Add all 23 GoF design pattern implementations

03-behavioral/mediator/ChatMediator.java

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+package mediator;
+
+/** Mediator interface — defines how colleagues communicate through the hub */
+public interface ChatMediator {
+    void sendMessage(String message, User sender);
+    void addUser(User user);
+}

03-behavioral/mediator/ChatRoom.java

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+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());
+            }
+        }
+    }
+}

03-behavioral/mediator/Main.java

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+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 ===");
+    }
+}

03-behavioral/mediator/README.md

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+# Mediator Design Pattern — Java Example
+
+**Pattern:** Behavioral → Mediator
+**Article:** https://ankurm.com/mediator-design-pattern-java/
+
+## What this example shows
+
+A chat room where users never talk to each other directly. `ChatMediator` declares the hub contract (`sendMessage()`/`addUser()`). `User` is a colleague — it knows only the mediator, never other users. `ChatRoom` is the concrete mediator: it holds the list of users and routes every message to everyone except the sender. `Main` adds three users and has each one broadcast a message, then prints the O(N) vs O(N²) connection-count comparison.
+
+## How to run
+
+```bash
+javac mediator/*.java -d out/mediator
+java -cp out/mediator mediator.Main
+```
+
+Requires Java 25.
+
+## Post Section ↔ File Mapping
+
+| Post Section | File(s) |
+|---|---|
+| Implementation: Chat Room — the Mediator interface | `ChatMediator.java` |
+| Implementation: Chat Room — the Colleague | `User.java` |
+| Implementation: Chat Room — the Concrete Mediator | `ChatRoom.java` |
+| Implementation: Chat Room — wiring it together | `Main.java` |
+
+Note: the "Mediator in Practice: MVC and Spring Events" snippet (`ApplicationEventPublisher`/`@EventListener` example) is illustrative only — it is not part of this repository's runnable example.
+
+Article: https://ankurm.com/mediator-design-pattern-java/
+All patterns: https://ankurm.com/design-patterns-java/

03-behavioral/mediator/User.java

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+package mediator;
+
+/**
+ * Colleague — knows only the mediator, not other users.
+ * Sends messages through the mediator; receives via receive().
+ */
+public class User {
+    private final String name;
+    private final ChatMediator mediator;
+
+    public User(String name, ChatMediator mediator) {
+        this.name     = name;
+        this.mediator = mediator;
+    }
+
+    public String getName() { return name; }
+
+    public void send(String message) {
+        System.out.println("[" + name + "] sends: " + message);
+        mediator.sendMessage(message, this);
+    }
+
+    public void receive(String message, String from) {
+        System.out.println("  [" + name + "] received from " + from + ": " + message);
+    }
+}