refactor pub sub system

solutions/java/src/LLDRunner.java

@@ -20,7 +20,7 @@
 import onlineshopping.OnlineShoppingServiceDemo;
 import onlinestockbrokeragesystem.StockBrokerageSystemDemo;
 import parkinglot.ParkingLotDemo;
-import pubsubsystem.PubSubDemo;
+import pubsubsystem.PubSubSystemDemo;
 import restaurantmanagementsystem.RestaurantManagementDemo;
 import ridesharingservice.RideSharingServiceDemo;
 import snakeandladdergame.SnakeAndLadderDemo;
@@ -58,7 +58,7 @@ public static void main(String[] args) {
 //        OnlineShoppingServiceDemo.run();
 //        StockBrokerageSystemDemo.run();
 //        ParkingLotDemo.run();
-//        PubSubDemo.run();
+//        PubSubSystemDemo.run();
 //        RestaurantManagementDemo.run();
 //        RideSharingServiceDemo.run();
 //        SnakeAndLadderDemo.run();

solutions/java/src/pubsubsystem/ConcreteSubscriber.java → solutions/java/src/pubsubsystem/PrintSubscriber.java

@@ -1,9 +1,9 @@
 package pubsubsystem;
 
-public class ConcreteSubscriber implements Subscriber {
+public class PrintSubscriber implements Subscriber {
     private final String name;
 
-    public ConcreteSubscriber(String name) {
+    public PrintSubscriber(String name) {
         this.name = name;
     }
 

solutions/java/src/pubsubsystem/PubSubSystemDemo.java

@@ -0,0 +1,40 @@
+package pubsubsystem;
+
+public class PubSubSystemDemo {
+    public static void run() {
+        // Create topics
+        Topic topic1 = new Topic("Topic1");
+        Topic topic2 = new Topic("Topic2");
+
+        // Create publishers
+        Publisher publisher1 = new Publisher();
+        Publisher publisher2 = new Publisher();
+
+        // Create subscribers
+        Subscriber subscriber1 = new PrintSubscriber("Subscriber1");
+        Subscriber subscriber2 = new PrintSubscriber("Subscriber2");
+        Subscriber subscriber3 = new PrintSubscriber("Subscriber3");
+
+        publisher1.registerTopic(topic1);
+        publisher2.registerTopic(topic2);
+
+        // Subscribe to topics
+        topic1.addSubscriber(subscriber1);
+        topic1.addSubscriber(subscriber2);
+        topic2.addSubscriber(subscriber2);
+        topic2.addSubscriber(subscriber3);
+
+        // Publish messages
+        publisher1.publish(topic1, new Message("Message1 for Topic1"));
+        publisher1.publish(topic1, new Message("Message2 for Topic1"));
+        publisher2.publish(topic2, new Message("Message1 for Topic2"));
+
+
+        // Unsubscribe from a topic
+        topic1.removeSubscriber(subscriber2);
+
+        // Publish more messages
+        publisher1.publish(topic1, new Message("Message3 for Topic1"));
+        publisher2.publish(topic2, new Message("Message2 for Topic2"));
+    }
+}

solutions/java/src/pubsubsystem/Publisher.java

@@ -1,13 +1,24 @@
 package pubsubsystem;
 
+import java.util.HashSet;
+import java.util.Set;
+
 public class Publisher {
-    private final Topic topic;
+    private final Set<Topic> topics;
+
+    public Publisher() {
+        this.topics = new HashSet<>();
+    }
 
-    public Publisher(Topic topic) {
-        this.topic = topic;
+    public void registerTopic(Topic topic) {
+        topics.add(topic);
     }
 
-    public void publish(Message message) {
+    public void publish(Topic topic, Message message) {
+        if(!topics.contains(topic)) {
+            System.out.println("This publisher can't publish to topic: " + topic.getName());
+            return;
+        }
         topic.publish(message);
     }
 }