Code cleanup (#1461)

* Code cleanup

* Fix flux tests

* Fix checkstyle errors

* Fix compile error

half-sync-half-async/src/main/java/com/iluwatar/halfsynchalfasync/App.java

@@ -95,7 +95,7 @@ public class App {
    * ArithmeticSumTask.
    */
   static class ArithmeticSumTask implements AsyncTask<Long> {
-    private long numberOfElements;
+    private final long numberOfElements;
 
     public ArithmeticSumTask(long numberOfElements) {
       this.numberOfElements = numberOfElements;

half-sync-half-async/src/main/java/com/iluwatar/halfsynchalfasync/AsynchronousService.java

@@ -48,7 +48,7 @@ public class AsynchronousService {
    * tasks should be performed in the background which does not affect the performance of main
    * thread.
    */
-  private ExecutorService service;
+  private final ExecutorService service;
 
   /**
    * Creates an asynchronous service using {@code workQueue} as communication channel between