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

03-behavioral/template-method/DataMigration.java

@@ -0,0 +1,50 @@
+package template;
+
+/**
+ * Abstract Class with Template Method.
+ * The overall migration algorithm is fixed here; subclasses fill in
+ * the source-specific steps (connect, read, transform).
+ */
+public abstract class DataMigration {
+
+    // THE TEMPLATE METHOD — defines the fixed algorithm skeleton
+    public final void migrate() {
+        System.out.println("\n[" + getSourceName() + "] Starting migration...");
+        connect();
+        validate();
+        int rowCount = readData();
+        int transformed = transformData(rowCount);
+        writeData(transformed);
+        if (sendNotification()) {
+            notifyStakeholders();
+        }
+        disconnect();
+        System.out.println("[" + getSourceName() + "] Migration complete.\n");
+    }
+
+    // Abstract steps — must be implemented by each subclass
+    protected abstract String getSourceName();
+    protected abstract void   connect();
+    protected abstract int    readData();
+    protected abstract int    transformData(int rawCount);
+
+    // Concrete steps — common to all migrations
+    protected void validate() {
+        System.out.println("  Validating schema compatibility...");
+    }
+
+    protected void writeData(int count) {
+        System.out.println("  Writing " + count + " rows to target...");
+    }
+
+    protected void disconnect() {
+        System.out.println("  Closing source connection.");
+    }
+
+    // Hook — subclasses may override to change behaviour
+    protected boolean sendNotification() { return true; }
+
+    protected void notifyStakeholders() {
+        System.out.println("  Sending completion email to team.");
+    }
+}