SOLID Principles in Java 🚀

A comprehensive Java repository that demonstrates all SOLID Principles through practical, real-world examples.

Unlike theory-based notes, this repository focuses on understanding each principle by first writing bad code, identifying the problem, and then refactoring it into a clean, maintainable solution.

"The best way to learn software design is by improving bad code."

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📖 About

This repository contains implementation-based examples of the five SOLID principles using plain Java.

Each principle includes:

• ❌ Bad Design (Violation)
• ✅ Good Design (Solution)
• 💻 Complete Java Source Code
• 📚 Detailed Notes & Explanation
• 🏗️ Real-World Examples
• 🎯 Interview Notes

The examples are intentionally built around different domains such as:

• 🛒 Order Management System
• 💳 Payment Gateway
• 🚗 Vehicle Rental System
• 🍽️ Restaurant Management
• 📩 Notification System

instead of using a single Employee Management example repeatedly.

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📂 Repository Structure

solid-principles-java
│
├── a_SRP
│   ├── before/
│   ├── after/
│   └── README.md
│
├── b_OCP
│   ├── before/
│   ├── after/
│   └── README.md
│
├── c_LSP
│   ├── before/
│   ├── after/
│   └── README.md
│
├──d_ISP
│   ├── before/
│   ├── after/
│   └── README.md
│
├── e_DIP
│   ├── before/
│   ├── after/
│   └── README.md
│
└── README.md

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📚 SOLID Principles

Principle
S - Single Responsibility Principle
O - Open/Closed Principle
L - Liskov Substitution Principle
I - Interface Segregation Principle
D - Dependency Inversion Principle

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🎯 Learning Approach

Each principle follows the same structured learning path.

Problem Statement
        ↓
Bad Design
        ↓
Problems in the Design
        ↓
Refactoring
        ↓
Good Design
        ↓
Execution Output
        ↓
Explanation
        ↓
Interview Notes

This approach helps understand why a principle exists instead of simply memorizing its definition.

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🛠 Technologies Used

• Java
• Object-Oriented Programming (OOP)
• SOLID Principles
• IntelliJ IDEA
• Git & GitHub

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🎯 Who Is This Repository For?

This repository is useful for:

• Java Beginners
• Backend Developers
• Spring Boot Learners
• College Students
• Placement Preparation
• Software Design Enthusiasts
• Technical Interview Preparation

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📌 Learning Outcome

After completing this repository, you will be able to:

• Write clean and maintainable Java code.
• Identify violations of SOLID principles.
• Refactor bad code into better designs.
• Understand the relationship between OOP and SOLID.
• Build scalable applications with proper object-oriented design.
• Apply these principles in Spring Boot and enterprise applications.

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🤝 Contributions

Suggestions, improvements, and pull requests are always welcome.

If you find this repository helpful, consider giving it a ⭐ on GitHub.

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📜 License

This project is licensed under the MIT License.

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👨‍💻 Author

Bhanu Prakash Salapu

• GitHub: https://github.com/prakashsalapu
• LinkedIn: https://linkedin.com/in/prakashsalapu