Add all 23 GoF design pattern implementations

03-behavioral/template-method/ApiMigration.java

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+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; }
+}

03-behavioral/template-method/CsvMigration.java

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+package template;
+
+public class CsvMigration extends DataMigration {
+
+    private final String filePath;
+
+    public CsvMigration(String filePath) { this.filePath = filePath; }
+
+    @Override protected String getSourceName() { return "CSV:" + filePath; }
+
+    @Override
+    protected void connect() {
+        System.out.println("  Opening CSV file: " + filePath);
+    }
+
+    @Override
+    protected int readData() {
+        System.out.println("  Parsing CSV rows...");
+        return 1_500;  // simulated row count
+    }
+
+    @Override
+    protected int transformData(int rawCount) {
+        System.out.println("  Mapping CSV columns to target schema (" + rawCount + " rows)...");
+        return rawCount;  // no rows lost
+    }
+}

03-behavioral/template-method/DataMigration.java

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+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.");
+    }
+}

03-behavioral/template-method/Main.java

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+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 ===");
+    }
+}

03-behavioral/template-method/README.md

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+# Template Method Design Pattern — Java Example
+
+**Pattern:** Behavioral → Template Method
+**Article:** https://ankurm.com/template-method-design-pattern-java/
+
+## What this example shows
+
+A data migration pipeline with a fixed skeleton and variable steps. `DataMigration` declares `migrate()` as `final` — the sequence (connect, validate, read, transform, write, notify, disconnect) can never be reordered or skipped by a subclass. `getSourceName()`, `connect()`, `readData()`, and `transformData()` are abstract steps each subclass must supply; `validate()`, `writeData()`, and `disconnect()` are concrete steps shared by everyone; `sendNotification()` is a hook with a default of `true` that a subclass can override. `CsvMigration` uses every default unchanged. `ApiMigration` overrides the hook to stay silent. `Main` runs both through the identical skeleton.
+
+## How to run
+
+```bash
+javac template-method/*.java -d out/template-method
+java -cp out/template-method template.Main
+```
+
+Requires Java 25.
+
+## Post Section ↔ File Mapping
+
+| Post Section | File(s) |
+|---|---|
+| Implementation: Data Migration Pipeline — the abstract class & template method | `DataMigration.java` |
+| Implementation: Data Migration Pipeline — CsvMigration | `CsvMigration.java` |
+| Implementation: Data Migration Pipeline — ApiMigration | `ApiMigration.java` |
+| Implementation: Data Migration Pipeline — wiring it together | `Main.java` |
+
+Article: https://ankurm.com/template-method-design-pattern-java/
+All patterns: https://ankurm.com/design-patterns-java/