1 /* 2 * Copyright (c) 1997, 2021, 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.lang; 27 import java.util.*; 28 29 /** 30 * This interface imposes a total ordering on the objects of each class that 31 * implements it. This ordering is referred to as the class's <i>natural 32 * ordering</i>, and the class's {@code compareTo} method is referred to as 33 * its <i>natural comparison method</i>.<p> 34 * 35 * Lists (and arrays) of objects that implement this interface can be sorted 36 * automatically by {@link Collections#sort(List) Collections.sort} (and 37 * {@link Arrays#sort(Object[]) Arrays.sort}). Objects that implement this 38 * interface can be used as keys in a {@linkplain SortedMap sorted map} or as 39 * elements in a {@linkplain SortedSet sorted set}, without the need to 40 * specify a {@linkplain Comparator comparator}.<p> 41 * 42 * The natural ordering for a class {@code C} is said to be <i>consistent 43 * with equals</i> if and only if {@code e1.compareTo(e2) == 0} has 44 * the same boolean value as {@code e1.equals(e2)} for every 45 * {@code e1} and {@code e2} of class {@code C}. Note that {@code null} 46 * is not an instance of any class, and {@code e.compareTo(null)} should 47 * throw a {@code NullPointerException} even though {@code e.equals(null)} 48 * returns {@code false}.<p> 49 * 50 * It is strongly recommended (though not required) that natural orderings be 51 * consistent with equals. This is so because sorted sets (and sorted maps) 52 * without explicit comparators behave "strangely" when they are used with 53 * elements (or keys) whose natural ordering is inconsistent with equals. In 54 * particular, such a sorted set (or sorted map) violates the general contract 55 * for set (or map), which is defined in terms of the {@code equals} 56 * method.<p> 57 * 58 * For example, if one adds two keys {@code a} and {@code b} such that 59 * {@code (!a.equals(b) && a.compareTo(b) == 0)} to a sorted 60 * set that does not use an explicit comparator, the second {@code add} 61 * operation returns false (and the size of the sorted set does not increase) 62 * because {@code a} and {@code b} are equivalent from the sorted set's 63 * perspective.<p> 64 * 65 * Virtually all Java core classes that implement {@code Comparable} 66 * have natural orderings that are consistent with equals. One 67 * exception is {@link java.math.BigDecimal}, whose {@linkplain 68 * java.math.BigDecimal#compareTo natural ordering} equates {@code 69 * BigDecimal} objects with equal numerical values and different 70 * representations (such as 4.0 and 4.00). For {@link 71 * java.math.BigDecimal#equals BigDecimal.equals()} to return true, 72 * the representation and numerical value of the two {@code 73 * BigDecimal} objects must be the same.<p> 74 * 75 * For the mathematically inclined, the <i>relation</i> that defines 76 * the natural ordering on a given class C is:<pre>{@code 77 * {(x, y) such that x.compareTo(y) <= 0}. 78 * }</pre> The <i>quotient</i> for this total order is: <pre>{@code 79 * {(x, y) such that x.compareTo(y) == 0}. 80 * }</pre> 81 * 82 * It follows immediately from the contract for {@code compareTo} that the 83 * quotient is an <i>equivalence relation</i> on {@code C}, and that the 84 * natural ordering is a <i>total order</i> on {@code C}. When we say that a 85 * class's natural ordering is <i>consistent with equals</i>, we mean that the 86 * quotient for the natural ordering is the equivalence relation defined by 87 * the class's {@link Object#equals(Object) equals(Object)} method:<pre> 88 * {(x, y) such that x.equals(y)}. </pre><p> 89 * 90 * In other words, when a class's natural ordering is consistent with 91 * equals, the equivalence classes defined by the equivalence relation 92 * of the {@code equals} method and the equivalence classes defined by 93 * the quotient of the {@code compareTo} method are the same. 94 * 95 * <p>This interface is a member of the 96 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework"> 97 * Java Collections Framework</a>. 98 * 99 * @param <T> the type of objects that this object may be compared to 100 * 101 * @author Josh Bloch 102 * @see java.util.Comparator 103 * @since 1.2 104 */ 105 public interface Comparable<T> { 106 /** 107 * Compares this object with the specified object for order. Returns a 108 * negative integer, zero, or a positive integer as this object is less 109 * than, equal to, or greater than the specified object. 110 * 111 * <p>The implementor must ensure {@link Integer#signum 112 * signum}{@code (x.compareTo(y)) == -signum(y.compareTo(x))} for 113 * all {@code x} and {@code y}. (This implies that {@code 114 * x.compareTo(y)} must throw an exception if and only if {@code 115 * y.compareTo(x)} throws an exception.) 116 * 117 * <p>The implementor must also ensure that the relation is transitive: 118 * {@code (x.compareTo(y) > 0 && y.compareTo(z) > 0)} implies 119 * {@code x.compareTo(z) > 0}. 120 * 121 * <p>Finally, the implementor must ensure that {@code 122 * x.compareTo(y)==0} implies that {@code signum(x.compareTo(z)) 123 * == signum(y.compareTo(z))}, for all {@code z}. 124 * 125 * @apiNote 126 * It is strongly recommended, but <i>not</i> strictly required that 127 * {@code (x.compareTo(y)==0) == (x.equals(y))}. Generally speaking, any 128 * class that implements the {@code Comparable} interface and violates 129 * this condition should clearly indicate this fact. The recommended 130 * language is "Note: this class has a natural ordering that is 131 * inconsistent with equals." 132 * 133 * @param o the object to be compared. 134 * @return a negative integer, zero, or a positive integer as this object 135 * is less than, equal to, or greater than the specified object. 136 * 137 * @throws NullPointerException if the specified object is null 138 * @throws ClassCastException if the specified object's type prevents it 139 * from being compared to this object. 140 */ compareTo(T o)141 public int compareTo(T o); 142 } 143