Single-responsibility Principle (SRP) states: A class should have one and only one reason to change, meaning that a class should have only one job. Open-closed Principle (OCP) states: Objects or entities should be open for extension but closed for modification. Liskov Substitution Principle states: Let q(x) be a property provable about objects of x of type T. Then q(y) should be provable for objects y of type S where S is a subtype of T. Interface segregation principle states: A client should never be forced to implement an interface that it doesn’t use, or clients shouldn’t be forced to depend on methods they do not use. Dependency inversion principle states: Entities must depend on abstractions, not on concretions. It states that the high-level module must not depend on the low-level module, but they should depend on abstractions.