Introduction to SOLID Principles

 

SOLID is an acronym that represents five key principles of object-oriented software design. Following these principles helps developers create robust, maintainable, and scalable systems.

1. S – Single Responsibility Principle (SRP)

A class should have only one reason to change.

• Each class should do one thing and do it well.

• Changes in requirements should affect only one class, reducing risk of bugs.

Example:

• A ReportGenerator class should generate reports, but not handle saving them to disk. Saving can be a separate ReportSaver class.

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2. O – Open/Closed Principle (OCP)

Software entities should be open for extension, but closed for modification.

• You should be able to add new functionality without changing existing code.

• Achieved via inheritance or interfaces.

Example:

• A PaymentProcessor interface allows adding new payment methods (Credit Card, PayPal) without modifying existing processors.

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3. L – Liskov Substitution Principle (LSP)

Subtypes must be replaceable for their base types without affecting correctness.

• Derived classes should behave consistently with their base class.

• Prevents unexpected behavior when substituting subclasses.

Example:

• If Bird has a Fly() method, a subclass Penguin should not inherit it if penguins cannot fly. Better to redesign the hierarchy.

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4. I – Interface Segregation Principle (ISP)

Clients should not be forced to depend on interfaces they do not use.

• Split large interfaces into smaller, specific ones.

• Makes code easier to implement and maintain.

Example:

• Instead of a Machine interface with Print(), Scan(), and Fax(), create separate IPrinter, IScanner, IFax interfaces.

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5. D – Dependency Inversion Principle (DIP)

High-level modules should not depend on low-level modules. Both should depend on abstractions.

• Reduces coupling and increases flexibility.

• Use interfaces or abstract classes to decouple dependencies.

Example:

• A ReportService depends on an ILogger interface rather than a concrete FileLogger. You can switch to DatabaseLogger without changing ReportService.

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Summary

• SOLID principles help make code clean, flexible, and maintainable.

• Applying them reduces bugs, simplifies testing, and allows safe changes.