📌 Why You Need a Framework
Object-Oriented Design interviews can feel open-ended and ambiguous. Without a clear structure, it's easy to spiral into naming classes or sketching random UML diagrams without purpose.
This guide walks you through a repeatable 6-step approach to break down any OOD question clearly, collaboratively, and confidently.
✅ Step 1: Clarify the Problem
What to do:
- Ask clarifying questions about the system's scope
- Understand the core goals: Who are the users? What are the main actions?
- Nail down functional vs. non-functional requirements
Why it matters:
You don’t want to waste time designing features that aren't needed. Clear communication up front makes the rest of the conversation smoother.
✅ Step 2: List Functional & Non-Functional Requirements
Functional Examples:
- What can users do? (search books, borrow elevator, pay for parking)
- What entities are involved and how do they interact?
Non-Functional Examples:
- Scalability, extensibility, security, maintainability
- Does it need to support concurrent users or be event-driven?
Pro tip: Write these down clearly so you can reference them while designing.
✅ Step 3: Identify Core Entities and Relationships
What to do:
- Brainstorm the nouns: vehicles, users, spots, books, tickets
- Group them into entities (classes) and note their relationships:
- Is-a → use inheritance
- Has-a → use composition
Deliverables:
- Rough list of classes and their relationships
- Think in terms of object responsibilities, not just data fields
✅ Step 4: Define Core Classes and Interfaces
What to cover:
- Define core classes with attributes and behaviors
- Separate concerns clearly:
ParkingSpotvs.ParkingLotControllerElevatorvs.ElevatorController
Bonus points:
- Use interfaces or abstract classes where behavior varies
- Keep class responsibilities single and clear
✅ Step 5: Model Object Interactions
What to show:
- Describe how objects work together during key flows
- E.g., What happens when a vehicle enters a parking lot?
- What does the library do when a book is borrowed?
Visuals help:
- Draw sequence diagrams, or talk through object collaboration
- Use state transitions for things like
Loan,Elevator, orSpot
✅ Step 6: Discuss Edge Cases, Extensions, and Trade-Offs
What to do:
- Bring up failure scenarios:
- What if all elevators are full?
- What if a book is reserved and then returned late?
Discuss extensibility:
- Could we add online payments? digital books? dynamic pricing?
Talk trade-offs:
- Inheritance vs. composition
- Centralized vs. distributed scheduling
- Pre-calculation vs. real-time computation
This shows you're not just designing for today, but for the system’s future.
🧠Final Tips for Success
- Think out loud: Show your process, not just the result
- Start broad, then go deep: Don’t rush into coding classes
- Stay user-focused: Always tie designs back to use cases
- Practice classic questions: Parking Lot, Elevator, Library, Vending Machine
✅ Summary
| Step | Action |
|---|---|
| 1 | Clarify scope and requirements |
| 2 | List functional and non-functional goals |
| 3 | Identify entities and relationships |
| 4 | Define core classes and their responsibilities |
| 5 | Model key object interactions and flows |
| 6 | Handle edge cases, trade-offs, and extensibility |
A structured approach shows maturity in design thinking. It helps you stay calm under pressure, think modularly, and collaborate effectively with interviewers.