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   * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   * or visit www.oracle.com if you need additional information or have any
   * questions.
   */
  
  package java.util;
  
  import java.io.InvalidObjectException;
  
  /**
   * This class implements the <tt>Set</tt> interface, backed by a hash table
   * (actually a <tt>HashMap</tt> instance).  It makes no guarantees as to the
   * iteration order of the set; in particular, it does not guarantee that the
   * order will remain constant over time.  This class permits the <tt>null</tt>
   * element.
   *
   * <p>This class offers constant time performance for the basic operations
   * (<tt>add</tt>, <tt>remove</tt>, <tt>contains</tt> and <tt>size</tt>),
   * assuming the hash function disperses the elements properly among the
   * buckets.  Iterating over this set requires time proportional to the sum of
   * the <tt>HashSet</tt> instance's size (the number of elements) plus the
   * "capacity" of the backing <tt>HashMap</tt> instance (the number of
   * buckets).  Thus, it's very important not to set the initial capacity too
   * high (or the load factor too low) if iteration performance is important.
   *
   * <p><strong>Note that this implementation is not synchronized.</strong>
   * If multiple threads access a hash set concurrently, and at least one of
   * the threads modifies the set, it <i>must</i> be synchronized externally.
   * This is typically accomplished by synchronizing on some object that
   * naturally encapsulates the set.
   *
   * If no such object exists, the set should be "wrapped" using the
   * {@link Collections#synchronizedSet Collections.synchronizedSet}
   * method.  This is best done at creation time, to prevent accidental
   * unsynchronized access to the set:<pre>
   *   Set s = Collections.synchronizedSet(new HashSet(...));</pre>
   *
   * <p>The iterators returned by this class's <tt>iterator</tt> method are
   * <i>fail-fast</i>: if the set is modified at any time after the iterator is
   * created, in any way except through the iterator's own <tt>remove</tt>
   * method, the Iterator throws a {@link ConcurrentModificationException}.
   * Thus, in the face of concurrent modification, the iterator fails quickly
   * and cleanly, rather than risking arbitrary, non-deterministic behavior at
   * an undetermined time in the future.
   *
   * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
   * as it is, generally speaking, impossible to make any hard guarantees in the
   * presence of unsynchronized concurrent modification.  Fail-fast iterators
   * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
   * Therefore, it would be wrong to write a program that depended on this
   * exception for its correctness: <i>the fail-fast behavior of iterators
   * should be used only to detect bugs.</i>
   *
   * <p>This class is a member of the
   * <a href="{@docRoot}/../technotes/guides/collections/index.html">
   * Java Collections Framework</a>.
   *
   * @param <E> the type of elements maintained by this set
   *
   * @author  Josh Bloch
   * @author  Neal Gafter
   * @see     Collection
   * @see     Set
   * @see     TreeSet
   * @see     HashMap
   * @since   1.2
   */
  
  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
       */
      @SuppressWarnings("unchecked")
      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(e);
          }
      }
  
      /**
       * 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 capacity and verify non-negative.
          int capacity = s.readInt();
          if (capacity < 0) {
              throw new InvalidObjectException("Illegal capacity: " +
                                               capacity);
          }
  
          // Read load factor and verify positive and non NaN.
          float loadFactor = s.readFloat();
          if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
              throw new InvalidObjectException("Illegal load factor: " +
                                               loadFactor);
          }
  
          // Read size and verify non-negative.
          int size = s.readInt();
          if (size < 0) {
              throw new InvalidObjectException("Illegal size: " +
                                               size);
          }
  
          // Set the capacity according to the size and load factor ensuring that
          // the HashMap is at least 25% full but clamping to maximum capacity.
          capacity = (int) Math.min(size * Math.min(1 / loadFactor, 4.0f),
                  HashMap.MAXIMUM_CAPACITY);
  
          // Create backing HashMap
          map = (((HashSet<?>)this) instanceof LinkedHashSet ?
                 new LinkedHashMap<E,Object>(capacity, loadFactor) :
                 new HashMap<E,Object>(capacity, loadFactor));
  
          // Read in all elements in the proper order.
          for (int i=0; i<size; i++) {
              @SuppressWarnings("unchecked")
                  E e = (E) s.readObject();
              map.put(e, PRESENT);
          }
      }
  
      /**
       * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
       * and <em>fail-fast</em> {@link Spliterator} over the elements in this
       * set.
       *
       * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
       * {@link Spliterator#DISTINCT}.  Overriding implementations should document
       * the reporting of additional characteristic values.
       *
       * @return a {@code Spliterator} over the elements in this set
       * @since 1.8
       */
      public Spliterator<E> spliterator() {
          return new HashMap.KeySpliterator<E,Object>(map, 0, -1, 0, 0);
      }
  }
  