java.util.Comparator Java Source 1.6.0


001/*
002 * @(#)Comparator.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.util;
009
010/**
011 * A comparison function, which imposes a <i>total ordering</i> on some
012 * collection of objects.  Comparators can be passed to a sort method (such
013 * as {@link Collections#sort(List,Comparator) Collections.sort} or {@link
014 * Arrays#sort(Object[],Comparator) Arrays.sort}) to allow precise control
015 * over the sort order.  Comparators can also be used to control the order of
016 * certain data structures (such as {@link SortedSet sorted sets} or {@link
017 * SortedMap sorted maps}), or to provide an ordering for collections of
018 * objects that don't have a {@link Comparable natural ordering}.<p>
019 *
020 * The ordering imposed by a comparator <tt>c</tt> on a set of elements
021 * <tt>S</tt> is said to be <i>consistent with equals</i> if and only if
022 * <tt>c.compare(e1, e2)==0</tt> has the same boolean value as
023 * <tt>e1.equals(e2)</tt> for every <tt>e1</tt> and <tt>e2</tt> in
024 * <tt>S</tt>.<p>
025 *
026 * Caution should be exercised when using a comparator capable of imposing an
027 * ordering inconsistent with equals to order a sorted set (or sorted map).
028 * Suppose a sorted set (or sorted map) with an explicit comparator <tt>c</tt>
029 * is used with elements (or keys) drawn from a set <tt>S</tt>.  If the
030 * ordering imposed by <tt>c</tt> on <tt>S</tt> is inconsistent with equals,
031 * the sorted set (or sorted map) will behave "strangely."  In particular the
032 * sorted set (or sorted map) will violate the general contract for set (or
033 * map), which is defined in terms of <tt>equals</tt>.<p>
034 *
035 * For example, suppose one adds two elements {@code a} and {@code b} such that
036 * {@code (a.equals(b) && c.compare(a, b) != 0)}
037 * to an empty {@code TreeSet} with comparator {@code c}.
038 * The second {@code add} operation will return
039 * true (and the size of the tree set will increase) because {@code a} and
040 * {@code b} are not equivalent from the tree set's perspective, even though
041 * this is contrary to the specification of the
042 * {@link Set#add Set.add} method.<p>
043 *
044 * Note: It is generally a good idea for comparators to also implement
045 * <tt>java.io.Serializable</tt>, as they may be used as ordering methods in
046 * serializable data structures (like {@link TreeSet}, {@link TreeMap}).  In
047 * order for the data structure to serialize successfully, the comparator (if
048 * provided) must implement <tt>Serializable</tt>.<p>
049 *
050 * For the mathematically inclined, the <i>relation</i> that defines the
051 * <i>imposed ordering</i> that a given comparator <tt>c</tt> imposes on a
052 * given set of objects <tt>S</tt> is:<pre>
053 *       {(x, y) such that c.compare(x, y) <= 0}.
054 * </pre> The <i>quotient</i> for this total order is:<pre>
055 *       {(x, y) such that c.compare(x, y) == 0}.
056 * </pre>
057 *
058 * It follows immediately from the contract for <tt>compare</tt> that the
059 * quotient is an <i>equivalence relation</i> on <tt>S</tt>, and that the
060 * imposed ordering is a <i>total order</i> on <tt>S</tt>.  When we say that
061 * the ordering imposed by <tt>c</tt> on <tt>S</tt> is <i>consistent with
062 * equals</i>, we mean that the quotient for the ordering is the equivalence
063 * relation defined by the objects' {@link Object#equals(Object)
064 * equals(Object)} method(s):<pre>
065 *     {(x, y) such that x.equals(y)}. </pre><p>
066 *
067 * This interface is a member of the
068 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
069 * Java Collections Framework</a>.
070 *
071 * @param <T> the type of objects that may be compared by this comparator
072 *
073 * @author  Josh Bloch
074 * @author  Neal Gafter
075 * @version 1.26, 04/21/06
076 * @see Comparable
077 * @see java.io.Serializable
078 * @since 1.2
079 */
080
081public interface Comparator<T> {
082    /**
083     * Compares its two arguments for order.  Returns a negative integer,
084     * zero, or a positive integer as the first argument is less than, equal
085     * to, or greater than the second.<p>
086     *
087     * In the foregoing description, the notation
088     * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical
089     * <i>signum</i> function, which is defined to return one of <tt>-1</tt>,
090     * <tt>0</tt>, or <tt>1</tt> according to whether the value of
091     * <i>expression</i> is negative, zero or positive.<p>
092     *
093     * The implementor must ensure that <tt>sgn(compare(x, y)) ==
094     * -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>.  (This
095     * implies that <tt>compare(x, y)</tt> must throw an exception if and only
096     * if <tt>compare(y, x)</tt> throws an exception.)<p>
097     *
098     * The implementor must also ensure that the relation is transitive:
099     * <tt>((compare(x, y)>0) && (compare(y, z)>0))</tt> implies
100     * <tt>compare(x, z)>0</tt>.<p>
101     *
102     * Finally, the implementor must ensure that <tt>compare(x, y)==0</tt>
103     * implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all
104     * <tt>z</tt>.<p>
105     *
106     * It is generally the case, but <i>not</i> strictly required that
107     * <tt>(compare(x, y)==0) == (x.equals(y))</tt>.  Generally speaking,
108     * any comparator that violates this condition should clearly indicate
109     * this fact.  The recommended language is "Note: this comparator
110     * imposes orderings that are inconsistent with equals."
111     *
112     * @param o1 the first object to be compared.
113     * @param o2 the second object to be compared.
114     * @return a negative integer, zero, or a positive integer as the
115     * 	       first argument is less than, equal to, or greater than the
116     *	       second.
117     * @throws ClassCastException if the arguments' types prevent them from
118     * 	       being compared by this comparator.
119     */
120    int compare(T o1, T o2);
121
122    /**
123     *
124     * Indicates whether some other object is "equal to" this
125     * comparator.  This method must obey the general contract of
126     * {@link Object#equals(Object)}.  Additionally, this method can return
127     * <tt>true</tt> <i>only</i> if the specified object is also a comparator
128     * and it imposes the same ordering as this comparator.  Thus,
129     * <code>comp1.equals(comp2)</code> implies that <tt>sgn(comp1.compare(o1,
130     * o2))==sgn(comp2.compare(o1, o2))</tt> for every object reference
131     * <tt>o1</tt> and <tt>o2</tt>.<p>
132     *
133     * Note that it is <i>always</i> safe <i>not</i> to override
134     * <tt>Object.equals(Object)</tt>.  However, overriding this method may,
135     * in some cases, improve performance by allowing programs to determine
136     * that two distinct comparators impose the same order.
137     *
138     * @param   obj   the reference object with which to compare.
139     * @return  <code>true</code> only if the specified object is also
140     *		a comparator and it imposes the same ordering as this
141     *		comparator.
142     * @see Object#equals(Object)
143     * @see Object#hashCode()
144     */
145    boolean equals(Object obj);
146}