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| 1 | /* | ||
| 2 | * Copyright (c) 1997, 2007, 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 <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 | * <tt>(!a.equals(b) && a.compareTo(b) == 0)</tt> 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}/../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 | */ | ||
| 96 | |||
| 97 | public interface Comparable<T> { | ||
| 98 | /** | ||
| 99 | * Compares this object with the specified object for order. Returns a | ||
| 100 | * negative integer, zero, or a positive integer as this object is less | ||
| 101 | * than, equal to, or greater than the specified object. | ||
| 102 | * | ||
| 103 | * <p>The implementor must ensure <tt>sgn(x.compareTo(y)) == | ||
| 104 | * -sgn(y.compareTo(x))</tt> for all <tt>x</tt> and <tt>y</tt>. (This | ||
| 105 | * implies that <tt>x.compareTo(y)</tt> must throw an exception iff | ||
| 106 | * <tt>y.compareTo(x)</tt> throws an exception.) | ||
| 107 | * | ||
| 108 | * <p>The implementor must also ensure that the relation is transitive: | ||
| 109 | * <tt>(x.compareTo(y)>0 && y.compareTo(z)>0)</tt> implies | ||
| 110 | * <tt>x.compareTo(z)>0</tt>. | ||
| 111 | * | ||
| 112 | * <p>Finally, the implementor must ensure that <tt>x.compareTo(y)==0</tt> | ||
| 113 | * implies that <tt>sgn(x.compareTo(z)) == sgn(y.compareTo(z))</tt>, for | ||
| 114 | * all <tt>z</tt>. | ||
| 115 | * | ||
| 116 | * <p>It is strongly recommended, but <i>not</i> strictly required that | ||
| 117 | * <tt>(x.compareTo(y)==0) == (x.equals(y))</tt>. Generally speaking, any | ||
| 118 | * class that implements the <tt>Comparable</tt> 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 | * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical | ||
| 125 | * <i>signum</i> function, which is defined to return one of <tt>-1</tt>, | ||
| 126 | * <tt>0</tt>, or <tt>1</tt> according to whether the value of | ||
| 127 | * <i>expression</i> is negative, zero or positive. | ||
| 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 | */ | ||
| 137 | public int compareTo(T o); | ||
| 138 | } | ||