Easy Explanation of SOLID Principles for Software Developers

Many of us struggle to remember the SOLID principles in coding. Here is a simple and creative explanation of each principle. I hope this helps you remember them.

S: Single Responsibility Principle (SRP)

Metaphor: The Swiss Army Knife vs. a Set of Tools
Imagine trying to fix an entire car with just a Swiss Army knife. Sure, it has many tools, but none of them are ideal for the specific tasks at hand. Instead, you use a proper set of tools—each tool designed for a single purpose, like a wrench for bolts or a screwdriver for screws.

Story:
Emma is a chef at a busy restaurant. If Emma had to cook, clean, manage accounts, and take orders, she’d quickly burn out. Instead, each staff member has one responsibility: chefs cook, servers take orders, and cleaners clean. This division of labour ensures efficiency and prevents chaos, just like each class in your code should have one job and one job only.

Key Lesson: A class should have one reason to change—its single responsibility.

O: Open/Closed Principle (OCP)

Metaphor: A Modular Lego House
When you build a Lego house, you can add new rooms or modify the design without breaking the structure. You just attach new blocks rather than rebuilding the entire thing. Similarly, your code should allow extensions without modifying existing functionality.

Story:
A theme park wants to add a new roller coaster to its offerings. Instead of demolishing the park, they simply reserve space and construct the ride without affecting other attractions. Your code should work the same way—be open to adding features but closed to altering stable, working parts.

Key Lesson: Classes should be open for extension but closed for modification.

L: Liskov Substitution Principle (LSP)

Metaphor: The Reliable Replacement Worker
Imagine you run a café. You hire a replacement barista for a sick employee. The replacement should be able to perform the same tasks—make coffee, take orders—without requiring the customers to adjust. If they can’t, the workflow breaks.

Story:
Sarah runs a dog day-care. She has a rule: “Every animal should be walked using the same leash method.” Whether it’s a golden retriever or a dachshund, Sarah’s employees use the same process for walking them. If a new exotic animal (say, a snake) can’t follow this method, it doesn’t fit the daycare’’s system. Similarly, in your code, derived classes should be able to replace base classes without breaking functionality.

Key Lesson: Subclasses should behave like their parent class without introducing unexpected behaviour.

I: Interface Segregation Principle (ISP)

Metaphor: The Right-Sized Menus
Imagine a restaurant offering separate menus: one for breakfast, one for lunch, and one for drinks. Customers only need to look at the menu relevant to their needs. If the restaurant had one massive menu with everything on it, customers would feel overwhelmed.

Story:
A library has different user groups—readers, authors, and admins. Readers only care about borrowing books, authors care about submitting books, and admins care about managing the library. If the library gave everyone a giant manual covering all features, it would be confusing. Instead, they tailor guides for each role. Similarly, your interfaces should be focused and specific, so classes only implement what they actually need.

Key Lesson: Break larger interfaces into smaller, more focused ones to avoid forcing unnecessary implementation.

D: Dependency Inversion Principle (DIP)

Metaphor: The Universal Power Adapter
When traveling abroad, you carry a universal power adapter that works with any socket. You don’t need to carry separate chargers for each country. Similarly, your code should depend on abstractions (universal solutions) rather than concrete implementations (specific chargers).

Story:
John is building a toy car factory. Instead of hardwiring each machine to a specific power source, he uses sockets compatible with any machine. Now, he can easily swap or upgrade machines without redesigning the factory. Similarly, your classes should rely on interfaces or abstractions so the underlying implementation can change without affecting the rest of the system.

Key Lesson: Depend on abstractions, not concrete implementations, for flexibility and scalability.

Summary Table of Metaphors and Lessons: