Re: Custom Iterable, Issue with standard Interface?

Robert Klemme wrote:


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();
         }
         this.target = target;
     }
     public boolean hasNext() {
         return target.hasNext();
     }
     public E next() {
         return target.next();
     }
     public void remove() {
         target.hasNext();
     }
}

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