#567 Marker Interface pull request

marker/.gitignore

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+/target/

marker/README.md

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

marker/etc/MarkerDiagram.ucls

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+<?xml version="1.0" encoding="UTF-8"?>
+<class-diagram version="1.2.0" icons="true" automaticImage="PNG" always-add-relationships="false" generalizations="true" 
+  realizations="true" associations="true" dependencies="false" nesting-relationships="true" router="FAN">  
+  <class id="1" language="java" name="Guard" project="marker" file="/marker/src/main/java/Guard.java" binary="false" 
+    corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="416" y="348"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <interface id="2" language="java" name="Permission" project="marker" file="/marker/src/main/java/Permission.java" 
+    binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="261" y="175"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </interface>  
+  <class id="3" language="java" name="Thief" project="marker" file="/marker/src/main/java/Thief.java" binary="false" 
+    corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="236" y="355"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <realization id="4">    
+    <end type="SOURCE" refId="1"/>    
+    <end type="TARGET" refId="2"/>  
+  </realization>  
+  <classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+    sort-features="false" accessors="true" visibility="true">    
+    <attributes public="true" package="true" protected="true" private="true" static="true"/>    
+    <operations public="true" package="true" protected="true" private="true" static="true"/>  
+  </classifier-display>  
+  <association-display labels="true" multiplicity="true"/>
+</class-diagram>

marker/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+    The MIT License
+    Copyright (c) 2014-2016 Ilkka Seppälä
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <parent>
+    <artifactId>java-design-patterns</artifactId>
+    <groupId>com.iluwatar</groupId>
+    <version>1.16.0-SNAPSHOT</version>
+  </parent>
+  <modelVersion>4.0.0</modelVersion>
+
+  <artifactId>marker</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+
+
+</project>

marker/src/main/java/App.java

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

marker/src/main/java/Guard.java

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

marker/src/main/java/Permission.java

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

marker/src/main/java/Thief.java

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

marker/src/test/java/AppTest.java

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

marker/src/test/java/GuardTest.java

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

marker/src/test/java/ThiefTest.java

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

pom.xml

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