Code cleanup (#1461)

* Code cleanup

* Fix flux tests

* Fix checkstyle errors

* Fix compile error

double-checked-locking/src/test/java/com/iluwatar/doublechecked/locking/InventoryTest.java

@@ -109,7 +109,7 @@ public class InventoryTest {
 
 
   private class InMemoryAppender extends AppenderBase<ILoggingEvent> {
-    private List<ILoggingEvent> log = new LinkedList<>();
+    private final List<ILoggingEvent> log = new LinkedList<>();
 
     public InMemoryAppender(Class clazz) {
       ((Logger) LoggerFactory.getLogger(clazz)).addAppender(this);