Re: Custom Iterable, Issue with standard Interface?

Thomas Hawtin <>
Mon, 16 Oct 2006 17:15:20 +0100
Robert Klemme wrote:

I have a similar situation as demonstrated below. I cannot find a
proper solution to make method iterator() work properly. It works with
the custom interface Iterable2 (see below). I tried several
alternatives I could think of but to no avail (see comments in code).
Any other ideas? Or am I running into a limitation of Iterable?
Research did not turn up much useful information for this situation.

So you have an Iterator<D> but want to return it as type Iterator<B>,
where D extends B?

I suggest you write a proxy:

class IteratorTypeProxy<E> implements Iterator<E> {
     private final Iterator<? extends E> target;
     public static <E> Iterator<E> create(Iterator<? extends E> target) {
         return new IteratorTypeProxy<E>(target);
     private IteratorTypeProxy(Iterator<? extends E> target) {
         if (target == null) {
             throw new NullPointerException();
         } = target;
     public boolean hasNext() {
         return target.hasNext();
     public E next() {
     public void remove() {

Note, you can't implement, say, the full ListIterator interface this
way. add(E) and set(E) wouldn't work.

As Iterable<B>.iterator gives an Iterator<B> instead of Iterator<?
extends B>, the method can be overridden to return ListIterator<B>. (It
also helps to make client code easier to read - as a rule, don't return
wildcarded types.)

Tom Hawtin

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