📌 Why Principles Matter
Object-Oriented Design (OOD) interviews aren’t just about writing classes—they’re about demonstrating your understanding of how to structure complex systems in a way that's clean, modular, extensible, and testable.
Mastering these 5 foundational principles will help you reason through design trade-offs and build systems that reflect real-world complexity with simplicity and elegance.
1. 🧱 Encapsulation
Definition:
Encapsulation is the practice of hiding internal state and exposing behavior through a clear interface.
Why it matters in interviews:
Encapsulation helps you protect class invariants and ensures that your system’s components interact in predictable ways. It also keeps logic changeable without affecting the rest of the codebase.
Example:
In a Parking Lot system, a ParkingSpot object shouldn't expose direct control over its isOccupied flag to external classes. Instead, it should have controlled methods like assignVehicle() and removeVehicle().
2. 🧬 Inheritance
Definition:
Inheritance allows one class (child) to acquire properties and behaviors of another class (parent), promoting code reuse and modeling "is-a" relationships.
When to use:
Use inheritance when multiple classes share a common base behavior, and you want to build specialized types on top of that.
Example:
In a Library System:
User→ base classMemberandLibrarian→ subclasses with additional responsibilities
Pitfall to avoid:
Don’t use inheritance when the relationship isn’t truly “is-a.” Favor composition if behaviors differ significantly.
3. 🧠 Polymorphism
Definition:
Polymorphism allows you to interact with different object types through a common interface, letting behavior vary dynamically at runtime.
Why it’s powerful in OOD:
It enables extensibility and flexibility—you can add new types without changing client code.
Example:
A PaymentProcessor interface could have multiple implementations:
CreditCardPaymentProcessorCashPaymentProcessorPayPalPaymentProcessor
Your system just calls processPayment()—it doesn't care how the implementation works under the hood.
4. 🎭 Abstraction
Definition:
Abstraction means focusing on essential behavior while hiding irrelevant details.
How it helps in design:
It lets you reason at the right level. By using interfaces and abstract classes, you can decouple what something does from how it does it.
Example:
Instead of exposing low-level control over how an Elevator moves, expose simple methods like goToFloor(floorNumber) and let the object manage internal logic (doors, speed, direction).
Interviewer bonus:
Show that you design to interfaces and let concrete implementations vary.
5. ⚙️ Composition Over Inheritance
Definition:
Composition is when one class includes instances of other classes to reuse their functionality, rather than inheriting from them.
Why it’s preferred:
Composition gives you more control and flexibility, avoiding deep, rigid inheritance hierarchies.
Example:
In a Snake Game:
Snakehas a list ofSnakeSegmentobjects (composition)- You wouldn’t create a
LongSnakeSegmentthat inherits fromSnake—that’s poor modeling
Use "has-a" relationships where appropriate instead of defaulting to "is-a."
✅ Summary
| Principle | Key Benefit | Interview Insight |
|---|---|---|
| Encapsulation | Protect internal state | Cleaner APIs, safer object behavior |
| Inheritance | Reuse common logic | Builds type hierarchies (when truly related) |
| Polymorphism | Add new types without changing callers | Use interfaces and override behavior cleanly |
| Abstraction | Model only what matters | Focus on system boundaries and core methods |
| Composition > Inheritance | Better flexibility and testability | Avoid overuse of deep inheritance trees |
By deeply understanding and correctly applying these principles, you’ll not only impress interviewers, but also build systems that are easier to reason about, test, extend, and debug.