Comparator


1   /*
2    * @(#)Comparator.java  1.22 03/12/19
3    *
4    * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
5    * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6    */
7   
8   package java.util;
9   
10  /**
11   * A comparison function, which imposes a <i>total ordering</i> on some
12   * collection of objects.  Comparators can be passed to a sort method (such as
13   * <tt>Collections.sort</tt>) to allow precise control over the sort order.
14   * Comparators can also be used to control the order of certain data
15   * structures (such as <tt>TreeSet</tt> or <tt>TreeMap</tt>).<p>
16   *
17   * The ordering imposed by a Comparator <tt>c</tt> on a set of elements
18   * <tt>S</tt> is said to be <i>consistent with equals</i> if and only if
19   * <tt>(compare((Object)e1, (Object)e2)==0)</tt> has the same boolean value as
20   * <tt>e1.equals((Object)e2)</tt> for every <tt>e1</tt> and <tt>e2</tt> in
21   * <tt>S</tt>.<p>
22   *
23   * Caution should be exercised when using a comparator capable of imposing an
24   * ordering inconsistent with equals to order a sorted set (or sorted map).
25   * Suppose a sorted set (or sorted map) with an explicit Comparator <tt>c</tt>
26   * is used with elements (or keys) drawn from a set <tt>S</tt>.  If the
27   * ordering imposed by <tt>c</tt> on <tt>S</tt> is inconsistent with equals,
28   * the sorted set (or sorted map) will behave "strangely."  In particular the
29   * sorted set (or sorted map) will violate the general contract for set (or
30   * map), which is defined in terms of <tt>equals</tt>.<p>
31   * 
32   * For example, if one adds two keys <tt>a</tt> and <tt>b</tt> such that
33   * <tt>(a.equals((Object)b) && c.compare((Object)a, (Object)b) != 0)</tt> to a
34   * sorted set with comparator <tt>c</tt>, the second <tt>add</tt> operation
35   * will return false (and the size of the sorted set will not increase)
36   * because <tt>a</tt> and <tt>b</tt> are equivalent from the sorted set's
37   * perspective.<p>
38   *
39   * Note: It is generally a good idea for comparators to implement
40   * <tt>java.io.Serializable</tt>, as they may be used as ordering methods in
41   * serializable data structures (like <tt>TreeSet</tt>, <tt>TreeMap</tt>).  In
42   * order for the data structure to serialize successfully, the comparator (if
43   * provided) must implement <tt>Serializable</tt>.<p>
44   *
45   * For the mathematically inclined, the <i>relation</i> that defines
46   * the <i>total order</i> that a given comparator <tt>c</tt> imposes on a
47   * given set of objects <tt>S</tt> is:<pre>
48   *       {(x, y) such that c.compare((Object)x, (Object)y) &lt;= 0}.
49   * </pre> The <i>quotient</i> for this total order is:<pre>
50   *       {(x, y) such that c.compare((Object)x, (Object)y) == 0}.
51   * </pre>
52   *
53   * It follows immediately from the contract for <tt>compare</tt> that the
54   * quotient is an <i>equivalence relation</i> on <tt>S</tt>, and that the
55   * natural ordering is a <i>total order</i> on <tt>S</tt>.  When we say that
56   * the ordering imposed by <tt>c</tt> on <tt>S</tt> is <i>consistent with
57   * equals</i>, we mean that the quotient for the natural ordering is the
58   * equivalence relation defined by the objects' <tt>equals(Object)</tt>
59   * method(s):<pre>
60   *       {(x, y) such that x.equals((Object)y)}.
61   * </pre><p>
62   *
63   * This interface is a member of the 
64   * <a HREF="{@docRoot}/../guide/collections/index.html">
65   * Java Collections Framework</a>.
66   *
67   * @author  Josh Bloch
68   * @author  Neal Gafter
69   * @version 1.22, 12/19/03
70   * @see Comparable
71   * @see Arrays#sort(Object[], Comparator)
72   * @see TreeMap
73   * @see TreeSet
74   * @see SortedMap
75   * @see SortedSet
76   * @see java.io.Serializable
77   * @since 1.2
78   */
79  
80  public interface Comparator<T> {
81      /**
82       * Compares its two arguments for order.  Returns a negative integer,
83       * zero, or a positive integer as the first argument is less than, equal
84       * to, or greater than the second.<p>
85       *
86       * The implementor must ensure that <tt>sgn(compare(x, y)) ==
87       * -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>.  (This
88       * implies that <tt>compare(x, y)</tt> must throw an exception if and only
89       * if <tt>compare(y, x)</tt> throws an exception.)<p>
90       *
91       * The implementor must also ensure that the relation is transitive:
92       * <tt>((compare(x, y)&gt;0) &amp;&amp; (compare(y, z)&gt;0))</tt> implies
93       * <tt>compare(x, z)&gt;0</tt>.<p>
94       *
95       * Finally, the implementer must ensure that <tt>compare(x, y)==0</tt>
96       * implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all
97       * <tt>z</tt>.<p>
98       *
99       * It is generally the case, but <i>not</i> strictly required that 
100      * <tt>(compare(x, y)==0) == (x.equals(y))</tt>.  Generally speaking,
101      * any comparator that violates this condition should clearly indicate
102      * this fact.  The recommended language is "Note: this comparator
103      * imposes orderings that are inconsistent with equals."
104      * 
105      * @param o1 the first object to be compared.
106      * @param o2 the second object to be compared.
107      * @return a negative integer, zero, or a positive integer as the
108      *         first argument is less than, equal to, or greater than the
109      *         second. 
110      * @throws ClassCastException if the arguments' types prevent them from
111      *         being compared by this Comparator.
112      */
113     int compare(T o1, T o2);
114 
115     /**
116      * 
117      * Indicates whether some other object is &quot;equal to&quot; this
118      * Comparator.  This method must obey the general contract of
119      * <tt>Object.equals(Object)</tt>.  Additionally, this method can return
120      * <tt>true</tt> <i>only</i> if the specified Object is also a comparator
121      * and it imposes the same ordering as this comparator.  Thus,
122      * <code>comp1.equals(comp2)</code> implies that <tt>sgn(comp1.compare(o1,
123      * o2))==sgn(comp2.compare(o1, o2))</tt> for every object reference
124      * <tt>o1</tt> and <tt>o2</tt>.<p>
125      *
126      * Note that it is <i>always</i> safe <i>not</i> to override
127      * <tt>Object.equals(Object)</tt>.  However, overriding this method may,
128      * in some cases, improve performance by allowing programs to determine
129      * that two distinct Comparators impose the same order.
130      *
131      * @param   obj   the reference object with which to compare.
132      * @return  <code>true</code> only if the specified object is also
133      *      a comparator and it imposes the same ordering as this
134      *      comparator.
135      * @see     java.lang.Object#equals(java.lang.Object)
136      * @see java.lang.Object#hashCode()
137      */
138     boolean equals(Object   obj);
139 }
140
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