java.lang.Comparable Java Source 1.6.0


001/*
002 * @(#)Comparable.java	1.26 06/04/21
003 *
004 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
005 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
006 */
007
008package java.lang;
009import java.util.*;
010
011/**
012 * This interface imposes a total ordering on the objects of each class that
013 * implements it.  This ordering is referred to as the class's <i>natural
014 * ordering</i>, and the class's <tt>compareTo</tt> method is referred to as
015 * its <i>natural comparison method</i>.<p>
016 *
017 * Lists (and arrays) of objects that implement this interface can be sorted
018 * automatically by {@link Collections#sort(List) Collections.sort} (and
019 * {@link Arrays#sort(Object[]) Arrays.sort}).  Objects that implement this
020 * interface can be used as keys in a {@linkplain SortedMap sorted map} or as
021 * elements in a {@linkplain SortedSet sorted set}, without the need to
022 * specify a {@linkplain Comparator comparator}.<p>
023 *
024 * The natural ordering for a class <tt>C</tt> is said to be <i>consistent
025 * with equals</i> if and only if <tt>e1.compareTo(e2) == 0</tt> has
026 * the same boolean value as <tt>e1.equals(e2)</tt> for every
027 * <tt>e1</tt> and <tt>e2</tt> of class <tt>C</tt>.  Note that <tt>null</tt>
028 * is not an instance of any class, and <tt>e.compareTo(null)</tt> should
029 * throw a <tt>NullPointerException</tt> even though <tt>e.equals(null)</tt>
030 * returns <tt>false</tt>.<p>
031 *
032 * It is strongly recommended (though not required) that natural orderings be
033 * consistent with equals.  This is so because sorted sets (and sorted maps)
034 * without explicit comparators behave "strangely" when they are used with
035 * elements (or keys) whose natural ordering is inconsistent with equals.  In
036 * particular, such a sorted set (or sorted map) violates the general contract
037 * for set (or map), which is defined in terms of the <tt>equals</tt>
038 * method.<p>
039 *
040 * For example, if one adds two keys <tt>a</tt> and <tt>b</tt> such that
041 * <tt>(!a.equals(b) && a.compareTo(b) == 0)</tt> to a sorted
042 * set that does not use an explicit comparator, the second <tt>add</tt>
043 * operation returns false (and the size of the sorted set does not increase)
044 * because <tt>a</tt> and <tt>b</tt> are equivalent from the sorted set's
045 * perspective.<p>
046 *
047 * Virtually all Java core classes that implement <tt>Comparable</tt> have natural
048 * orderings that are consistent with equals.  One exception is
049 * <tt>java.math.BigDecimal</tt>, whose natural ordering equates
050 * <tt>BigDecimal</tt> objects with equal values and different precisions
051 * (such as 4.0 and 4.00).<p>
052 *
053 * For the mathematically inclined, the <i>relation</i> that defines
054 * the natural ordering on a given class C is:<pre>
055 *       {(x, y) such that x.compareTo(y) <= 0}.
056 * </pre> The <i>quotient</i> for this total order is: <pre>
057 *       {(x, y) such that x.compareTo(y) == 0}.
058 * </pre>
059 *
060 * It follows immediately from the contract for <tt>compareTo</tt> that the
061 * quotient is an <i>equivalence relation</i> on <tt>C</tt>, and that the
062 * natural ordering is a <i>total order</i> on <tt>C</tt>.  When we say that a
063 * class's natural ordering is <i>consistent with equals</i>, we mean that the
064 * quotient for the natural ordering is the equivalence relation defined by
065 * the class's {@link Object#equals(Object) equals(Object)} method:<pre>
066 *     {(x, y) such that x.equals(y)}. </pre><p>
067 *
068 * This interface is a member of the
069 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
070 * Java Collections Framework</a>.
071 *
072 * @param <T> the type of objects that this object may be compared to
073 *
074 * @author  Josh Bloch
075 * @version 1.26, 04/21/06
076 * @see java.util.Comparator
077 * @since 1.2
078 */
079
080public interface Comparable<T> {
081    /**
082     * Compares this object with the specified object for order.  Returns a
083     * negative integer, zero, or a positive integer as this object is less
084     * than, equal to, or greater than the specified object.
085     *
086     * <p>The implementor must ensure <tt>sgn(x.compareTo(y)) ==
087     * -sgn(y.compareTo(x))</tt> for all <tt>x</tt> and <tt>y</tt>.  (This
088     * implies that <tt>x.compareTo(y)</tt> must throw an exception iff
089     * <tt>y.compareTo(x)</tt> throws an exception.)
090     *
091     * <p>The implementor must also ensure that the relation is transitive:
092     * <tt>(x.compareTo(y)>0 && y.compareTo(z)>0)</tt> implies
093     * <tt>x.compareTo(z)>0</tt>.
094     *
095     * <p>Finally, the implementor must ensure that <tt>x.compareTo(y)==0</tt>
096     * implies that <tt>sgn(x.compareTo(z)) == sgn(y.compareTo(z))</tt>, for
097     * all <tt>z</tt>.
098     *
099     * <p>It is strongly recommended, but <i>not</i> strictly required that
100     * <tt>(x.compareTo(y)==0) == (x.equals(y))</tt>.  Generally speaking, any
101     * class that implements the <tt>Comparable</tt> interface and violates
102     * this condition should clearly indicate this fact.  The recommended
103     * language is "Note: this class has a natural ordering that is
104     * inconsistent with equals."
105     *
106     * <p>In the foregoing description, the notation
107     * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical
108     * <i>signum</i> function, which is defined to return one of <tt>-1</tt>,
109     * <tt>0</tt>, or <tt>1</tt> according to whether the value of
110     * <i>expression</i> is negative, zero or positive.
111     *
112     * @param   o the object to be compared.
113     * @return  a negative integer, zero, or a positive integer as this object
114     *		is less than, equal to, or greater than the specified object.
115     *
116     * @throws ClassCastException if the specified object's type prevents it
117     *         from being compared to this object.
118     */
119    public int compareTo(T o);
120}