Template method to allow subclasses to supply elements for the iteration.

  Template method to allow subclasses to supply elements for the
iteration. The directory iterator maintains a stack of iterators
covering each level in the directory hierarchy. The current iterator
covers the current directory being scanned. If the next entry in that
directory is a subdirectory, the current iterator is pushed onto the
stack and a new iterator is created for the subdirectory. If the
entry is a file, it is returned as the next element and the iterator
remains valid. If there are no more entries in the current directory,
the topmost iterator on the stack is popped off to become the
current iterator.
Returns: the next ClassFile in the iteration.