Adjust checkstyle rules. Make checkstyle fail the build when violations are found. Correct all current checkstyle violations.

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

@@ -9,6 +9,9 @@ package com.iluwatar.callback;
  */
 public class App {
 
+  /**
+   * Program entry point
+   */
   public static void main(String[] args) {
     Task task = new SimpleTask();
     Callback callback = new Callback() {

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

@@ -7,6 +7,9 @@ package com.iluwatar.callback;
  */
 public abstract class Task {
 
+  /**
+   * Execute with callback
+   */
   public final void executeWith(Callback callback) {
     execute();
     if (callback != null) {