What Is SOLID? Five Object-Oriented Design Principles Every Developer Should Know

Why SOLID Matters

As a software project grows, code that started out simple can quickly turn into a tangled web of dependencies. SOLID is a set of five object-oriented design principles that make code easier to change, simpler to test, and cheaper to maintain over time.

The Five Principles, Explained Simply

S – Single Responsibility: A class should have only one reason to change. An Invoice class, for example, shouldn't both calculate tax and print the invoice.

O – Open/Closed: Code should be open for extension but closed for modification. Interfaces and patterns like Strategy make this possible without touching existing, working code.

L – Liskov Substitution: A subclass should be usable anywhere its parent class is expected, without breaking the program.

I – Interface Segregation: Prefer several small, focused interfaces over one large, do-everything interface.

D – Dependency Inversion: High-level classes shouldn't depend directly on low-level implementation details — both should depend on abstractions instead.

Takeaway

Following SOLID isn't about making code more complicated — it's the opposite: it keeps long-term maintenance simple. On the Laravel and Next.js projects I work on, these principles directly affect how fast new features ship and how few bugs slip through.