This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I don't know if I've really understood your question, but I could give you this example:
A Hashtable collection is indexed by keys and IS NOT ordered or "iteratable" (don't know if this term really exists) by itself. Therefore, to iterate through a Hashtable (traverse its elements) you may call the method elements() of the class Hashtable, and this method will return a Enumeration object. Now, with a Enumeration object, you are able to traverse the elements of your Hashtable collection just using the methods of that interface, that are: nextElement() and hasMoreElements().