1 /*
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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10 *
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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).
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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 */
137 public int compareTo(T o);
138 }