1 /*
2 * Copyright (c) 1997, 2018, 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.base/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 | NullPointerException unused) {
97 return false;
98 }
99 }
100
101 /**
102 * Returns the hash code value for this set. The hash code of a set is
103 * defined to be the sum of the hash codes of the elements in the set,
104 * where the hash code of a {@code null} element is defined to be zero.
105 * This ensures that {@code s1.equals(s2)} implies that
106 * {@code s1.hashCode()==s2.hashCode()} for any two sets {@code s1}
107 * and {@code s2}, as required by the general contract of
108 * {@link Object#hashCode}.
109 *
110 * <p>This implementation iterates over the set, calling the
111 * {@code hashCode} method on each element in the set, and adding up
112 * the results.
113 *
114 * @return the hash code value for this set
115 * @see Object#equals(Object)
116 * @see Set#equals(Object)
117 */
118 public int hashCode() {
119 int h = 0;
120 Iterator<E> i = iterator();
121 while (i.hasNext()) {
122 E obj = i.next();
123 if (obj != null)
124 h += obj.hashCode();
125 }
126 return h;
127 }
128
129 /**
130 * Removes from this set all of its elements that are contained in the
131 * specified collection (optional operation). If the specified
132 * collection is also a set, this operation effectively modifies this
133 * set so that its value is the <i>asymmetric set difference</i> of
134 * the two sets.
135 *
136 * <p>This implementation determines which is the smaller of this set
137 * and the specified collection, by invoking the {@code size}
138 * method on each. If this set has fewer elements, then the
139 * implementation iterates over this set, checking each element
140 * returned by the iterator in turn to see if it is contained in
141 * the specified collection. If it is so contained, it is removed
142 * from this set with the iterator's {@code remove} method. If
143 * the specified collection has fewer elements, then the
144 * implementation iterates over the specified collection, removing
145 * from this set each element returned by the iterator, using this
146 * set's {@code remove} method.
147 *
148 * <p>Note that this implementation will throw an
149 * {@code UnsupportedOperationException} if the iterator returned by the
150 * {@code iterator} method does not implement the {@code remove} method.
151 *
152 * @param c collection containing elements to be removed from this set
153 * @return {@code true} if this set changed as a result of the call
154 * @throws UnsupportedOperationException if the {@code removeAll} operation
155 * is not supported by this set
156 * @throws ClassCastException if the class of an element of this set
157 * is incompatible with the specified collection
158 * (<a href="Collection.html#optional-restrictions">optional</a>)
159 * @throws NullPointerException if this set contains a null element and the
160 * specified collection does not permit null elements
161 * (<a href="Collection.html#optional-restrictions">optional</a>),
162 * or if the specified collection is null
163 * @see #remove(Object)
164 * @see #contains(Object)
165 */
166 public boolean removeAll(Collection<?> c) {
167 Objects.requireNonNull(c);
168 boolean modified = false;
169
170 if (size() > c.size()) {
171 for (Object e : c)
172 modified |= remove(e);
173 } else {
174 for (Iterator<?> i = iterator(); i.hasNext(); ) {
175 if (c.contains(i.next())) {
176 i.remove();
177 modified = true;
178 }
179 }
180 }
181 return modified;
182 }
183
184 }