Detailed explanation of visitor mode of Java design mode

The beginning of Dr. Yan Hong's Book Java and patterns describes the visitor pattern as follows:

Visitor patterns are the behavior patterns of objects. The purpose of visitor pattern is to encapsulate some operations imposed on some data structure element. Once these operations need to be modified, the data structure that accepts the operation can remain unchanged.

Concept of assignment

The type of a variable when it is declared is called the static type of the variable, and some people call the static type explicit type; The real type of the object referenced by the variable is also called the actual type of the variable. For example: