The cornerstone of good software design is focus. A class should do one thing, do it well, and not rely on external assumptions to fulfill its responsibilities. This is the essence of the Single Responsibility Principle (SRP). As I explained to my mentee during a mentoring session: “Think about this component as if it’s the only thing in the universe. It needs to meet its own requirements without relying on something else to clean up after it.” This article explores how to design classes that are self-sufficient, maintainable, and aligned with SRP.

What Is the Single Responsibility Principle?

The SRP states that a class should have one reason to change — essentially, it should own a single responsibility. This doesn’t just make theoretical sense; it leads to practical benefits:

  • Easier Maintenance: A focused class is easier to understand, test, and modify.
  • Improved Reusability: When a class does one thing, it’s more likely to be reusable in different contexts.
  • Reduced Coupling: A self-sufficient class doesn’t depend heavily on other components, making your codebase more modular.

In a mentoring session, I emphasized: “You have to compartmentalize the requirements of each component. Don’t assume that something else will handle synchronization or updates — every class must take care of its own responsibilities.”

Defining a Class’s Responsibility

Start by asking, “What is this class responsible for?” For example, let’s consider a class that manages articles, synchronizes them with a backing store.

When my mentee and I reviewed such a class, I said: “The responsibility of this class is to create and manage articles. That’s its primary purpose. Anything outside of that should be handled by another component.” This clarity helps prevent a class from taking on too many unrelated tasks.

How to Keep a Class Focused

  1. Clearly Define Boundaries

Identify what your class is meant to do and stick to it. If you find yourself adding methods or properties that don’t align with the primary purpose, it’s a sign that functionality belongs elsewhere. As I advised: “All we’re talking about is this class. Forget about anything outside of it. What’s its job?”

  1. Encapsulate Related Logic

Group related functionality within the class, but avoid mixing concerns. For instance, in our session, I noted: “If your class is responsible for synchronizing state, it should handle both the in-memory structure and the file, but not other processes like authentication.”

  1. Rely on Composition, Not Assumptions

When a class needs to interact with other components, use composition to inject dependencies explicitly. Avoid hidden assumptions about what other parts of the system will handle. I explained: “You can’t assume that another component will synchronize the file or update dependencies. This class must meet its own requirements.”

An Example of SRP in Action

Let’s revisit the article management class. Initially, the class attempted to manage articles, validate them, summarize them, and proofread them. This violated SRP, as it bundled unrelated responsibilities into one component.

Before Refactoring:

public class ArticleManager
{
    public void CreateArticle(Article newArticle)
    {
        // Validate
        Validate(newArticle);
        // Create Summary
        CreateSummary(newArticle);
        // Proofread
        Proofread(newArticle);
        // save the article to the database
        SaveToDatabase(newArticle);
    }
}

After Refactoring:

  • ArticleManager handles saving of articles to the database.
  • ArticleValidator validates an articles contents to make sure it is complete before we begin processing it.
  • ArticleSummarizer creates a short summary (or summaries) of the article for different purposes.
  • ArticleProofreader creates a list of editing suggestions.
public class ArticleManager
{
    private readonly IArticleValidator _validator;
    private readonly IArticleSummarizer _summarizer;
    private readonly IArticleProofreader _proofreader;

    public ArticleManager(IArticleValidator validator, IArticleSummarizer summarizer, IArticleProofreader proofreader)
    {
        _validator = validator;
        _summarizer = summarizer;
        _proofreader = proofreader;
    }

    public void CreateArticle(Article newArticle)
    {
        var validationResults = _validator.Validate(newArticle);
        var summarizationResults = _summarizer.GenerateSummary(newArticle);
        var proofReaderResults = _proofreader.Proofread(newArticle);
        SaveToDatabase(newArticle);
    }
}

By offloading these other responsabilities to separate classes, ArticleManager is getting closer to a single responsability. This refactoring process should have opened our eyes to a new responsability — or a deliniation between two distinct responsabilities — Saving Articles to the database and orchestrating all the business logic around the creation of a new article.

We’ve already got some of these other responsabilities isolated but somebody needs to know what to do with their results. What should we do once an article is validated? What should we do when we have generated an articles summary? What should happen when we have proof read the article — do other components need to kick off if foul language was detected or a myriad of other responsibilities?

The point is that this is a process — a process by which you tease out the responsibilities as they make themselves aware to you.

When Responsibilities Blur

Sometimes, responsibilities overlap or evolve over time, making it difficult to maintain focus. This is where iteration comes into play. As I mentioned: “You might make a design choice that seems best at the time, but two weeks later, you realize it wasn’t the right one. That’s okay — design is an iterative process.”

The Benefits of Adhering to SRP

  • Improved Maintainability

Focused classes are easier to debug and modify. As I said: “If you treat each piece of code as if it’s the only thing in the universe, you’ll build something that’s easier to understand and maintain.”

  • Enhanced Testability

When a class has a single responsibility, its tests can focus on that specific behavior without mocking unrelated functionality.

  • Greater Reusability

Classes that do one thing well are more likely to be reused across projects or systems. By breaking functionality into smaller, focused components, you create building blocks for future development.

Conclusion

The Single Responsibility Principle isn’t just a theoretical guideline — it’s a practical approach to writing cleaner, more reliable code. By defining clear boundaries, encapsulating related logic, and avoiding external assumptions, you create classes that are easier to maintain and scale. As I told my mentee: “There’s no single right answer in design, but adhering to principles like SRP will always steer you in the right direction.” By keeping your classes focused, you’ll build systems that are not only functional but elegant and enduring.