Now days, writing the clean code is just the starting point. What really matters is how your code is structured—it should be readable, maintainable, extensible, and scalable. That is where Object-Oriented Design (OOD) comes into play.
Whether you are preparing for interviews, working in a product team, or building large systems, OOD is foundational. Every software/firmware engineer, at some point, needs to design systems—not just write code.
This is the first article in a series on Object-Oriented Design (OOD). In this introduction, we will explore what OOD is, why it’s essential in real-world software development, and how you can start improving your design skills.
What is Object-Oriented Design?
Object-Oriented Design (OOD) is an approach to designing software systems by organizing them into objects. Objects are self-contained components that combine both data (state) and behavior (methods).
OOD is built on the foundation of Object-Oriented Programming (OOP), which models real-world entities using classes and objects.
Unlike procedural programming, which relies on writing long sequences of instructions, OOD focuses on structuring code around interacting objects. Each object is responsible for managing its own state and behavior, making the system more modular, flexible, and easier to maintain.
In OOD, objects (or entities) are the building blocks. Each entity represents a specific concept or thing in your system, and it comes with clearly defined responsibilities—what it knows (data) and what it can do (behavior).
🧠 Why Learn OOD?
Understanding Object-Oriented Design (OOD) is not just a nice-to-have skill, but it is essential for growing as a software engineer. So let’s understand why we should learn LLD:
1.🎯 In Interviews
Many technical interviews, especially for mid to senior roles, include at least one system or object design round. Common questions include:
- Entry level (0–2 years): Might be asked to design a small class hierarchy (e.g., Design a Book Library).
- Mid-level (2–5 years): Design a Parking Lot, Rate Limiter, or TTT Game.
- Senior level (5+ years): Design scalable, distributed systems that are production-ready
A strong grasp of OOD helps you approach these problems with structure and confidence.
2.🏗️ In Jobs
OOD becomes even more important on the job:
- Designing modules in a scalable architecture.
- Collaborating with PMs and designers to translate features into code.
- Debugging complex systems with well-designed class boundaries.
- Writing tests for modular, reusable components.
- Extending systems without breaking existing.
⚙️ Key OOD Principles to Learn:
To get better at Object-Oriented Design, it’s important to understand the core principles that guide well-structured, maintainable software. These principles help you design systems that are modular, reusable, and easy to change.
Here are the most important ones to learn and apply:
🔹 1. SOLID Principles
These are five foundational principles for writing maintainable and scalable object-oriented software:
- S – Single Responsibility Principle
A class should have only one reason to change. - O – Open/Closed Principle
Classes should be open for extension but closed for modification. - L – Liskov Substitution Principle
Subtypes must be replaceable by their base types without altering program correctness. - I – Interface Segregation Principle
No client should be forced to depend on methods it does not use. - D – Dependency Inversion Principle
Depend on abstractions, not concrete implementations.
🔹 2. Design Patterns
Reusable solutions to common design problems. They fall into three categories:
A. Creational Patterns:
Handle object creation. Examples:
- Singleton.
- Factory Method.
- Abstract Factory.
- Prototype.
- Builder.
B. Structural Patterns
Deal with object composition. Examples:
- Adapter.
- Decorator.
- Composite.
- Flyweight.
- Proxy.
C. Behavioral Patterns
Handle communication between objects. Examples:
- Observer.
- Strategy.
- Command.
- State.
- Template Method.
🔹 3. UML Diagrams:
Visual representations of system design that help communicate structure and behavior:
- Class Diagram – Shows classes, relationships, and responsibilities.
- Sequence Diagram – Visualizes the flow of messages between objects over time.
- Component Diagram – Shows high-level organization and dependencies among software components.
🧪 What Makes Good Code?
A well-designed codebase should be:
- Readable: Easy for others to understand
- Modular: Broken into independent, manageable parts
- Testable: Easy to write unit tests
- Maintainable: Easy to debug and update
- Extensible: Easy to add new features
🧗 How to Get Better at OOD
Read System Design Books:
- “Designing Data-Intensive Applications” by Martin Kleppmann
- “Head First Design Patterns”
Practice on Real Problems:
- Design a Parking Lot
- Design an API Rate Limiter
- Design a Movie Ticket Booking System
Use Online Platforms:
- Udemy.
- YouTube.