HashSet和HashMap的关系

public class HashSet<E>    extends AbstractSet<E>    implements Set<E>, Cloneable, java.io.Serializable{    static final long serialVersionUID = -5024744406713321676L;    private transient HashMap<E,Object> map;    // Dummy value to associate with an Object in the backing Map    private static final Object PRESENT = new Object();    /**     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has     * default initial capacity (16) and load factor (0.75).     */    public HashSet() {map = new HashMap<E,Object>();    }.........    /**     * Returns an iterator over the elements in this set.  The elements     * are returned in no particular order.     *     * @return an Iterator over the elements in this set     * @see ConcurrentModificationException     */    public Iterator<E> iterator() {return map.keySet().iterator();    }

我貌似也遇到过这样的面试官 ，问这两者的关系，原来是HashSet就是用的HashMap啊，只是省略了value，只用了key