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

03-behavioral/mediator/User.java

@@ -0,0 +1,26 @@
+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);
+    }
+}