1 /*
2 * Copyright (c) 1997, 2019, 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 // FIXME: the first paragraph below is copied from Set.equals, because
69 // {@inheritDoc} inherits text from superclasses, not interfaces (see JDK-6376959).
70 /**
71 * Compares the specified object with this set for equality. Returns
72 * {@code true} if the specified object is also a set, the two sets
73 * have the same size, and every member of the specified set is
74 * contained in this set. This operation uses the membership semantics
75 * of this set. This definition ensures that the {@code equals} method
76 * works properly across different implementations of the
77 * {@code Set} interface.
78 *
79 * @implSpec
80 * This implementation first checks if the specified object is this
81 * set; if so it returns {@code true}. Then, it checks if the
82 * specified object is a set whose size is identical to the size of
83 * this set; if not, it returns {@code false}. If so, it returns
84 * {@code containsAll((Collection) o)}.
85 *
86 * @implNote
87 * {@inheritDoc}
88 *
89 * @param o object to be compared for equality with this set
90 * @return {@code true} if the specified object is equal to this set
91 */
92 public boolean equals(Object o) {
93 if (o == this)
94 return true;
95
96 if (!(o instanceof Set))
97 return false;
98 Collection<?> c = (Collection<?>) o;
99 if (c.size() != size())
100 return false;
101 try {
102 return containsAll(c);
103 } catch (ClassCastException | NullPointerException unused) {
104 return false;
105 }
106 }
107
108 /**
109 * Returns the hash code value for this set. The hash code of a set is
110 * defined to be the sum of the hash codes of the elements in the set,
111 * where the hash code of a {@code null} element is defined to be zero.
112 * This ensures that {@code s1.equals(s2)} implies that
113 * {@code s1.hashCode()==s2.hashCode()} for any two sets {@code s1}
114 * and {@code s2}, as required by the general contract of
115 * {@link Object#hashCode}.
116 *
117 * <p>This implementation iterates over the set, calling the
118 * {@code hashCode} method on each element in the set, and adding up
119 * the results.
120 *
121 * @return the hash code value for this set
122 * @see Object#equals(Object)
123 * @see Set#equals(Object)
124 */
125 public int hashCode() {
126 int h = 0;
127 Iterator<E> i = iterator();
128 while (i.hasNext()) {
129 E obj = i.next();
130 if (obj != null)
131 h += obj.hashCode();
132 }
133 return h;
134 }
135 }