Visitor mode and Double Dispatch

I. Concepts
Static Dispatch occurs during the compilation period. The assignment is based on the Static type information, and the method overload is Static assignment.
Dynamic Dispatch (Dynamic Dispatch) occurs during the running period. Dynamic Dispatch dynamically replaces a method. Object-oriented languages use dynamic allocation to achieve polymorphism.
The Java language supports static multi-assignment and dynamic single-assignment. The design mode Java can be used to implement Double Dispatch, that is, the visitor mode.

Ii. Visitor Pattern
Purpose: encapsulate operations that are applied to a certain data structure element.
UML diagram:
 
Main principle: "anti-Pass", Element is used for two rounds of calls between visitors, and sinlge dispatch is used to determine the type during the call.