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

03-behavioral/iterator/Main.java

@@ -0,0 +1,37 @@
+package iterator;
+
+/**
+ * Iterator Design Pattern — Runnable Demo
+ * Run: javac iterator/*.java -d out/iterator && java -cp out/iterator iterator.Main
+ * Article: https://ankurm.com/iterator-design-pattern-java/
+ */
+public class Main {
+    public static void main(String[] args) {
+        System.out.println("=== Iterator Design Pattern Demo ===\n");
+
+        BookShelf shelf = new BookShelf();
+        shelf.addBook(new Book("Clean Code",              "Robert Martin", 2008));
+        shelf.addBook(new Book("The Pragmatic Programmer","Andrew Hunt",   1999));
+        shelf.addBook(new Book("Effective Java",          "Joshua Bloch",  2001));
+        shelf.addBook(new Book("Design Patterns",         "Gang of Four",  1994));
+        shelf.addBook(new Book("Refactoring",             "Martin Fowler", 2018));
+        shelf.addBook(new Book("Working Effectively with Legacy Code", "Michael Feathers", 2004));
+
+        System.out.println("-- All books (forward iterator) --");
+        BookIterator it = shelf.iterator();
+        while (it.hasNext()) {
+            System.out.println("  " + it.next());
+        }
+
+        System.out.println("\n-- Books from the 2000s --");
+        BookIterator it2000s = shelf.iteratorByDecade(2000);
+        while (it2000s.hasNext()) {
+            System.out.println("  " + it2000s.next());
+        }
+
+        System.out.println("\n-- JDK Iterable: same pattern, different vocabulary --");
+        System.out.println("  java.util.Iterator is our BookIterator; for-each uses it under the hood");
+
+        System.out.println("\n=== Demo complete ===");
+    }
+}