SOLID in programming: definitive guide with examples and practical tips

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Software quality is not a whim or a passing fad: it is the foundation upon which any technological project that seeks to scale, evolve, and survive in a changing market is built. Have you ever maintained or tried to evolve an application without structure or best practices? You probably know what it's like to pray every time you touch a line of code. This is where the principles SOLID They are game-changers: they offer clear guidance that, when applied with common sense, turns fragile, tangled systems into robust solutions that are easy to understand and maintain over the long term.

SOLID, beyond being an easy-to-remember acronym, represents five fundamentals capable of completely transforming the way we design classes, modules, and architectures in object-oriented programming. If you work in the development world, applying them is a turning point for both teams and individual projects. In this article, you'll discover everything essential and advanced about SOLID, its history, each principle in depth, practical examples, advantages, limitations, and how to integrate them to achieve clean, testable, and robust code.

What is SOLID and how did this approach come about?

SOLID It is much more than an acronym: it encapsulates five key principles that form the basis for the Object-oriented programming modern. These principles emerged at the end of the 20th century, amidst the increasing complexity of software systems and the urgent need to address problems such as spaghetti code, rigidity in the face of changes and the dreaded coupling between components.

The origin is located in the year 2000, when Robert C. Martin—known as Uncle Bob— compiled in his influential article “Design Principles and Design Patterns” several recommendations based on his experience and observation of successful and resoundingly unsuccessful systems. Shortly after, Michael Feathers coined the term SOLID to group and give visibility to these five principles which, applied jointly, provide systems:

• Maintainable: future development is agile and secure
• Scalable: prepared to grow without falling into chaos
• Easy to understand: Any team member can join and contribute
• Low coupling and high cohesion: Changes have the least possible impact and the code is reusable.

Since then, SOLID has been considered the cornerstone of object-oriented software design and its influence reaches both classical architecture and contemporary approaches (microservices, functional programming and multiparadigm).

Breaking down the acronym: What does each SOLID principle mean?

the acronym SOLID It is made up of five letters, each representing a principle:

• S – Single Responsibility Principle (SRP): Single Responsibility Principle
• O – Open/Closed Principle (OCP): Open/closed principle
• L – Liskov Substitution Principle (LSP): Liskov substitution principle
• I – Interface Segregation Principle (ISP): Principle of interface segregation
• D – Dependency Inversion Principle (DIP): Dependency Inversion Principle

Each of these principles plays a specific role in minimizing complexity, maximizing error protection, and achieving a truly professional code structure.

The importance of SOLID in modern development

The use of SOLID goes far beyond academic orthodoxy; Applying these principles is synonymous with survival in real projectsProblems with rigid designs, circular dependencies, testing difficulties, and the famous "this works, but no one knows why" are symptoms of ignoring good practices.

Do you want to work as a team, share code, and evolve your architecture? Adopting SOLID will allow you to avoid:

• “God” classes: objects that do everything and end up being sources of bugs
• Uncontrolled modifications: Each improvement or correction triggers a “butterfly effect” of errors
• Inability to reuse code: Copy and paste ends up being the recurring (bad) solution
• Inability to apply unit tests: the code is never decoupled enough
• Low motivation and high costs: Every new development or integration is a source of uncertainty and frustration

Therefore, Adopting SOLID not only makes life easier for developers, but it also allows you to build robust projects that are ready to grow and capable of adapting to ever-changing business requirements.

Principle S: Single Responsibility (SRP)

El Single Responsibility Principle establishes that Each class or module should have one, and only one, reason to changeIts essential objective is to promote the cohesion, that is, each component of the system focuses on a single task.

Why is this so difficult in practice? Because it's often tempting to add more methods or functionality to an existing class rather than creating a new one. However, when a class has multiple responsibilities, changes to one can negatively affect the rest, generating side effects that are impossible to anticipate.

Classic example: Think of a class that represents a book and that incorporates, in addition to the data of the book itself, logic to print it, save it, send notifications, etc. Each of these functionalities responds to different reasons for changing, which ends up breaking the SRP.

The solution is in separate each responsibility into its own class: one class for workbook data, another for printing, another for saving, another for notifications, and so on. This facilitates maintenance, reduces conflicts between teams, and makes version control much simpler.

Key phrase: “Gather things that change for the same reasons. Separate things that change for different reasons.”

