Add all 23 GoF design pattern implementations

01-creational/factory-method/Main.java

@@ -0,0 +1,23 @@
+package factorymethod;
+
+public class Main {
+
+    public static void main(String[] args) {
+
+        // The client only knows about NotificationSender (the Creator)
+        NotificationSender sender;
+
+        sender = new EmailSender();
+        sender.notify("alice@example.com", "Your order has shipped!");
+
+        sender = new SmsSender();
+        sender.notify("+1-555-0100", "OTP: 482910");
+
+        sender = new PushSender();
+        sender.notify("device-token-xyz", "New message from Bob");
+
+        // Adding Slack: just swap the creator. Client code doesn't change.
+        sender = new SlackSender();  // new class, nothing else touched
+        sender.notify("#alerts", "CPU usage above 90%");
+    }
+}