SOLID Principles are well-known Object-Oriented Design (OOD)principles summarized by Uncle Bob (Robert C. Martin). The word SOLID comes from the initials of the five principles it refers to, including Single responsibility principle, Open-closed principle, Liskov substitution principle, Interface segregation principle and Dependency inversion principle. Those principles are closely related to each other, and can be a great guidance in practice.
Here is a widely used summary of SOLID principles from Uncle Bob:
Single responsibility principle: A class should have one, and only one, reason to change
Open-closed principle: You should be able to extend a classes behavior, without modifying it
Liskov substitution principle: Derived classes must be substitutable for their base classes
Interface segregation principle: Make fine-grained interfaces that are client specific
Dependency inversion principle: Depend on abstractions, not on concretions
In this chapter, we will...