# 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/