Hey buddy, did you imagine that this software is as interconnected as kofta and potatoes? 🤔 Sir, one day I was in an interview for my current company, and suddenly they asked me a question that made me pause and think. We always say that OOP and SOLID are very important principles, but do we really know how each of them aligns with the other? As developers, we all know these things: the four principles of OOP—Encapsulation, Inheritance, Polymorphism, and Abstraction—and the SOLID principles that we always try to apply. But the question that reset my brain was: Which OOP principle does each SOLID principle depend on or achieve? At that moment, I realized that sometimes we treat OOP and SOLID as if they were a memorization list or checkboxes in a code review. The truth is, they complement each other, and you can't fully understand one without the other. 1️⃣ Single Responsibility Principle (SRP) ⬅️ Encapsulation The idea here, my friend, is that a class should have only one reason to change. That's why proper encapsulation is essential. You need to group the data and methods that have a single responsibility in one place and hide any details that are not related to the primary responsibility. If you find a class that does validation, persistence, logging, and business logic, it means it's violating SRP and encapsulation at the same time. Encapsulation is not just about private variables; it's about separating responsibilities first and foremost. 2️⃣ Open/Closed Principle (OCP) ⬅️ Inheritance + Polymorphism This principle requires both inheritance and polymorphism to work together. Inheritance allows you to add new behavior without modifying the old code. Polymorphism enables the system to treat child classes as if they were parent classes without using if/else statements or "breaking" the system. In other words, the old code remains unchanged, and the new code integrates seamlessly. If you're breaking OCP, you'll likely see if/else statements based on class type or switch statements on enums, which means you're not using polymorphism correctly. 3️⃣ Liskov Substitution Principle (LSP) ⬅️ Inheritance (but done right) LSP tells you that any child class must be able to replace its parent without corrupting the system's behavior. If you inherit a class and override methods in a way that breaks the contract or changes the expected behavior, it means you're doing superficial inheritance, essentially copy-pasting. LSP is what distinguishes logical inheritance from something that looks good but isn't. The risk is significant. Inheritance without LSP = a ticking time bomb 💣 4️⃣ Interface Segregation Principle (ISP) 5️⃣ Dependency Inversion Principle (DIP) ⬅️ Abstraction Both of these principles are entirely based on Abstraction. ISP tells you not to create bloated Interfaces and not to force a Class to implement Methods it doesn't need. Each Interface should be small and have a single, clear purpose. DIP advises that High-level modules should not depend on Low-level modules; both should depend on Abstractions. This means, rely on the Interface, not the Implementation. This opens the door to Dependency Injection, Mocking, and genuine Unit Testing. 💡 Summary OOP (Object-Oriented Programming) is a set of tools in your hands, and SOLID is the guide for using these tools. OOP without SOLID is like having object-oriented code that is superficially correct but fragile and prone to growing errors. SOLID without a proper understanding of OOP leads to misapplied principles. However, when both work together, you can build software that lasts for years, not just code that works today but becomes legacy tomorrow 🧱 If you want more examples of code, comparisons of right/wrong practices, or interview questions like these, let me know. I genuinely enjoyed sharing this information with you!