1/* 2 * Copyright (c) 1997, 2013, 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 26package java.lang; 27import 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 <tt>compareTo</tt> 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 <tt>C</tt> is said to be <i>consistent 43 * with equals</i> if and only if <tt>e1.compareTo(e2) == 0</tt> has 44 * the same boolean value as <tt>e1.equals(e2)</tt> for every 45 * <tt>e1</tt> and <tt>e2</tt> of class <tt>C</tt>. Note that <tt>null</tt> 46 * is not an instance of any class, and <tt>e.compareTo(null)</tt> should 47 * throw a <tt>NullPointerException</tt> even though <tt>e.equals(null)</tt> 48 * returns <tt>false</tt>.<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 <tt>equals</tt> 56 * method.<p> 57 * 58 * For example, if one adds two keys <tt>a</tt> and <tt>b</tt> 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 <tt>add</tt> 61 * operation returns false (and the size of the sorted set does not increase) 62 * because <tt>a</tt> and <tt>b</tt> are equivalent from the sorted set's 63 * perspective.<p> 64 * 65 * Virtually all Java core classes that implement <tt>Comparable</tt> have natural 66 * orderings that are consistent with equals. One exception is 67 * <tt>java.math.BigDecimal</tt>, whose natural ordering equates 68 * <tt>BigDecimal</tt> 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> 73 * {(x, y) such that x.compareTo(y) <= 0}. 74 * </pre> The <i>quotient</i> for this total order is: <pre> 75 * {(x, y) such that x.compareTo(y) == 0}. 76 * </pre> 77 * 78 * It follows immediately from the contract for <tt>compareTo</tt> that the 79 * quotient is an <i>equivalence relation</i> on <tt>C</tt>, and that the 80 * natural ordering is a <i>total order</i> on <tt>C</tt>. 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}openjdk-redirect.html?v=8&path=/technotes/guides/collections/index.html"> 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 */ 96public 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 <tt>sgn(x.compareTo(y)) == 103 * -sgn(y.compareTo(x))</tt> for all <tt>x</tt> and <tt>y</tt>. (This 104 * implies that <tt>x.compareTo(y)</tt> must throw an exception iff 105 * <tt>y.compareTo(x)</tt> throws an exception.) 106 * 107 * <p>The implementor must also ensure that the relation is transitive: 108 * <tt>(x.compareTo(y)>0 && y.compareTo(z)>0)</tt> implies 109 * <tt>x.compareTo(z)>0</tt>. 110 * 111 * <p>Finally, the implementor must ensure that <tt>x.compareTo(y)==0</tt> 112 * implies that <tt>sgn(x.compareTo(z)) == sgn(y.compareTo(z))</tt>, for 113 * all <tt>z</tt>. 114 * 115 * <p>It is strongly recommended, but <i>not</i> strictly required that 116 * <tt>(x.compareTo(y)==0) == (x.equals(y))</tt>. Generally speaking, any 117 * class that implements the <tt>Comparable</tt> interface and violates 118 * this condition should clearly indicate this fact. The recommended 119 * language is "Note: this class has a natural ordering that is 120 * inconsistent with equals." 121 * 122 * <p>In the foregoing description, the notation 123 * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical 124 * <i>signum</i> function, which is defined to return one of <tt>-1</tt>, 125 * <tt>0</tt>, or <tt>1</tt> according to whether the value of 126 * <i>expression</i> is negative, zero or positive. 127 * 128 * @param o the object to be compared. 129 * @return a negative integer, zero, or a positive integer as this object 130 * is less than, equal to, or greater than the specified object. 131 * 132 * @throws NullPointerException if the specified object is null 133 * @throws ClassCastException if the specified object's type prevents it 134 * from being compared to this object. 135 */ 136 public int compareTo(T o); 137} 138