Object Oriented Design Interview: A step-by-step Guide

The Object-Oriented Design interview isn’t a LeetCode-style challenge. It’s an exploration of how you think about architecture, how you break down a system into responsibilities, abstract behavior into interfaces, and model complexity in a way that’s flexible, maintainable, and testable. 

It’s one of the most revealing parts of any senior engineering interview loop, especially at FAANG and top-tier product companies.

While traditional data structure problems test algorithmic skill, the Object-Oriented Design interview is about architectural thinking:

• Can you take a vague product prompt and turn it into modular, extensible classes?
• Do you know when to use inheritance and when to avoid it?
• Can your system grow without breaking?

Unlike System Design interviews (which emphasize infrastructure, scale, and distributed systems), the Object-Oriented Design interview zooms into the code-level blueprint. You’ll be expected to reason through:

• Abstract base classes vs interfaces
• Real-world polymorphism
• Reusable components and separation of concerns
• Design patterns (when to use Strategy, Observer, Composite, etc.)

The best candidates treat the Object-Oriented Design interview as a conversation, not a solo act. You’ll ask thoughtful questions, explain your class structure out loud, and iterate as feedback is given. Interviewers don’t expect perfection. They want to see how you navigate trade-offs, organize logic, and design for change.

7 Steps to Crack an Object-Oriented Design Interview

Step 1: Clarify Functional and Behavioral Requirements 

The most common mistake in an Object-Oriented Design interview is jumping straight into class declarations without asking any questions. When you do that, you’re guessing instead of designing.

Your first job is to clarify the system’s expected behavior. This not only helps you scope the problem, but also tells the interviewer you think like a product-minded engineer. Ask questions like:

• What are the core user actions?
• What are the system’s boundaries?
• Are there performance constraints or concurrency concerns?
• Should this be built for extensibility, or is it a one-off solution?

Let’s say the prompt is:

“Design a parking lot system.”

You should immediately follow up with:

• Is this a single-level lot or multi-level?
• Should we support different vehicle types (bike, car, truck)?
• Do we need to track entry/exit times and calculate fees?
• Will the lot support digital reservations or real-time availability?

Without these answers, your class design will be either overcomplicated or miss essential use cases. You’ll struggle to justify relationships like composition vs inheritance or explain why you added (or omitted) certain responsibilities.

The goal of this step is to narrow ambiguity into a concrete feature set so that every class you build later has a clear purpose.

Pro tip: As you clarify requirements, start listing key nouns and verbs. Nouns often become classes (Car, Ticket, Level), while verbs turn into methods (parkVehicle(), assignSpot()).

In your Object-Oriented Design interview, taking time to scope early isn’t wasted time. It’s what sets strong candidates apart. Interviewers want to see you break down problems, not rush through them.

Remember, clarity drives structure. Once you understand what the system should do, you’ll be ready to define how it should do it.

Step 2: Identify Key Entities, Behaviors, and Relationships

Now that you’ve scoped your system, the next step in your Object-Oriented Design interview is to identify its core building blocks. These are the entities (classes), their responsibilities (methods), and how they relate to each other.

Start with a simple rule:

• Nouns → candidate classes
• Verbs → candidate methods

For a library system, for example, the nouns might be: Book, User, Librarian, Loan, Library, and SearchEngine.

Verbs could be: borrow(), return(), reserve(), search().

Once you have these, ask:

• Who owns what?
• Which objects interact with each other?
• Are there “is-a” relationships (inheritance)?
• Are there “has-a” relationships (composition)?

Here’s how that plays out in practice:

• Book might be a base class.
  • Ebook and Audiobook could inherit from it.
• User could be abstract, with Member and Admin as subclasses.
• Library could contain books and handle user registration.
• Loan could act as a transaction object between User and Book.

Use this phase to group responsibilities:

• Single Responsibility: Every class should have one job.
• High Cohesion: Methods in a class should work toward a common goal.
• Low Coupling: Classes should talk to each other via interfaces or exposed APIs, not by accessing each other’s internals.

Pro tip: If the class names or relationships feel ambiguous, go back to the use cases and ask: “Does this class reflect a real-world responsibility?”

Interviewers love it when candidates narrate their reasoning:

“I’m thinking PaymentMethod could be an interface, and we’d have CreditCard and PayPal implement it. That way, the checkout flow depends only on the interface, not the concrete types.”

By clearly identifying objects and their behaviors, you create the foundation for a scalable, extensible system.

In the next section, we’ll organize these entities into a clean class hierarchy that balances flexibility with maintainability, which is a skill every Object-Oriented Design interview tests deeply.

Step 3: Define Class Hierarchies and Interfaces

Once you’ve mapped out the core entities and their behaviors, it’s time to organize them into a clean, intentional structure. In your Object-Oriented Design interview, this means defining how objects relate to each other through inheritance, interfaces, and composition.

