Code cleanup (#1461)

* Code cleanup

* Fix flux tests

* Fix checkstyle errors

* Fix compile error

producer-consumer/src/main/java/com/iluwatar/producer/consumer/Item.java

@@ -28,9 +28,9 @@ package com.iluwatar.producer.consumer;
  */
 public class Item {
 
-  private String producer;
+  private final String producer;
 
-  private int id;
+  private final int id;
 
   public Item(String producer, int id) {
     this.id = id;

producer-consumer/src/main/java/com/iluwatar/producer/consumer/ItemQueue.java

@@ -31,7 +31,7 @@ import java.util.concurrent.LinkedBlockingQueue;
  */
 public class ItemQueue {
 
-  private BlockingQueue<Item> queue;
+  private final BlockingQueue<Item> queue;
 
   public ItemQueue() {