Building Maintainable Software: The Real World Guide to SOLID Principles (That Saved My Career)

There was a time early in my career when every new feature felt like defusing a bomb, one wrong move and the whole application would break.

If you’ve ever opened a legacy codebase and instantly wanted to close your laptop, this article is for you.

The SOLID principles quite literally saved my career.
They turned my spaghetti code into structured, testable, and actually enjoyable software.

Solid principles

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What Are SOLID Principles?

SOLID is an acronym for five design principles that help developers build maintainable, scalable, and flexible software.

Letter	Principle	Meaning
S	Single Responsibility Principle	Every class should have one and only one reason to change.
O	Open/Closed Principle	Classes should be open for extension, but closed for modification.
L	Liskov Substitution Principle	Subclasses should be replaceable for their parent classes.
I	Interface Segregation Principle	Don’t force classes to implement what they don’t need.
D	Dependency Inversion Principle	Depend on abstractions, not on concrete implementations.

Why SOLID Matters in the Real World

You will rarely get fired for missing a semicolon.
But you will lose credibility when your team can’t maintain or extend your code.

The SOLID principles solve this by:

• Making your code easier to change without breaking things.
• Helping teams collaborate without stepping on each other’s toes.
• Allowing you to add features faster with fewer bugs.

1.    Single Responsibility Principle (SRP)

“A class should have only one reason to change.”

Imagine a restaurant where the chef also handles deliveries, accounting, and social media.
If anything changes in marketing or finances, the poor chef has to change too. That’s a mess.

The same goes for code. 

Before (Violation): Bad code

public class OrderService {

    public void placeOrder(Order order) {
        // Save order to DB
        System.out.println("Saving order...");

        // Send confirmation email
        System.out.println("Sending confirmation email...");
    }
}

This class has two responsibilities, managing orders and sending emails.

After (Following SRP): Fixed code

public class OrderService {
    private final EmailService emailService;

    public OrderService(EmailService emailService) {
        this.emailService = emailService;
    }

    public void placeOrder(Order order) {
        System.out.println("Saving order...");
        emailService.sendConfirmation(order);
    }
}

public class EmailService {
    public void sendConfirmation(Order order) {
        System.out.println("Sending email to customer...");
    }
}

Now, OrderService only handles orders, while EmailService takes care of emails. Both can change independently, that’s maintainability.

graph TD A[OrderService] --> B[EmailService] A --> C[Database] B --> D[Customer]

SRP makes each class focused on one responsibility.

2.    Open/Closed Principle (OCP)

“Software entities should be open for extension, but closed for modification.”

You shouldn’t need to rebuild a car’s engine just to add Bluetooth.
Likewise, your class should be extendable without altering the existing code.

Before (Violation): Bad code

public class PaymentProcessor {

    public void process(String type) {
        if (type.equals("CREDIT")) {
            System.out.println("Processing credit card payment...");
        } else if (type.equals("PAYPAL")) {
            System.out.println("Processing PayPal payment...");
        }
    }
}

If a new payment type comes in say “CRYPTO”, you must modify this class. That’s risky and prone to errors

After (Following OCP) Fixed code

public interface Payment {
    void process();
}

public class CreditCardPayment implements Payment {
    public void process() {
        System.out.println("Processing credit card payment...");
    }
}

public class PayPalPayment implements Payment {
    public void process() {
        System.out.println("Processing PayPal payment...");
    }
}

public class PaymentProcessor {
    public void processPayment(Payment payment) {
        payment.process();
    }
}

Now you can simply create a CryptoPayment class without touching the existing code!

Criteria	Before (Bad)	After (Good)
New payment type	Modify existing class	Add new class
Risk of breaking code	High	Low
Testability	Poor	Excellent

3.    Liskov Substitution Principle (LSP)

“Subclasses should be replaceable for their base classes without breaking functionality.”

If your boss sends a dog to a “Pet” daycare, the daycare should still work fine  because a dog is a type of pet. 

Before (Violation): Bad code

public class Bird {
    public void fly() {
        System.out.println("Flying...");
    }
}

public class Penguin extends Bird {
    @Override
    public void fly() {
        throw new UnsupportedOperationException("Penguins can’t fly!");
    }
}

When code expects a Bird, it might call .fly(), but the Penguin breaks this contract.

After (Following LSP): Fixed code

public class Bird {
    public void move() {
        System.out.println("Moving...");
    }
}

public interface Flyable {
    void fly();
}

public class Eagle extends Bird implements Flyable {
    public void fly() {
        System.out.println("Eagle flying high...");
    }
}

public class Penguin extends Bird {
    @Override
    public void move() {
        System.out.println("Penguin waddling on ice...");
    }
}

Now both classes respect the behavior expected of their type, no broken promises.

classDiagram class Bird { +move() } class Flyable { +fly() } Bird <|-- Eagle Bird <|-- Penguin Eagle ..|> Flyable

4.    Interface Segregation Principle (ISP)

“Don’t force clients to depend on interfaces they don’t use.”

Imagine being hired as a chef, but your job description includes plumbing, accounting, and babysitting. Ridiculous, right?

 Before (Violation): Bad code

public interface Worker {
    void cook();
    void serve();
    void fixPlumbing();
}

public class Chef implements Worker {
    public void cook() { System.out.println("Cooking..."); }
    public void serve() { System.out.println("Serving food..."); }
    public void fixPlumbing() {
        throw new UnsupportedOperationException("I’m not a plumber!");
    }
}

After (Following ISP): Fixed code

public interface Cook {
    void cook();
}

public interface Server {
    void serve();
}

public class Chef implements Cook {
    public void cook() {
        System.out.println("Cooking delicious meals...");
    }
}

Now each role does only what’s relevant. Smaller, focused interfaces are easier to maintain.

Concept	Before	After
Interface size	Big and messy	Small and precise
Unused methods	Many	None
Flexibility	Low	High

5.    Dependency Inversion Principle (DIP)

“High level modules should not depend on low level modules. Both should depend on abstractions.”

You plug your phone charger into a socket, not directly into the power grid.
The socket (abstraction) makes your system flexible and replaceable.

 Before (Violation): Bad code

public class NotificationService {
    private final EmailSender emailSender = new EmailSender();

    public void notifyUser(String message) {
        emailSender.send(message);
    }
}

If tomorrow you need to send SMS, you have to modify this class which is not great.

After (Following DIP: Fixed code)

public interface MessageSender {
    void send(String message);
}

public class EmailSender implements MessageSender {
    public void send(String message) {
        System.out.println("Sending Email: " + message);
    }
}

public class SMSSender implements MessageSender {
    public void send(String message) {
        System.out.println("Sending SMS: " + message);
    }
}

public class NotificationService {
    private final MessageSender sender;

    public NotificationService(MessageSender sender) {
        this.sender = sender;
    }

    public void notifyUser(String message) {
        sender.send(message);
    }
}

Now NotificationService depends only on the abstraction, not the concrete class. You can easily switch from email to SMS, or even WhatsApp, without touching existing logic.

graph LR A[NotificationService] -->|depends on| B[MessageSender Interface] B --> C[EmailSender] B --> D[SMSSender]

How the principles work together

Principle	Core Idea	Benefit
SRP	One class, one responsibility	Easier to test & maintain
OCP	Extend without modifying	Adds flexibility
LSP	Subtypes behave like base types	Prevents broken inheritance
ISP	Split big interfaces	Avoids unnecessary dependencies
DIP	Depend on abstractions	Decouples high & low level code

The SOLID principles makes your software flexible, testable, and future proof.