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

03-behavioral/memento/Main.java

@@ -0,0 +1,38 @@
+package memento;
+
+/**
+ * Memento Design Pattern — Runnable Demo
+ * Run: javac memento/*.java -d out/memento && java -cp out/memento memento.Main
+ * Article: https://ankurm.com/memento-design-pattern-java/
+ */
+public class Main {
+    public static void main(String[] args) {
+        System.out.println("=== Memento Design Pattern Demo ===\n");
+
+        Editor editor = new Editor();
+        History history = new History();
+
+        System.out.println("Initial: " + editor);
+
+        editor.type("Hello");
+        history.save(editor.save());
+        System.out.println("After type: " + editor);
+
+        editor.type(", World");
+        history.save(editor.save());
+        System.out.println("After type: " + editor);
+
+        editor.type("! How are you?");
+        System.out.println("After type: " + editor);
+
+        System.out.println("\n-- Undo --");
+        editor.restore(history.undo());
+        System.out.println("After undo: " + editor);
+
+        editor.restore(history.undo());
+        System.out.println("After undo: " + editor);
+
+        System.out.println("\nHistory empty: " + (history.size() == 0));
+        System.out.println("\n=== Demo complete ===");
+    }
+}