SOLID Principles: Ultimate Guide to Clean and Scalable Code

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SOLID Principles & LLD HLD

The SOLID principles LLD HLD

S — Single Responsibility Principle (SRP)
O — Open/Closed Principle (OCP)
L — Liskov Substitution Principle (LSP)
I — Interface Segregation Principle (ISP)

✅ Final Expert Notes – SOLID Principles Summary (with Ideal Code Flow)

Principle	Principle Core	Clean Code Tip (As You Mentioned)
S	Each class should do one thing only	🔹 Make classes small and focused 🔹 Don't mix responsibilities
O	Extend behavior without modifying existing code	🔹 Allow adding/removing logic without touching current logic
L	Subclass must behave like superclass	🔹 Superclass reference should safely hold any subclass
I	Don't force unnecessary method implementation	🔹 Create granular interfaces for exact behavior
D	High-level should depend on abstraction, not concrete	🔹 Inject interface (superclass), not implementation

💡 Your Thought	✅ Matched Explanation
S: Make classes small and single-responsibility	✔️ SRP section breaks logic, printing, and persistence into separate focused classes
O: Design in a way that new logic can be added without changing existing code	✔️ OCP example lets you add new shapes without modifying `AreaCalculator`
L: Superclass should hold subclasses properly in inheritance	✔️ LSP example uses `Bird` superclass and replaces safely with `Sparrow`, `Crow`
I: Don’t force classes to implement unnecessary interface methods	✔️ ISP example breaks down `Animal` into `Flyable`, `Swimmable`, etc., used only as needed
DIP: Inject interfaces, not concrete classes. Let interface hold subclasses	✔️ DIP uses `MessageService` as interface, injected into high-level `NotificationSender` class

Rule:

Principle	Design Pattern Synergy	Gaming Context

SRP

MVC, Clean Architecture

Separate combat logic from rendering and persistence

OCP

Strategy, Decorator

Add power-ups or attack modes easily

LSP

Polymorphism

Switch between NPC types safely

ISP

Role Interfaces, Adapter

Build modular components: shooter, jumper, defender

DIP

Dependency Injection, Inversion of Control

Easily plug different services (DB, cache, APIs)

eg:→

Principle	Key Rule	Gaming Analogy
SRP	One reason to change	`PlayerManager` shouldn't handle UI and DB
OCP	Extend, don’t modify	Add new weapons without changing `WeaponManager`
LSP	Subtypes should behave correctly	`Mage` shouldn't break game when used as a `Player`
ISP	Prefer small, specific interfaces	Don't force a tank to fly
DIP	Depend on abstractions	Use `AudioService`, not `MP3Player` directly

1)Single Responsibility Principle (SRP)

Meaning:

A class should have only one reason to change, meaning it should have only one responsibility.

Key Points:

A class should focus on a single task or functionality.
Reduces coupling and increases cohesion.
Simplifies debugging and testing.

Use Case:

A system that separates data access logic, business logic, and presentation logic.

Real-Time Complex Example:

A user management module where one class handles user data and another class handles email notifications.

Code Example (Without SRP):

class UserManager {
   public void createUser(String name, String email) {
       System.out.println("User created with name: " + name);
       sendEmail(email);
   }
   public void sendEmail(String email) {
       System.out.println("Email sent to: " + email);
   }
}

Refactored Code (With SRP):

class UserManager {
   private EmailService emailService;
   public UserManager(EmailService emailService) {
       this.emailService = emailService;
   }
   public void createUser(String name, String email) {
       System.out.println("User created with name: " + name);
       emailService.sendEmail(email);
   }
}
class EmailService {
   public void sendEmail(String email) {
       System.out.println("Email sent to: " + email);
   }
}

2. Open-Closed Principle (OCP)

Meaning:

Software entities (classes, modules, functions) should be open for extension but closed for modification.

Key Points:

You can add new functionality without altering existing code.
Achieved through abstraction and polymorphism.
Helps prevent regression errors.

Use Case:

A payment processing system that supports multiple payment methods (credit card, PayPal, etc.).

Real-Time Complex Example:

Adding a new payment type without modifying the existing code.

