Design Patterns

Creational Patterns

• Singleton — one instance globally (use sparingly, hard to test)
• Factory Method — delegate instantiation to subclasses
• Abstract Factory — families of related objects
• Builder — step-by-step complex object construction
• Prototype — clone existing objects

Structural Patterns

• Adapter — incompatible interfaces compatibility
• Decorator — add behavior without modifying class
• Facade — simplified interface to complex subsystem
• Proxy — control access to another object
• Composite — tree structures, treat individual/group uniformly

Behavioral Patterns

• Observer — event subscription/notification
• Strategy — swap algorithms at runtime
• Command — encapsulate actions as objects
• Chain of Responsibility — pass request through handler chain
• Template Method — define skeleton, let subclasses fill in
• State — change behavior when internal state changes
• Iterator — traverse collection without exposing internals

Modern Patterns

• Repository — abstract data layer from business logic
• Unit of Work — batch DB operations into a transaction
• CQRS — separate read and write models
• Saga — distributed transaction management
• Circuit Breaker — prevent cascading failures

Anti-Patterns to Avoid

• God Object — class that knows too much
• Spaghetti Code — no clear structure
• Golden Hammer — using one pattern for everything
• Premature Optimization — optimizing before profiling