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If you are a student learning programming or a developer spending time reading about clean code or software design, you’ve probably heard this advice more times than you can count: “Follow the Single Responsibility Principle.”
It sounds simple. Almost obvious. Yet in real projects, the Single Responsibility Principle (SRP) is one of the most misunderstood and frequently violated principles. Classes slowly take on more responsibilities, functions start doing “just one more thing,” and before you know it, a small change in one place breaks something completely unrelated.
It sounds simple. Almost obvious. Yet in real projects, the Single Responsibility Principle (SRP) is one of the most misunderstood and frequently violated principles. Classes slowly take on more responsibilities, functions start doing “just one more thing,” and before you know it, a small change in one place breaks something completely unrelated.
This blog is part of a SOLID Principles series, where we break down each principle one at a time, using practical examples and real-world reasoning. The ideas in this series build on each other, so understanding SRP properly will make the rest of the principles much easier to grasp.
If you’re aiming to write code that’s easier to change, test, and scale, this is the right place to start. Now, let’s clearly understand what SRP actually means.
The Single Responsibility Principle (SRP) states that:
“A class, module, or function should have only one reason to change.”
At first glance, this sounds straightforward but the key phrase here is “reason to change.” SRP is not about counting methods or lines of code. It’s about understanding responsibility. A responsibility represents one business or technical concern. If a piece of code handles more than one concern, it has more than one reason to change. And that’s where problems begin.
Ask yourself this question: “If this requirement changes, how many different reasons could force me to modify this code?” If the answer is more than one, SRP is being violated. Let’s understand this with one practical example.Imagine you have a class called InvoiceService in an application. This class does the following
At first glance, this might feel reasonable. After all, everything here is related to invoices. But now ask the SRP question: How many different reasons could force this class to change?
This single class would need to change if:
These are four completely different reasons, coming from different parts of the business and technical system. That means SRP is violated.
With SRP, the responsibilities are split:
Now, if the finance team updates tax rules, only the calculation logic changes. If marketing updates the invoice design, only the PDF generator changes. Each unit of code has one clear responsibility and one reason to change.
Following the Single Responsibility Principle is not about writing “perfect” code. It’s about making your software easier to change, safer to maintain, and simpler to understand as it grows. Here are a few benefits of SRP:
When each class or module has only one responsibility, a change affects only one part of the system. For example, changing invoice calculation rules should not require touching email or database logic. SRP ensures that when requirements change, which they always do, the impact remains limited and predictable
A class with a single responsibility has a clear purpose. Developers don’t need to mentally parse unrelated logic to understand what the code does. This is especially valuable in team environments, where someone new to the codebase should be able to understand a component quickly without fear of breaking something.
With SRP:
Instead of testing everything together, you can test one responsibility at a time, leading to more reliable and faster feedback.
One of the biggest problems in large codebases is unexpected side effects where fixing one issue accidentally breaks another. SRP minimizes this risk by ensuring unrelated logic does not live in the same place. When you change something, you know exactly what it affects.
When responsibilities are well-defined, multiple developers can work in parallel without stepping on each other’s code. One developer can improve business logic while another updates reporting or notifications without merge conflicts or coordination overhead.
Using SRP starts with a mindset shift, stop grouping code by “entity” and start grouping it by “responsibility.” Let’s understand this with one clear code example.
Consider this class:
class InvoiceService {
public double calculateTotal(double amount) {
return amount + (amount * 0.18);
}
public void saveInvoice(String invoiceData) {
System.out.println("Saving invoice to database");
}
public void generateInvoicePdf(String invoiceData) {
System.out.println("Generating PDF");
}
public void sendInvoiceEmail(String email) {
System.out.println("Sending invoice email");
}
}At first glance, this looks fine. But this class has multiple responsibilities:
That means it has multiple reasons to change, which violates SRP.
Now let’s apply SRP by separating concerns.
Invoice Calculation
class InvoiceCalculator {
public double calculateTotal(double amount) {
return amount + (amount * 0.18);
}
}
Invoice Persistence
class InvoiceRepository {
public void save(String invoiceData) {
System.out.println("Saving invoice to database");
}
}
Invoice Document Generation
class InvoicePdfGenerator {
public void generate(String invoiceData) {
System.out.println("Generating PDF");
}
}
Invoice Notification
class InvoiceEmailService {
public void send(String email) {
System.out.println("Sending invoice email");
}
}Now we have three different classes. Each class has one responsibility and only a single reason to change. Hence, changes remain isolated and predictable, and one change does not impact the entire system. This is SRP in action.
While the Single Responsibility Principle is powerful, blindly applying it can do more harm than good. SRP is a guideline, not a law, and context always matters. Understanding when not to split is just as important as knowing when to do it.
If a class has minimal logic, rarely changes, and is unlikely to grow, splitting it into multiple classes can introduce unnecessary complexity. For example, separating a simple data transformation into multiple layers may make the code harder to follow, not easier. Here is a rule of thumb: If splitting code makes it harder to understand, you’re probably overengineering.
SRP is about reasons to change, not just responsibilities. If two pieces of logic always change together, are tightly coupled by nature, and share the same lifecycle, then separating them doesn’t add value. In such cases, keeping them together actually reduces cognitive load and improves maintainability.
One common mistake is breaking SRP so far that you end up with dozens of one-method classes, deep call chains, and code that’s difficult to trace. SRP should simplify your system, not fragment it.
The Single Responsibility Principle is often introduced as a simple rule, but its real value lies in how it changes the way you think about code. SRP teaches you to stop grouping logic by features and start organizing it by responsibilities and reasons to change. When applied thoughtfully, it leads to systems that are easier to understand, safer to modify, and more resilient as requirements evolve.
At the same time, SRP is not about splitting code endlessly or following design rules blindly. The goal is clarity, knowing what each part of your system is responsible for, and keeping changes localized when the system grows.
This blog is part of a SOLID Principles series, where each principle builds on the previous one. SRP lays the foundation for everything that follows. In the next blog, we’ll explore the Open/Closed Principle (OCP). It will teach you how to design software that can grow with new requirements without constantly modifying existing code.
If you want to build software that scales beyond just working code, keep following the series.
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