SOLID Principles Explained Simply

This content originally appeared on DEV Community and was authored by Mohsen Fallahnejad

The SOLID principles are five guidelines that help developers write software that’s easy to maintain, flexible, and easy to change.

1. S — Single-Responsibility Principle (SRP)

Each module or class should have one reason to change.

In simple terms: do one job, and do it well.

Bad:

class Report {
  generate() {/* ... */}
  saveToDatabase() {/* ... */} // ❌ different responsibility
}

Good:

class ReportGenerator { generate() {/* ... */} }
class ReportSaver { saveToDatabase() {/* ... */} }

2. O — Open-Closed Principle (OCP)

Software entities should be open for extension but closed for modification.

You should be able to add new behavior without touching existing, stable code.

Example:

Instead of editing a payment processor class every time you add a new method, create a new class implementing the same interface.

3. L — Liskov Substitution Principle (LSP)

Subclasses should behave like their parent classes.

Replacing a parent object with a child should not break your program.

4. I — Interface Segregation Principle (ISP)

Clients should not depend on methods they don’t use.

Create many small, specific interfaces instead of a single “god” interface.

5. D — Dependency Inversion Principle (DIP)

High-level modules should depend on abstractions, not on low-level details.

Abstractions shouldn’t depend on details—details should depend on abstractions.

Why SOLID Matters

• Easier to maintain – changes in one place have fewer ripple effects.
• More testable – smaller, focused units are easier to test.
• Reusable & scalable – modular code can grow without chaos.

Quick Reference

That’s SOLID—five simple habits that make your code cleaner, safer, and easier to grow.

Originally published on: SniplyBlog

This content originally appeared on DEV Community and was authored by Mohsen Fallahnejad