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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 | +} | |