> For the complete documentation index, see [llms.txt](https://hanfak.gitbook.io/workspace/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hanfak.gitbook.io/workspace/general-paradigms/patterns/gangoffour/visitor.md).

# Visitor Pattern

In object-oriented programming and software engineering, the visitor design pattern is a way of separating an algorithm from an object structure on which it operates. A practical result of this separation is the ability to add new operations to existing object structures without modifying those structures. It is one way to follow the open/closed principle.

We could use inheritance to do this, but it will mean that each object will need to be modified (OCP)

### Links

* <https://softwareengineering.stackexchange.com/questions/333692/understanding-the-need-of-visitor-pattern>
* <https://medium.com/factory-mind/visitor-design-pattern-demystified-940bd3903d56>
* <http://webcache.googleusercontent.com/search?q=cache:http://butunclebob.com/ArticleS.UncleBob.IuseVisitor>
* <https://www.codeproject.com/Articles/872151/Visitor-Pattern-ReExplained>
* <https://www.journaldev.com/1769/visitor-design-pattern-java>
* <https://stackoverflow.com/questions/9759141/overloading-in-java-and-multiple-dispatch>
* Designing functional and fluent API: example of the Visitor Pattern by José Paumard <https://www.youtube.com/watch?v=gq23w9nycBs>
  * <https://github.com/JosePaumard/devoxx-belgium-2019-visitor-lambda/tree/master/src/main/java/org/paumard/visitor>
* Understanding The Visitor Design Pattern <https://www.youtube.com/watch?v=TeZqKnC2gvA>
* <https://nipafx.dev/java-visitor-pattern-pointless/>
*

## Generified visitor

<https://stackoverflow.com/questions/36363142/generified-implementation-of-visitor-pattern-in-java>

## Double dispatch

* dynamic dispatch. The actual implementation of a method is selected in runtime by the target object

  ```java
  interface Greetings {
    void hello();
  }

  class Foo extends Greetings {
    public void hello() { System.out.println("Hello"); }
  }

  class Bar extends Greetings {
    public void hello() { System.out.println("Hi!"); }
  }

  Greetings g = /* ... */;
  g.hello();
  ```
* Double dispatch
  * The ability to choose the method implementation based on this object and also one of the arguments passed to the function.
  * the Visitor pattern is a way to simulate the lack of support for double dispatch
* To simulate multiple dispatch is a language feature known as pattern matching.

## Issues

* <https://blog.jooq.org/the-visitor-pattern-re-visited/>

## Anti pattern

* <https://typeinference.com/oop/2015/08/17/anti\\_oop\\_design\\_patterns.html>

## Improved via Java pattern matching

* <https://youtu.be/10dn\\_-TBzLE?t=1516> Revisiting Design Patterns after 20 by Edson Yanaga