Introduction
In software development, one of the biggest challenges is maintaining code quality as systems grow larger and more complex. Poorly structured code often leads to problems such as duplication, high dependency between modules, and difficulties in adding new features. To address these issues, Robert C. Martin (widely known as Uncle Bob) introduced the concept of SOLID principles, a set of five object-oriented programming (OOP) design rules that help developers write cleaner, more maintainable, and scalable code.
What is SOLID
These principles are not strict laws but best practices. When applied properly, they provide a framework for building software that adapts well to changes, remains easy to test, and is less likely to break when extended.
What Are SOLID Principles?
The acronym SOLID represents five key principles of OOP design:
S — Single Responsibility Principle (SRP)
Each class should have one and only one reason to change. In other words, a class should focus on a single responsibility. This prevents classes from becoming bloated with multiple, unrelated functions.
O — Open/Closed Principle (OCP)
Classes should be open for extension but closed for modification. This means we can add new features by creating new classes or methods instead of altering existing ones, reducing the risk of breaking stable code.
L — Liskov Substitution Principle (LSP)
Subtypes must be substitutable for their base types without altering the correctness of the program. If class B inherits from class A, then objects of class B should be usable anywhere class A is expected.
I — Interface Segregation Principle (ISP)
No client should be forced to depend on methods it does not use. Instead of one large, general-purpose interface, we should design smaller, more specific interfaces tailored to client needs.
D — Dependency Inversion Principle (DIP)
High-level modules should not depend on low-level modules; both should depend on abstractions. This reduces coupling and increases flexibility by ensuring that changes in one part of the codebase don’t ripple uncontrollably through the system.
How SOLID Principles Work in Practice
1. Single Responsibility Principle (SRP)
Imagine a class that handles both generating invoices and sending email notifications. If we need to change the email system, the invoice functionality could unintentionally break. By splitting responsibilities into two separate classes, we ensure that changes in one area do not affect the other.
2. Open/Closed Principle (OCP)
Suppose we have a payment system that currently supports only credit card payments. Instead of modifying the existing class to add PayPal or cryptocurrency, we can extend the system by adding new classes that follow the same interface. This keeps existing code stable while allowing new features.
3. Liskov Substitution Principle (LSP)
Consider the classic example of squares and rectangles. If a Square class inherits from a Rectangle class, problems can arise because changing one side of a square automatically changes the other side, which violates the expected behavior of rectangles. This example highlights why proper substitution is critical.
4. Interface Segregation Principle (ISP)
A common mistake is creating a large interface that forces implementing classes to include unused methods. For example, if a Printer interface requires methods for print, scan, and fax, but some printers only support printing, those classes will have unnecessary code. By splitting into smaller interfaces, each printer class only implements what it needs.
5. Dependency Inversion Principle (DIP)
Instead of having a NotificationService depend directly on an EmailSender class, we can depend on an abstraction (such as a MessageSender interface). This way, we can easily switch between email, SMS, or push notifications without altering the core service logic.
Benefits of Using SOLID Principles
- Maintainability: Code becomes easier to update when new requirements appear.
- Flexibility: Developers can extend the system without breaking existing functionality.
- Reusability: Components are more modular and reusable across projects.
- Testability: Smaller, single-purpose classes and abstractions are simpler to unit test.
- Collaboration: Teams working on different modules face fewer integration issues.
When to Apply (and When Not To)
While SOLID principles offer many advantages, they are not always necessary.
When to apply: In medium to large projects, especially those that need long-term maintenance and scalability. For enterprise systems or collaborative projects with multiple developers, SOLID can prevent future headaches.
When not to apply strictly: For small projects, prototypes, or Minimum Viable Products (MVPs), focusing too much on SOLID can lead to overengineering. In such cases, it may be better to prioritize speed and simplicity, applying SOLID gradually as the project grows.
SOLID in Relation to Other Principles
SOLID does not exist in isolation. It often works hand in hand with other software design principles such as:
- KISS (Keep It Simple, Stupid): Encourages simplicity in design.
- DRY (Don’t Repeat Yourself): Avoids code duplication by centralizing logic.
- YAGNI (You Aren’t Gonna Need It): Prevents adding unnecessary features.
- GRASP (General Responsibility Assignment Software Patterns): Provides guidelines for responsibility distribution in object-oriented design. Together, these principles help build a software ecosystem that is robust, scalable, and easy to adapt to future changes.
Conclusion
The SOLID principles provide developers with a proven framework to design clean, extensible, and maintainable object-oriented systems. While not rules that must be followed blindly, they serve as guiding lights that prevent common pitfalls in software design. By applying these five principles carefully and balancing them with simplicity, developers can create systems that remain resilient in the face of change.
Reference for Further Reading
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
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Top comments (1)
There are some aspects of this article I do not agree with. In particular, when initially developing an application, you have the best opportunity to follow SOLID principals. I have worked on too many applications that you can see sections of code copied over and over. Variable names that lead you to believe that the developer had originally written the code for something else. While the code is fresh in your mind, refactoring should become as common as unit testing.