Re: Iterators. Why Bother?

Eric Sosman <esosman@ieee-dot-org.invalid>
Wed, 06 Oct 2010 21:59:14 -0400
On 10/6/2010 11:25 AM, Steve wrote:

I know that with Java 6 it is easy to loop through a collection object
with the enhanced for loop.

I know that before Java 6 people used the Iterator class, but I am not
sure I understand why. If you can get a number of elements in the
collection and there is a method for pulling an element out by an
index, you can just use a standard for loop for iteration.

Is my answer that some collection classes do not let you pull elements
out by an index?

     Exactly. List defines an order: 0th thing, 1st thing, 2nd thing,
and so on. Set does not, and neither does Map (some specializations
of these interfaces have order, but the general cases don't).

     Also, even for List there are flavors for which access-by-index
is possible but expensive. For example, LinkedList must step over
Math.min(k, list.size()-k-1) elements to find the k'th. Traversing
an entire LinkedList by way of indices is therefore an O(N*N) process,
which an Iterator reduces to O(N).

Eric Sosman

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