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package java.util;
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<>();
     * Constructs a new set containing the elements in the specified
     * collection.  The <tt>HashMap</tt> is created with default load factor
     * (0.75) and an initial capacity sufficient to contain the elements in
     * the specified collection.
     * @param c the collection whose elements are to be placed into this set
     * @throws NullPointerException if the specified collection is null
    public HashSet(Collection<? extends E> c) {
        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
        addAll(c);
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * the specified initial capacity and the specified load factor.
     * @param      initialCapacity   the initial capacity of the hash map
     * @param      loadFactor        the load factor of the hash map
     * @throws     IllegalArgumentException if the initial capacity is less
     *             than zero, or if the load factor is nonpositive
    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * the specified initial capacity and default load factor (0.75).
     * @param      initialCapacity   the initial capacity of the hash table
     * @throws     IllegalArgumentException if the initial capacity is less
     *             than zero
    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
     * Constructs a new, empty linked hash set.  (This package private
     * constructor is only used by LinkedHashSet.) The backing
     * HashMap instance is a LinkedHashMap with the specified initial
     * capacity and the specified load factor.
     * @param      initialCapacity   the initial capacity of the hash map
     * @param      loadFactor        the load factor of the hash map
     * @param      dummy             ignored (distinguishes this
     *             constructor from other int, float constructor.)
     * @throws     IllegalArgumentException if the initial capacity is less
     *             than zero, or if the load factor is nonpositive
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
     * 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();
     * Returns the number of elements in this set (its cardinality).
     * @return the number of elements in this set (its cardinality)
    public int size() {
        return map.size();
     * Returns <tt>true</tt> if this set contains no elements.
     * @return <tt>true</tt> if this set contains no elements
    public boolean isEmpty() {
        return map.isEmpty();
     * Returns <tt>true</tt> if this set contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this set
     * contains an element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     * @param o element whose presence in this set is to be tested
     * @return <tt>true</tt> if this set contains the specified element
    public boolean contains(Object o) {
        return map.containsKey(o);
     * Adds the specified element to this set if it is not already present.
     * More formally, adds the specified element <tt>e</tt> to this set if
     * this set contains no element <tt>e2</tt> such that
     * <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>.
     * If this set already contains the element, the call leaves the set
     * unchanged and returns <tt>false</tt>.
     * @param e element to be added to this set
     * @return <tt>true</tt> if this set did not already contain the specified
     * element
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
     * Removes the specified element from this set if it is present.
     * More formally, removes an element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>,
     * if this set contains such an element.  Returns <tt>true</tt> if
     * this set contained the element (or equivalently, if this set
     * changed as a result of the call).  (This set will not contain the
     * element once the call returns.)
     * @param o object to be removed from this set, if present
     * @return <tt>true</tt> if the set contained the specified element
    public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
     * Removes all of the elements from this set.
     * The set will be empty after this call returns.
    public void clear() {
        map.clear();
     * Returns a shallow copy of this <tt>HashSet</tt> instance: the elements
     * themselves are not cloned.
     * @return a shallow copy of this set
    public Object clone() {
        try {
            HashSet<E> newSet = (HashSet<E>) super.clone();
            newSet.map = (HashMap<E, Object>) map.clone();
            return newSet;
        } catch (CloneNotSupportedException e) {
            throw new InternalError();
     * Save the state of this <tt>HashSet</tt> instance to a stream (that is,
     * serialize it).
     * @serialData The capacity of the backing <tt>HashMap</tt> instance
     *             (int), and its load factor (float) are emitted, followed by
     *             the size of the set (the number of elements it contains)
     *             (int), followed by all of its elements (each an Object) in
     *             no particular order.
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        // Write out any hidden serialization magic
        s.defaultWriteObject();
        // Write out HashMap capacity and load factor
        s.writeInt(map.capacity());
        s.writeFloat(map.loadFactor());
        // Write out size
        s.writeInt(map.size());
        // Write out all elements in the proper order.
        for (E e : map.keySet())
            s.writeObject(e);
     * Reconstitute the <tt>HashSet</tt> instance from a stream (that is,
     * deserialize it).
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Read in any hidden serialization magic
        s.defaultReadObject();
        // Read in HashMap capacity and load factor and create backing HashMap
        int capacity = s.readInt();
        float loadFactor = s.readFloat();
        map = (((HashSet)this) instanceof LinkedHashSet ?
               new LinkedHashMap<E,Object>(capacity, loadFactor) :
               new HashMap<E,Object>(capacity, loadFactor));
        // Read in size
        int size = s.readInt();
        // Read in all elements in the proper order.
        for (int i=0; i<size; i++) {
            E e = (E) s.readObject();
            map.put(e, PRESENT);
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