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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   * 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
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   */
  
  package java.util;
  
  /**
   * This class provides a skeletal implementation of the <tt>Set</tt>
   * interface to minimize the effort required to implement this
   * interface. <p>
   *
   * The process of implementing a set by extending this class is identical
   * to that of implementing a Collection by extending AbstractCollection,
   * except that all of the methods and constructors in subclasses of this
   * class must obey the additional constraints imposed by the <tt>Set</tt>
   * interface (for instance, the add method must not permit addition of
   * multiple instances of an object to a set).<p>
   *
   * Note that this class does not override any of the implementations from
   * the <tt>AbstractCollection</tt> class.  It merely adds implementations
   * for <tt>equals</tt> and <tt>hashCode</tt>.<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 AbstractCollection
   * @see Set
   * @since 1.2
   */
  
  public abstract class AbstractSet<E> extends AbstractCollection<E> implements Set<E> {
      /**
       * Sole constructor.  (For invocation by subclass constructors, typically
       * implicit.)
       */
      protected AbstractSet() {
      }
  
      // Comparison and hashing
  
      /**
       * Compares the specified object with this set for equality.  Returns
       * <tt>true</tt> if the given object is also a set, the two sets have
       * the same size, and every member of the given set is contained in
       * this set.  This ensures that the <tt>equals</tt> method works
       * properly across different implementations of the <tt>Set</tt>
       * interface.<p>
       *
       * This implementation first checks if the specified object is this
       * set; if so it returns <tt>true</tt>.  Then, it checks if the
       * specified object is a set whose size is identical to the size of
       * this set; if not, it returns false.  If so, it returns
       * <tt>containsAll((Collection) o)</tt>.
       *
       * @param o object to be compared for equality with this set
       * @return <tt>true</tt> if the specified object is equal to this set
       */
      public boolean equals(Object o) {
          if (o == this)
              return true;
  
          if (!(o instanceof Set))
              return false;
          Collection<?> c = (Collection<?>) o;
          if (c.size() != size())
              return false;
          try {
              return containsAll(c);
          } catch (ClassCastException unused)   {
              return false;
          } catch (NullPointerException unused) {
              return false;
          }
      }
  
      /**
       * Returns the hash code value for this set.  The hash code of a set is
       * defined to be the sum of the hash codes of the elements in the set,
       * where the hash code of a <tt>null</tt> element is defined to be zero.
       * This ensures that <tt>s1.equals(s2)</tt> implies that
       * <tt>s1.hashCode()==s2.hashCode()</tt> for any two sets <tt>s1</tt>
       * and <tt>s2</tt>, as required by the general contract of
       * {@link Object#hashCode}.
       *
       * <p>This implementation iterates over the set, calling the
       * <tt>hashCode</tt> method on each element in the set, and adding up
       * the results.
       *
       * @return the hash code value for this set
       * @see Object#equals(Object)
       * @see Set#equals(Object)
       */
      public int hashCode() {
          int h = 0;
          Iterator<E> i = iterator();
          while (i.hasNext()) {
              E obj = i.next();
              if (obj != null)
                  h += obj.hashCode();
          }
          return h;
      }
  
      /**
       * Removes from this set all of its elements that are contained in the
       * specified collection (optional operation).  If the specified
       * collection is also a set, this operation effectively modifies this
       * set so that its value is the <i>asymmetric set difference</i> of
       * the two sets.
       *
       * <p>This implementation determines which is the smaller of this set
       * and the specified collection, by invoking the <tt>size</tt>
       * method on each.  If this set has fewer elements, then the
       * implementation iterates over this set, checking each element
       * returned by the iterator in turn to see if it is contained in
       * the specified collection.  If it is so contained, it is removed
       * from this set with the iterator's <tt>remove</tt> method.  If
       * the specified collection has fewer elements, then the
       * implementation iterates over the specified collection, removing
       * from this set each element returned by the iterator, using this
       * set's <tt>remove</tt> method.
       *
       * <p>Note that this implementation will throw an
       * <tt>UnsupportedOperationException</tt> if the iterator returned by the
       * <tt>iterator</tt> method does not implement the <tt>remove</tt> method.
       *
       * @param  c collection containing elements to be removed from this set
       * @return <tt>true</tt> if this set changed as a result of the call
       * @throws UnsupportedOperationException if the <tt>removeAll</tt> operation
       *         is not supported by this set
       * @throws ClassCastException if the class of an element of this set
       *         is incompatible with the specified collection
       * (<a href="Collection.html#optional-restrictions">optional</a>)
       * @throws NullPointerException if this set contains a null element and the
       *         specified collection does not permit null elements
       * (<a href="Collection.html#optional-restrictions">optional</a>),
       *         or if the specified collection is null
       * @see #remove(Object)
       * @see #contains(Object)
       */
      public boolean removeAll(Collection<?> c) {
          Objects.requireNonNull(c);
          boolean modified = false;
  
          if (size() > c.size()) {
              for (Iterator<?> i = c.iterator(); i.hasNext(); )
                  modified |= remove(i.next());
          } else {
              for (Iterator<?> i = iterator(); i.hasNext(); ) {
                  if (c.contains(i.next())) {
                      i.remove();
                      modified = true;
                  }
              }
          }
          return modified;
      }
  
  }
  