Characteristics of inheritance
1. You can't force the use of the Extensions mechanism. The inheritance mechanism is an "is-a" relationship that exists in itself, and it is not artificially and deliberately created.
2. Inheritance is a mechanism that enables one class to obtain all the properties and methods in another through this mechanism.
3. Subclasses can directly use the existing properties and methods in the parent class, but also add their own properties and methods.
4. Advantages: improving the reusability and expansibility of the code.