Add all 23 GoF design pattern implementations

01-creational/factory-method/NotificationSender.java

@@ -0,0 +1,31 @@
+package factorymethod;
+
+public abstract class NotificationSender {
+
+    // THE factory method — this is the one thing subclasses override.
+    // It returns a Notification (the Product interface), never a concrete class.
+    protected abstract Notification createNotification();
+
+    // The shared workflow — marked final so subclasses cannot change it.
+    // It calls createNotification() to get the right notification,
+    // then runs the same steps every time regardless of which channel was created.
+    public final void notify(String recipient, String message) {
+        Notification notification = createNotification();     // delegate creation
+        System.out.println("Sending " + notification.getType() + " notification...");
+        notification.send(recipient, message);                // use the Product interface
+        System.out.println("Delivered.");
+    }
+
+    // Shared retry logic — works for all channels without change
+    public void notifyWithRetry(String recipient, String message, int maxRetries) {
+        for (int attempt = 1; attempt <= maxRetries; attempt++) {
+            try {
+                notify(recipient, message);
+                return; // success — exit
+            } catch (Exception e) {
+                System.err.printf("Attempt %d failed: %s%n", attempt, e.getMessage());
+            }
+        }
+        throw new RuntimeException("All " + maxRetries + " attempts failed");
+    }
+}