SOLID Principles: Writing Code That Survives the Real World

Source: Dev.to

Introduction

Writing software is not just about making things work.
It’s about making things work today, tomorrow, and one year from now.

Last week, we explored Event‑Driven Architecture — how systems communicate and scale.
This week, we’re stepping into something even more foundational: SOLID Principles.

SOLID is not a framework or a library; it’s a way of thinking. Each letter represents a principle that helps us design clean, flexible, and maintainable software.

Single Responsibility Principle (SRP)

One thing = One job
A class should have only one responsibility and only one reason to change.

If a class handles user registration, sending emails, and saving data to the database, it has too many responsibilities. Just as a spoon shouldn’t brush your teeth, a class shouldn’t try to do everything.

Real‑world analogy

• A chef cooks.
• A driver drives.
• A teacher teaches.

Diagram

classDiagram
    class UserService {
        +registerUser()
    }
    class EmailService {
        +sendEmail()
    }
    class UserRepository {
        +save()
    }
    UserService --> EmailService
    UserService --> UserRepository

Figure 1: Single Responsibility Principle – each class has only one responsibility.

Open/Closed Principle (OCP)

You can add new toys to your toy box without breaking your old ones.
Extend behavior without modifying existing code.

Instead of changing old code every time you need something new, you extend it. Good software design allows growth without destruction.

Real‑world analogy

You don’t break your house walls to add a new device; you plug something into an existing socket.

Diagram

classDiagram
    class Payment {
        <> 
        +pay(amount)
    }
    class CreditCardPayment {
        +pay(amount)
    }
    class PayPalPayment {
        +pay(amount)
    }
    class PaymentProcessor {
        +process(payment)
    }
    Payment  Payment

Figure 2: Open–Closed Principle – extend functionality without modifying existing code.

Liskov Substitution Principle (LSP)

If you replace chocolate milk with strawberry milk, it should still behave like milk.
If something is a “type of” something else, it should behave properly as that type.

Example

“A penguin is a bird,” but “all birds can fly” is false for penguins. The problem is the design assumption, not the penguin.

Rule: Don’t force subclasses to break expectations. If a subclass cannot fully behave like its parent, the inheritance design is wrong.

Diagram

classDiagram
    class Bird
    class FlyingBird {
        <> 
        +fly()
    }
    class Sparrow
    class Penguin

Interface Segregation Principle (ISP)

Don’t force someone to do things they don’t need to do.

Example

If you give a kid responsibilities like “cook” and “drive,” that’s too much. Give only what they actually need.

Analogy

A TV remote has only TV buttons; an AC remote has only AC buttons. Large interfaces are like giant remotes, while small, specific interfaces are clean and focused.

Diagram

classDiagram
    class Workable {
        <> 
        +work()
    }
    class Eatable {
        <> 
        +eat()
    }
    class Human
    class Robot
    Workable  
        +save()
    }
    class MySQLDatabase
    class MongoDatabase
    class OrderService {
        +processOrder()
    }
    Database  Database

Figure 5: Dependency Inversion Principle – depend on abstractions, not concrete implementations.

Dependency Inversion Principle (DIP)

High‑level modules should not depend on low‑level modules; both should depend on abstractions. This reduces coupling and makes the system more flexible.

(The diagram above illustrates this principle.)

Final Thoughts

SOLID principles are not just theory; they help you write code that:

• Is easier to understand
• Is easier to extend
• Is easier to maintain
• Doesn’t break easily

Good design today saves you from pain tomorrow. When you follow SOLID, your software grows cleanly — just like a well‑organized toy box.

What about you?
Have you ever worked on a project where poor design caused problems later? Which SOLID principle do you struggle with the most? Let’s discuss in the comments.