diff --git a/03-behavioral/iterator/README.md b/03-behavioral/iterator/README.md
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+# Iterator Design Pattern — Java Example
+
+**Pattern:** Behavioral → Iterator
+**Article:** https://ankurm.com/iterator-design-pattern-java/
+
+## What this example shows
+
+A `BookShelf` stores `Book` objects in an internal `ArrayList` but never exposes it. Callers only see the `BookIterator` interface (`hasNext()`/`next()`). `BookShelf` hands out two different concrete iterators: `ForwardIterator` walks every book in order, and `DecadeIterator` filters to only books published in a given decade, computing the next match lazily as `advance()` is called. `Main` exercises both iterators, then prints a line noting that `java.util.Iterator`/for-each work the same way under the hood.
+
+## How to run
+
+```bash
+javac iterator/*.java -d out/iterator
+java -cp out/iterator iterator.Main
+```
+
+Requires Java 25.
+
+## Post Section ↔ File Mapping
+
+| Post Section | File(s) |
+|---|---|
+| Custom Iterator Implementation — the Iterator interface | `BookIterator.java` |
+| Custom Iterator Implementation — the element type | `Book.java` |
+| Custom Iterator Implementation — the Aggregate (BookShelf, ForwardIterator, DecadeIterator) | `BookShelf.java` |
+| Custom Iterator Implementation — wiring it together | `Main.java` |
+
+Note: the "How Java's For-Each Loop Uses Iterator" snippet (`Iterable<Book>` example) is illustrative only — it is not part of this repository's runnable example.
+
+Article: https://ankurm.com/iterator-design-pattern-java/
+All patterns: https://ankurm.com/design-patterns-java/
diff --git a/README.md b/README.md
index 6e5197a..86e6e97 100644
--- a/README.md
+++ b/README.md
@@ -338,6 +338,23 @@ javac 03-behavioral/command/*.java -d out/command
 java -cp out/command command.Main
 ```
 
+### Iterator (`03-behavioral/iterator/`)
+
+| Post Section | File(s) |
+|---|---|
+| Custom Iterator Implementation — the Iterator interface | `BookIterator.java` |
+| Custom Iterator Implementation — the element type | `Book.java` |
+| Custom Iterator Implementation — the Aggregate (BookShelf, ForwardIterator, DecadeIterator) | `BookShelf.java` |
+| Custom Iterator Implementation — wiring it together | `Main.java` |
+
+Note: the "How Java's For-Each Loop Uses Iterator" snippet (`Iterable<Book>` example) is illustrative only — it is not part of this repository's runnable example.
+
+Run it:
+```bash
+javac 03-behavioral/iterator/*.java -d out/iterator
+java -cp out/iterator iterator.Main
+```
+
 ## Reference
 
 - *Design Patterns: Elements of Reusable Object-Oriented Software* — Gamma, Helm, Johnson, Vlissides