diff --git a/acyclic-visitor/README.md b/acyclic-visitor/README.md index f293e4393..19e886505 100644 --- a/acyclic-visitor/README.md +++ b/acyclic-visitor/README.md @@ -9,12 +9,126 @@ tags: --- ## Intent -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. + +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. + +## Explanation + +Real world example + +> We have a hierarchy of modem classes. The modems in this hierarchy need to be visited by an external algorithm based +> on filtering criteria (is it Unix or DOS compatible modem). + +In plain words + +> Acyclic Visitor allows functions to be added to existing class hierarchies without modifying the hierarchies. + +[WikiWikiWeb](https://wiki.c2.com/?AcyclicVisitor) says + +> The Acyclic Visitor pattern allows new functions to be added to existing class hierarchies without affecting those +> hierarchies, and without creating the dependency cycles that are inherent to the GangOfFour VisitorPattern. + +**Programmatic Example** + +Here's the `Modem` hierarchy. + +```java +public abstract class Modem { + public abstract void accept(ModemVisitor modemVisitor); +} + +public class Zoom extends Modem { + ... + @Override + public void accept(ModemVisitor modemVisitor) { + if (modemVisitor instanceof ZoomVisitor) { + ((ZoomVisitor) modemVisitor).visit(this); + } else { + LOGGER.info("Only ZoomVisitor is allowed to visit Zoom modem"); + } + } +} + +public class Hayes extends Modem { + ... + @Override + public void accept(ModemVisitor modemVisitor) { + if (modemVisitor instanceof HayesVisitor) { + ((HayesVisitor) modemVisitor).visit(this); + } else { + LOGGER.info("Only HayesVisitor is allowed to visit Hayes modem"); + } + } +} +``` + +Next we introduce the `ModemVisitor` hierarchy. + +```java +public interface ModemVisitor { +} + +public interface HayesVisitor extends ModemVisitor { + void visit(Hayes hayes); +} + +public interface ZoomVisitor extends ModemVisitor { + void visit(Zoom zoom); +} + +public interface AllModemVisitor extends ZoomVisitor, HayesVisitor { +} + +public class ConfigureForDosVisitor implements AllModemVisitor { + ... + @Override + public void visit(Hayes hayes) { + LOGGER.info(hayes + " used with Dos configurator."); + } + @Override + public void visit(Zoom zoom) { + LOGGER.info(zoom + " used with Dos configurator."); + } +} + +public class ConfigureForUnixVisitor implements ZoomVisitor { + ... + @Override + public void visit(Zoom zoom) { + LOGGER.info(zoom + " used with Unix configurator."); + } +} +``` + +Finally here are the visitors in action. + +```java + var conUnix = new ConfigureForUnixVisitor(); + var conDos = new ConfigureForDosVisitor(); + var zoom = new Zoom(); + var hayes = new Hayes(); + hayes.accept(conDos); + zoom.accept(conDos); + hayes.accept(conUnix); + zoom.accept(conUnix); +``` + +Program output: + +``` + // Hayes modem used with Dos configurator. + // Zoom modem used with Dos configurator. + // Only HayesVisitor is allowed to visit Hayes modem + // Zoom modem used with Unix configurator. +``` ## Class diagram + ![alt text](./etc/acyclic-visitor.png "Acyclic Visitor") ## Applicability + This pattern can be used: * When you need to add a new function to an existing hierarchy without the need to alter or affect that hierarchy. @@ -24,6 +138,7 @@ This pattern can be used: * When the recompilation, relinking, retesting or redistribution of the derivatives of Element is very expensive. ## Consequences + The good: * No dependency cycles between class hierarchies. @@ -32,11 +147,14 @@ The good: The bad: -* 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. +* 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. * Parallel hierarchy of visitors has to be created for all members in visitable class hierarchy. ## Related patterns -* [Visitor Pattern](../visitor/) + +* [Visitor Pattern](https://java-design-patterns.com/patterns/visitor/) ## Credits -* [Acyclic Visitor](http://condor.depaul.edu/dmumaugh/OOT/Design-Principles/acv.pdf) + +* [Acyclic Visitor by Robert C. Martin](http://condor.depaul.edu/dmumaugh/OOT/Design-Principles/acv.pdf) +* [Acyclic Visitor in WikiWikiWeb](https://wiki.c2.com/?AcyclicVisitor)