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‎acyclic-visitor/README.md

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---
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layout: pattern
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title: Acyclic Visitor
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folder: acyclic-visitor
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permalink: /patterns/acyclic-visitor/
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categories: Behavioral
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tags:
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- Java
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- Difficulty-Intermediate
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---
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![alt text](./etc/acyclic-visitor.png "Acyclic Visitor")
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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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## 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 there are functions that operate upon a hierarchy, but which do not belong in the hierarchy itself. e.g. the ConfigureForDOS / ConfigureForUnix / ConfigureForX issue.
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* When you need to perform very different operations on an object depending upon its type.
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* When the visited class hierarchy will be frequently extended with new derivatives of the Element class.
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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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* No need to recompile all the visitors if a new one is added.
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* Does not cause compilation failure in existing visitors if class hierarchy has a new member.
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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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* 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/README.md)
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## Credits
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* [Acyclic Visitor](http://condor.depaul.edu/dmumaugh/OOT/Design-Principles/acv.pdf)

‎acyclic-visitor/etc/Acyclic Visitor.ucls

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<?xml version="1.0" encoding="UTF-8"?>
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<class-diagram version="1.2.2" icons="true" always-add-relationships="false" generalizations="true" realizations="true"
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associations="true" dependencies="false" nesting-relationships="true" router="FAN">
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<interface id="1" language="java" name="com.iluwatar.acyclicvisitor.ModemVisitor" project="acyclic-visitor"
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file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ModemVisitor.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="860" y="67"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</interface>
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<class id="2" language="java" name="com.iluwatar.acyclicvisitor.Modem" project="acyclic-visitor"
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file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/Modem.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="327" y="77"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<class id="3" language="java" name="com.iluwatar.acyclicvisitor.ConfigureForUnixVisitor" project="acyclic-visitor"
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file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ConfigureForUnixVisitor.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="124" width="196" x="647" y="225"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<class id="4" language="java" name="com.iluwatar.acyclicvisitor.Zoom" project="acyclic-visitor"
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file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/Zoom.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="203" y="305"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<interface id="5" language="java" name="com.iluwatar.acyclicvisitor.HayesVisitor" project="acyclic-visitor"
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file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/HayesVisitor.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="1019" y="468"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</interface>
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<interface id="6" language="java" name="com.iluwatar.acyclicvisitor.ZoomVisitor" project="acyclic-visitor"
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file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ZoomVisitor.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="758" y="467"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</interface>
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<class id="7" language="java" name="com.iluwatar.acyclicvisitor.Hayes" project="acyclic-visitor"
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file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/Hayes.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="479" y="307"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<class id="8" language="java" name="com.iluwatar.acyclicvisitor.ConfigureForDosVisitor" project="acyclic-visitor"
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file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ConfigureForDosVisitor.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="142" width="192" x="883" y="225"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<generalization id="9">
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<end type="SOURCE" refId="7"/>
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<end type="TARGET" refId="2"/>
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</generalization>
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<realization id="10">
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<end type="SOURCE" refId="8"/>
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<end type="TARGET" refId="6"/>
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</realization>
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<realization id="11">
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<end type="SOURCE" refId="8"/>
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<end type="TARGET" refId="5"/>
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</realization>
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<realization id="12">
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<end type="SOURCE" refId="3"/>
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<end type="TARGET" refId="6"/>
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</realization>
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<realization id="13">
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<end type="SOURCE" refId="3"/>
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<end type="TARGET" refId="1"/>
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</realization>
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<realization id="14">
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<end type="SOURCE" refId="8"/>
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<end type="TARGET" refId="1"/>
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</realization>
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<generalization id="15">
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<end type="SOURCE" refId="4"/>
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<end type="TARGET" refId="2"/>
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</generalization>
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<classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</classifier-display>
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<association-display labels="true" multiplicity="true"/>
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</class-diagram>

