Application extension of polymorphism
1. Polymorphism is applied to the return value: subtype+this type can be returned as rhetorical value
2 . Polymorphism is applied to formal parameters: subtype+this type can be passed as parameters.
Example：
1. Polymorphism is applied to the return value: subtype+this type can be returned as rhetorical value
2 . Polymorphism is applied to formal parameters: subtype+this type can be passed as parameters.
Example：