Start with a guiding principle:

Design for extensibility, not cleverness.

Use Inheritance to Model “Is-a” Relationships

Use inheritance only when one class is a strict subtype of another. For example:

• ElectricVehicle is a Vehicle
• AdminUser is a User

But don’t overuse it. If two classes share behavior but don’t form a subtype relationship, extract an interface or use composition instead.

Bad example: Printer inherits from Scanner just because they both share powerOn()

Good solution: Create a PowerDevice interface with powerOn()

Use Interfaces to Abstract Capabilities

Interfaces let you define contracts without locking in implementation. For example:

In your Object-Oriented Design interview, this tells the interviewer:

• You’re thinking about decoupling
• You’re enabling polymorphism
• You understand the Open/Closed Principle: your system can grow without modifying existing code

Apply Composition When Behavior Varies

Example:

Instead of subclassing Vehicle for every type (Truck, SUV, Sedan), have Vehicle contain a DrivingBehavior:

This gives you the flexibility to mix and match behaviors at runtime—ideal for systems that evolve frequently.

Interview-ready narrative:

“I’m choosing an abstract base class for User, with Member and Admin as subclasses. But for payment types, I’ll go with an interface, so we can add new methods like crypto or rewards without refactoring existing classes.”

The best Object-Oriented Designs respect responsibility boundaries and reduce surprise. When each class does only one job and speaks through contracts, your system is easier to test, extend, and understand.

Step 4: Handle Object Lifecycle, Dependencies, and Extensibility

A system isn’t just a diagram of classes. It’s a living, evolving application. In your Object-Oriented Design interview, showing how you manage object creation, decouple dependencies, and future-proof your design is what separates a passable answer from a great one.

Managing Object Creation

Avoid tight coupling by introducing flexibility into how objects are created. Options include:

• Factory Method: Defer object instantiation to a subclass or factory.
• Builder Pattern: Ideal for constructing complex objects with optional fields.
• Dependency Injection: Inject dependencies via the constructor or the setter, rather than instantiating inside a class.

Example:

This makes OrderProcessor easy to test, mock, and extend.

Designing for Extensibility

Interviewers may ask:

“How would you add feature X later without modifying the current system?”

That’s your cue to:

• Point out the interface use
• Leverage inheritance hierarchies or abstract classes
• Mention the use of Strategy or Command pattern
• Show where you’d plug in new classes (e.g., new vehicle types, new notification methods)

Maintain Testability

Good design makes testing easy. Your system should:

• Expose behavior through interfaces (for mocking)
• Avoid static state and global variables
• Support dependency injection for components like databases, payment gateways, or loggers

Interview-ready summary:

“I designed Cart to accept a PricingEngine interface via its constructor, so we can swap in a new pricing algorithm, like promotions or loyalty rewards, without modifying core logic.”

In real-world systems, change is constant. Interviewers want to see that you’re building architecture that adapts without breaking.

Step 5: Incorporate Design Patterns Where Needed

Design patterns aren’t required in every Object-Oriented Design interview, but when used thoughtfully, they show architectural fluency. The key is to choose the right pattern for the right problem, not to force one in just to impress.

Common Patterns to Know

1. Strategy Pattern
   • Replaces large if-else logic with interchangeable behaviors
   • Use for pricing rules, shipping strategies, and access controls
   • Promotes the Open/Closed Principle
2. Decorator Pattern
   • Dynamically adds responsibilities to an object at runtime
   • Use for adding discounts, taxes, or analytics wrappers
3. Observer Pattern
   • Use when an event in one object should trigger behavior in others
   • Great for building notification systems, audit logs, or reactive UIs
4. Factory Pattern
   • Centralizes object creation
   • Useful when you want to hide complex construction logic
5. Composite Pattern
   • Models tree-like structures (e.g., file systems, UI components)
   • Treats individual objects and groups uniformly

When to Avoid Patterns

• Don’t force a Factory when a simple new will do.
• Don’t use Observer unless multiple listeners actually exist.
• Don’t reach for Singleton unless global state is required, and even then, use with caution.

Interview framing example:

In this Object-Oriented Design interview, I’d use the Strategy Pattern to decouple payment logic from the checkout process. That way, we can support multiple payment types and even inject behavior like fraud checks or logging without modifying core business logic.

Final Tip

Know when to use a pattern, but more importantly, know why. Your interviewer will care more about your reasoning than your jargon.

When used well, design patterns show that your system is not only functional but also scalable, elegant, and battle-tested.

Step 6: Code the Core Classes and Method Interactions

Once you’ve laid out your class hierarchy and architecture strategy, the interviewer will want to see how you translate that design into real, usable code. This is the point in your Object-Oriented Design interview where you move from abstract structure to concrete implementation.

