SOLID Principles
Five design principles for object-oriented programming that promote maintainable, flexible, and understandable code: Single Responsibility, Open-Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion.
Definition
SOLID is an acronym for: Single Responsibility (a class should have one reason to change), Open-Closed (open for extension, closed for modification), Liskov Substitution (subtypes must be substitutable for base types), Interface Segregation (many specific interfaces are better than one general), and Dependency Inversion (depend on abstractions, not concretions). These principles were promoted by Robert Martin.
Why It Matters
SOLID principles reduce coupling, increase cohesion, and make code easier to test, maintain, and extend. Violations often lead to fragile code where changes cascade unexpectedly. Code review tools like diffray can flag SOLID violations in architecture reviews.
Example
Violating Single Responsibility: a UserService that handles authentication, database operations, and email sending. Following SOLID: separate AuthService, UserRepository, and EmailService classes.