LLD Object-Oriented Design: Classes & Objects (Deep Understanding)

After shifting your thinking toward objects and responsibilities, the next step is to understand the most basic building block of object-oriented design:

Classes and objects are not just syntax — they are a way to model real-world systems.

Many people learn them early in programming, but never truly understand their design meaning in LLD.

What is a Class Really?

A class is not just a template for code.

A class is a blueprint for behavior and state of a real-world entity.

It defines:

• what data an entity holds (state)
• what it can do (behavior)

Example

class BankAccount:
    def __init__(self, balance):
        self.balance = balance

    def deposit(self, amount):
        self.balance += amount

Here:

• balance → state
• deposit() → behavior

What is an Object?

An object is a real instance created from a class.

It represents a specific entity in the system with actual data.

Example

account1 = BankAccount(5000)
account2 = BankAccount(12000)

Now:

• account1 → has its own state (5000)
• account2 → has its own state (12000)

Same class, different reality.

Key Insight: Class vs Object

A class defines structure.
An object carries actual data.

Why This Matters in LLD

In low-level design, systems are built using objects that represent real entities.

Example:

Food Delivery System

• User → object
• Order → object
• Restaurant → object
• DeliveryPartner → object

Each object:

• holds state
• performs behavior
• interacts with other objects

State vs Behavior (Critical Distinction)

State

Represents data held by an object.

Examples:

• balance in bank account
• order status
• user details

Behavior

Represents actions an object can perform.

Examples:

• deposit money
• place order
• assign delivery

Rule of Thumb

State tells “what it is”, behavior tells “what it does”.

Common Mistake: Passive Data Classes

Beginners often design classes like this:

class Order:
    def __init__(self, id, status):
        self.id = id
        self.status = status

And then put all logic outside.

This leads to:

• anemic models
• scattered business logic
• hard-to-maintain systems

Better Approach: Active Objects

class Order:
    def mark_paid(self):
        if self.status != "CREATED":
            raise Exception("Invalid state")
        self.status = "PAID"

Now:

• object controls its state
• rules are enforced locally
• invalid transitions are prevented

Core Design Principle

Objects should manage their own state and behavior.

Not:

• external services manipulating internal state freely

Why Objects Make Systems Scalable

When systems grow:

• functions become unmanageable
• shared state becomes dangerous
• logic becomes scattered

Objects solve this by:

• encapsulating state
• bundling behavior
• reducing global complexity

Real-World Mapping

Think of objects as real entities:

Each object has:

• identity
• state
• behavior

Design Intuition

When designing a system, always ask:

“What does this entity know, and what can it do?”

If the answer is unclear, the design is incomplete.

One-Line Takeaway

Classes define structure, objects represent real instances, and together they model real-world systems through state and behavior.