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

03-behavioral/iterator/BookShelf.java

@@ -0,0 +1,57 @@
+package iterator;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Aggregate — the collection. Exposes iterators without
+ * revealing its internal storage structure.
+ */
+public class BookShelf {
+    private final List<Book> books = new ArrayList<>();
+
+    public void addBook(Book book) { books.add(book); }
+
+    /** Standard forward iterator */
+    public BookIterator iterator() {
+        return new ForwardIterator();
+    }
+
+    /** Filtered iterator — only books from a specific decade */
+    public BookIterator iteratorByDecade(int decade) {
+        return new DecadeIterator(decade);
+    }
+
+    // --- Inner iterator implementations ---
+
+    private class ForwardIterator implements BookIterator {
+        private int index = 0;
+
+        @Override public boolean hasNext() { return index < books.size(); }
+        @Override public Book    next()    { return books.get(index++); }
+    }
+
+    private class DecadeIterator implements BookIterator {
+        private final int decade;
+        private int index = 0;
+        private Book nextBook;
+
+        DecadeIterator(int decade) {
+            this.decade = decade;
+            advance();
+        }
+
+        private void advance() {
+            nextBook = null;
+            while (index < books.size()) {
+                Book b = books.get(index++);
+                if (b.getYear() / 10 * 10 == decade) { nextBook = b; break; }
+            }
+        }
+
+        @Override public boolean hasNext() { return nextBook != null; }
+        @Override public Book next() {
+            Book b = nextBook; advance(); return b;
+        }
+    }
+}