SOLID Principles in LLD: Writing Code That Doesn’t Collapse Under Pressure

There’s a moment every developer faces.

The code works.
The feature ships.
Everyone’s happy.

And then… requirements change.

Suddenly your once-beautiful code becomes fragile, tangled, and hard to maintain. That’s where SOLID principles step in. Not as theory, but as survival tools.

Or as Harvey Specter might put it: > “You don’t prepare for the easy cases. You prepare for the hard ones.”

SOLID is exactly that preparation.

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What is SOLID?

SOLID is an acronym representing five design principles:

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

These principles help you write code that is:

• Maintainable
• Scalable
• Testable
• Flexible

Let’s walk through each — with intuition, real-life examples, and code.

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1. Single Responsibility Principle (SRP)

Definition

A class should have only one reason to change.

Meaning: One job. One purpose.

Real-life example

Think of a waiter in a restaurant.
A waiter takes orders and serves food.
He doesn’t cook, manage inventory, and do accounting.

Bad Design ❌

class Invoice {
    void calculateTotal() {}
    void printInvoice() {}
    void saveToDB() {}
}

Good Design ✅

class Invoice {
    void calculateTotal() {}
}

class InvoicePrinter {
    void printInvoice() {}
}

class InvoiceRepository {
    void saveToDB() {}
}

“Focus is what separates winners from everyone else.”

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2. Open/Closed Principle (OCP)

Definition

Software entities should be open for extension but closed for modification.

Bad Design ❌

class PaymentService {
    void pay(String type) {
        if(type.equals("UPI")) {}
        else if(type.equals("CARD")) {}
    }
}

Good Design ✅

interface Payment {
    void pay();
}

class UpiPayment implements Payment {
    public void pay() {}
}

class CardPayment implements Payment {
    public void pay() {}
}

“When the rules change, adapt — don’t panic.”

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3. Liskov Substitution Principle (LSP)

Definition

Objects of a superclass should be replaceable with subclass objects without breaking behavior.

Example ❌

Rectangle vs Square inheritance issue.

“Consistency builds trust — in people and in code.”

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4. Interface Segregation Principle (ISP)

Definition

Clients should not be forced to depend on interfaces they don’t use.

Bad Design ❌

interface Machine {
    void print();
    void scan();
    void fax();
}

Good Design ✅

interface Printer {
    void print();
}

interface Scanner {
    void scan();
}

“Don’t carry baggage that isn’t yours.”

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5. Dependency Inversion Principle (DIP)

Definition

High-level modules should depend on abstractions, not concrete implementations.

Bad Design ❌

class Notification {
    EmailService email = new EmailService();
}

Good Design ✅

interface MessageService {
    void send();
}

class EmailService implements MessageService {
    public void send() {}
}

class Notification {
    MessageService service;
    Notification(MessageService service) {
        this.service = service;
    }
}

“Power comes from control. Control comes from abstraction.”

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Final Thoughts

SOLID principles are not rules to memorize.
They’re lenses to think with.

SOLID helps you build systems that evolve without collapsing.

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Build clean. Build scalable. Build with intent.