Single Responsibility

Class should have only one job (have only one reason for change)

Open-Closed

Liskov Substitution

An object (such as a class) may be replaced by a sub-object (such as a class that extends the first class) without breaking the program

• if S is a subtype of T, then objects of type T in a program may be replaced with objects of type S without altering any of the desirable properties of that program

Interface Segregation

Dependency Inversion