Object Oriented Design (OOD) Interview Preparation

Other Parts of This Series:

• Part 16: Software Design Patterns and Principles - Part 16 (Template Method Design Pattern)

What is OOD (Object-Oriented Design)?

Object-Oriented Design (OOD) is the process of designing a system by breaking it down into objects (classes) that interact with each other to solve a problem.

In Simple Terms, OOD is the modeling of real-world problems using objects, behaviors, and relationships.

Key Components of OOD

1. Objects: Represent real-world entities like: User, Car, Order
2. Classes: Blueprint of objects
3. Encapsulation: Hide internal data, expose behavior
4. Relationships: How objects interact? Means is it “has-a” (composition) or “is-a” (inheritance)

Goal of OOD

• Manage complexity
• Improve maintainability
• Make system extensible

How Are OOD Interviews Different From Other Interviews?

What Interviewers Look For in OOD?

• Clear thinking
• Proper abstraction
• Clean class design
• Use of principles (Like: SOLID)
• Communication
• Extensibility
• Balance of Cohesion & Coupling

Difference Between OOD and System Design

How To Prepare For An OOD Interview?

• Step 1: Master OOP Fundamentals
• Step 2: Learn OOP Principles
• Step 3: Learn Key Design Patterns
• Step 4: Practice Core OOD Problems
• Step 5: Add Advanced Thinking
• Step 6: Practice Communication
• Step 7: Do Mock Interviews
• Step 8: Build Real Projects (Best ROI)

Let’s discuss each step in brief.

Step 1: Master OOP Fundamentals

You must be very comfortable with:

• Encapsulation: Hide internal state and expose only necessary behavior
• Abstraction: Focus on what, not how
• Inheritance: “is-a” relationship
• Polymorphism: Same interface, different behavior

I had some detailed articles on OOP fundamentals. Please read if you are really interested.

• Common Programming Style
• Java Crash Course - Part 3 (Object Oriented Programming)

Step 2: Learn OOP Principles

Design principles provide high-level guidelines to design better software applications. They do not provide implementation guidelines and are not bound to any programming language. SOLID is a well known design principle term.

I had a detailed article on OOP design principles. Please read if you are really interested.

• Software Design Patterns and Principles - Part 1 (Introduction)

Step 3: Learn Key Design Patterns

Design patterns provide low-level solutions related to the implementation of commonly occurring object-oriented problems.

I had a detailed article series on OOP design patterns. Please read if you are really interested.

• Software Design Patterns and Principles - Part 2 (Classification and Most Used Design Patterns)

Step 4: Practice Core OOD Problems

Start with:

• Parking Lot
• Elevator System
• Library Management
• Ride Sharing (Uber)
• ATM System
• Notification System
• Movie System
• Vending Machine
• Restaurant Management System

Step 5: Add Advanced Thinking

Once basic design is clear:

• Concurrency
• Scalability
• Fault Tolerance & Recovery
• Persistence

Step 6: Practice Communication

This is critical. Always structure your answer:

• Clarify requirements
• Define use cases
• Identify classes
• Add relationships
• Apply patterns
• Discuss edge cases

I had some detailed articles on presenting and soft skills. Please read if you are really interested.

• Soft Skills
• Present Like a Pro

Step 7: Do Mock Interviews

• Speak out loud
• Time yourself (30–45 mins)
• Explain decisions

Step 8: Build Real Projects (Best ROI)

Learn by doing is the best learning approach. So, practice by build real-world project.

Common Mistakes to Avoid

• Assuming requirements & not asking clarification questions
• Jumping into coding too early
• Overengineering
• Ignoring edge cases
• Not explaining decisions
• Using patterns blindly
• Ignoring advance cases like: concurrency, scalability
• Not discussing future improvement scopes of the solution

OOD Interview Framework

Having a clear framework for the OOD interview is more important than many realize. Without structure, the interview can feel disorganized and difficult for you and the interviewer to follow.

1. Understand & Clarify the Problem (Very First Step)

Goal: Avoid wrong assumptions.

Ask questions like:

• What are the core features?
• Who are the users/actors?
• What are the inputs/outputs?
• Any constraints

Example:

“Are we designing a parking lot for a single building or multiple locations?”

✅ Tips:

• Never jump into coding/design immediately.
• Repeat the problem in your own words.
• Identify functional vs non-functional requirements.

2. Identify Use Cases (Behavior First)

Think in terms of what the system does (verb), not classes (noun) yet.

Example (Parking Lot):

• Vehicle enters
• Vehicle exits
• Assign parking spot
• Calculate fee

✅ Tips:

• Start with happy path, then edge cases.
• Mention at least 1–2 edge cases:
• Full parking
• Invalid ticket

3. Identify Core Entities/Objects (Nouns → Classes)

Extract nouns from the problem.

Example:

• ParkingLot
• Vehicle
• ParkingSpot
• Ticket

✅ Tips:

• Don’t over-design early.
• Keep it minimal and extendable.

4. Define Relationships (Linking)

Think in terms of:

• Association
• Composition
• Inheritance

Example:

• ParkingLot has many ParkingSpots (Composition)
• Vehicle → Car, Bike (Inheritance)

✅ Tips:

• Prefer composition over inheritance unless there’s a strong “is-a” relationship.
• Avoid deep inheritance chains.

5. Define Class Responsibilities (SRP)

Each class should have one clear responsibility.

Example:

• ParkingLot → manages spots
• Ticket → stores entry/exit info
• FeeCalculator → calculates cost

✅ Tips:

• Follow Single Responsibility Principle (SRP).
• If a class is doing too much → split it.

6. Define Key Methods & Interfaces

Now define behavior.

Example:

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class ParkingLot {
    ParkingSpot assignSpot(Vehicle v);
    void releaseSpot(Ticket t);
}

✅ Tips:

• Keep methods high-level first, refine later.
• Use interfaces for flexibility.

7. Apply Design Patterns (Only When Needed)

Use patterns only when justified.

Common OOD Patterns:

• Factory Pattern → Object creation
• Strategy Pattern → Replace conditional logic
• Singleton → Shared resource (e.g., ParkingLot instance)
• Observer → Event-driven systems

Example:

• Fee calculation → Strategy Pattern

✅ Tips:

• Don’t force patterns.

Say:

“We can use Strategy Pattern here to make fee calculation extensible.”

8. Handle Edge Cases & Constraints (Deep-dive)

Think deeper:

• What if system scales?
• What if concurrency happens?

Example:

• Two cars trying to take the same spot → need locking

✅ Tips:

• Mention thread safety
• Mention data consistency
• Mention failures

9. Persistence (Optional but Strong Signal)

Mention how data is stored:

• In-memory vs DB
• Caching

Example:

• Store tickets in DB
• Cache available spots

✅ Tips:

• Keep it simple (simple language data type like map) unless interviewer pushes deeper.

10. Write Clean Code (If Asked)

Focus on:

• Readability
• Modularity

Example:

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interface FeeStrategy {
    double calculateFee(Ticket t);
}

11. Walk Through Example

Simulate:

“Car enters → spot assigned → ticket created → exit → fee calculated”

✅ Tips:

• This shows practical correctness.

12. Discuss Trade-offs

This is what senior engineers do.

Example:

• Why composition over inheritance?
• Why Strategy pattern?

Conclusion

OOD interviews are not about perfection, they are about how you think, structure, and evolve a design.

So follow the 4 step rules: Learn → Practice → Explain → Improve.

Further Readings

• Book: Object-Oriented Design Interview by Alex Xu (ByteByteGo)