added Callback pattern

callback/etc/callback.ucls

@@ -0,0 +1,41 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<class-diagram version="1.1.8" icons="true" automaticImage="JPEG" always-add-relationships="false" 
+  generalizations="true" realizations="true" associations="true" dependencies="false" nesting-relationships="true">  
+  <class id="1" language="java" name="com.iluwatar.SimpleTask" project="callback" 
+    file="/callback/src/main/java/com/iluwatar/SimpleTask.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="358" y="315"/>    
+    <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="com.iluwatar.Callback" project="callback" 
+    file="/callback/src/main/java/com/iluwatar/Callback.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="529" y="165"/>    
+    <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="com.iluwatar.Task" project="callback" 
+    file="/callback/src/main/java/com/iluwatar/Task.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="356" y="163"/>    
+    <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>  
+  <generalization id="4">    
+    <end type="SOURCE" refId="1"/>    
+    <end type="TARGET" refId="3"/>  
+  </generalization>  
+  <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>

callback/pom.xml

@@ -0,0 +1,18 @@
+<?xml version="1.0"?>
+<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>com.iluwatar</groupId>
+    <artifactId>java-design-patterns</artifactId>
+    <version>1.0-SNAPSHOT</version>
+  </parent>
+  <artifactId>callback</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+</project>

callback/src/main/java/com/iluwatar/App.java

@@ -0,0 +1,19 @@
+package com.iluwatar;
+
+/**
+ * Callback pattern is more native for dynamic languages where function are first-class citizen.
+ * Prior to Java8 can be simulated using simple (alike command) interfaces.
+ */
+public class App {
+
+	public static void main(String[] args) {
+		Task task = new SimpleTask();
+		Callback callback = new Callback() {
+			@Override
+			public void call() {
+				System.out.println("I'm done now.");
+			}
+		};
+		task.executeWith(callback);
+	}
+}

callback/src/main/java/com/iluwatar/Callback.java

@@ -0,0 +1,9 @@
+package com.iluwatar;
+
+/**
+ * Callback interface
+ */
+public interface Callback {
+
+	public void call();
+}

callback/src/main/java/com/iluwatar/SimpleTask.java

@@ -0,0 +1,13 @@
+package com.iluwatar;
+
+/**
+ * Implementation of task that need to be executed
+ */
+public class SimpleTask extends Task {
+
+	@Override
+	public void execute() {
+		System.out.println("Perform some important activity.");
+	}
+
+}

callback/src/main/java/com/iluwatar/Task.java

@@ -0,0 +1,16 @@
+package com.iluwatar;
+
+/**
+ * Template-method class for callback hook execution
+ */
+public abstract class Task {
+
+	public final void executeWith(Callback callback) {
+		execute();
+		if (callback != null) {
+			callback.call();
+		}
+	}
+
+	public abstract void execute();
+}

callback/src/test/java/com/iluwatar/AppTest.java

@@ -0,0 +1,12 @@
+package com.iluwatar;
+
+import org.junit.Test;
+
+public class AppTest {
+
+	@Test
+	public void test() {
+		String[] args = {};
+		App.main(args);
+	}
+}