Foundations for design interviews.

Five timeless object-oriented design principles, with worked examples.

Encapsulation, abstraction, inheritance, polymorphism — in modern code.

Decouple construction from use. Practical DI without a framework.

When to compose and when to inherit. Real interview tradeoffs.

Map of GoF patterns interviewers actually ask about.

Swap algorithms at runtime. The most-asked pattern in LLD rounds.

Hide construction. Choose abstract factory vs simple factory.

Event-driven design. Foundation of pub/sub systems.

Construct complex objects step-by-step. Fluent APIs done right.

Bridge incompatible interfaces. Classic third-party integration tool.

Why interviewers expect you to avoid it — and when it's fine.

Reduce ripple effects. Measure coupling in your designs.