Stop Writing “Spaghetti Code”: The SOLID Principles Explained in 5 Minutes

Let’s be honest: we have all written code that “works” but looks like a crime scene. You fix a bug in the Login screen, and suddenly the Settings page crashes. Why? Because your code is tightly coupled, rigid, and fragile.

Whether you are building a mobile app in React Native or a native app in Kotlin, the cure to spaghetti code is the same: The SOLID Principles.

These aren’t just academic theories; they are the survival kit for any developer wanting to move from “Junior” to “Senior.” Here is the definitive cheat sheet for 2025.

1. S — Single Responsibility Principle (SRP)

The Rule: A class (or component) should have one, and only one, reason to change.

The Mistake: creating a “God Component” that fetches data, formats dates, handles validation, and renders UI.

The Fix: Imagine a Swiss Army Knife. It’s cool, but if the knife breaks, you lose the scissors too. Instead, use a toolbox where the hammer is just a hammer.

// ❌ BAD: One component doing everything
const UserProfile = () => {
  const [data, setData] = useState(null);

  useEffect(() => {
    fetch('/api/user').then(d => setData(d)); // Data Fetching
  }, []);

  const formatDate = (date) => { ... } // Helper Logic

  return <div>{formatDate(data.date)}</div>; // UI Rendering
};

// ✅ GOOD: Separate concerns
// useUser.ts handles fetching
// DateFormatter.ts handles logic
// UserProfile.tsx handles ONLY UI

2. O — Open/Closed Principle (OCP)

The Rule: Software entities should be open for extension, but closed for modification.

The Analogy: Think of your phone. If you want to add functionality (like a new camera filter), you install an app (Extension). You don’t unscrew the back of the phone and solder new wires to the motherboard (Modification).

In Code: If you have to change a massive switch statement every time you add a new feature, you are breaking this rule. Use polymorphism or interfaces instead.

3. L — Liskov Substitution Principle (LSP)

The Rule: Subtypes must be substitutable for their base types.

The “Duck” Test: If it looks like a duck and quacks like a duck but requires batteries, you have the wrong abstraction.

The Mistake (Kotlin Example): Creating a Bird class with a fly() method, and then creating a Penguin class that extends Bird. Penguins can't fly, so you have to throw an exception. This breaks LSP.

The Fix: Separate the interfaces. Have a Bird class and a FlyingBird interface. Penguins remain birds, but they don't implement the flying interface.

4. I — Interface Segregation Principle (ISP)

The Rule: Clients should not be forced to depend on interfaces they do not use.

The Reality: Don’t create massive interfaces (bloated contracts).

The Scenario: You have an interface Worker with methods code(), test(), and eatLunch().

• A Human Worker needs all three.
• A Robot Worker needs code() and test(), but if it implements Worker, it is forced to implement eatLunch() (which returns an error).

The Fix: Split it up! Create Workable and Feedable. The Robot only implements Workable.

5. D — Dependency Inversion Principle (DIP)

The Rule: Depend on abstractions, not concretions.

The Translation: Your high-level code (Business Logic) shouldn’t care about the low-level details (Database, API, Bluetooth).

If your App directly imports MySQLDatabase, you are stuck with MySQL forever. If you instead import DatabaseInterface, you can swap MySQL for PostgreSQL, MongoDB, or Firebase without changing a single line of your business logic.

Pro Tip: This is exactly why we use Dependency Injection libraries like Hilt in Android!

Final Thoughts: Writing SOLID code takes a little more time initially, but it saves you hours of debugging later. Start by refactoring just one component today using the Single Responsibility Principle. Your future self will thank you.