Over the last few articles, we've explored the core concepts of Object-Oriented Programming.
We discussed why OOP was introduced, how objects help us organize code, and how concepts like Encapsulation, Abstraction, Inheritance, Composition, and Polymorphism make software easier to build and maintain.
At this point, it might seem like we've found the perfect solution.
So why did developers feel the need to introduce another set of principles called SOLID?
Wasn't OOP enough?
The short answer is:
OOP gave us powerful tools, but it didn't tell us how to use them well.
Let's understand why.
The Early Days of OOP
When Object-Oriented Programming became popular, developers finally had a better way to organize software.
Instead of scattering related data and functions across an application, they could model real-world entities as objects.
It was a huge improvement.
Applications became easier to understand.
Code became more reusable.
Responsibilities felt more organized.
For a while, everything seemed to be moving in the right direction.
Then Applications Started Growing
As software evolved, projects became much larger than anyone had imagined.
Applications that once had a few dozen classes now had hundreds.
Large enterprise systems contained thousands of classes maintained by multiple teams.
And a new kind of problem started appearing.
The problem was no longer:
How do we organize code?
The problem became:
How do we organize large codebases?
Simply using classes and objects wasn't enough anymore.
OOP Doesn't Prevent Bad Design
Let's imagine we're building an e-commerce application.
We create a class called Order.
At first, it seems reasonable.
It stores order details.
Then we add price calculations.
Later, invoice generation.
Then payment processing.
Email notifications.
Inventory updates.
Discount calculations.
Audit logging.
Before we realize it, our class looks something like this:
Order
├── calculateTotal()
├── processPayment()
├── sendConfirmationEmail()
├── generateInvoice()
├── updateInventory()
├── applyDiscount()
├── writeAuditLog()
├── exportPDF()
└── ...
Technically, it's Object-Oriented.
It uses a class.
It encapsulates data.
But is it well-designed?
Probably not.
The class has become responsible for almost every part of the order lifecycle.
Changing one feature increases the risk of breaking another.
Testing becomes harder.
Understanding the class becomes harder.
Maintaining it becomes harder.
The problem wasn't OOP.
The problem was how we were using OOP.
Classes Became Too Dependent on Each Other
Another issue appeared.
Imagine a payment service.
OrderService
│
▼
StripePaymentService
The OrderService creates and depends directly on StripePaymentService.
Everything works.
Until the business says:
"We're moving to Razorpay."
Now changes are needed in multiple places.
The higher-level business logic is tightly coupled to a specific implementation.
Again...
OOP didn't prevent this.
It allowed it.
Reusing Code Sometimes Made Things Worse
Inheritance was introduced to reduce duplication.
But many applications started building deep inheritance hierarchies.
Employee
│
├── Manager
│ ├── RegionalManager
│ │ └── SeniorRegionalManager
Initially, it looked elegant.
Over time, it became difficult to understand.
A change in a parent class unexpectedly affected several child classes.
Developers realized that simply because something can inherit doesn't always mean it should.
This realization eventually led to the idea:
Favor Composition Over Inheritance.
The Missing Piece
At this point, developers had all the building blocks.
They had:
- Classes
- Objects
- Encapsulation
- Abstraction
- Inheritance
- Polymorphism
But something was still missing.
There were no clear guidelines for questions like:
- How many responsibilities should a class have?
- When should existing code be modified?
- How should dependencies be managed?
- How should objects communicate?
- How do we build software that's easy to extend?
OOP answered:
What tools do we have?
Developers now needed answers to:
How should we use those tools?
🏋️ Enter SOLID
This is where the SOLID principles come in.
SOLID isn't a replacement for Object-Oriented Programming.
It builds on top of it.
Think of it this way.
OOP gives you the bricks, cement, and steel to construct a building.
SOLID gives you architectural guidelines for using those materials wisely.
Without those guidelines, you can still build a structure.
It just might not be easy to maintain.
What SOLID Tries to Achieve
The SOLID principles encourage us to build software that is:
- Easier to understand.
- Easier to modify.
- Easier to test.
- Easier to extend.
- Less tightly coupled.
- More maintainable over time.
None of these principles are magical rules.
They're lessons learned from years of building—and maintaining—large software systems.
Most of them were born from mistakes developers repeatedly encountered in real-world projects.
The Key Takeaway
Object-Oriented Programming solved many problems.
It gave developers a better way to organize software.
But it couldn't guarantee good design.
You can write terrible Object-Oriented code.
You can also write excellent Object-Oriented code.
The difference isn't the programming paradigm.
It's the design decisions you make.
That's exactly what the SOLID principles try to improve.
They don't teach us how to write Object-Oriented code.
They teach us how to write better Object-Oriented code.
What's Next?
Now that we understand why SOLID was introduced, we can finally explore the first principle:
The Single Responsibility Principle (SRP).
We'll answer a deceptively simple question:
How many responsibilities should a class really have?
As you'll see, the answer isn't "one method" or "one task."
It's much more interesting than that.
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