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iluwatar#590 explanation for Acyclic Visitor
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‎acyclic-visitor/README.md‎

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## Intent
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Allow new functions to be added to existing class hierarchies without affecting those hierarchies, and without creating the troublesome dependency cycles that are inherent to the GOF VISITOR Pattern.
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Allow new functions to be added to existing class hierarchies without affecting those hierarchies, and without creating
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the troublesome dependency cycles that are inherent to the GoF Visitor Pattern.
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## Explanation
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Real world example
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> We have a hierarchy of modem classes. The modems in this hierarchy need to be visited by an external algorithm based
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> on filtering criteria (is it Unix or DOS compatible modem).
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In plain words
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> Acyclic Visitor allows functions to be added to existing class hierarchies without modifying the hierarchies.
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[WikiWikiWeb](https://wiki.c2.com/?AcyclicVisitor) says
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> The Acyclic Visitor pattern allows new functions to be added to existing class hierarchies without affecting those
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> hierarchies, and without creating the dependency cycles that are inherent to the GangOfFour VisitorPattern.
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**Programmatic Example**
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Here's the `Modem` hierarchy.
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```java
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public abstract class Modem {
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public abstract void accept(ModemVisitor modemVisitor);
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}
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public class Zoom extends Modem {
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...
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@Override
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public void accept(ModemVisitor modemVisitor) {
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if (modemVisitor instanceof ZoomVisitor) {
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((ZoomVisitor) modemVisitor).visit(this);
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} else {
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LOGGER.info("Only ZoomVisitor is allowed to visit Zoom modem");
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}
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}
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}
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public class Hayes extends Modem {
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...
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@Override
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public void accept(ModemVisitor modemVisitor) {
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if (modemVisitor instanceof HayesVisitor) {
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((HayesVisitor) modemVisitor).visit(this);
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} else {
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LOGGER.info("Only HayesVisitor is allowed to visit Hayes modem");
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}
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}
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}
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```
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Next we introduce the `ModemVisitor` hierarchy.
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```java
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public interface ModemVisitor {
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}
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public interface HayesVisitor extends ModemVisitor {
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void visit(Hayes hayes);
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}
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public interface ZoomVisitor extends ModemVisitor {
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void visit(Zoom zoom);
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}
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public interface AllModemVisitor extends ZoomVisitor, HayesVisitor {
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}
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public class ConfigureForDosVisitor implements AllModemVisitor {
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...
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@Override
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public void visit(Hayes hayes) {
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LOGGER.info(hayes + " used with Dos configurator.");
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}
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@Override
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public void visit(Zoom zoom) {
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LOGGER.info(zoom + " used with Dos configurator.");
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}
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}
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public class ConfigureForUnixVisitor implements ZoomVisitor {
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...
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@Override
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public void visit(Zoom zoom) {
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LOGGER.info(zoom + " used with Unix configurator.");
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}
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}
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```
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Finally here are the visitors in action.
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```java
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var conUnix = new ConfigureForUnixVisitor();
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var conDos = new ConfigureForDosVisitor();
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var zoom = new Zoom();
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var hayes = new Hayes();
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hayes.accept(conDos);
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zoom.accept(conDos);
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hayes.accept(conUnix);
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zoom.accept(conUnix);
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```
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Program output:
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```
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// Hayes modem used with Dos configurator.
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// Zoom modem used with Dos configurator.
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// Only HayesVisitor is allowed to visit Hayes modem
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// Zoom modem used with Unix configurator.
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```
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## Class diagram
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![alt text](./etc/acyclic-visitor.png "Acyclic Visitor")
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## Applicability
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This pattern can be used:
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* When you need to add a new function to an existing hierarchy without the need to alter or affect that hierarchy.
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* When the recompilation, relinking, retesting or redistribution of the derivatives of Element is very expensive.
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## Consequences
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The good:
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* No dependency cycles between class hierarchies.
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The bad:
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* Violates the principle of least surprise or Liskov's Substitution principle by showing that it can accept all visitors but actually only being interested in particular visitors.
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* Violates [Liskov's Substitution Principle](https://java-design-patterns.com/principles/#liskov-substitution-principle) by showing that it can accept all visitors but actually only being interested in particular visitors.
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* Parallel hierarchy of visitors has to be created for all members in visitable class hierarchy.
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## Related patterns
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* [Visitor Pattern](../visitor/)
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* [Visitor Pattern](https://java-design-patterns.com/patterns/visitor/)
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## Credits
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* [Acyclic Visitor](http://condor.depaul.edu/dmumaugh/OOT/Design-Principles/acv.pdf)
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* [Acyclic Visitor by Robert C. Martin](http://condor.depaul.edu/dmumaugh/OOT/Design-Principles/acv.pdf)
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* [Acyclic Visitor in WikiWikiWeb](https://wiki.c2.com/?AcyclicVisitor)

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