Please help, how to implement hasNext() and next() of a HashSet iterator?

Question

Hi guys,
Can someone help me out with this? I'm struggling to write an iterator (the hasNext() and next() methods) for a SimpleHashSet class and I keep failing. I have a basic unit test, which goes like this:
@Test
    public void testIterator() {
        SimpleHashSet<String> instance = new SimpleHashSet<String>();
        assertTrue(instance.iterator() != null);
        Iterator<String> it = instance.iterator();
        assertFalse(it.hasNext()); //Should return false on an empty HashSet
        assertTrue(it.next() == null); //Should return null on an empty HashSet
        instance.add("First");
        instance.add("Second");
        assertTrue(it.hasNext()); // Return true if has first element
        assertTrue(it.next() != null); //Should return first element
        assertTrue(it.hasNext()); //Return true if has second element
        assertTrue(it.next() != null); //Should return second element
        assertFalse(it.hasNext()); //Return false, no third element
        assertTrue(it.next() == null); //Return true, no third element         
    }and I can't figure out a way to pass all of these tests. I have written several variants and none of them works completely, only partially, inconsistently. I just don't know how to write it.
The SimpleHashSet is declared like this:
public class SimpleHashSet<E> extends AbstractSet<E> {
    private static final int SIZE = 11;
    LinkedList<E>[] buckets = new LinkedList[SIZE];
}I appreciate all help and guidance,
PR.

Answer 1

Awesome, great answers, very helpful, thanks very much. Careful debugging and testing led me to a working solution. It passes my tests. This is what I have:
    @Override
    public Iterator<E> iterator() {
        final SimpleHashSet<E> here = this;
        Iterator<E> it = new Iterator<E>() {
            private final SimpleHashSet<E> mySet = here;
            private List<LinkedList<E>> bucketList = Arrays.asList(mySet.buckets);
            private Iterator<LinkedList<E>> bucketsIterator = bucketList.iterator();
            private LinkedList<E> currentBucket;
            private Iterator<E> elements;
                currentBucket = bucketsIterator.next();
                elements = currentBucket.iterator();  
            @Override
            public boolean hasNext() {
                while (bucketsIterator.hasNext()) {
                    if (elements.hasNext()) {
                        return true;
                    } else {
                        currentBucket = bucketsIterator.next();
                        elements = currentBucket.iterator();
                if (elements.hasNext())
                    return true;
                return false;
            @Override
            public E next() {
                while (bucketsIterator.hasNext()) {
                    if (elements.hasNext()) {
                          return elements.next();
                    } else {
                        currentBucket = bucketsIterator.next();
                        elements = currentBucket.iterator();
                if (elements.hasNext())
                    return elements.next();
                if (!bucketsIterator.hasNext())
                    throw new NoSuchElementException();
                return null;
        return it;
    }I'm only not sure if it behaves the same as an original HashSet iterator. I doubt. The problem with my implementation is that both hasNext() and next() progress the internal iterators forward, so i.e. calling hasNext() on an empty set will progress the internal iterators to the end, so then adding some elements and calling hasNext() again will return false. I couldn't figure out a way that hasNext() does not progress the iterators, so that only next does that. Both hasNext() and next() are dependent on each other in my example. But it works.
Thanks,
PR.