Object-Oriented Design (Low-Level Design) Interview Prep

Object-oriented design (OOD) rounds, also called low-level design (LLD) rounds, ask you to model a small system in code: classes, their responsibilities and how they interact. Unlike high-level system design, the focus is on clean abstractions, not servers and databases.

A repeatable approach

1. Clarify the scope. Which features are in? Who are the actors (customer, admin, delivery agent)?
2. Identify the core objects. Nouns in the requirements usually become classes (Locker, Package, Slot, Order).
3. Assign responsibilities. Each class should have one clear job. Keep data and the behavior that uses it together.
4. Define relationships. Composition vs inheritance, interfaces for things that vary (payment methods, sizes).
5. Walk through a use case. Trace one request end to end through your classes and adjust.
6. Handle edge cases and extensibility. Concurrency, invalid states, and how a new feature would fit.

Patterns that come up often: Strategy (swappable behavior), State (vending machines, elevators), Observer (notifications), Factory (creating the right subtype) and Singleton (used sparingly).

Watch classic OOD problems worked live

These are recordings from SDE Skills sessions, our most-watched object-oriented design videos.

Amazon Locker (16,000+ views)

Vending Machine system design

Meeting Scheduler

Blackjack game

More problems to practice

Parking lot system, elevator controller, library management, chess, ride sharing, rate limiter (as a class), LRU cache, and a notification service. Try each with the 6-step approach above, then compare notes with others.

Practice with other engineers, free

SDE Skills is a volunteer-run 501(c)(3) nonprofit with free weekly design sessions and daily coding practice on Zoom. See the schedule or read How to Prepare for a System Design Interview.

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