The SOLID principles are a set of five design principles that are commonly used in object-oriented programming to create more maintainable, flexible, and scalable software. The acronym “SOLID” stands for:

• Single Responsibility Principle (SRP): A class should have only one reason to change.
• Open-Closed Principle (OCP): Software entities (classes, modules, functions, etc.) should be open for extension but closed for modification.
• Liskov Substitution Principle (LSP): Objects of a superclass should be able to be replaced with objects of a subclass without causing errors or unexpected behavior.
• Interface Segregation Principle (ISP): A client should not be forced to depend on methods it does not use.
• Dependency Inversion Principle (DIP): High-level modules should not depend on low-level modules; both should depend on abstractions.

Together, these principles provide a framework for designing software that is modular, testable, and easy to maintain and extend over time. By adhering to the SOLID principles, developers can create software that is more robust, adaptable, and efficient, and that can be updated or modified more easily without breaking existing functionality.

Criticism

Relatively recently, Dan North has posted a rebuttal of each of the SOLID principles on several forums and posted his own take on a set of modern principles he called CUPID. While the ideas of CUPID are fairly sound, arguably there is still merit in understanding SOLID, if not actively adopting them.