Every developer has faced the frustration of working on a codebase that feels messy, fragile, and difficult to change. The bigger the project grows, the harder it becomes to add new features without breaking old ones. That’s why software engineers rely on the SOLID principles, a set of guidelines designed to keep code clean, structured, and adaptable.
In this article, we’ll walk through the SOLID principles, explain their significance, and show how they can be applied in everyday programming. Think of this as your roadmap to writing code that lasts.
What Are the SOLID Principles?
The term SOLID is an acronym representing five core principles of object-oriented design. Popularized by Robert C. Martin, these principles have become a standard foundation for building high-quality software. They are:
- S — Single Responsibility Principle (SRP)
- O — Open/Closed Principle (OCP)
- L — Liskov Substitution Principle (LSP)
- I — Interface Segregation Principle (ISP)
- D — Dependency Inversion Principle (DIP)
Together, these principles help developers create software that is easier to understand, modify, and extend.
1. Single Responsibility Principle (SRP)
The Single Responsibility Principle is straightforward: a class should only have one job.
When a class takes on multiple tasks, changes in one area may unintentionally affect another. For instance, if a single class handles both invoice calculations and file storage, altering the storage logic could break the calculation functionality.
By splitting responsibilities — such as having InvoiceCalculator and InvoiceStorage as separate classes—you ensure changes remain isolated. This makes your system more reliable and easier to maintain.
2. Open/Closed Principle (OCP)
The Open/Closed Principle states that code should be open to extension but closed to modification.
This principle encourages developers to design systems where new features can be added without rewriting existing code. A common example is using interfaces or abstract classes to handle multiple implementations.
For instance, a logging system can be extended to support file logging, database logging, or cloud logging by simply creating new classes that implement a common interface, without modifying the existing logging logic.
3. Liskov Substitution Principle (LSP)
The Liskov Substitution Principle ensures that subclasses can be used in place of their parent classes without causing issues.
Violating LSP often happens when subclasses break the expectations set by their parent class. For example, if you design a Shape class with a calculateArea() method, all subclasses should provide a consistent way to calculate an area. If a subclass throws an error instead of returning a valid value, the principle is broken.
Properly following LSP means structuring your class hierarchy so that each subclass fully supports the behavior of its parent.
4. Interface Segregation Principle (ISP)
The Interface Segregation Principle warns against creating overly large interfaces. Instead, you should design smaller, more specific ones.
Consider an interface IVehicle with methods like drive(), fly(), and sail(). A car class that implements this interface would be forced to include irrelevant methods like fly() or sail(), which makes no sense.
By splitting the interface into smaller ones — such as IDriveable, IFlyable, and ISailable—each class only implements what it needs. This leads to cleaner, more modular code.
5. Dependency Inversion Principle (DIP)
The Dependency Inversion Principle highlights that high-level modules should not depend directly on low-level modules. Instead, both should rely on abstractions.
Take a notification system as an example. If your NotificationManager class directly uses an EmailService, switching to an SMS or push notification service would require changing the main class. By introducing an interface like INotificationService, you can plug in new services without altering the core logic.
DIP improves flexibility and reduces the risk of tight coupling in your system.
Why Developers Should Care About SOLID
At first, these principles might feel abstract. But as projects evolve, their value becomes obvious. Teams that apply SOLID principles consistently benefit from:
Codebases that are easier to test and debug
Faster adaptation to new requirements
Lower risks when adding new features
More logical and organized architecture
Simply put, SOLID principles are not just academic theory — they are practical tools for everyday software development.
Real-World Applications of SOLID
Here’s where SOLID principles truly shine:
Web Applications — Keeping controllers, services, and repositories separated improves readability and flexibility.
Microservices — Each service can follow SRP, ensuring it does one job well.
Mobile Development — Frequent updates require a flexible structure, which OCP and DIP support effectively.
Team Collaboration — When code is clean and modular, multiple developers can work on different parts without conflict.
By applying SOLID, you reduce complexity while making your software adaptable to change.
Conclusion
The SOLID principles provide a blueprint for writing better code — code that is structured, adaptable, and built to last. While they may require some extra thought during development, the payoff is significant: cleaner design, smoother collaboration, and more resilient applications.
For the complete and detailed version of this topic, you can read the original article here:
👉 What is SOLID? Principles, how it works, and practical applications
If you’d love to explore more programming insights and resources, feel free to visit my knowledge hub at https://kienthucmo.com/.
Top comments (0)