Why 99% of Developers use interfaces wrong and how you can fix it now

Coding (Software architecture)

Designing Interfaces with Precision: The Interface Segregation Principle Demystified

In the world of software development, there are often moments of innovation that shape the way we design and build our systems.

One such moment occurred during Robert C. Martin's consultation with Xerox, where a new printer system was being developed.


This system had incredible capabilities, ranging from stapling to faxing, but as the software grew, it encountered a significant challenge.

The root of the problem lay in the design itself: a single Job class was utilized by almost all of the tasks, resulting in a "fat" class with numerous methods specific to different clients.


This meant that even a simple stapling job would have knowledge of unnecessary methods associated with printing tasks.

Enter Robert C. Martin and his visionary solution.


Drawing upon the principles of the Interface Segregation Principle (ISP) and the Dependency Inversion Principle, a transformative approach was adopted.

An interface layer was introduced, creating separate interfaces for each job type: Print Job and Staple Job.


These interfaces were implemented by the Job class, allowing the respective Print and Staple classes to call specific methods without being burdened by irrelevant functionality.

In this blog post, we delve into the Interface Segregation Principle its origin, core concepts, and practical implications.

Understanding the Interface Segregation Principle

The Interface Segregation Principle (ISP) is one of the key principles in the SOLID set of guidelines for software design.

It emphasizes that clients should not be forced to depend on interfaces they do not use.


In practical terms, this means that interfaces should be tailored to the specific needs of clients, containing only the methods that are relevant to them.

By following the ISP, we can avoid creating bloated interfaces with unnecessary dependencies.

Applying the ISP leads to improved modularity, flexibility, and code reuse.


Clients can rely on interfaces that expose only the methods they need, resulting in cleaner and more focused code.

This approach reduces the risk of unintended dependencies and promotes better software design.


If you're interested in learning more about the SOLID principles, including the Open-Closed Principle and the Single Responsibility Principle, be sure to check out the previous articles on our blog.


They provide valuable insights into designing scalable and maintainable software systems.

Let's now explore the Interface Segregation Principle and discover how it can enhance your code design and promote better software development practices.

Benefits of Applying the Interface Segregation Principle

Applying the Interface Segregation Principle (ISP) offers several benefits that contribute to improved code quality and maintainability. 


Enhanced Code Modularity


By designing interfaces that are specific to clients' needs, ISP promotes a modular approach to code organization.

Each client can rely on a separate interface tailored to their requirements, reducing the chances of unnecessary dependencies. This modular design facilitates easier code maintenance and modification.


Increased Flexibility


ISP allows for greater flexibility in software development.

Clients can interact with interfaces that expose only the methods they require, without being burdened by unused functionality.

This flexibility enables developers to make changes or add new features to individual components without impacting unrelated parts of the codebase.


Clean and Focused Code


With ISP, interfaces become clean and focused on specific tasks.

They avoid the pitfall of "fat" interfaces that encompass unrelated methods.

This clean code approach improves readability, reduces complexity, and makes the codebase easier to understand and navigate.


Improved Maintainability


ISP plays a crucial role in enhancing code maintainability.

By adhering to ISP, developers can confidently make modifications to individual interfaces or add new interfaces without affecting unrelated components.


This decoupled nature of interfaces ensures that changes are localized, reducing the risk of unintended consequences and making the system easier to maintain over time.

By embracing ISP, you can create more modular, flexible, and maintainable code.


The result is a cleaner codebase that is easier to understand, modify, and extend.

Applying the Interface Segregation Principle in Practice

I hope you understand the theory now,

There are many benefit in appliing the ISP to your project.

Below I have described the situation Robert C. Martin was in and how he fixed the issue at Xeros


// Interface segregation principle (without ISP)

interface Job {
    public function printJob();
    public function stapleJob();
    public function faxJob();

class NewPrinter implements Job {
    public function printJob() {
        // NewPrinter-specific work logic

    public function stapleJob() {
        // NewPrinter-specific eating routine

    public function faxJob() {
        // NewPrinter-specific sleeping routine

class oldPrinter implements Job {
    public function printJob() {
        // oldPrinter-specific work logic

    public function stapleJob() {
        // oldPrinter-specific eating routine

    public function faxJob() {
        // oldPrinter-specific sleeping routine



In the above code, the `Job` interface defines three methods: `printJob()`, `stapleJob()`, and `faxJob()`.

However, both the `NewPrinter` and `oldPrinter` classes are forced to implement all three methods, even though they might not need or use them all.


This violates the Interface Segregation Principle.


Let's refactor the code to adhere to ISP:

// Interface segregation principle (with ISP)

interface Printable {
    public function printJob();

interface Stapleable {
    public function stapleJob();

interface Faxable {
    public function faxJob();

class NewPrinter implements Printable, Stapleable, Faxable {
    public function printJob() {
        // NewPrinter-specific work logic

    public function stapleJob() {
        // NewPrinter-specific eating routine

    public function faxJob() {
        // NewPrinter-specific sleeping routine

class oldPrinter implements Printable {
    public function printJob() {
        // oldPrinter-specific work logic


In the refactored code, we have segregated the `Job` interface into three separate interfaces: `Printable`, `Stapleable`, and `Faxable`.

Each class now implements only the interfaces that are relevant to its responsibilities.


The `oldPrinter` class, for example, only implements the `Printable` interface since it doesn't require eating or sleeping functionality.

This segregation of interfaces provides increased flexibility.


NewPrinters can now choose and implement only the interfaces that are needed by a particular class, avoiding unnecessary dependencies and ensuring a more flexible and maintainable codebase.

Common Pitfalls and Challenges

When applying the Interface Segregation Principle (ISP), it's important to be aware of some common pitfalls and challenges that can arise.

