Object-oriented design principles by Robert C. Martin aka Uncle Bob. These are principles, not rules or goals. They are meant to guide design but not dictate it.
A class should have one and only one reason to change, meaning that a class should have only one job.
Objects or entities should be open for extension, but closed for modification. By coding to interfaces and having well-defined, single responsibility methods, they can be re-used or combined with other logic to extend the functionality of a class without necessarily modifying the internal logic of the class itself.
Every subclass/derived class should be substitutable for their base/parent class.
A client should never be forced to implement an interface that it doesn’t use, and clients shouldn’t be forced to depend on methods they do not use. Interfaces should be specific, single-responsibility to avoid this.
Entities must depend on abstractions not on concretions. High-level module should not depend on low-level modules, but they should depend on abstractions. So substituting for a different low-level module implementation shouldn’t break the high-level module.