1 /*
2 * Copyright (c) 2019, 2023, 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
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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
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24 */
25 package java.lang;
26
27 /**
28 * This is the common base class of all Java language record classes.
29 *
30 * <p>More information about records, including descriptions of the
31 * implicitly declared methods synthesized by the compiler, can be
32 * found in section 8.10 of
33 * <cite>The Java Language Specification</cite>.
34 *
35 * <p>A <em>record class</em> is a shallowly immutable, transparent carrier for
36 * a fixed set of values, called the <em>record components</em>. The Java
37 * language provides concise syntax for declaring record classes, whereby the
38 * record components are declared in the record header. The list of record
39 * components declared in the record header form the <em>record descriptor</em>.
40 *
41 * <p>A record class has the following mandated members: a <em>canonical
42 * constructor</em>, which must provide at least as much access as the record
43 * class and whose descriptor is the same as the record descriptor;
44 * a private final field corresponding to each component, whose name and
45 * type are the same as that of the component; a public accessor method
46 * corresponding to each component, whose name and return type are the same as
47 * that of the component. If not explicitly declared in the body of the record,
48 * implicit implementations for these members are provided.
49 *
50 * <p>The implicit declaration of the canonical constructor has the same accessibility
51 * as the record class and initializes the component fields from the corresponding
52 * constructor arguments. The implicit declaration of the accessor methods returns
53 * the value of the corresponding component field. The implicit declaration of the
54 * {@link Object#equals(Object)}, {@link Object#hashCode()}, and {@link Object#toString()}
55 * methods are derived from all of the component fields.
56 *
57 * <p>The primary reasons to provide an explicit declaration for the
58 * canonical constructor or accessor methods are to validate constructor
59 * arguments, perform defensive copies on mutable components, or normalize groups
60 * of components (such as reducing a rational number to lowest terms.)
61 *
62 * <p>For all record classes, the following invariant must hold: if a record R's
63 * components are {@code c1, c2, ... cn}, then if a record instance is copied
64 * as follows:
65 * <pre>
66 * R copy = new R(r.c1(), r.c2(), ..., r.cn());
67 * </pre>
68 * then it must be the case that {@code r.equals(copy)}.
69 *
70 * @apiNote
71 * A record class that {@code implements} {@link java.io.Serializable} is said
72 * to be a <i>serializable record</i>. Serializable records are serialized and
73 * deserialized differently than ordinary serializable objects. During
74 * deserialization the record's canonical constructor is invoked to construct
75 * the record object. Certain serialization-related methods, such as readObject
76 * and writeObject, are ignored for serializable records. More information about
77 * serializable records can be found in the
78 * <a href="{@docRoot}/../specs/serialization/serial-arch.html#serialization-of-records">
79 * <cite>Java Object Serialization Specification,</cite> Section 1.13,
80 * "Serialization of Records"</a>.
81 *
82 * @apiNote
83 * A record class structure can be obtained at runtime via reflection.
84 * See {@link Class#isRecord()} and {@link Class#getRecordComponents()} for more details.
85 *
86 * @spec serialization/index.html Java Object Serialization Specification
87 * @jls 8.10 Record Types
88 * @since 16
89 */
90 @jdk.internal.MigratedValueClass
91 @jdk.internal.ValueBased
92 public abstract class Record {
93 /**
94 * Constructor for record classes to call.
95 */
96 protected Record() {}
97
98 /**
99 * Indicates whether some other object is "equal to" this one. In addition
100 * to the general contract of {@link Object#equals(Object) Object.equals},
101 * record classes must further obey the invariant that when
102 * a record instance is "copied" by passing the result of the record component
103 * accessor methods to the canonical constructor, as follows:
104 * <pre>
105 * R copy = new R(r.c1(), r.c2(), ..., r.cn());
106 * </pre>
107 * then it must be the case that {@code r.equals(copy)}.
108 *
109 * @implSpec
110 * The implicitly provided implementation returns {@code true} if
111 * and only if the argument is an instance of the same record class
112 * as this record, and each component of this record is equal to
113 * the corresponding component of the argument; otherwise, {@code
114 * false} is returned. Equality of a component {@code c} is
115 * determined as follows:
116 * <ul>
117 *
118 * <li> If the component is of a reference type, the component is
119 * considered equal if and only if {@link
120 * java.util.Objects#equals(Object,Object)
121 * Objects.equals(this.c, r.c)} would return {@code true}.
122 *
123 * <li> If the component is of a primitive type, using the
124 * corresponding primitive wrapper class {@code PW} (the
125 * corresponding wrapper class for {@code int} is {@code
126 * java.lang.Integer}, and so on), the component is considered
127 * equal if and only if {@code
128 * PW.compare(this.c, r.c)} would return {@code 0}.
129 *
130 * </ul>
131 *
132 * Apart from the semantics described above, the precise algorithm
133 * used in the implicitly provided implementation is unspecified
134 * and is subject to change. The implementation may or may not use
135 * calls to the particular methods listed, and may or may not
136 * perform comparisons in the order of component declaration.
137 *
138 * @see java.util.Objects#equals(Object,Object)
139 *
140 * @param obj the reference object with which to compare.
141 * @return {@code true} if this record is equal to the
142 * argument; {@code false} otherwise.
143 */
144 @Override
145 public abstract boolean equals(Object obj);
146
147 /**
148 * Returns a hash code value for the record.
149 * Obeys the general contract of {@link Object#hashCode Object.hashCode}.
150 * For records, hashing behavior is constrained by the refined contract
151 * of {@link Record#equals Record.equals}, so that any two records
152 * created from the same components must have the same hash code.
153 *
154 * @implSpec
155 * The implicitly provided implementation returns a hash code value derived
156 * by combining appropriate hashes from each component.
157 * The precise algorithm used in the implicitly provided implementation
158 * is unspecified and is subject to change within the above limits.
159 * The resulting integer need not remain consistent from one
160 * execution of an application to another execution of the same
161 * application, even if the hashes of the component values were to
162 * remain consistent in this way. Also, a component of primitive
163 * type may contribute its bits to the hash code differently than
164 * the {@code hashCode} of its primitive wrapper class.
165 *
166 * @see Object#hashCode()
167 *
168 * @return a hash code value for this record.
169 */
170 @Override
171 public abstract int hashCode();
172
173 /**
174 * Returns a string representation of the record.
175 * In accordance with the general contract of {@link Object#toString()},
176 * the {@code toString} method returns a string that
177 * "textually represents" this record. The result should
178 * be a concise but informative representation that is easy for a
179 * person to read.
180 * <p>
181 * In addition to this general contract, record classes must further
182 * participate in the invariant that any two records which are
183 * {@linkplain Record#equals(Object) equal} must produce equal
184 * strings. This invariant is necessarily relaxed in the rare
185 * case where corresponding equal component values might fail
186 * to produce equal strings for themselves.
187 *
188 * @implSpec
189 * The implicitly provided implementation returns a string which
190 * contains the name of the record class, the names of components
191 * of the record, and string representations of component values,
192 * so as to fulfill the contract of this method.
193 * The precise format produced by this implicitly provided implementation
194 * is subject to change, so the present syntax should not be parsed
195 * by applications to recover record component values.
196 *
197 * @see Object#toString()
198 *
199 * @return a string representation of the object.
200 */
201 @Override
202 public abstract String toString();
203 }