Code Example (Without OCP):

class PaymentProcessor {
   public void processPayment(String type) {
       if (type.equals("CreditCard")) {
           System.out.println("Processing credit card payment...");
       } else if (type.equals("PayPal")) {
           System.out.println("Processing PayPal payment...");
       }
   }
}

Refactored Code (With OCP):

interface Payment {
   void pay();
}
class CreditCardPayment implements Payment {
   public void pay() {
       System.out.println("Processing credit card payment...");
   }
}
class PayPalPayment implements Payment {
   public void pay() {
       System.out.println("Processing PayPal payment...");
   }
}
class PaymentProcessor {
   public void processPayment(Payment payment) {
       payment.pay();
   }
}

3. Liskov Substitution Principle (LSP)

Meaning:

Derived classes should be substitutable for their base classes without affecting the correctness of the program.

Key Points:

Avoids using subclasses that break the functionality of a base class.
Helps ensure the "is-a" relationship is maintained.

Use Case:

A shape drawing application that works for both circles and squares.

Real-Time Complex Example:

A class hierarchy where a subclass doesn't override behavior that contradicts the base class.

Code Example (Violation of LSP):

class Rectangle {
   int width, height;
   public void setWidth(int width) {
       this.width = width;
   }
   public void setHeight(int height) {
       this.height = height;
   }
   public int getArea() {
       return width * height;
   }
}
class Square extends Rectangle {
   @Override
   public void setWidth(int width) {
       super.setWidth(width);
       super.setHeight(width);
   }
   @Override
   public void setHeight(int height) {
       super.setHeight(height);
       super.setWidth(height);
   }
}

Refactored Code (With LSP):

interface Shape {
   int getArea();
}
class Rectangle implements Shape {
   int width, height;
   public Rectangle(int width, int height) {
       this.width = width;
       this.height = height;
   }
   public int getArea() {
       return width * height;
   }
}
class Square implements Shape {
   int side;
   public Square(int side) {
       this.side = side;
   }
   public int getArea() {
       return side * side;
   }
}

4. Interface Segregation Principle (ISP)

Meaning:

A class should not be forced to implement interfaces it does not use.

Key Points:

Split large interfaces into smaller, specific ones.
Ensures only relevant methods are implemented.

Use Case:

A printer interface that segregates basic printing and advanced faxing/scanning functionalities.

Real-Time Complex Example:

A multifunctional printer (MFP) where some models don't support scanning or faxing.

Code Example (Without ISP):

interface Printer {
   void print();
   void scan();
   void fax();
}
class BasicPrinter implements Printer {
   public void print() {
       System.out.println("Printing...");
   }
   public void scan() {
       throw new UnsupportedOperationException("Scan not supported");
   }
   public void fax() {
       throw new UnsupportedOperationException("Fax not supported");
   }
}

Refactored Code (With ISP):

interface Print {
   void print();
}
interface Scan {
   void scan();
}
interface Fax {
   void fax();
}
class BasicPrinter implements Print {
   public void print() {
       System.out.println("Printing...");
   }
}
class MultiFunctionPrinter implements Print, Scan, Fax {
   public void print() {
       System.out.println("Printing...");
   }
   public void scan() {
       System.out.println("Scanning...");
   }
   public void fax() {
       System.out.println("Faxing...");
   }
}

5. Dependency Inversion Principle (DIP)

Meaning:

High-level modules should not depend on low-level modules. Both should depend on abstractions.

Key Points:

Increases modularity and flexibility.
Dependency injection is a common implementation.

Use Case:

A notification system where the high-level module doesn't depend on specific email or SMS classes.

Real-Time Complex Example:

Switching from email notifications to SMS notifications without changing the core business logic.

Code Example (Without DIP):

class EmailService {
   public void sendEmail(String message) {
       System.out.println("Sending email: " + message);
   }
}
class NotificationManager {
   private EmailService emailService;
   public NotificationManager() {
       this.emailService = new EmailService();
   }
   public void notify(String message) {
       emailService.sendEmail(message);
   }
}

Refactored Code (With DIP):

interface NotificationService {
   void notify(String message);
}
class EmailService implements NotificationService {
   public void notify(String message) {
       System.out.println("Sending email: " + message);
   }
}
class SMSService implements NotificationService {
   public void notify(String message) {
       System.out.println("Sending SMS: " + message);
   }
}
class NotificationManager {
   private NotificationService notificationService;
   public NotificationManager(NotificationService notificationService) {
       this.notificationService = notificationService;
   }
   public void notify(String message) {
       notificationService.notify(message);
   }
}

Why Are SOLID Principles Important?

