Common Pitfalls in Object-Oriented Design Interviews (and How to Avoid Them)

📌 Why This Matters

You might know classes, inheritance, and design patterns—but still struggle to communicate and structure your ideas effectively in an OOD interview.

This guide uncovers the most common mistakes candidates make, and how to avoid them with clarity, structure, and confidence.

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❌ Pitfall 1: Jumping Straight into Code or UML

What happens:
Candidates start defining classes or writing methods before even understanding the requirements.

Why it’s bad:
It shows you’re thinking tactically, not strategically. You might end up with unnecessary features or the wrong abstractions.

What to do instead:

• Spend the first 5–10 minutes clarifying requirements
• List functional and non-functional needs
• Confirm scope with the interviewer before diving in

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❌ Pitfall 2: “God Class” Syndrome

What happens:
You put too much responsibility into one class (e.g., a ParkingLot that manages spots, users, payments, and state transitions).

Why it’s bad:
Violates the Single Responsibility Principle, makes testing and future changes harder.

What to do instead:

• Break responsibilities into focused classes (e.g., ParkingFloor, PaymentService, TicketManager)
• Ask: “If this class changes, will other things break?”

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❌ Pitfall 3: Overusing Inheritance Instead of Composition

What happens:
You model everything with deep class hierarchies, even when behavior could be better shared via composition.

Why it’s bad:
Inheritance locks you into rigid trees. It’s hard to mix and match behaviors or change direction later.

What to do instead:

• Use composition when one object has-a another
• Example: A Snake has a list of SnakeSegments (composition), not extends Segment

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❌ Pitfall 4: Ignoring Object Lifecycles and State Transitions

What happens:
You define static relationships but forget how objects evolve (e.g., a book being borrowed, returned, or reserved).

Why it’s bad:
It makes your design feel incomplete or disconnected from real-world behavior.

What to do instead:

• Track state transitions: borrowed → returned, idle → moving → door open
• Consider modeling these with enums, state machines, or lifecycle diagrams

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❌ Pitfall 5: Forgetting Extensibility and Trade-Offs

What happens:
You focus only on the immediate use case and ignore how your design would adapt to new features or constraints.

Why it’s bad:
Interviewers want to see that your system isn’t fragile.

What to do instead:

• Ask yourself: “What if we add…?”

  • Online payments?
  • Support for electric vehicles?
  • VIP floors or reservations?

• Discuss trade-offs honestly:

  • Inheritance vs. interfaces
  • Precomputed vs. real-time values
  • Simplicity vs. flexibility

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❌ Pitfall 6: Not Thinking From the User's Perspective

What happens:
You get lost in technical details and forget who will use the system and how.

Why it’s bad:
It disconnects your design from actual use cases and weakens your communication.

What to do instead:

• Walk through core user flows
• Narrate how a Member searches for a book, or how a Driver enters a parking lot
• Design systems that feel real and interactive

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✅ Summary: Design with Awareness

Pitfall	What to Do Instead
Premature class definitions	Clarify requirements first
One class doing everything	Break responsibilities cleanly
Inheritance overuse	Prefer composition when possible
Missing state transitions	Model lifecycle clearly
No thought to extensibility	Discuss trade-offs and future growth
Technical-first thinking	Center your design around user actions

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Avoiding these pitfalls doesn’t require brilliance—just awareness, practice, and a willingness to pause and think before you design.