Object-Oriented Programming and SOLID Principles

Low-Level System Design relies heavily on Object-Oriented Programming (OOP) to model real-world systems through classes and objects. OOP enables encapsulation of data and behavior, making systems more modular, reusable, and easier to understand.

To write truly maintainable and extensible designs—especially in interviews—it's essential to go one step further and apply the SOLID principles, a set of five best practices that guide object-oriented design.

Core OOP Concepts

• Encapsulation - Group related data and behavior inside objects.
• Abstraction - Hide internal details and expose only what’s necessary.
• Inheritance - Allow one class to inherit from another for reusability.
• Polymorphism - Enable objects to be treated as instances of their parent class.

SOLID Principles

S - Single Responsibility Principle (SRP)
A class should only have one responsibility or reason to change.

O - Open/Closed Principle (OCP)
Classes should be open for extension, but closed for modification.

L - Liskov Substitution Principle (LSP)
Subtypes must be substitutable for their base types.

I - Interface Segregation Principle (ISP)
Interfaces should be client-specific, not one-size-fits-all.

D - Dependency Inversion Principle (DIP)
High-level modules should depend on abstractions, not concrete implementations.

Why This Matters in Interviews

Demonstrating both OOP concepts and SOLID principles shows that you don’t just know how to code—you know how to design systems that scale, adapt, and evolve. It's a major signal to interviewers that you’re a thoughtful and capable engineer.

In upcoming sections, we’ll apply these principles to real-world LLD problems and show how they help create elegant, maintainable solutions.