Advantages of applying SRP

• Simplified maintenance: You know exactly where to modify the code in response to new requirements.
• Conflict reduction: Several developers can work on different areas without stepping on each other's toes.
• Clean version control: Each class reflects changes related to a single responsibility
• Fewer errors: errors are limited to the functionality in question

Common mistakes and how to avoid them

A common mistake is mixing different logics (for example, business logic, presentation logic, and persistence logic) into a single class. The solution is to divide the workload by creating separate classes for each topic, although this may be more laborious at first.

Principle O: Open/Closed (OCP)

This principle reads: “Software entities should be open to extension but closed to modification.”The central idea is to allow add new features without having to touch existing code, which minimizes the risk of introducing errors and facilitates system evolution.

In practice, it is recommended that classes and modules be designed so that, when requirements change or new needs arise, extensions (for example, new classes that implement an interface) can be added instead of modifying the tested and productive code.

Tip: take advantage of the use of interfaces and abstract classes to achieve this goal. This way, you can develop new features simply by creating new implementations, without fear of breaking what already works.

Example: In an invoicing system, if you need to store invoices in different locations (file, database, external services, etc.), first define a persistence interface. Each new storage method is implemented as a separate class, avoiding the need to modify the underlying logic.

Key points when applying the OCP

• Reduce the risk of bugs: the tested codebase remains intact
• Promotes extensibility: the system grows without collapsing in complexity
• Promotes polymorphism: You can replace or combine implementations at runtime

Be careful, this principle can backfire if taken to the extreme, creating countless interfaces and extensions without a clear approach, resulting in overengineering. Balance and common sense above all!

Liskov Substitution Principle (LSP)

Established by Barbara Liskov, this principle requires that Subclasses must be completely substitutable by their base classes. without altering the expected functioning of the system.

The key is that if a method or function expects an object of a base class, it can also work with any object of its subclasses. without unexpected behavior or errors.

Revealing example: If you have a Rectangle class and a Square subclass (where both sides are equal), methods that work with Rectangle should also work with Square without any problem. If a subclass starts breaking or modifying the base class's contracts (e.g., by creatively overriding setters), the LSP is being violated.

Why is LSP so important?

• Safety: prevents hard-to-detect errors and unexpected behavior
• Clarity: ensures consistency of inheritance and polymorphism
• Flexibility: makes it possible to extend the system without rewriting or breaking existing code

Principle I: Interface Segregation Principle (ISP)

"Many small, specific interfaces are better than one giant, general interface.”. The ISP encourages defining precise interfaces adapted to each need, instead of forcing all implementers to inherit methods they will never use.

Simple example: Imagine a parking interface that includes payment and reservation methods. To model free parking, you'd be forced to implement payment methods you'll never use, creating unnecessary code and confusion.

The solution is in split the interface into several more specialized ones: one for managing places and another for billing. This way, each class implements only what it needs.

In the long run, this reduces complexity, facilitates the integration of new, specific features, and eliminates "noise" in implementations. Remember: The fewer absurd dependencies, the cleaner and more maintainable the system will be..

Principle D: Dependency Inversion Principle (DIP)

Perhaps the most profound of all, this principle establishes two fundamental rules:

• High-level modules should not depend on low-level modules: both must depend on abstractions
• Abstractions should not depend on details, details should depend on abstractions.

And what does this mean in practice? Your core components (for example, business logic controllers) should always work with interfaces or abstract classes, never directly with concrete implementations (databases, external services, etc.). This way, if the technology or requirements change tomorrow, your core code will barely change.

Dependency injection It's one of the patterns that best helps fulfill this principle. This makes the system more flexible, decoupled, and easier to test with mocks or simulations.

How to apply SOLID principles in popular languages

These principles are not exclusive to any language. You can apply them in both Java, C#, Python or any other object-oriented environment. Let's see how they materialize with practical examples:

Example in Java

Suppose you have a notification service. Instead of directly depending on a specific notification type, create an interface and have the various implementations inherit from it:

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

public class EmailNotificationService implements NotificationService {
  @Override
  public void send(String message) {
    // Enviar email
  }
}

public class UserController {
  private NotificationService notificationService;
  public UserController(NotificationService notificationService) {
    this.notificationService = notificationService;
  }
  public void notifyUser() {
    notificationService.send("Bienvenido a la inversión de dependencias");
  }
}

Thus, UserController It depends on the abstraction, i.e. the interface, not the concrete implementation. For a deeper understanding of dependency management, you can read about What is OpenTitan and its influence on hardware and software security.

