SOLID Principles | OOPS Programming

When it comes to building maintainable, scalable, and testable software, writing “just working code” doesn’t cut it anymore—especially in large projects with multiple contributors. That’s where SOLID When building software, principles step in to guide us. Originally introduced by Robert C. Martin (Uncle Bob) and later shaped into the SOLID acronym by Michael Feathers, these five object-oriented design principles have quietly revolutionized how we architect software. Whether you’re coding alone or leading a development team, understanding SOLID can save you from future refactors, tangled logic, and frustrating bugs. In this blog, I’ll break down each principle with simple, real-world examples—because clean code isn’t just a practice; it’s a mindset.

These principles aren’t just some fancy academic ideas. In fact, SOLID principles are your secret sauce for writing understandable code, making your app more scalable, avoiding last-minute bugs, and keeping your QA team sane.

Therefore, if you’re tired of spaghetti code, mystery bugs, and classes that try to do everything (but fail at all), let’s dive in and explore how SOLID can help.

What Does SOLID Principles Stand For?

• S – Single Responsibility Principle
• O – Open/Closed Principle
• L – Liskov Substitution Principle
• I – Interface Segregation Principle
• D – Dependency Inversion Principle

Each principle is like a building block. On their own, they’re helpful; however, when combined, they form a powerful system for writing software that lasts. Together, they not only improve code quality but also make maintenance easier and development faster. So, understanding and applying these principles can truly transform how you build software.

[1] Single Responsibility Principle (SRP)

“A class should have only one reason to change.”

Think of it like this: your mobile app’s notification manager shouldn’t, at the same time, be formatting receipts and saving them to a file. In other words, that’s like asking your plumber to also do your taxes. Clearly, each role needs to stay focused on its own job. Otherwise, things get messy fast.

What not to do:

class Invoice {
    func calculateTotal() { ... }
    func printInvoice() { ... }
    func saveToFile() { ... }
}

This class is doing way too much—handling billing logic, printing, saving—everything under one roof. Because of this, it becomes hard to maintain and easy to break. So, to fix it, break the responsibilities into separate, focused classes. This way, each class has one clear job, making your code cleaner and easier to manage.

So Below Let’s Fix it:
Break the responsibilities:

class Invoice { ... }
class InvoicePrinter { ... }
class InvoicePersistence { ... }

This class is doing way too much—handling billing logic, printing, saving—everything under one roof. Because of this, it becomes hard to maintain and easy to break. So, to fix it, break the responsibilities into separate, focused classes. Each class now has one job. That’s it. SRP: ✅

[2] Open/Closed Principle (OCP)

“Software entities should be open for extension, but closed for modification.”

This principle becomes your best friend when a client requests a “tiny change” that quickly spirals into a complete rewrite. Instead of changing what already works, you extend the existing code. That way, you keep stability while adding new features seamlessly.

Bad approach:

class InvoicePersistence {
    func saveToFile() { ... }
    func saveToDatabase() { ... } // Now you've modified production code!
}

Better:

protocol Persistence {
    func save()
}

class FilePersistence: Persistence { ... }
class DatabasePersistence: Persistence { ... }

No need to touch old code; instead, just plug in new functionality like a USB—resulting in no bugs and no drama.

[3] Liskov Substitution Principle (LSP)

“Subtypes must be substitutable for their base types.”

If your child class breaks your parent class’s promise, then clearly, you’ve got a problem that needs fixing.

Bad inheritance:

class Rectangle { ... }
class Square: Rectangle { ... } // Feels right, but logically wrong

When you pass a Square where a Rectangle is expected, calculations can quickly explode. This isn’t true inheritance; rather, it’s a ticking time bomb waiting to go off. So, instead, ask yourself: “If I pass this child class, will everything still work exactly the same?” If the answer is no, then don’t subclass—it’s better to rethink your design.

[4] Interface Segregation Principle (ISP)

“No client should be forced to depend on methods it does not use.”

Have you ever implemented an interface only to find that half the methods just throw errors? If so, that’s a big red flag. It means the interface is trying to do too much, and it’s time to split it up into more focused, smaller interfaces.

Bad design:

protocol Worker {
    func eat()
    func work()
}

class Robot: Worker {
    func eat() {
        fatalError("Robots don't eat.")
    }
}

Better:
Split it up.

protocol Workable {
    func work()
}

protocol Feedable {
    func eat()
}

class Robot: Workable { ... }
class Human: Workable, Feedable { ... }

Now, no class is forced to pretend it can do something it can’t. No faking. No hacks.

[5] Dependency Inversion Principle (DIP)

“High-level modules should not depend on low-level modules. Both should depend on abstractions.”

Your app shouldn’t rely directly on some low-level file system class to save stuff. What if tomorrow you want to save to the cloud or a NoSQL DB?

Hard dependencies = hard to change.

Wrong way:

class InvoiceManager {
    let fileManager = FileManager() // Tight coupling
}

Right way:

protocol Storage {
    func save(data: String)
}

class FileStorage: Storage { ... }
class CloudStorage: Storage { ... }

class InvoiceManager {
    let storage: Storage
}

Abstractions win. Your high-level modules stay blissfully unaware of the implementation details. That’s clean architecture.

Final Thoughts on SOLID Principles

SOLID Principles isn’t just theory—it’s practical.

If you’ve ever inherited code that made you cry inside, chances are someone ignored these principles. And if you want your next project to survive changes, team members, or that one QA guy who finds every bug—start applying SOLID Principles today.

It’ll save you time. It’ll save your clients money. And it might just save your sanity.

Need help implementing SOLID Principles in your next app or codebase?

Let’s talk. I’ve been doing this for 11+ years and I’ve seen how small tweaks can create big results.

🚀 Clean code starts here.

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