AbstractSet.java   [plain text]

/* AbstractSet.java -- Abstract implementation of most of Set
   Copyright (C) 1998, 2000, 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
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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.

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Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
02111-1307 USA.

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As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
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terms of your choice, provided that you also meet, for each linked
independent module, the terms and conditions of the license of that
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this exception to your version of the library, but you are not
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exception statement from your version. */


package java.util;

/**
 * An abstract implementation of Set to make it easier to create your own
 * implementations. In order to create a Set, subclass AbstractSet and
 * implement the same methods that are required for AbstractCollection
 * (although these methods must of course meet the requirements that Set puts
 * on them - specifically, no element may be in the set more than once). This
 * class simply provides implementations of equals() and hashCode() to fulfil
 * the requirements placed on them by the Set interface.
 *
 * @author Original author unknown
 * @author Eric Blake <ebb9@email.byu.edu>
 * @see Collection
 * @see AbstractCollection
 * @see Set
 * @see HashSet
 * @see TreeSet
 * @see LinkedHashSet
 * @since 1.2
 * @status updated to 1.4
 */
public abstract class AbstractSet extends AbstractCollection implements Set
{
  /**
   * The main constructor, for use by subclasses.
   */
  protected AbstractSet()
  {
  }

  /**
   * Tests whether the given object is equal to this Set. This implementation
   * first checks whether this set <em>is</em> the given object, and returns
   * true if so. Otherwise, if o is a Set and is the same size as this one, it
   * returns the result of calling containsAll on the given Set. Otherwise, it
   * returns false.
   *
   * @param o the Object to be tested for equality with this Set
   * @return true if the given object is equal to this Set
   */
  public boolean equals(Object o)
  {
    return (o == this ||
            (o instanceof Set && ((Set) o).size() == size()
             && containsAll((Collection) o)));
  }

  /**
   * Returns a hash code for this Set. The hash code of a Set is the sum of the
   * hash codes of all its elements, except that the hash code of null is
   * defined to be zero. This implementation obtains an Iterator over the Set,
   * and sums the results.
   *
   * @return a hash code for this Set
   */
  public int hashCode()
  {
    Iterator itr = iterator();
    int hash = 0;
    int pos = size();
    while (--pos >= 0)
      hash += hashCode(itr.next());
    return hash;
  }

  /**
   * Removes from this set all elements in the given collection (optional
   * operation). This implementation uses <code>size()</code> to determine
   * the smaller collection.  Then, if this set is smaller, it iterates
   * over the set, calling Iterator.remove if the collection contains
   * the element.  If this set is larger, it iterates over the collection,
   * calling Set.remove for all elements in the collection. Note that
   * this operation will fail if a remove methods is not supported.
   *
   * @param c the collection of elements to remove
   * @return true if the set was modified as a result
   * @throws UnsupportedOperationException if remove is not supported
   * @throws NullPointerException if the collection is null
   * @see AbstractCollection#remove(Object)
   * @see Collection#contains(Object)
   * @see Iterator#remove()
   */
  public boolean removeAll(Collection c)
  {
    int oldsize = size();
    int count = c.size();
    Iterator i;
    if (oldsize < count)
      {
	for (i = iterator(), count = oldsize; count > 0; count--)
          if (c.contains(i.next()))
            i.remove();
      }
    else
      for (i = c.iterator(); count > 0; count--)
        remove(i.next());
    return oldsize != size();
  }

}