The Real Signs You Need SOLID Principles in Your Code

SOLID principles are foundational to maintainable, scalable software design.

A colleague once dropped a line that stuck with me:

“If you’re implementing an interface and returning null or throwing UnsupportedOperationException, you probably need Interface Segregation.”

That one sentence nailed it better than most books.

It made me realize: we overcomplicate design principles. We memorize definitions but don’t know when or why to actually use them.

So here’s SOLID explained in plain language, with real signals that tell you “Yo, it’s time to apply this.”

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🧱 S – Single Responsibility Principle (SRP)

One reason to change. One job. That’s it.

If your class is juggling different concerns, reading files, validating inputs, sending emails then that’s a massive red flag.

Test yourself:
“Would two different people ask for changes to this class for different reasons?”

Example

```java // BAD: One class doing multiple unrelated jobs class UserManager { void saveUser(User user) { … } void sendWelcomeEmail(User user) { … } // Shouldn’t live here void logActivity(User user) { … } // Nope }

// BETTER: Separate classes by responsibility
class UserRepository { void save(User user) { ... } }
class EmailService { void sendWelcome(User user) { ... } }
class AuditLogger { void logActivity(User user) { ... } }

🔌 O – Open/Closed Principle (OCP)

Open for extension, closed for modification.

If you keep cracking open the same class every time there’s a new requirement, you’re violating OCP.

Ask this: “Can I add a new behavior without editing existing working code?”

Example

// BAD: Have to modify this class every time a new shape is added
class AreaCalculator {
    double calculate(Object shape) {
        if (shape instanceof Circle) { ... }
        else if (shape instanceof Rectangle) { ... }
        // Keep adding more else-ifs?
    }
}

// BETTER: Use polymorphism
interface Shape {
    double calculateArea();
}

class Circle implements Shape {
    public double calculateArea() { ... }
}

class Rectangle implements Shape {
    public double calculateArea() { ... }
}

🐧 L – Liskov Substitution Principle (LSP)

Subclasses should behave like their parent class - no surprises.

If your subclass throws exceptions or breaks assumptions just to “fit in,” it doesn’t belong.

Quick test: “If I use this subclass anywhere the parent is expected, will it work without weird behavior?”

Example

// BAD: Violates LSP
class Bird { void fly() { ... } }

class Penguin extends Bird {
    void fly() { throw new UnsupportedOperationException(); } // uh-oh
}

// BETTER: Use better abstraction
interface Bird { void eat(); }

interface FlyingBird extends Bird { void fly(); }

class Penguin implements Bird {
    public void eat() { ... }
}

class Eagle implements FlyingBird {
    public void fly() { ... }
    public void eat() { ... }
}

🎯 I – Interface Segregation Principle (ISP)

Don’t make classes implement methods they don’t care about.

If you have to write return null; or throw new UnsupportedOperationException();, your interface is too damn fat. Ask yourself: “Am I implementing methods that have nothing to do with my object’s job?”

Example

// BAD: Too broad
interface Worker {
    void work();
    void eat();
}

// Robot is not hungry
class Robot implements Worker {
    public void work() { ... }
    public void eat() { throw new UnsupportedOperationException(); } // Red flag
}

// BETTER: Split it
interface Workable { void work(); }
interface Eatable { void eat(); }

class Human implements Workable, Eatable { ... }
class Robot implements Workable { ... }

🔌 D – Dependency Inversion Principle (DIP)

High-level modules shouldn’t depend on low-level details. Both should depend on abstractions.

If you hardcode your service to use a specific class (like new MySQLDatabase()), swapping or testing becomes painful.

Check this: “If I wanted to replace this dependency with something else (like a mock or a different DB), can I do it easily?”

Example

// BAD: Tight coupling to low-level details
class OrderService {
    private MySQLDatabase db = new MySQLDatabase(); // Hard to test/mock
}

// BETTER: Depend on abstraction
interface Database {
    void save(Order order);
}

class OrderService {
    private final Database db;

    public OrderService(Database db) {
        this.db = db;
    }
}

Final Thought

SOLID isn’t about sounding smart. It’s about writing code that survives growth, change, and real-world stress.

If you ever catch yourself:

-Editing the same class for multiple features -Dreading changes because everything’s tangled -Implementing methods you don’t need -Struggling to test or swap dependencies

Then yes, you need SOLID. And you need it now.

📌 Follow along weekly right here or catch me on LinkedIn. I’m documenting the grind so you don’t have to make the same mistakes I did.