001: /*
002: * @(#)Comparator.java 1.24 06/10/10
003: *
004: * Copyright 1990-2006 Sun Microsystems, Inc. All Rights Reserved.
005: * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER
006: *
007: * This program is free software; you can redistribute it and/or
008: * modify it under the terms of the GNU General Public License version
009: * 2 only, as published by the Free Software Foundation.
010: *
011: * This program is distributed in the hope that it will be useful, but
012: * WITHOUT ANY WARRANTY; without even the implied warranty of
013: * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
014: * General Public License version 2 for more details (a copy is
015: * included at /legal/license.txt).
016: *
017: * You should have received a copy of the GNU General Public License
018: * version 2 along with this work; if not, write to the Free Software
019: * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
020: * 02110-1301 USA
021: *
022: * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa
023: * Clara, CA 95054 or visit www.sun.com if you need additional
024: * information or have any questions.
025: *
026: */
027:
028: package java.util;
029:
030: /**
031: * A comparison function, which imposes a <i>total ordering</i> on some
032: * collection of objects. Comparators can be passed to a sort method (such as
033: * <tt>Collections.sort</tt>) to allow precise control over the sort order.
034: * Comparators can also be used to control the order of certain data
035: * structures (such as <tt>TreeSet</tt> or <tt>TreeMap</tt>).<p>
036: *
037: * The ordering imposed by a Comparator <tt>c</tt> on a set of elements
038: * <tt>S</tt> is said to be <i>consistent with equals</i> if and only if
039: * <tt>(compare((Object)e1, (Object)e2)==0)</tt> has the same boolean value as
040: * <tt>e1.equals((Object)e2)</tt> for every <tt>e1</tt> and <tt>e2</tt> in
041: * <tt>S</tt>.<p>
042: *
043: * Caution should be exercised when using a comparator capable of imposing an
044: * ordering inconsistent with equals to order a sorted set (or sorted map).
045: * Suppose a sorted set (or sorted map) with an explicit Comparator <tt>c</tt>
046: * is used with elements (or keys) drawn from a set <tt>S</tt>. If the
047: * ordering imposed by <tt>c</tt> on <tt>S</tt> is inconsistent with equals,
048: * the sorted set (or sorted map) will behave "strangely." In particular the
049: * sorted set (or sorted map) will violate the general contract for set (or
050: * map), which is defined in terms of <tt>equals</tt>.<p>
051: *
052: * For example, if one adds two keys <tt>a</tt> and <tt>b</tt> such that
053: * <tt>(a.equals((Object)b) && c.compare((Object)a, (Object)b) != 0)</tt> to a
054: * sorted set with comparator <tt>c</tt>, the second <tt>add</tt> operation
055: * will return false (and the size of the sorted set will not increase)
056: * because <tt>a</tt> and <tt>b</tt> are equivalent from the sorted set's
057: * perspective.<p>
058: *
059: * Note: It is generally a good idea for comparators to implement
060: * <tt>java.io.Serializable</tt>, as they may be used as ordering methods in
061: * serializable data structures (like <tt>TreeSet</tt>, <tt>TreeMap</tt>). In
062: * order for the data structure to serialize successfully, the comparator (if
063: * provided) must implement <tt>Serializable</tt>.<p>
064: *
065: * For the mathematically inclined, the <i>relation</i> that defines
066: * the <i>total order</i> that a given comparator <tt>c</tt> imposes on a
067: * given set of objects <tt>S</tt> is:<pre>
068: * {(x, y) such that c.compare((Object)x, (Object)y) <= 0}.
069: * </pre> The <i>quotient</i> for this total order is:<pre>
070: * {(x, y) such that c.compare((Object)x, (Object)y) == 0}.
071: * </pre>
072: *
073: * It follows immediately from the contract for <tt>compare</tt> that the
074: * quotient is an <i>equivalence relation</i> on <tt>S</tt>, and that the
075: * natural ordering is a <i>total order</i> on <tt>S</tt>. When we say that
076: * the ordering imposed by <tt>c</tt> on <tt>S</tt> is <i>consistent with
077: * equals</i>, we mean that the quotient for the natural ordering is the
078: * equivalence relation defined by the objects' <tt>equals(Object)</tt>
079: * method(s):<pre>
080: * {(x, y) such that x.equals((Object)y)}.
081: * </pre><p>
082: *
083: * This interface is a member of the
084: * <a href="{@docRoot}/../guide/collections/index.html">
085: * Java Collections Framework</a>.
086: *
087: * @author Josh Bloch
088: * @version 1.15, 02/02/00
089: * @see Comparable
090: * @see Arrays#sort(Object[], Comparator)
091: * @see TreeMap
092: * @see TreeSet
093: * @see SortedMap
094: * @see SortedSet
095: * @see java.io.Serializable
096: * @since 1.2
097: */
098:
099: public interface Comparator {
100: /**
101: * Compares its two arguments for order. Returns a negative integer,
102: * zero, or a positive integer as the first argument is less than, equal
103: * to, or greater than the second.<p>
104: *
105: * The implementor must ensure that <tt>sgn(compare(x, y)) ==
106: * -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>. (This
107: * implies that <tt>compare(x, y)</tt> must throw an exception if and only
108: * if <tt>compare(y, x)</tt> throws an exception.)<p>
109: *
110: * The implementor must also ensure that the relation is transitive:
111: * <tt>((compare(x, y)>0) && (compare(y, z)>0))</tt> implies
112: * <tt>compare(x, z)>0</tt>.<p>
113: *
114: * Finally, the implementer must ensure that <tt>compare(x, y)==0</tt>
115: * implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all
116: * <tt>z</tt>.<p>
117: *
118: * It is generally the case, but <i>not</i> strictly required that
119: * <tt>(compare(x, y)==0) == (x.equals(y))</tt>. Generally speaking,
120: * any comparator that violates this condition should clearly indicate
121: * this fact. The recommended language is "Note: this comparator
122: * imposes orderings that are inconsistent with equals."
123: *
124: * @param o1 the first object to be compared.
125: * @param o2 the second object to be compared.
126: * @return a negative integer, zero, or a positive integer as the
127: * first argument is less than, equal to, or greater than the
128: * second.
129: * @throws ClassCastException if the arguments' types prevent them from
130: * being compared by this Comparator.
131: */
132: int compare(Object o1, Object o2);
133:
134: /**
135: *
136: * Indicates whether some other object is "equal to" this
137: * Comparator. This method must obey the general contract of
138: * <tt>Object.equals(Object)</tt>. Additionally, this method can return
139: * <tt>true</tt> <i>only</i> if the specified Object is also a comparator
140: * and it imposes the same ordering as this comparator. Thus,
141: * <code>comp1.equals(comp2)</code> implies that <tt>sgn(comp1.compare(o1,
142: * o2))==sgn(comp2.compare(o1, o2))</tt> for every object reference
143: * <tt>o1</tt> and <tt>o2</tt>.<p>
144: *
145: * Note that it is <i>always</i> safe <i>not</i> to override
146: * <tt>Object.equals(Object)</tt>. However, overriding this method may,
147: * in some cases, improve performance by allowing programs to determine
148: * that two distinct Comparators impose the same order.
149: *
150: * @param obj the reference object with which to compare.
151: * @return <code>true</code> only if the specified object is also
152: * a comparator and it imposes the same ordering as this
153: * comparator.
154: * @see java.lang.Object#equals(java.lang.Object)
155: * @see java.lang.Object#hashCode()
156: */
157: boolean equals(Object obj);
158: }
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