By understanding these challenges and adopting effective strategies, you can ensure a successful implementation of ISP in your codebase.


Overly granular interfaces


One common mistake is creating interfaces that are too fine-grained, leading to a proliferation of small interfaces.

While it's important to have interfaces that represent specific responsibilities, creating an excessive number of interfaces can make the codebase difficult to manage.


It's crucial to strike a balance between granularity and practicality.

To overcome this challenge, focus on defining interfaces that are cohesive and serve a meaningful purpose.


Identify common behavior and group them into logical interfaces.

Avoid creating interfaces with only one or two methods unless they provide clear value and are widely used.


Violating the Single Responsibility Principle (SRP)


It's possible to unintentionally violate the SRP while implementing ISP.

This happens when a class is forced to implement multiple interfaces, each representing a different responsibility.


This can lead to a class with a high degree of coupling and low cohesion.

To address this challenge, ensure that each class has a clear and single responsibility.


Analyze the interfaces and their dependencies carefully to avoid overloading a class with unrelated responsibilities.

Consider refactoring the class into smaller, more focused classes that adhere to the SRP.


Adapting existing code


Implementing ISP in an existing codebase can be challenging, especially if the codebase has tightly coupled dependencies and interfaces that don't align with the principle.

Adapting existing code to follow ISP requires careful planning and refactoring.


To overcome this challenge, start by identifying the high-level responsibilities of each class and extracting interfaces that represent those responsibilities.


Gradually introduce the new interfaces and refactor the code to depend on the interfaces rather than concrete implementations.

Use automated tests to ensure that refactoring doesn't introduce regressions.


By being mindful of these common pitfalls and challenges, you can implement the Interface Segregation Principle effectively in your codebase.

Strive for balanced and cohesive interfaces, ensure adherence to the Single Responsibility Principle, and carefully adapt existing code to follow ISP.


These strategies will help you achieve a more modular and maintainable codebase while reaping the benefits of ISP.

Implementing ISP in Your Projects

Implementing the Interface Segregation Principle (ISP) in your projects can lead to more modular and maintainable code.

By following a few practical steps and guidelines, you can effectively incorporate ISP into your development process.


Begin by identifying the distinct responsibilities of your classes.

Analyze the behavior and functionality of each class to determine the specific methods and properties that are essential for each responsibility.


This helps establish a clear understanding of the responsibilities and sets the foundation for designing cohesive interfaces.

Once you have identified the responsibilities, design interfaces that are cohesive and focused.


Each interface should represent a specific responsibility and contain only the methods and properties relevant to that responsibility.

By keeping interfaces focused, you ensure that clients only depend on the interfaces they require, following the core idea of ISP.


If you're working with an existing codebase, you may need to refactor the code to align with ISP.

Identify classes that have dependencies on interfaces that are not relevant to their responsibilities.


Extract those dependencies into separate interfaces that align with their respective responsibilities.

This refactoring step helps ensure that classes adhere to the interfaces that are most appropriate for their functionality.


Implement the interfaces on classes that specifically handle the corresponding responsibilities.

This approach ensures that each class only depends on the interfaces relevant to its functionality, avoiding unnecessary dependencies.


By adhering to ISP, you create a more modular and loosely coupled codebase that is easier to maintain and extend.

Utilizing dependency injection can be beneficial when implementing ISP.


Instead of instantiating concrete classes directly, rely on interfaces in your code.


This enables easy substitution of implementations, promoting flexibility and adherence to ISP.

Dependency injection containers or manual dependency injection techniques can assist in managing dependencies effectively.


To enforce ISP and validate adherence, employ automated testing.

Write unit tests that focus on the behavior and contracts defined by the interfaces.


These tests ensure that classes implement the required methods and adhere to the interfaces' contracts.

Automated testing provides confidence that changes to the codebase don't break the expected behavior and help maintain the integrity of the ISP implementation.


Additionally, there are tools and techniques that can aid in enforcing ISP.

Static code analysis tools, like PHP_CodeSniffer or PHPStan, can detect violations of ISP in your codebase and provide valuable insights.


Integrated development environments (IDEs) with code inspection capabilities can also highlight code areas that violate ISP principles, assisting you in making corrections.


By following these practical steps, designing cohesive interfaces, refactoring existing code, separating interface implementation, utilizing dependency injection, and employing automated testing, you can effectively implement the Interface Segregation Principle in your projects.


These practices promote modularity, maintainability, and flexibility, resulting in cleaner and more robust code.



In conclusion, the Interface Segregation Principle plays a crucial role in creating modular and maintainable software systems.

By focusing on designing cohesive interfaces and ensuring that clients only depend on the interfaces they require, we can achieve code that is flexible, extensible, and easier to maintain.


Throughout this blog post, we explored the basics of ISP and its core idea: clients should not be forced to depend on interfaces they do not use.

We discussed the benefits of adhering to ISP, such as improved code modularity and flexibility, as well as how it promotes clean code and enhances maintainability.


Implementing ISP in your software development practices requires identifying responsibilities, designing focused interfaces, refactoring existing code, and utilizing techniques like dependency injection and automated testing.

By following these practical steps, you can incorporate ISP into your projects and reap the rewards of a more modular and loosely coupled codebase.


It's important to remember that ISP is just one of the SOLID principles,

a set of guidelines that promote good software design and development practices.


Adhering to these principles as a whole helps us build software systems that are robust, maintainable, and scalable.

Stay tuned for upcoming blog posts in our SOLID principles series, where we'll delve deeper into related topics.


We'll continue exploring how these principles can be applied in real-world scenarios, providing practical examples and insights to further enhance your software development skills.


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