Collection Classes - Beginning Java 2 SDK - page 496

A much better approach that can be used to process the elements from a collection sequentially involves
something called an iterator .

It is worth noting at this point that Java also provides something called an enumerator. An
enumerator provides essentially the same capability as an iterator, but it is recommended in the Java
documentation that you should use an iterator in preference to an enumerator for collections.

In general an iterator is an object that you can use to retrieve all the objects in a collection one by one.
Someone dealing cards from a deck one by one is acting as an iterator for the card deck - without the
shuffle, of course.

In Java, an iterator is an interface that can be implemented by a collection class. Any collection object
can create an object of type Iterator that encapsulates references to all the objects in the original
collection in some sequence, and that can be accessed using the Iterator interface methods. In other
words an iterator provides an easy way to get at all the objects in a collection one at a time. The basic
mechanism for using an iterator in Java is illustrated below.

Collection object (set or list)

collection

Calling its iterator() method
creates an object that is an
iterator.

An iterator is for one time
use. To access the
objects from a collection
again, you just obtain
another iterator object.

Iterator
object

iterator

Each successive call of the next() method
for the iterator returns the next object

Collection

object1 object2 object3 object4

The Iterator interface in java.util declares just three methods:

Method Description

next()

Returns an object as type Object starting with the first, and sets the
Iterator object to return the next object on the next call of this method. If
there is no object to be returned the method throws a
NoSuchElementException exception.

hasNext() Returns true if there is a next object to be retrieved by a call to next() .