HashSet.java

/* HashSet.java -- a class providing a HashMap-backed Set
   Copyright (C) 1998, 1999, 2001 Free Software Foundation, Inc.

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2, or (at your option)
any later version.

GNU Classpath 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 for more details.

You should have received a copy of the GNU General Public License
along with GNU Classpath; see the file COPYING.  If not, write to the
Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
02111-1307 USA.

As a special exception, if you link this library with other files to
produce an executable, this library does not by itself cause the
resulting executable to be covered by the GNU General Public License.
This exception does not however invalidate any other reasons why the
executable file might be covered by the GNU General Public License. */


package java.util;

import java.io.IOException;
import java.io.Serializable;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

/**
 * This class provides a HashMap-backed implementation of the Set interface.
 * <p>
 *
 * Most operations are O(1), assuming no hash collisions.  In the worst
 * case (where all hashes collide), operations are O(n). Setting the
 * initial capacity too low will force many resizing operations, but
 * setting the initial capacity too high (or loadfactor too low) leads
 * to wasted memory and slower iteration.
 * <p>
 *
 * HashSet accepts the null key and null values.  It is not synchronized,
 * so if you need multi-threaded access, consider using:<br>
 * <code>Set s = Collections.synchronizedSet(new HashSet(...));</code>
 * <p>
 *
 * The iterators are <i>fail-fast</i>, meaning that any structural
 * modification, except for <code>remove()</code> called on the iterator
 * itself, cause the iterator to throw a
 * {@link ConcurrentModificationException} rather than exhibit
 * non-deterministic behavior.
 *
 * @author Jon Zeppieri
 * @author Eric Blake <ebb9@email.byu.edu>
 * @see Collection
 * @see Set
 * @see TreeSet
 * @see Collections#synchronizedSet(Set)
 * @see HashMap
 * @see LinkedHashSet
 * @since 1.2
 * @status updated to 1.4
 */
public class HashSet extends AbstractSet
  implements Set, Cloneable, Serializable
{
  /**
   * Compatible with JDK 1.2.
   */
  private static final long serialVersionUID = -5024744406713321676L;

  /**
   * The HashMap which backs this Set.
   */
  private transient HashMap map;

  /**
   * Construct a new, empty HashSet whose backing HashMap has the default
   * capacity (11) and loadFacor (0.75).
   */
  public HashSet()
  {
    this(HashMap.DEFAULT_CAPACITY, HashMap.DEFAULT_LOAD_FACTOR);
  }

  /**
   * Construct a new, empty HashSet whose backing HashMap has the supplied
   * capacity and the default load factor (0.75).
   *
   * @param initialCapacity the initial capacity of the backing HashMap
   * @throws IllegalArgumentException if the capacity is negative
   */
  public HashSet(int initialCapacity)
  {
    this(initialCapacity, HashMap.DEFAULT_LOAD_FACTOR);
  }

  /**
   * Construct a new, empty HashSet whose backing HashMap has the supplied
   * capacity and load factor.
   *
   * @param initialCapacity the initial capacity of the backing HashMap
   * @param loadFactor the load factor of the backing HashMap
   * @throws IllegalArgumentException if either argument is negative, or
   *         if loadFactor is POSITIVE_INFINITY or NaN
   */
  public HashSet(int initialCapacity, float loadFactor)
  {
    map = init(initialCapacity, loadFactor);
  }

  /**
   * Construct a new HashSet with the same elements as are in the supplied
   * collection (eliminating any duplicates, of course). The backing storage
   * has twice the size of the collection, or the default size of 11,
   * whichever is greater; and the default load factor (0.75).
   *
   * @param c a collection of initial set elements
   * @throws NullPointerException if c is null
   */
  public HashSet(Collection c)
  {
    this(Math.max(2 * c.size(), HashMap.DEFAULT_CAPACITY));
    addAll(c);
  }

  /**
   * Adds the given Object to the set if it is not already in the Set.
   * This set permits a null element.
   *
   * @param o the Object to add to this Set
   * @return true if the set did not already contain o
   */
  public boolean add(Object o)
  {
    return map.put(o, "") == null;
  }

  /**
   * Empties this Set of all elements; this takes constant time.
   */
  public void clear()
  {
    map.clear();
  }

  /**
   * Returns a shallow copy of this Set. The Set itself is cloned; its
   * elements are not.
   *
   * @return a shallow clone of the set
   */
  public Object clone()
  {
    HashSet copy = null;
    try
      {
        copy = (HashSet) super.clone();
      }
    catch (CloneNotSupportedException x)
      {
        // Impossible to get here.
      }
    copy.map = (HashMap) map.clone();
    return copy;
  }

  /**
   * Returns true if the supplied element is in this Set.
   *
   * @param o the Object to look for
   * @return true if it is in the set
   */
  public boolean contains(Object o)
  {
    return map.containsKey(o);
  }

  /**
   * Returns true if this set has no elements in it.
   *
   * @return <code>size() == 0</code>.
   */
  public boolean isEmpty()
  {
    return map.size == 0;
  }

  /**
   * Returns an Iterator over the elements of this Set, which visits the
   * elements in no particular order.  For this class, the Iterator allows
   * removal of elements. The iterator is fail-fast, and will throw a
   * ConcurrentModificationException if the set is modified externally.
   *
   * @return a set iterator
   * @see ConcurrentModificationException
   */
  public Iterator iterator()
  {
    // Avoid creating intermediate keySet() object by using non-public API.
    return map.iterator(HashMap.KEYS);
  }

  /**
   * Removes the supplied Object from this Set if it is in the Set.
   *
   * @param o the object to remove
   * @return true if an element was removed
   */
  public boolean remove(Object o)
  {
    return (map.remove(o) != null);
  }

  /**
   * Returns the number of elements in this Set (its cardinality).
   *
   * @return the size of the set
   */
  public int size()
  {
    return map.size;
  }

  /**
   * Helper method which initializes the backing Map. Overridden by
   * LinkedHashSet for correct semantics.
   *
   * @param capacity the initial capacity
   * @param load the initial load factor
   * @return the backing HashMap
   */
  HashMap init(int capacity, float load)
  {
    return new HashMap(capacity, load);
  }

  /**
   * Serializes this object to the given stream.
   *
   * @param s the stream to write to
   * @throws IOException if the underlying stream fails
   * @serialData the <i>capacity</i> (int) and <i>loadFactor</i> (float)
   *             of the backing store, followed by the set size (int),
   *             then a listing of its elements (Object) in no order
   */
  private void writeObject(ObjectOutputStream s) throws IOException
  {
    s.defaultWriteObject();
    // Avoid creating intermediate keySet() object by using non-public API.
    Iterator it = map.iterator(HashMap.KEYS);
    s.writeInt(map.buckets.length);
    s.writeFloat(map.loadFactor);
    s.writeInt(map.size);
    while (it.hasNext())
      s.writeObject(it.next());
  }

  /**
   * Deserializes this object from the given stream.
   *
   * @param s the stream to read from
   * @throws ClassNotFoundException if the underlying stream fails
   * @throws IOException if the underlying stream fails
   * @serialData the <i>capacity</i> (int) and <i>loadFactor</i> (float)
   *             of the backing store, followed by the set size (int),
   *             then a listing of its elements (Object) in no order
   */
  private void readObject(ObjectInputStream s)
    throws IOException, ClassNotFoundException
  {
    s.defaultReadObject();

    map = init(s.readInt(), s.readFloat());
    for (int size = s.readInt(); size > 0; size--)
      map.put(s.readObject(), "");
  }
}