1 /* 2 * Copyright (c) 2019, 2020, 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 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 * @jls 8.10 Record Types 87 * @since 16 88 */ 89 public abstract class Record { 90 /** 91 * Constructor for record classes to call. 92 */ Record()93 protected Record() {} 94 95 /** 96 * Indicates whether some other object is "equal to" this one. In addition 97 * to the general contract of {@link Object#equals(Object) Object.equals}, 98 * record classes must further obey the invariant that when 99 * a record instance is "copied" by passing the result of the record component 100 * accessor methods to the canonical constructor, as follows: 101 * <pre> 102 * R copy = new R(r.c1(), r.c2(), ..., r.cn()); 103 * </pre> 104 * then it must be the case that {@code r.equals(copy)}. 105 * 106 * @implSpec 107 * The implicitly provided implementation returns {@code true} if 108 * and only if the argument is an instance of the same record class 109 * as this record, and each component of this record is equal to 110 * the corresponding component of the argument; otherwise, {@code 111 * false} is returned. Equality of a component {@code c} is 112 * determined as follows: 113 * <ul> 114 * 115 * <li> If the component is of a reference type, the component is 116 * considered equal if and only if {@link 117 * java.util.Objects#equals(Object,Object) 118 * Objects.equals(this.c, r.c} would return {@code true}. 119 * 120 * <li> If the component is of a primitive type, using the 121 * corresponding primitive wrapper class {@code PW} (the 122 * corresponding wrapper class for {@code int} is {@code 123 * java.lang.Integer}, and so on), the component is considered 124 * equal if and only if {@code 125 * PW.compare(this.c, r.c)} would return {@code 0}. 126 * 127 * </ul> 128 * 129 * Apart from the semantics described above, the precise algorithm 130 * used in the implicitly provided implementation is unspecified 131 * and is subject to change. The implementation may or may not use 132 * calls to the particular methods listed, and may or may not 133 * perform comparisons in the order of component declaration. 134 * 135 * @see java.util.Objects#equals(Object,Object) 136 * 137 * @param obj the reference object with which to compare. 138 * @return {@code true} if this record is equal to the 139 * argument; {@code false} otherwise. 140 */ 141 @Override equals(Object obj)142 public abstract boolean equals(Object obj); 143 144 /** 145 * Returns a hash code value for the record. 146 * Obeys the general contract of {@link Object#hashCode Object.hashCode}. 147 * For records, hashing behavior is constrained by the refined contract 148 * of {@link Record#equals Record.equals}, so that any two records 149 * created from the same components must have the same hash code. 150 * 151 * @implSpec 152 * The implicitly provided implementation returns a hash code value derived 153 * by combining appropriate hashes from each component. 154 * The precise algorithm used in the implicitly provided implementation 155 * is unspecified and is subject to change within the above limits. 156 * The resulting integer need not remain consistent from one 157 * execution of an application to another execution of the same 158 * application, even if the hashes of the component values were to 159 * remain consistent in this way. Also, a component of primitive 160 * type may contribute its bits to the hash code differently than 161 * the {@code hashCode} of its primitive wrapper class. 162 * 163 * @see Object#hashCode() 164 * 165 * @return a hash code value for this record. 166 */ 167 @Override hashCode()168 public abstract int hashCode(); 169 170 /** 171 * Returns a string representation of the record. 172 * In accordance with the general contract of {@link Object#toString()}, 173 * the {@code toString} method returns a string that 174 * "textually represents" this record. The result should 175 * be a concise but informative representation that is easy for a 176 * person to read. 177 * <p> 178 * In addition to this general contract, record classes must further 179 * participate in the invariant that any two records which are 180 * {@linkplain Record#equals(Object) equal} must produce equal 181 * strings. This invariant is necessarily relaxed in the rare 182 * case where corresponding equal component values might fail 183 * to produce equal strings for themselves. 184 * 185 * @implSpec 186 * The implicitly provided implementation returns a string which 187 * contains the name of the record class, the names of components 188 * of the record, and string representations of component values, 189 * so as to fulfill the contract of this method. 190 * The precise format produced by this implicitly provided implementation 191 * is subject to change, so the present syntax should not be parsed 192 * by applications to recover record component values. 193 * 194 * @see Object#toString() 195 * 196 * @return a string representation of the object. 197 */ 198 @Override toString()199 public abstract String toString(); 200 } 201