1 /*
   2  * Copyright (c) 1997, 2013, 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 {@code Set}
  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 {@code Set}
  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 {@code AbstractCollection} class.  It merely adds implementations
  42  * for {@code equals} and {@code hashCode}.<p>
  43  *
  44  * This class is a member of the
  45  * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
  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      * {@code true} 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 {@code equals} method works
  73      * properly across different implementations of the {@code Set}
  74      * interface.<p>
  75      *
  76      * This implementation first checks if the specified object is this
  77      * set; if so it returns {@code true}.  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      * {@code containsAll((Collection) o)}.
  81      *
  82      * @param o object to be compared for equality with this set
  83      * @return {@code true} 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 {@code null} element is defined to be zero.
 107      * This ensures that {@code s1.equals(s2)} implies that
 108      * {@code s1.hashCode()==s2.hashCode()} for any two sets {@code s1}
 109      * and {@code s2}, as required by the general contract of
 110      * {@link Object#hashCode}.
 111      *
 112      * <p>This implementation iterates over the set, calling the
 113      * {@code hashCode} 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 {@code size}
 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 {@code remove} 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 {@code remove} method.
 149      *
 150      * <p>Note that this implementation will throw an
 151      * {@code UnsupportedOperationException} if the iterator returned by the
 152      * {@code iterator} method does not implement the {@code remove} method.
 153      *
 154      * @param  c collection containing elements to be removed from this set
 155      * @return {@code true} if this set changed as a result of the call
 156      * @throws UnsupportedOperationException if the {@code removeAll} 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         Objects.requireNonNull(c);
 170         boolean modified = false;
 171 
 172         if (size() > c.size()) {
 173             for (Object e : c)
 174                 modified |= remove(e);
 175         } else {
 176             for (Iterator<?> i = iterator(); i.hasNext(); ) {
 177                 if (c.contains(i.next())) {
 178                     i.remove();
 179                     modified = true;
 180                 }
 181             }
 182         }
 183         return modified;
 184     }
 185 
 186 }