how is an iterator created?

[william kane]

How do iterator and enumerator work?Do they actually copy the the contents of the source data structure(collection that returns this enumerator/iterator) into a diffenent data structure?

[Michael Matola]

For typical Iterator implementations take a look at the source code for AbstractList, specifically the iterator() and listIterator() methods and the private classes Itr and ListItr that they rely on.
For Enumeration, take a look at the source code for Vector, specifically the elements() method.
In both cases, the data structures aren't copied; the Iterator and Enumeration instances just hold index variable(s) that indicate the current state of the iterator/enumeration in its traversal of the list.
Although I believe these are typical implementations, I don't think iterator and enumeration implementors are under any contract to implement this way.

[Thomas Paul]

You should note that every current implementation of iterator is fail-fast. That is, the iterator will throw a ConcurrentModificationException if the underlying Collection object is changed in some way.
Sun does add this to their doc:

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.