Unit 6 — Design Principles & Patterns

Learning objectives

• Explain cohesion, coupling, information hiding, and dependency direction.
• Apply SOLID principles as reasoning tools rather than slogans.
• Recognize when a design pattern clarifies a recurring design problem.

Localizing change

Good modular design aims to keep related responsibilities together and reduce unnecessary knowledge between modules. When a requirement changes, fewer components should need coordinated modification.

Design principles

• Single Responsibility: group things that change for the same reason.
• Open/Closed: enable common extensions without repeatedly editing stable code.
• Liskov Substitution: subtypes should preserve client expectations.
• Interface Segregation: avoid forcing clients to depend on operations they do not need.
• Dependency Inversion: high-level policy should not be tightly bound to volatile low-level detail.

Patterns

Patterns are named design knowledge, not code templates. Examples include Strategy, Observer, Adapter, Factory Method, and Dependency Injection.

Warning

Do not add patterns merely to make a design look sophisticated. Additional indirection has a cost.