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.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} have natural 66 * orderings that are consistent with equals. One exception is 67 * {@code java.math.BigDecimal}, whose natural ordering equates 68 * {@code BigDecimal} objects with equal values and different precisions 69 * (such as 4.0 and 4.00).<p> 70 * 71 * For the mathematically inclined, the <i>relation</i> that defines 72 * the natural ordering on a given class C is:<pre>{@code 73 * {(x, y) such that x.compareTo(y) <= 0}. 74 * }</pre> The <i>quotient</i> for this total order is: <pre>{@code 75 * {(x, y) such that x.compareTo(y) == 0}. 76 * }</pre> 77 * 78 * It follows immediately from the contract for {@code compareTo} that the 79 * quotient is an <i>equivalence relation</i> on {@code C}, and that the 80 * natural ordering is a <i>total order</i> on {@code C}. When we say that a 81 * class's natural ordering is <i>consistent with equals</i>, we mean that the 82 * quotient for the natural ordering is the equivalence relation defined by 83 * the class's {@link Object#equals(Object) equals(Object)} method:<pre> 84 * {(x, y) such that x.equals(y)}. </pre><p> 85 * 86 * This interface is a member of the 87 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework"> 88 * Java Collections Framework</a>. 89 * 90 * @param <T> the type of objects that this object may be compared to 91 * 92 * @author Josh Bloch 93 * @see java.util.Comparator 94 * @since 1.2 95 */ 96 public interface Comparable<T> { 97 /** 98 * Compares this object with the specified object for order. Returns a 99 * negative integer, zero, or a positive integer as this object is less 100 * than, equal to, or greater than the specified object. 101 * 102 * <p>The implementor must ensure 103 * {@code sgn(x.compareTo(y)) == -sgn(y.compareTo(x))} 104 * for all {@code x} and {@code y}. (This 105 * implies that {@code x.compareTo(y)} must throw an exception iff 106 * {@code y.compareTo(x)} throws an exception.) 107 * 108 * <p>The implementor must also ensure that the relation is transitive: 109 * {@code (x.compareTo(y) > 0 && y.compareTo(z) > 0)} implies 110 * {@code x.compareTo(z) > 0}. 111 * 112 * <p>Finally, the implementor must ensure that {@code x.compareTo(y)==0} 113 * implies that {@code sgn(x.compareTo(z)) == sgn(y.compareTo(z))}, for 114 * all {@code z}. 115 * 116 * <p>It is strongly recommended, but <i>not</i> strictly required that 117 * {@code (x.compareTo(y)==0) == (x.equals(y))}. Generally speaking, any 118 * class that implements the {@code Comparable} interface and violates 119 * this condition should clearly indicate this fact. The recommended 120 * language is "Note: this class has a natural ordering that is 121 * inconsistent with equals." 122 * 123 * <p>In the foregoing description, the notation 124 * {@code sgn(}<i>expression</i>{@code )} designates the mathematical 125 * <i>signum</i> function, which is defined to return one of {@code -1}, 126 * {@code 0}, or {@code 1} according to whether the value of 127 * <i>expression</i> is negative, zero, or positive, respectively. 128 * 129 * @param o the object to be compared. 130 * @return a negative integer, zero, or a positive integer as this object 131 * is less than, equal to, or greater than the specified object. 132 * 133 * @throws NullPointerException if the specified object is null 134 * @throws ClassCastException if the specified object's type prevents it 135 * from being compared to this object. 136 */ compareTo(T o)137 public int compareTo(T o); 138 } 139