added Callback pattern

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);
+	}
+}