Add all 23 GoF design pattern implementations

03-behavioral/template-method/Main.java

@@ -0,0 +1,24 @@
+package template;
+
+/**
+ * Template Method Design Pattern — Runnable Demo
+ * Run: javac template/*.java -d out/template && java -cp out/template template.Main
+ * Article: https://ankurm.com/template-method-design-pattern-java/
+ */
+public class Main {
+    public static void main(String[] args) {
+        System.out.println("=== Template Method Design Pattern Demo ===");
+
+        DataMigration csvJob = new CsvMigration("customers_export.csv");
+        csvJob.migrate();
+
+        DataMigration apiJob = new ApiMigration("https://api.partner.com/v2/orders");
+        apiJob.migrate();
+
+        System.out.println("-- Both jobs used the same migrate() skeleton --");
+        System.out.println("-- Each provided its own connect/read/transform implementation --");
+        System.out.println("-- ApiMigration overrode the sendNotification() hook: no email --");
+
+        System.out.println("\n=== Demo complete ===");
+    }
+}