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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
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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
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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  * {@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) &lt;= 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="https://docs.oracle.com/javase/8/docs/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 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 <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)&gt;0 &amp;&amp; y.compareTo(z)&gt;0)</tt> implies
109      * <tt>x.compareTo(z)&gt;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      */
compareTo(T o)136     public int compareTo(T o);
137 }
138