Code cleanup (#1461)

* Code cleanup

* Fix flux tests

* Fix checkstyle errors

* Fix compile error

event-driven-architecture/src/main/java/com/iluwatar/eda/event/UserCreatedEvent.java

@@ -32,7 +32,7 @@ import com.iluwatar.eda.model.User;
  */
 public class UserCreatedEvent extends AbstractEvent {
 
-  private User user;
+  private final User user;
 
   public UserCreatedEvent(User user) {
     this.user = user;

event-driven-architecture/src/main/java/com/iluwatar/eda/event/UserUpdatedEvent.java

@@ -32,7 +32,7 @@ import com.iluwatar.eda.model.User;
  */
 public class UserUpdatedEvent extends AbstractEvent {
 
-  private User user;
+  private final User user;
 
   public UserUpdatedEvent(User user) {
     this.user = user;

event-driven-architecture/src/main/java/com/iluwatar/eda/framework/EventDispatcher.java

@@ -32,7 +32,7 @@ import java.util.Map;
  */
 public class EventDispatcher {
 
-  private Map<Class<? extends Event>, Handler<? extends Event>> handlers;
+  private final Map<Class<? extends Event>, Handler<? extends Event>> handlers;
 
   public EventDispatcher() {
     handlers = new HashMap<>();

event-driven-architecture/src/main/java/com/iluwatar/eda/model/User.java

@@ -32,7 +32,7 @@ import com.iluwatar.eda.event.UserUpdatedEvent;
  */
 public class User {
 
-  private String username;
+  private final String username;
 
   public User(String username) {
     this.username = username;