Example in C#

public interface IPrinter { void Print(); }
public interface IScanner { void Scan(); }
public class MultifunctionDevice : IPrinter, IScanner {
  public void Print() { /* ... */ }
  public void Scan() { /* ... */ }
}

In this way, each interface is specific and only what is necessary for the device is implemented.

Python example

class User:
    def __init__(self, name):
        self.name = name

class UserRepository:
    def save(self, user):
        # Guardar en la base de datos
        pass

class UserNotificationService:
    def send_welcome_email(self, user):
        # Enviar correo
        pass

Relationship between SOLID and Clean Code

The concept of CleanCode (clean code) is naturally intertwined with the SOLID principles. Writing clean code involves using meaningful names, keeping methods small and focused, reducing unnecessary complexity, avoiding repetition, and structuring software to be human-readable.

In fact, Uncle Bob's own books and articles (such as "Clean Code: Practical Agile Software Skills") present SOLID as a fundamental guide to achieving truly "clean" and sustainable systems. If you want to delve deeper into related aspects, you can consult good practices in software development.

How does SOLID help clean code? By making every part of the software easy to understand, modify, test, and extend, avoiding unnecessary comments, breaking the copy/paste habit, and making the code more understandable for every team member.

Advantages and benefits of implementing SOLID

• Ease of maintenance and evolution: Changes are managed in a localized and secure manner
• Scalability: The system can grow both in functionality and development teams without collapsing
• Code reuse: Components and modules are used in different contexts, avoiding duplication.
• Effective collaboration: Teams work in parallel without the risk of overlapping critical changes
• Error reduction: Clarity and segmentation minimize the appearance of bugs and make them easier to locate.
• Testability: Segmented and decoupled code is easier to test, ideal for unit and integration testing

Limitations and possible criticisms of SOLID

Like any approach, applying SOLID in an overly dogmatic way can lead to problems:

• Unnecessary complexity: Too many interfaces or fragmented classes can hinder development, especially in small projects.
• Overdesign: Anticipating all possible extensions can end up complicating the simple.
• Learning curve: Beginner developers can feel overwhelmed when structuring their first applications.
• Rigidity: Following the principles strictly may, in some scenarios, go against the required agility or efficiency

The key is in apply SOLID with common senseConsider the size, budget, and context of your projects. In smaller applications, you probably don't need to invest in complex patterns, but you should embrace the concept of single responsibility to avoid major problems in the future.

When (and when not) to apply SOLID

These principles are designed for systems that require high maintainability, scalability, and flexibility. However, there are contexts where applying them strictly can be counterproductive:

• Small projects or prototypes: prioritizes speed and simplicity over future extensibility
• New teams: impose principles progressively, not as dogma
• High performance situations: evaluate whether adding additional layers really compensates
• Legacy systems: Adapting legacy code may require considerable investment; prioritize incremental improvements

Ultimately, apply what makes sense, but keep in mind that premature optimization overload can be as damaging as the absence of good practices.

Relationship of SOLID with other principles and patterns

In addition to SOLID, there are other complementary principles and good practices:

• DRY (Don't Repeat Yourself): avoid code duplication
• KISS (Keep It Simple, Stupid): keep the solutions simple
• YAGNI (You Aren't Gonna Need It): Don't implement features until they are really needed
• GRASP: patterns of assigning responsibilities to objects

Integrating SOLID with these approaches multiplies the benefits in any modern object-oriented architecture.

Practical tips for adopting SOLID in your daily life

• Start small: implements the single responsibility principle first
• Refactor wisely: Periodically review your code looking for code smells and opportunities for improvement.
• Adapt theory to practice: adjust the principles to the reality of your team and project
• Share knowledge: promotes internal debates, reviews and training to lay the foundations

Do not forget that SOLID principles are guidelines, not inflexible lawsUse them to improve, but don't sacrifice pragmatics or speed of delivery when the context requires it.

This approach provides a solid framework for developers to create more robust, flexible, and maintainable systems over the long term, preventing software from aging prematurely and facilitating adaptation to changing market and business requirements.