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    1   /*
    2    * Copyright (c) 1997, 2006, Oracle and/or its affiliates. All rights reserved.
    3    * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
    4    *
    5    * This code is free software; you can redistribute it and/or modify it
    6    * under the terms of the GNU General Public License version 2 only, as
    7    * published by the Free Software Foundation.  Oracle designates this
    8    * particular file as subject to the "Classpath" exception as provided
    9    * by Oracle in the LICENSE file that accompanied this code.
   10    *
   11    * This code is distributed in the hope that it will be useful, but WITHOUT
   12    * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   13    * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   14    * version 2 for more details (a copy is included in the LICENSE file that
   15    * accompanied this code).
   16    *
   17    * You should have received a copy of the GNU General Public License version
   18    * 2 along with this work; if not, write to the Free Software Foundation,
   19    * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   20    *
   21    * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   22    * or visit www.oracle.com if you need additional information or have any
   23    * questions.
   24    */
   25   
   26   package java.util;
   27   
   28   /**
   29    * This class provides a skeletal implementation of the <tt>Set</tt>
   30    * interface to minimize the effort required to implement this
   31    * interface. <p>
   32    *
   33    * The process of implementing a set by extending this class is identical
   34    * to that of implementing a Collection by extending AbstractCollection,
   35    * except that all of the methods and constructors in subclasses of this
   36    * class must obey the additional constraints imposed by the <tt>Set</tt>
   37    * interface (for instance, the add method must not permit addition of
   38    * multiple instances of an object to a set).<p>
   39    *
   40    * Note that this class does not override any of the implementations from
   41    * the <tt>AbstractCollection</tt> class.  It merely adds implementations
   42    * for <tt>equals</tt> and <tt>hashCode</tt>.<p>
   43    *
   44    * This class is a member of the
   45    * <a href="{@docRoot}/../technotes/guides/collections/index.html">
   46    * Java Collections Framework</a>.
   47    *
   48    * @param <E> the type of elements maintained by this set
   49    *
   50    * @author  Josh Bloch
   51    * @author  Neal Gafter
   52    * @see Collection
   53    * @see AbstractCollection
   54    * @see Set
   55    * @since 1.2
   56    */
   57   
   58   public abstract class AbstractSet<E> extends AbstractCollection<E> implements Set<E> {
   59       /**
   60        * Sole constructor.  (For invocation by subclass constructors, typically
   61        * implicit.)
   62        */
   63       protected AbstractSet() {
   64       }
   65   
   66       // Comparison and hashing
   67   
   68       /**
   69        * Compares the specified object with this set for equality.  Returns
   70        * <tt>true</tt> if the given object is also a set, the two sets have
   71        * the same size, and every member of the given set is contained in
   72        * this set.  This ensures that the <tt>equals</tt> method works
   73        * properly across different implementations of the <tt>Set</tt>
   74        * interface.<p>
   75        *
   76        * This implementation first checks if the specified object is this
   77        * set; if so it returns <tt>true</tt>.  Then, it checks if the
   78        * specified object is a set whose size is identical to the size of
   79        * this set; if not, it returns false.  If so, it returns
   80        * <tt>containsAll((Collection) o)</tt>.
   81        *
   82        * @param o object to be compared for equality with this set
   83        * @return <tt>true</tt> if the specified object is equal to this set
   84        */
   85       public boolean equals(Object o) {
   86           if (o == this)
   87               return true;
   88   
   89           if (!(o instanceof Set))
   90               return false;
   91           Collection c = (Collection) o;
   92           if (c.size() != size())
   93               return false;
   94           try {
   95               return containsAll(c);
   96           } catch (ClassCastException unused)   {
   97               return false;
   98           } catch (NullPointerException unused) {
   99               return false;
  100           }
  101       }
  102   
  103       /**
  104        * Returns the hash code value for this set.  The hash code of a set is
  105        * defined to be the sum of the hash codes of the elements in the set,
  106        * where the hash code of a <tt>null</tt> element is defined to be zero.
  107        * This ensures that <tt>s1.equals(s2)</tt> implies that
  108        * <tt>s1.hashCode()==s2.hashCode()</tt> for any two sets <tt>s1</tt>
  109        * and <tt>s2</tt>, as required by the general contract of
  110        * {@link Object#hashCode}.
  111        *
  112        * <p>This implementation iterates over the set, calling the
  113        * <tt>hashCode</tt> method on each element in the set, and adding up
  114        * the results.
  115        *
  116        * @return the hash code value for this set
  117        * @see Object#equals(Object)
  118        * @see Set#equals(Object)
  119        */
  120       public int hashCode() {
  121           int h = 0;
  122           Iterator<E> i = iterator();
  123           while (i.hasNext()) {
  124               E obj = i.next();
  125               if (obj != null)
  126                   h += obj.hashCode();
  127           }
  128           return h;
  129       }
  130   
  131       /**
  132        * Removes from this set all of its elements that are contained in the
  133        * specified collection (optional operation).  If the specified
  134        * collection is also a set, this operation effectively modifies this
  135        * set so that its value is the <i>asymmetric set difference</i> of
  136        * the two sets.
  137        *
  138        * <p>This implementation determines which is the smaller of this set
  139        * and the specified collection, by invoking the <tt>size</tt>
  140        * method on each.  If this set has fewer elements, then the
  141        * implementation iterates over this set, checking each element
  142        * returned by the iterator in turn to see if it is contained in
  143        * the specified collection.  If it is so contained, it is removed
  144        * from this set with the iterator's <tt>remove</tt> method.  If
  145        * the specified collection has fewer elements, then the
  146        * implementation iterates over the specified collection, removing
  147        * from this set each element returned by the iterator, using this
  148        * set's <tt>remove</tt> method.
  149        *
  150        * <p>Note that this implementation will throw an
  151        * <tt>UnsupportedOperationException</tt> if the iterator returned by the
  152        * <tt>iterator</tt> method does not implement the <tt>remove</tt> method.
  153        *
  154        * @param  c collection containing elements to be removed from this set
  155        * @return <tt>true</tt> if this set changed as a result of the call
  156        * @throws UnsupportedOperationException if the <tt>removeAll</tt> operation
  157        *         is not supported by this set
  158        * @throws ClassCastException if the class of an element of this set
  159        *         is incompatible with the specified collection
  160        * (<a href="Collection.html#optional-restrictions">optional</a>)
  161        * @throws NullPointerException if this set contains a null element and the
  162        *         specified collection does not permit null elements
  163        * (<a href="Collection.html#optional-restrictions">optional</a>),
  164        *         or if the specified collection is null
  165        * @see #remove(Object)
  166        * @see #contains(Object)
  167        */
  168       public boolean removeAll(Collection<?> c) {
  169           boolean modified = false;
  170   
  171           if (size() > c.size()) {
  172               for (Iterator<?> i = c.iterator(); i.hasNext(); )
  173                   modified |= remove(i.next());
  174           } else {
  175               for (Iterator<?> i = iterator(); i.hasNext(); ) {
  176                   if (c.contains(i.next())) {
  177                       i.remove();
  178                       modified = true;
  179                   }
  180               }
  181           }
  182           return modified;
  183       }
  184   
  185   }

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