System DesignInterview questions and answers

SOLID is five OOP principles for maintainable software: Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion. Together they push you toward small, focused, swappable, and testable components — the foundation of clean object-oriented design.

A class/module should have exactly one reason to change — one job, one stakeholder. If you find yourself describing a class with 'and' (handles users AND sends emails), it's doing too much. Split it. Smaller, focused units are easier to test, change, and reuse.

Software should be open for extension but closed for modification. You should be able to add new behavior without editing existing stable code. Achieved via interfaces and polymorphism — add a new implementation instead of branching the original with if/else for every new case.

Subtypes must be substitutable for their base type without breaking behavior. If Square extends Rectangle but disallows different width/height, code that worked on Rectangle breaks on Square. Inheritance must preserve contracts, not just signatures.

Many small client-specific interfaces beat one fat general-purpose one. Consumers shouldn't depend on methods they don't use. Splitting interfaces reduces the surface area each component must care about and makes them easier to mock and test.