Maintainability: Easier to debug and modify code.
Scalability: Supports new requirements without breaking existing code.
Testability: Enhances unit testing by promoting loose coupling.
Reusability: Encourages modular design.
Readability: Clear structure simplifies understanding for new developers.

By adhering to these principles, developers create robust, flexible, and efficient software systems.

LLD (Low-Level Design) Syllabus – From Basics to Expert

Stage 1: Fundamentals of LLD

Learn core principles, design basics, and OOP application.

Topic	Focus
✅ OOP Principles	Encapsulation, Abstraction, Inheritance, Polymorphism
✅ SOLID Principles	SRP, OCP, LSP, ISP, DIP (already covered)
✅ UML Class Diagrams	Association, Aggregation, Composition, Inheritance
✅ Design Process	Requirements → Class/Interface Design → Sequence Diagrams
✅ Composition vs Inheritance	Prefer Composition (with real examples)
✅ Object Modeling	Model real-world entities using classes and relationships

Stage 2: Core Design Patterns

Must-know patterns for modular, extensible, and testable code.

Category	Patterns
Creational	Singleton, Factory, Abstract Factory, Builder, Prototype
Structural	Adapter, Decorator, Composite, Proxy, Bridge, Facade
Behavioral	Strategy, Observer, Command, Chain of Responsibility, State, Template Method, Mediator, Visitor

Focus on:

When to use each pattern
UML + Code
Pros, cons, trade-offs

Stage 3: Object Modeling Problems

Practice converting product ideas to class designs.

Problem	Concepts Tested
Design a Pen / Elevator / Vending Machine	Basic abstraction, state, hierarchy
Design a Parking Lot	Composition, OOP modeling, capacity rules
Design a BookMyShow / Splitwise	Real-world flows, service modeling
Design a Cab Booking System	Location tracking, driver-rider matching, status
Design a Chess / Tic-Tac-Toe / Snake game	Board logic, players, turn, game engine
Design a Notification System	Observer, Strategy, Queueing, async logic
Design a Rate Limiter	Token bucket, sliding window, multithreading
Design Amazon Locker	Locking, location, delivery flow

Stage 4: Advanced Design Concepts

Go beyond class diagrams. Introduce concurrency, scalability, and extensibility.

Topic	Concepts
Thread-safe Design	Locks, synchronization, atomic operations
Caching Strategies	LRU, LFU, Time-based, Write-through, In-memory vs distributed
Rate Limiting	Fixed window, sliding window, token bucket
Pub-Sub Architecture	Decouple producer/consumer using observer/event model
Feature Toggle	Interface-based injection, strategy patterns
Extensible Framework Design	Plugin architecture, interface registries

Stage 5: Refactoring & Code Review

Demonstrate ability to fix and evolve existing systems.

Topic	Skills
Refactor Monolith to Modular Design	Extract classes, split responsibilities
Decouple via Interfaces	Use DIP to remove tight coupling
Improve Testability	Use mocks, stubs, dependency injection
Handle Legacy Code	Add wrappers, refactor in steps, use Facade

Stage 6: Tooling & UML Support

Tools	Purpose
Draw.io / Lucidchart	UML diagrams
PlantUML	Text-based UML
VS Code + Java/Python	Practice LLD patterns
GitHub + JUnit/TestNG	Codebase organization + testing

Stage 7: Expert-Level Case Studies (LLD + HLD Hybrid)

Full product-level design breakdowns.

System	Why It’s Asked
Design Netflix/YouTube Player	Streaming, caching, state machines
Design WhatsApp/Slack Backend	Message queueing, delivery status, group chats
Design Instagram	Feed, user stories, media service
Design Zomato/UberEats	Menu, delivery flow, search
Design Logging/Monitoring System	Observer, Buffering, File I/O, rotation
Design GitHub	Branching, merging, repository modeling
Design Google Docs (collaboration)	CRDT, shared document edits

Master LLD Interview Thinking

Technique	Description
Start with Use-Cases	What should the system do? (e.g., booking, canceling)
Identify Core Objects	Nouns → classes, Verbs → methods
Think Extensibility	Ask: "How to add new feature without rewriting old code?"
Apply Patterns Sparingly	Don’t force all 23 patterns—apply based on need
Show Trade-offs	Memory vs speed, extensibility vs simplicity

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9 min read

nov 23, 2024

By Nitesh Synergy