SOLID Principles Explained with Real World Analogies

Introduction

When we build software, we often run into problems caused by unclear communication between components, messy variable declarations, and confusing class designs. It’s common to wonder why a class was written in a certain way or why it became hard to change later.

In this post, we’ll address these issues using one of the most well known foundations of good software design: SOLID principles.

What is it?

SOLID principles are a set of guidelines, rules, concepts, and definitions that help you write code that is clean, readable, and maintainable for you and your team.

How it works

SOLID is an acronym (a word formed from the initial letters of a multi word phrase, like NASA or PIN). It stands for:

Single Responsibility Principle (SRP)

This is one of the simplest principles. Basically, it says:

“A class, method, function, or whatever you write should have only one responsibility (one reason to change).”

Imagine you work at a busy restaurant and you are the only waiter during the dinner rush. It is intense, but you can keep things under control. Then the cook gets injured and has to leave, so you step into the kitchen while still taking orders and handling payments. Service slows down, mistakes pile up, and nothing gets done well because one person is trying to do too many jobs at once.

Open/Close Principle (OCP)

The Open/Close principle is straightforward. It says:

“A class, module, or function should be open for extension, but closed for modification.”

Imagine you’re playing a soccer match and your team has practiced a set play all week. Mid game, someone changes the rulebook so goals only count if the shot is taken from 20 meters away. Every existing strategy breaks, and nobody knows what “a goal” means anymore.

Liskov Substitution Principle (LSP)

The Liskov Substitution could be a little weird but is to related to Dependency Inversion Principle. Its says:

“Objects of a superclass should be replaceable with objects of its subclasses without breaking the application”

You’re at a restaurant 🍔. You order a classic hamburger. You don’t care who makes it, you just expect it to have a bun, meat, lettuce, etc., and to be eaten like any normal burger.

Now, the chef decides to give you a vegan burger instead of the classic one. According to the Liskov Substitution Principle, that should be fine, as long as the vegan burger behaves like a burger: it’s served the same way, it’s eaten the same way, and it fulfills the same purpose.

But… what if the “vegan burger” comes in a cup and you have to drink it with a straw?

Then the expectation is broken. You can’t use it like a normal burger anymore, even if it’s “technically” a variation.

Interface Segregation Principle (ISP)

The Interface Segretation, the concept explain itsefl. Its says:

“Clients should not be forced to depend on interfaces they do not use”

Imagine a TV remote 📺. You do not want a remote with 100 buttons if you only use five. It would be uncomfortable and confusing.

👉 It is better to have smaller one, more specific remotes: one for the TV, one for sound, and one for streaming.

Each device uses only what it needs.

Dependency Inversion Principle (DIP)

Dependency Inversion is the hardest one, but here’s a simple way to think about it. It says:

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

Think about charging your phone 🔋.

Your phone is not built for just one specific wall plug.

Instead, it relies on a common interface (a standard cable/port) that many chargers can use.

👉 That’s why you can:

• Borrow someone else’s charger
• Use a different adapter
• Charge from a wall outlet, a laptop, or a power bank

The key idea is that what matters is the shared “connection” (the interface), not the exact charger you have.

Advantages

SOLID helps you build systems that are easier to grow and easier to understand.

• Maintainability: changes stay local, so you don’t “break everything” when you update one feature.
• Readability: code has clearer intent (smaller classes, focused methods, explicit dependencies).
• Testability: when responsibilities are separated and dependencies are abstracted, unit tests become simple and fast.
• Extensibility: you can add new behavior by adding new classes, not rewriting existing ones.
• Team scalability: different people can work on different parts with fewer merge conflicts and fewer hidden couplings.

Disadvantages

SOLID is not free. Common trade offs include:

• More files / more indirection: interfaces, abstractions, and small classes can increase the amount of code.
• Over engineering risk: applying patterns too early can make a simple project harder than necessary.
• Harder navigation for beginners: understanding “where the real work happens” can take longer when there are many layers.
• Wrong abstractions are expensive: if you abstract the wrong thing, you may end up with complexity and still need changes.

When to use it

SOLID is most useful when you expect change and want to keep it under control:

• Medium to large codebases where multiple features evolve over time.
• Projects with multiple developers and long term maintenance.
• Domains with variability: multiple payment providers, notification channels, storage backends, etc.
• When you need strong testing: business logic should be unit testable without databases, frameworks, or external APIs.

Example

Here’s a simple example using a “payment” use case to show how the principles work together.

Bad (tight coupling): the high level service depends directly on a concrete gateway.

class OrderService {
  private final StripePaymentGateway gateway = new StripePaymentGateway();

  public void checkout(Order order) {
    gateway.charge(order.total());
  }
}

Better (DIP + OCP): depend on an abstraction, and extend by adding new implementations.

interface PaymentGateway {
  void charge(Money amount);
}

class StripeGateway implements PaymentGateway {
  public void charge(Money amount) { 
	  System.out.println("New Stripe balance amount: " + amount);
  }
}

class PaypalGateway implements PaymentGateway {
  public void charge(Money amount) {
	  System.out.println("New Paypal balance amount: " + amount);
	}
}

class OrderService {
  private final PaymentGateway gateway;

  OrderService(PaymentGateway gateway) {
    this.gateway = gateway;
  }

  public void checkout(Order order) {
    gateway.charge(order.total());
  }
}

Now:

• SR: OrderService coordinates checkout; gateways only handle payment.
• OCP: you add PaypalGateway without modifying OrderService.
• LSP: every gateway can replace another because they respect the PaymentGateway contract.
• ISP: the interface is small (only what clients need).
• DIP: the high level service depends on the abstraction (PaymentGateway), not a concrete class.

Conclusion

When responsibilities are clear, extensions don’t require rewriting existing code.