Object-Oriented Programming (OOP) in Low-Level Design

Low-Level System Design (LLD) relies heavily on Object-Oriented Programming (OOP) because it allows us to model real-world systems as a collection of classes and objects. OOP principles help in building modular, reusable, and maintainable code—qualities that interviewers look for in low-level system design interviews.

By mastering OOP, you can encapsulate behavior, hide complexity, reuse components, and adapt systems without rewriting everything.

Core OOP Principles for LLD

1. Encapsulation – Group related data and behavior inside objects

Encapsulation ensures that the internal state of an object is hidden and can only be changed through controlled methods.

Example:

class BankAccount {
  private balance: number;
  
  constructor(initial: number) {
    this.balance = initial;
  }
  
  deposit(amount: number) {
    this.balance += amount;
  }
  
  getBalance() {
    return this.balance;
  }
}

Here, balance is private and cannot be accessed directly from outside the class.

2. Abstraction – Hide implementation details and expose only what’s needed

Abstraction allows you to focus on what an object does instead of how it does it.

Example:

abstract class Animal {
  abstract makeSound(): void;
}

class Dog extends Animal {
  makeSound() {
    console.log('Woof!');
  }
}

Animal defines a contract (makeSound()), but the actual implementation is hidden inside Dog.

3. Inheritance – Reuse code by creating a parent-child class hierarchy

Inheritance lets you build new classes on top of existing ones, reducing duplication.

Example:

class Vehicle {
  move() {
    console.log('Moving...');
  }
}

class Car extends Vehicle {
  honk() {
    console.log('Beep!');
  }
}

Car inherits move() from Vehicle and adds its own honk() method.

4. Polymorphism – Treat objects of different classes uniformly

Polymorphism enables you to write code that works with objects through a shared interface or base class.

Example:

function makeItSound(animal: Animal) {
  animal.makeSound();
}

makeItSound(new Dog()); // Woof!

Here, makeItSound() works for any class that extends Animal.

Why OOP Mastery Matters in LLD Interviews

A strong grasp of OOP principles shows interviewers that you can:

• Design scalable and adaptable systems
• Minimize duplication through reusability
• Handle change gracefully without breaking existing code
• Communicate design ideas clearly using standard terminology

In the upcoming sections, we’ll see how Encapsulation, Abstraction, Inheritance, and Polymorphism come together in real-world LLD problems—turning abstract requirements into elegant, production-ready solutions.