Created a Lambdas example for the callback pattern, using the same ou…

…tput of the traditional version. Have also added another unit test for issue iluwatar#334

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

@@ -0,0 +1,19 @@
+package com.iluwatar.callback;
+
+/**
+ *
+ * This example generates the exact same output as {@link App} however the callback has been
+ * defined as a Lambdas expression.
+ *
+ */
+public class LambdasApp {
+
+  /**
+   * Program entry point
+   */
+  public static void main(String[] args){
+    Task task = new SimpleTask();
+    Callback c = () -> System.out.println("I'm done now.");
+    task.executeWith(c);
+  }
+}

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

@@ -36,4 +36,22 @@ public void call() {
     assertEquals("Callback called twice", new Integer(2), callingCount);
 
   }
+
+  @Test
+  public void testWithLambdasExample() {
+    Callback callback = () -> callingCount++;
+
+    Task task = new SimpleTask();
+
+    assertEquals("Initial calling count of 0", new Integer(0), callingCount);
+
+    task.executeWith(callback);
+
+    assertEquals("Callback called once", new Integer(1), callingCount);
+
+    task.executeWith(callback);
+
+    assertEquals("Callback called twice", new Integer(2), callingCount);
+
+  }
 }