SOLID Principle: Roadmap to Software Development

4 min readOct 23, 2023

The SOLID principles are a set of five design principles in object-oriented programming and software engineering. These principles are guidelines that help you write more maintainable, flexible, and robust code. In this blog post, we will dive into each of the SOLID principles, explaining what they are and how they can be applied to write better software.

Single Responsibility Principle (SRP)

The Single Responsibility Principle states that a class should have only one reason to change. In other words, it should have a single responsibility. When a class has more than one responsibility, it becomes tightly coupled, making it challenging to make changes without affecting other parts of the code.

To apply SRP, break down your code into smaller, focused classes or modules, each responsible for a single task. This not only makes your code more maintainable but also enhances its reusability.

Suppose you have a NotificationService that is responsible for both formatting notifications and sending them.

class NotificationService {
    public void sendNotification(String message) {
        // Format the message
        // Send the notification
    }
}

To adhere to SRP, you can separate the responsibilities:

class MessageFormatter {
    public String formatMessage(String message) {
        // Format the message
        return message;
    }
}

class NotificationSender {
    public void sendNotification(String formattedMessage) {
        // Send the notification
    }
}

Open/Closed Principle (OCP)

The Open/Closed Principle suggests that software entities (classes, modules, functions) should be open for extension but closed for modification. This means that you should be able to add new features or behaviors without altering existing code.

To follow the OCP, use inheritance, interfaces, and abstractions to allow for easy extension. By doing this, you can add new functionality by creating new classes or implementing new interfaces instead of modifying existing code.

Imagine you have a NotificationSender the class that sends notifications via email. To extend it for sending SMS notifications, you can create a new class without modifying the existing code:

class EmailNotificationSender {
    public void sendEmail(String message) {
        // Send an email
    }
}

class SmsNotificationSender {
    public void sendSms(String message) {
        // Send an SMS
    }
}

Liskov Substitution Principle (LSP)

The Liskov Substitution Principle focuses on the relationship between a base class and its derived classes. It states that objects of derived classes should be able to replace objects of the base class without affecting the correctness of the program.

In practical terms, this means that derived classes should not override or modify the behavior of the base class in a way that breaks the program’s functionality. Violating the LSP can lead to unexpected and erroneous behavior.

Interface Segregation Principle (ISP)

The Interface Segregation Principle encourages the creation of specific, client-focused interfaces rather than large, monolithic ones. It states that no client should be forced to depend on methods it does not use.

To follow ISP, create interfaces tailored to the needs of the clients that implement them. This reduces the burden on clients to implement unnecessary methods, leading to more cohesive and maintainable code.

Imagine you have an NotificationProvider interface with multiple methods, but not all notification providers need all of them.

interface NotificationProvider {
    void sendEmail();
    void sendSMS();
}

To follow ISP, you can create specific interfaces for each type of notification:

interface EmailNotificationProvider {
    void sendEmail();
}

interface SMSNotificationProvider {
    void sendSMS();
}

Dependency Inversion Principle (DIP)

The Dependency Inversion Principle promotes the decoupling of high-level modules from low-level modules. It suggests that both should depend on abstractions rather than concrete implementations. This way, changes to low-level modules won’t directly impact high-level modules.

To implement DIP, use dependency injection, inversion of control containers, and abstractions to separate high-level and low-level components. This allows for greater flexibility, easier testing, and reduced coupling.

Suppose you have a NotificationService that directly depends on concrete implementations of notification providers.

class NotificationService {
    private NotificationProvider provider;

    public NotificationService(NotificationProvider provider) {
        this.provider = provider;
    }

    public void sendNotification() {
        // Use the provider to send the notification
    }
}

To follow DIP, depend on abstractions instead of concrete implementations:

interface NotificationProvider {
    void sendNotification();
}

class EmailNotificationProvider implements NotificationProvider {
    public void sendNotification() {
        // Send an email
    }
}

class SMSNotificationProvider implements NotificationProvider {
    public void sendNotification() {
        // Send an SMS
    }
}

class NotificationService {
    private NotificationProvider provider;

    public NotificationService(NotificationProvider provider) {
        this.provider = provider;
    }

    public void sendNotification() {
        provider.sendNotification();
    }
}

These examples demonstrate how you can apply the SOLID principles to a simple notification system, making the code more modular and extensible.

Conclusion

The SOLID principles provide a solid foundation for writing maintainable and extensible code. By following these guidelines, you can create software that is more robust, easier to understand, and less prone to bugs. While they may require some practice to fully integrate into your coding habits, the benefits in terms of code quality and maintainability are well worth the effort. So, next time you start a new software project or refactor existing code, keep the SOLID principles in mind to build better, more reliable software.