Developers rely on SOLID, a set of five object‑oriented design rules first popularized by Robert C. Martin. The principles—Single Responsibility Principle, Open/Closed Principle, Liskov Substitution Principle, Interface Segregation Principle, and Dependency Inversion Principle—guide how classes interact, aiming to keep code modular and easy to extend.

A chef’s knife illustrates SRP: it performs one task, cutting, without trying to whisk. In Java, splitting an Employee class into PayCalculator and EmployeeDataSaver demonstrates the same idea. OCP treats a Lego brick as a closed unit that can be combined with new bricks, prompting developers to use stable interfaces like Shape so new shapes add without touching existing code.

LSP warns against inheritance tricks that break expected behavior; replacing a Rectangle‑Square hierarchy with a Shape interface restores substitutability. ISP advises against “fat” interfaces such as a Worker that forces robots to implement an eat method, recommending smaller, purpose‑specific contracts. DIP flips dependencies so high‑level modules like UserManager depend on abstractions (MessageService) rather than concrete EmailService, improving testability and flexibility.

Adopting SOLID helps teams scale, adapt to changing requirements, and maintain reliable systems.