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Merge pull request iluwatar#572 from 4lexis/master
iluwatar#567 Marker Interface pull request
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‎marker/.gitignore

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/target/

‎marker/README.md

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---
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layout: pattern
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title: Marker Interface
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folder: marker
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permalink: /patterns/marker/
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categories: Design
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tags:
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- Java
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- Difficulty-Beginner
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---
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## Intent
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Using empty interfaces as markers to distinguish special treated objects.
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![alt text](./etc/MarkerDiagram.png "Marker Interface")
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## Applicability
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Use the Marker Interface pattern when
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* you want to identify the special objects from normal objects (to treat them differently)
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* you want to mark that some object is available for certain sort of operations
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## Real world examples
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* [javase.8.docs.api.java.io.Serializable](https://docs.oracle.com/javase/8/docs/api/java/io/Serializable.html)
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* [javase.8.docs.api.java.lang.Cloneable](https://docs.oracle.com/javase/8/docs/api/java/lang/Cloneable.html)
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## Credits
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* [Effective Java 2nd Edition by Joshua Bloch](https://www.amazon.com/Effective-Java-2nd-Joshua-Bloch/dp/0321356683)

‎marker/etc/MarkerDiagram.png

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‎marker/etc/MarkerDiagram.ucls

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<?xml version="1.0" encoding="UTF-8"?>
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<class-diagram version="1.2.0" icons="true" automaticImage="PNG" always-add-relationships="false" generalizations="true"
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realizations="true" associations="true" dependencies="false" nesting-relationships="true" router="FAN">
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<class id="1" language="java" name="Guard" project="marker" file="/marker/src/main/java/Guard.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="416" y="348"/>
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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="2" language="java" name="Permission" project="marker" file="/marker/src/main/java/Permission.java"
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binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="261" y="175"/>
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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="3" language="java" name="Thief" project="marker" file="/marker/src/main/java/Thief.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="236" y="355"/>
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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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<realization id="4">
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<end type="SOURCE" refId="1"/>
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<end type="TARGET" refId="2"/>
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</realization>
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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>

‎marker/pom.xml

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<?xml version="1.0" encoding="UTF-8"?>
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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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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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<parent>
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<artifactId>java-design-patterns</artifactId>
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<groupId>com.iluwatar</groupId>
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<version>1.16.0-SNAPSHOT</version>
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</parent>
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<modelVersion>4.0.0</modelVersion>
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<artifactId>marker</artifactId>
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<dependencies>
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<dependency>
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<groupId>junit</groupId>
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<artifactId>junit</artifactId>
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<scope>test</scope>
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</dependency>
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</dependencies>
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</project>

‎marker/src/main/java/App.java

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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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/**
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* Created by Alexis on 28-Apr-17.
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* With Marker interface idea is to make empty interface and extend it.
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* Basically it is just to identify the special objects from normal objects.
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* Like in case of serialization , objects that need to be serialized must implement serializable interface
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* (it is empty interface) and down the line writeObject() method must be checking
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* if it is a instance of serializable or not.
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* <p>
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* Marker interface vs annotation
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* Marker interfaces and marker annotations both have their uses,
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* neither of them is obsolete or always better then the other one.
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* If you want to define a type that does not have any new methods associated with it,
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* a marker interface is the way to go.
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* If you want to mark program elements other than classes and interfaces,
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* to allow for the possibility of adding more information to the marker in the future,
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* or to fit the marker into a framework that already makes heavy use of annotation types,
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* then a marker annotation is the correct choice
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*/
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public class App {
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/**
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* Program entry point
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*
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* @param args command line args
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*/
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public static void main(String[] args) {
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final Logger logger = LoggerFactory.getLogger(App.class);
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Guard guard = new Guard();
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Thief thief = new Thief();
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if (guard instanceof Permission) {
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guard.enter();
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} else {
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logger.info("You have no permission to enter, please leave this area");
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}
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if (thief instanceof Permission) {
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thief.steal();
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} else {
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thief.doNothing();
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}
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}
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}
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‎marker/src/main/java/Guard.java

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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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/**
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* Class defining Guard
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*/
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public class Guard implements Permission {
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private static final Logger LOGGER = LoggerFactory.getLogger(Guard.class);
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protected static void enter() {
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LOGGER.info("You can enter");
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}
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}

‎marker/src/main/java/Permission.java

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/**
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* Interface without any methods
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* Marker interface is based on that assumption
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*/
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public interface Permission {
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}

‎marker/src/main/java/Thief.java

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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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/**
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* Class defining Thief
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*/
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public class Thief {
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private static final Logger LOGGER = LoggerFactory.getLogger(Thief.class);
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protected static void steal() {
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LOGGER.info("Steal valuable items");
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}
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protected static void doNothing() {
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LOGGER.info("Pretend nothing happened and just leave");
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}
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}

‎marker/src/test/java/AppTest.java

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import org.junit.Test;
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/**
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* Application test
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*/
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public class AppTest {
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@Test
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public void test() {
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String[] args = {};
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App.main(args);
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}
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}

‎marker/src/test/java/GuardTest.java

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import org.junit.Test;
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import static org.hamcrest.CoreMatchers.instanceOf;
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import static org.junit.Assert.assertThat;
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/**
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* Guard test
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*/
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public class GuardTest {
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@Test
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public void testGuard() {
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Guard guard = new Guard();
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assertThat(guard, instanceOf(Permission.class));
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}
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}

‎marker/src/test/java/ThiefTest.java

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import org.junit.Test;
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import static org.junit.Assert.assertFalse;
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/**
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* Thief test
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*/
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public class ThiefTest {
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@Test
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public void testThief() {
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Thief thief = new Thief();
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assertFalse(thief instanceof Permission);
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}
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}

‎pom.xml

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<module>converter</module>
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<module>guarded-suspension</module>
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<module>balking</module>
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<module>marker</module>
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</modules>
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<dependencyManagement>

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