BaseIterator.java

import java.util.Iterator;
import java.util.NoSuchElementException;

public abstract class BaseIterator<T> implements Iterator<T> {

    private T nextItem;

    @Override
    public boolean hasNext() {
        return fetchNext() != null;
    }

    @Override
    public T next() {
        T item = fetchNext();
        if (item == null) {
            throw new NoSuchElementException("iterator exhausted");
        }
        else {
            nextItem = null;
            return item;
        }
    }

    private T fetchNext() {
        if (nextItem == null) {
            nextItem = loadNext();
        }
        return nextItem;
    }

    // try to load next.  return null if no item found
    public abstract T loadNext();

}