What are the internals of the Iterator pattern?

The Iterator pattern defines two key interfaces: Iterator (with hasNext(), next(), and optionally remove()) and Iterable/Aggregate (with createIterator()). The concrete iterator holds a reference to the collection and a current position cursor. When next() is called, it returns the current element and advances the cursor. Importantly, the iterator is stateful and independent from the collection — you can have two iterators on the same collection traversing at different speeds simultaneously. In Java, the for-each loop desugars to an iterator call. For tree collections, you can have different iterator implementations for pre-order, in-order, and post-order traversal without changing the tree class.