/**

 * A Red-Black tree based {@link NavigableMap} implementation.

 * The map is sorted according to the {@linkplain Comparable natural

 * ordering} of its keys, or by a {@link Comparator} provided at map

 * creation time, depending on which constructor is used.

 *

 * <p>This implementation provides guaranteed log(n) time cost for the

 * {@code containsKey}, {@code get}, {@code put} and {@code remove}

 * operations.  Algorithms are adaptations of those in Cormen, Leiserson, and

 * Rivest's <em>Introduction to Algorithms</em>.

*/

static final class Entry<K,V> implements Map.Entry<K,V> {

        K key;

        V value;

        Entry<K,V> left;

        Entry<K,V> right;

        Entry<K,V> parent;

        boolean color = BLACK;

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 {@code HashMap} instance has

     * default initial capacity (16) and load factor (0.75).

     */

    public HashSet() {

        map = new HashMap<>();

    }

.............

　　public boolean add(E e) {

        return map.put(e, PRESENT)==null;

    }

........

}