Add all 23 GoF design pattern implementations

03-behavioral/mediator/ChatRoom.java

@@ -0,0 +1,27 @@
+package mediator;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Concrete Mediator — the chat room that routes messages between users.
+ * Users talk to ChatRoom; ChatRoom talks to users. Nobody else talks to anybody.
+ */
+public class ChatRoom implements ChatMediator {
+    private final List<User> users = new ArrayList<>();
+
+    @Override
+    public void addUser(User user) {
+        users.add(user);
+        System.out.println("  [ChatRoom] " + user.getName() + " joined the room");
+    }
+
+    @Override
+    public void sendMessage(String message, User sender) {
+        for (User user : users) {
+            if (user != sender) {  // don't echo back to sender
+                user.receive(message, sender.getName());
+            }
+        }
+    }
+}