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

  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