Add all 23 GoF design pattern implementations (2026-06-13)

03-behavioral/mediator/Main.java

@@ -0,0 +1,35 @@
+package mediator;
+
+/**
+ * Mediator Design Pattern — Runnable Demo
+ * Run: javac mediator/*.java -d out/mediator && java -cp out/mediator mediator.Main
+ * Article: https://ankurm.com/mediator-design-pattern-java/
+ */
+public class Main {
+    public static void main(String[] args) {
+        System.out.println("=== Mediator Design Pattern Demo ===\n");
+
+        ChatRoom room = new ChatRoom();
+
+        User alice = new User("Alice", room);
+        User bob   = new User("Bob",   room);
+        User carol = new User("Carol", room);
+
+        room.addUser(alice);
+        room.addUser(bob);
+        room.addUser(carol);
+
+        System.out.println();
+        alice.send("Hey everyone!");
+        System.out.println();
+        bob.send("Hi Alice and Carol!");
+        System.out.println();
+        carol.send("Good morning!");
+
+        System.out.println("\n-- Connections without Mediator: " + 3 + " users need " + (3 * 2) + " direct links --");
+        System.out.println("-- With Mediator: " + 3 + " users each connect only to the room --");
+        System.out.println("-- With N users: O(N) connections instead of O(N²) --");
+
+        System.out.println("\n=== Demo complete ===");
+    }
+}