This video introduces the solid principles, explaining how code changes create complexity, rigidity, and fragility, and outlines each principle—single responsibility, open/closed, Liskov substitution, interface segregation, and dependency inversion.

Explore the Liskov substitution principle by evaluating substitutability of subclasses for base classes, maintain contracts, avoid side effects, and use unit tests to enforce pre and post conditions.

Learn the interface segregation principle and why clients should not depend on unused methods. See living thing and door examples and solutions using adapters or a timer service.

Decouple high-level and low-level modules by depending on abstractions rather than concrete implementations, and invert dependencies so details depend on abstractions. Swap implementations via interfaces to improve flexibility and reuse.