But that doesn’t mean writing 200 lines of boilerplate. Instead, focus on writing core classes and key method interactions that showcase your understanding of:

• Encapsulation
• Polymorphism
• Data flow
• Behavioral design

Focus on the Most Critical Classes

Pick 2–4 of your highest-value classes and sketch out:

• Attributes
• Constructors
• Public methods
• Relationships

For example, in a shopping cart design:

This short snippet shows:

• Encapsulation: Fields are private, accessed through methods
• Responsibility: Item handles its own price, Cart knows how to add and total
• Behavior: You’re modeling real-world usage with clear method names and logic

Demonstrate Interaction

In your Object-Oriented Design interview, show how objects collaborate:

This shows the system in action, which is more valuable than sketching 20 classes you never use.

Design Tips

• Prefer composition: Let classes collaborate via methods, not inheritance.
• Limit class size: One class = one job.
• Keep visibility strict: default to private, expose only what’s needed.
• Follow naming clarity: addItem() > insert(); getTotal() > calculate().

Interview Framing

“Let me write a few key classes that demonstrate our design in motion. I’ll keep them focused and show how the components interact, especially how the Cart uses a PricingEngine interface to support extensibility.”

This section is your chance to show that your design is clean on paper and clean in code, too.

Step 7: Address Edge Cases, Constraints, and Testing 

If your design only works in ideal conditions, it’s not a real design, but a placeholder. That’s why in every strong Object-Oriented Design interview, interviewers will eventually ask:

“What could go wrong here?”

“How would you test this?”

“What edge cases should we worry about?”

This is your opportunity to show that your architecture is resilient, flexible, and production-aware.

Handle Real-World Edge Cases

Great systems anticipate misuse, scale stress, and invalid state transitions. For example:

• Parking Lot System:
  • What if no spots are available?
  • What if someone exits without a valid ticket?
• Library Checkout System:
  • What if a user tries to borrow more than their limit?
  • What if a reserved book is never returned?
• File System:
  • What happens with symbolic links or circular folders?
  • What if a file is deleted during an active read?

When you surface these proactively, you signal maturity. Use statements like:

“We should add a safeguard so that if getAvailableSpots() returns 0, assignSpot() throws a NoAvailableSpotsException.”

Handle Constraints

If the design includes constraints (e.g., rate limits, size limits, permission boundaries), model them explicitly:

• Add fields like maxBorrowLimit in the User class.
• Add permission checks in a FileAccessHandler.
• Add rate-limiting logic via a RequestThrottle interface for an API design.

Constraints make systems safer and extensible.

Make Your Design Testable

Testing should be baked into your Object-Oriented Design, not duct-taped afterward.

Here’s how:

• Use interfaces to mock dependencies
• Avoid static state and global variables
• Prefer constructor injection over hardcoded class creation

Example:

Now you can mock Channel in a unit test:

Interview Summary

“In the Object-Oriented Design interview, I’d validate my classes by unit testing with mocked dependencies. I’d also handle constraints like borrow limits and enforce proper state transitions to prevent invalid operations or data corruption.”

This section proves you’re designing not just for the whiteboard, but for real engineering teams that live with their designs in production.

Object-Oriented Design Interview Questions and Answers 

The best way to prepare for your Object-Oriented Design interview is to rehearse structured, deep answers to common prompts. They’re how top companies evaluate your ability to model systems that evolve, scale, and make sense under pressure.

Below are 6 common Object-Oriented Design interview questions, each with sample reasoning and architectural highlights.

1. Design a Ride-Sharing Platform (like Uber/Lyft)

Clarify:

• Are we supporting ride matching only, or full pricing and payments?
• Do riders and drivers have different app experiences?
• Do we need real-time location tracking?

Core Classes:

• User (abstract)
  • Rider
  • Driver
• Trip
• Location
• Vehicle
• PaymentMethod (interface)
• PricingEngine (Strategy Pattern)

Key Features:

• Trip lifecycle: start(), end(), rateTrip()
• Driver availability: toggle online/offline
• Surge pricing as an interchangeable pricing strategy

Bonus:

Use Observer pattern to notify drivers nearby when a rider requests a trip.

2. Design a Notification System

Clarify:

• Should the system support multiple delivery channels?
• Are notifications queued, batched, or instant?
• Do users have preferences?

Core Classes:

• Notification
• UserPreferences
• NotificationChannel (interface)
  • EmailChannel
  • SMSChannel
  • PushChannel

Pattern:

Use Observer Pattern to notify all subscribed channels when an event is triggered.

Sample logic:

Bonus:

Make channels plug-and-play using Dependency Injection so new ones can be added without breaking existing logic.

