Java Reference
In-Depth Information
it.next();
it.hasNext();
it.remove();
Imagine that this were to happen when there was one more short string
left in the iteration followed by some long ones. The invocation of
next
would return the last short string. Then
hasNext
would iterate through
the list of strings, and finding no more short ones, return
false
. When
remove
was invoked, it would invoke
remove
on the underlying iterator,
thereby removing the last (long) string that
hasNext
rejected. That would
be incorrect. Since the above code is valid, you cannot fix the problem
by forbidding the sequence of methods. You are effectively stuck. Be-
cause of this, you cannot build a filtering iterator on top of another
Iter-
ator
object. You can build one on top of a
ListIterator
though, since it
allows you to back up to the previously returned short string.
The methods of
ListIterator
have contracts similar to those of
Iterator
,
as you have learned earlier in this chapter. You can provide
ListIterator
objects in some circumstances where you might otherwise write an
Iter-
ator
. If you are writing a general utility class for others to use, you
should implement
ListIterator
instead of
Iterator
if possible.
Exercise 21.4
: Write a version of
ShortStrings
that implements
ListIterator
to filter a
ListIterator
object. Should your class extend
ShortStrings
?
