Code cleanup (#1461)

* Code cleanup

* Fix flux tests

* Fix checkstyle errors

* Fix compile error

twin/src/test/java/com/iluwatar/twin/BallItemTest.java

@@ -108,7 +108,7 @@ public class BallItemTest {
    * Logging Appender Implementation
    */
   public class InMemoryAppender extends AppenderBase<ILoggingEvent> {
-    private List<ILoggingEvent> log = new LinkedList<>();
+    private final List<ILoggingEvent> log = new LinkedList<>();
 
     public InMemoryAppender() {
       ((Logger) LoggerFactory.getLogger("root")).addAppender(this);