IsDone

CurrentItem

These operations define the basic services that an Iterator must provide in order to do its job. In more general terms, an Iterator should provide the following core capabilities:

Navigation – Moving forward or backward within the collection

Retrieval – Getting the currently referenced element

Validation – Determining if there are still elements in the collection, based on the Iterator's current position

Iterators may also provide extended operations. Some Iterators provide methods to move to the first or last element in the Iterator, for example.

Implementation

The Iterator class diagram is shown in Figure 2.7.

Figure 2.7. Iterator class diagram

To implement the Iterator pattern, you need:

Iterator – This interface defines the standard iteration methods. At a minimum, the interface defines methods for navigation, retrieval and validation (first, next, hasMoreElements and getCurrentItem)

ConcreteIterator – Classes that implement the Iterator. These classes reference the underlying collection. Normally, instances are created by the ConcreteAggregate. Because of the tight coupling with the

ConcreteAggregate, the ConcreteIterator often is an inner class of the ConcreteAggregate.

Aggregate – This interface defines a factory method to produce the Iterator.

ConcreteAggregate – This class implements the Aggregate, building a ConcreteIterator on demand. The ConcreteAggregate performs this task in addition to its fundamental responsibility of representing a collection of objects in a system. ConcreteAggregate creates the ConcreteIterator instance.

Benefits and Drawbacks

Many of the Iterator pattern's benefits stem from the advantages of defining a uniform interface for collection traversal. This greatly simplifies the use of collections, and allows you to use polymorphism when working with collections. To print the elements in any collection, for instance, you could obtain an Iterator, then call the toString method on any object, regardless of its underlying collection.

53