encapsulation, abstraction, inheritance, and polymorphism.

encapsulation(封裝)：
Each object keeps its state private. Other objects can only access by public methods.
abstraction:
Applying abstraction means that each object should only expose a high-level mechanism for using it.
This mechanism should hide internal implementation details. It should only reveal operations relevant for the other objects.
inheritance(繼承):
So how do we reuse the common logic and extract the unique logic into a separate class? One way to achieve this is inheritance.

It means that you create a (child) class by deriving from another (parent) class. This way, we form a hierarchy.

The child class reuses all fields and methods of the parent class (common part) and can implement its own (unique part).