3. Design a Chess Game

Clarify:

• Do we need AI/computer opponents?
• Should we support move validation, undo, and replay?
• What about multiplayer or saved state?

Core Classes:

• Game
• Board
• Cell
• Piece (abstract)
  • Pawn, Knight, Rook, etc.
• MoveValidator (Strategy per piece)

Behavior:

• Each piece implements getValidMoves(Board board)
• The game enforces turn alternation, a win condition, and a game state

Bonus:

Use Command Pattern for undo functionality, storing each move as a reversible action.

4. Design a File System

Clarify:

• Should we support symbolic links and permissions?
• Do we need search, compression, or sync?

Core Classes:

• FileSystemObject (abstract)
  • File
  • Directory
• Permission
• User

Pattern:

Use Composite Pattern: treat File and Directory uniformly.

• Directory maintains a list of FileSystemObject children
• getSize() recursively sums child sizes

Bonus:

Add role-based access controls with a PermissionEvaluator interface.

5. Design an E-Commerce Checkout System

Clarify:

• Do we support multiple payment methods?
• How do we handle coupons, taxes, and shipping?
• Should we model product inventory?

Core Classes:

• Cart
• Item
• Order
• Discount (interface)
• PaymentMethod (interface)
• ShippingStrategy (Strategy Pattern)

Sample:

Bonus:

Model extensibility by injecting Discount and ShippingStrategy using Strategy Pattern, allowing rules to vary by region, user tier, or cart value.

6. Design a Parking Lot System

Clarify:

• Single-level or multi-level?
• Support for car types (bike, car, truck)?
• Should it include payment and digital entry?

Core Classes:

• ParkingLot
• ParkingSpot
• Vehicle (abstract)
  • Car, Truck, Motorcycle
• Ticket
• PaymentProcessor

Key Relationships:

• ParkingLot contains Levels, which contain ParkingSpots
• SpotManager can use hash maps or priority queues to track availability

Pattern:

Use Factory Pattern for issuing tickets and assigning spots.

Bonus:

Add real-time availability display with an Observer on SpotManager to notify UI/dashboard services.

How to Deliver These Answers

Use a 5-step template:

1. Clarify the use case
2. Identify key entities and their behaviors
3. Show class hierarchy and interfaces
4. Walk through 1–2 usage flows with interactions
5. Discuss extensibility, testing, and trade-offs

Bonus Scenarios to Practice:

• Design a restaurant reservation system
• Design a calendar with event invitations
• Design a hotel booking system
• Design a music player or playlist manager

Interview-Winning Mindset

“I’ll start by clarifying the scope so we don’t overdesign. Then I’ll identify the key abstractions and highlight how I’d structure their interaction. I’ll code a few critical methods and then walk through edge cases and extensions.”

The goal in your Object-Oriented Design interview is not to draw every possible class. It’s to model behavior that makes sense and evolves safely.

Final Tips for the Object-Oriented Design Interview

The Object-Oriented Design interview is your chance to show that you’re an architect. A designer. A collaborator who writes software meant to be read, reused, and refactored.

Here’s how to stand out:

Lead With Structure

Before naming classes, pause and say:

“Let me clarify the use case before diving in.”

Clarify what’s in scope, what the edge cases are, and what the priorities should be, like performance, extensibility, maintainability, etc. This shows mature, product-first thinking.

Focus on Behavior Over Boilerplate

Don’t waste time creating dozens of empty classes. Instead, write 2–4 important ones and walk through how they interact in real-world flows:

• Cart checkout
• File read/write
• Trip start and rating
• Notification delivery

Emphasize methods and data relationships. Interviewers want to see your logic in motion.

Be Explicit About Trade-Offs

The best candidates say things like:

• “I chose composition over inheritance here because the behavior varies per instance.”
• “If we expect dozens of notification channels, we may want a plugin-based registry later.”
• “We could inject this validator to keep it testable.”

This proves you’re not chasing patterns. You’re thinking like a real system designer.

Design for Change

No interviewer expects a perfect answer. But they do want to know: if the business changes tomorrow, can your design handle it? Build to evolve, not to impress.

Conclusion: Design Like a Teammate, Not a Textbook 

You don’t need to memorize every pattern in the Gang of Four book to ace your Object-Oriented Design interview.

What you need is clarity. Empathy. Systems thinking.

Ask questions. Name your trade-offs. Write just enough code to show you understand how real objects collaborate. Use interfaces when behaviors vary. Use inheritance when types demand it. And always, always design like someone else will have to extend your work tomorrow.

Remember:

• Good code solves a problem.
• Great design solves the next five problems, too.

If you can communicate your decisions clearly, align them with user behavior, and show that your system is testable, flexible, and logical, you’ll do more than pass your Object-Oriented Design interview.

You’ll stand out as the kind of engineer teams love to work with.