‎acyclic-visitor/etc/acyclic-visitor.png

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‎acyclic-visitor/pom.xml

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<!--
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The MIT License
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Copyright (c) 2014 Ilkka Seppälä
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Permission is hereby granted, free of charge, to any person obtaining a copy
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of this software and associated documentation files (the "Software"), to deal
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in the Software without restriction, including without limitation the rights
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to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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copies of the Software, and to permit persons to whom the Software is
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furnished to do so, subject to the following conditions:
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The above copyright notice and this permission notice shall be included in
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all copies or substantial portions of the Software.
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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THE SOFTWARE.
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-->
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<project xmlns="http://maven.apache.org/POM/4.0.0"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<parent>
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<groupId>com.iluwatar</groupId>
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<artifactId>java-design-patterns</artifactId>
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<version>1.20.0-SNAPSHOT</version>
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</parent>
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<artifactId>acyclic-visitor</artifactId>
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<properties>
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<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
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<maven.compiler.source>1.8</maven.compiler.source>
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<maven.compiler.target>1.8</maven.compiler.target>
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</properties>
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<dependencies>
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<!-- https://mvnrepository.com/artifact/org.assertj/assertj-core -->
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<dependency>
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<groupId>org.assertj</groupId>
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<artifactId>assertj-core</artifactId>
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<version>3.9.1</version>
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<scope>test</scope>
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</dependency>
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<!-- https://mvnrepository.com/artifact/uk.org.lidalia/slf4j-test -->
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<dependency>
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<groupId>uk.org.lidalia</groupId>
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<artifactId>slf4j-test</artifactId>
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<version>1.0.0</version>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter-api</artifactId>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter-engine</artifactId>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>org.mockito</groupId>
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<artifactId>mockito-all</artifactId>
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<version>1.9.5</version>
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<scope>test</scope>
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</dependency>
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</dependencies>
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</project>

‎acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/AllModemVisitor.java

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/**
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* The MIT License
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* Copyright (c) 2014-2016 Ilkka Seppälä
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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package com.iluwatar.acyclicvisitor;
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/**
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* All ModemVisitor interface extends all visitor interfaces
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*/
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public interface AllModemVisitor extends ModemVisitor, ZoomVisitor, HayesVisitor{
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}

‎acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/App.java

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/**
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* The MIT License
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* Copyright (c) 2014-2016 Ilkka Seppälä
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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package com.iluwatar.acyclicvisitor;
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/**
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* The Acyclic Visitor pattern allows new functions to be added to existing class
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* hierarchies without affecting those hierarchies, and without creating the dependency
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* cycles that are inherent to the GoF Visitor pattern, by making the Visitor base class
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* degenerate
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* <p>
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* In this example the visitor base class is {@link ModemVisitor}. The base class of the
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* visited hierarchy is {@link Modem} and has two children {@link Hayes} and {@link Zoom}
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* each one having its own visitor interface {@link HayesVisitor} and {@link ZoomVisitor}
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* respectively. {@link ConfigureForUnixVisitor} and {@link ConfigureForDosVisitor}
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* implement each derivative's visit method only if it is required
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*/
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public class App {
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/**
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* Program's entry point
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*/
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public static void main(String[] args) {
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ConfigureForUnixVisitor conUnix = new ConfigureForUnixVisitor();
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ConfigureForDosVisitor conDos = new ConfigureForDosVisitor();
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Zoom zoom = new Zoom();
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Hayes hayes = new Hayes();
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hayes.accept(conDos); // Hayes modem with Unix configurator
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zoom.accept(conDos); // Zoom modem with Dos configurator
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hayes.accept(conUnix); // Hayes modem with Unix configurator
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zoom.accept(conUnix); // Zoom modem with Unix configurator
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}
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}

‎acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ConfigureForDosVisitor.java

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/**
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* The MIT License
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* Copyright (c) 2014-2016 Ilkka Seppälä
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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package com.iluwatar.acyclicvisitor;
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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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/**
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* ConfigureForDosVisitor class implements both zoom's and hayes' visit method
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* for Dos manufacturer
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*/
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public class ConfigureForDosVisitor implements AllModemVisitor {
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private static final Logger LOGGER = LoggerFactory.getLogger(ConfigureForDosVisitor.class);
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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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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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