Re: Generics - Is this possible?
Peter Duniho wrote:
On Tue, 15 Apr 2008 15:19:17 -0700, Patricia Shanahan <pats@acm.org> wrote:
Incidentally, does anyone understand *why* it has to be an Iterable, not
an Iterator? Of course, an Iterator based for loop would only process
the elements from the Iterator's current position on.
Only the language designers could tell you the actual reason why. But I
would agree with anyone who felt it wise to not allow user code access
to the actual iterator being used for the loop. The potential for
adding bugs seems to me to outweigh any potential convenience. It keeps
the semantics of the for(:) syntax nice, simple, and easy-to-predict.
I certainly agree that not allowing user code access to the actual
iterator would be desirable, if it were possible. It isn't. The
following, horrible, program prints "1", then "3", then gets a
java.util.NoSuchElementException.
import java.util.Arrays;
import java.util.Iterator;
import java.util.List;
public class WildFor {
public static void main(String[] args) {
List<Integer> myList = Arrays.asList(1, 2, 3, 4, 5);
RememberingIterable<Integer> myIterable =
new RememberingIterable<Integer>(myList);
for (Integer i : myIterable) {
myIterable.getLastIterator().next();
System.out.println(i);
}
}
static class RememberingIterable<T> implements
Iterable<T> {
private Iterable<T> baseIterable;
private Iterator<T> lastIterator;
public RememberingIterable(Iterable<T> iterable) {
baseIterable = iterable;
}
public Iterator<T> iterator() {
lastIterator = baseIterable.iterator();
return lastIterator;
}
public Iterator<T> getLastIterator() {
return lastIterator;
}
}
}