Implementing Iterator in Java

Iterator: A pattern that provides stepwise access to a collection or aggregate without exposing its internal representation.

In Java, custom iterator work usually starts by implementing Iterable<T> so the aggregate integrates with the language naturally.

1public final class TaskQueue implements Iterable<Task> {
2    private final List<Task> tasks = new ArrayList<>();
3
4    @Override
5    public Iterator<Task> iterator() {
6        return tasks.iterator();
7    }
8}

That is enough for many cases. A custom iterator becomes interesting when traversal is not identical to the backing structure.

Good Reasons For A Custom Iterator

• filtered traversal
• tree or graph traversal
• traversal over virtual or generated elements
• domain-specific order

Review Rule

If the collection is already well-modeled by a standard Java collection, prefer that first. Custom iterators should exist because traversal meaning is special, not because patterns are fun.