Add all 23 GoF design pattern implementations

03-behavioral/template-method/ApiMigration.java

@@ -0,0 +1,31 @@
+package template;
+
+public class ApiMigration extends DataMigration {
+
+    private final String endpoint;
+
+    public ApiMigration(String endpoint) { this.endpoint = endpoint; }
+
+    @Override protected String getSourceName() { return "API:" + endpoint; }
+
+    @Override
+    protected void connect() {
+        System.out.println("  Authenticating with API at " + endpoint + "...");
+    }
+
+    @Override
+    protected int readData() {
+        System.out.println("  Paginating through API responses...");
+        return 320;
+    }
+
+    @Override
+    protected int transformData(int rawCount) {
+        System.out.println("  Flattening JSON, deduplicating (" + rawCount + " records)...");
+        return rawCount - 15;  // 15 duplicates removed
+    }
+
+    // Override hook: silent migrations from APIs, no email spam
+    @Override
+    protected boolean sendNotification() { return false; }
+}