection has a justified benefit.