Java Collections Framework - Building Java Programs: A Back to Basics Approach - page 682

Iterator
An object that allows the efficient retrieval of the elements of a list in
sequential order.

Using an iterator, when we move from one element to the next, we don't have to
go back to the beginning of the list at each call and follow the links all the way from
the front of the list to the desired index. As we'll see in this chapter, iterators are cen-
tral to the Java Collections Framework. Every collection provides iterators to access
its elements. In other words, there's a familiar interface for examining the elements
of any collection.
An iterator object has the methods listed in Table 11.2. The methods next and
hasNext should be familiar from the Scanner class that you have already studied,
and their behavior is similar here.
To get an iterator from most collections, such as an ArrayList or a LinkedList ,
you call the method iterator on the list, which returns an object of type Iterator
for examining that list's elements. (You don't use the new keyword.) Generally, a
variable named list storing elements of type E uses an iterator in the following way:

Iterator<E> itr = list.iterator();
while (itr.hasNext()) {
<do something with itr.next()>;

}

The example of removing strings that have even length from a collection can be
implemented much more efficiently using an iterator:

// removes all strings of even length from the given linked list
public static void removeEvenLength(LinkedList<String> list) {
Iterator<String> i = list.iterator();
while (i.hasNext()) {
String element = i.next();
if (element.length() % 2 == 0) {
i.remove();

}

}

}

Table 11.2 Methods of Iterator Objects

Method Description

Returns true if there are more elements to be examined hasNext()

Returns the next element from the list and advances the position of the
iterator by one

next()

Removes the element most recently returned by next()

remove()