001/* Comparator.java -- Interface for objects that specify an ordering 002 Copyright (C) 1998, 2001, 2004, 2005 Free Software Foundation, Inc. 003 004This file is part of GNU Classpath. 005 006GNU Classpath is free software; you can redistribute it and/or modify 007it under the terms of the GNU General Public License as published by 008the Free Software Foundation; either version 2, or (at your option) 009any later version. 010 011GNU Classpath is distributed in the hope that it will be useful, but 012WITHOUT ANY WARRANTY; without even the implied warranty of 013MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 014General Public License for more details. 015 016You should have received a copy of the GNU General Public License 017along with GNU Classpath; see the file COPYING. If not, write to the 018Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 01902110-1301 USA. 020 021Linking this library statically or dynamically with other modules is 022making a combined work based on this library. Thus, the terms and 023conditions of the GNU General Public License cover the whole 024combination. 025 026As a special exception, the copyright holders of this library give you 027permission to link this library with independent modules to produce an 028executable, regardless of the license terms of these independent 029modules, and to copy and distribute the resulting executable under 030terms of your choice, provided that you also meet, for each linked 031independent module, the terms and conditions of the license of that 032module. An independent module is a module which is not derived from 033or based on this library. If you modify this library, you may extend 034this exception to your version of the library, but you are not 035obligated to do so. If you do not wish to do so, delete this 036exception statement from your version. */ 037 038 039package java.util; 040 041/** 042 * Interface for objects that specify an ordering between objects. The ordering 043 * should be <em>total</em>, such that any two objects of the correct type 044 * can be compared, and the comparison is reflexive, anti-symmetric, and 045 * transitive. It is also recommended that the comparator be <em>consistent 046 * with equals</em>, although this is not a strict requirement. A relation 047 * is consistent with equals if these two statements always have the same 048 * results (if no exceptions occur):<br> 049 * <code>compare((Object) e1, (Object) e2) == 0</code> and 050 * <code>e1.equals((Object) e2)</code><br> 051 * Comparators that violate consistency with equals may cause strange behavior 052 * in sorted lists and sets. For example, a case-sensitive dictionary order 053 * comparison of Strings is consistent with equals, but if it is 054 * case-insensitive it is not, because "abc" and "ABC" compare as equal even 055 * though "abc".equals("ABC") returns false. 056 * <P> 057 * In general, Comparators should be Serializable, because when they are passed 058 * to Serializable data structures such as SortedMap or SortedSet, the entire 059 * data structure will only serialize correctly if the comparator is 060 * Serializable. 061 * 062 * @author Original author unknown 063 * @author Eric Blake (ebb9@email.byu.edu) 064 * @see Comparable 065 * @see TreeMap 066 * @see TreeSet 067 * @see SortedMap 068 * @see SortedSet 069 * @see Arrays#sort(Object[], Comparator) 070 * @see java.io.Serializable 071 * @since 1.2 072 * @status updated to 1.4 073 */ 074public interface Comparator<T> 075{ 076 /** 077 * Return an integer that is negative, zero or positive depending on whether 078 * the first argument is less than, equal to or greater than the second 079 * according to this ordering. This method should obey the following 080 * contract: 081 * <ul> 082 * <li>if compare(a, b) < 0 then compare(b, a) > 0</li> 083 * <li>if compare(a, b) throws an exception, so does compare(b, a)</li> 084 * <li>if compare(a, b) < 0 and compare(b, c) < 0 then compare(a, c) 085 * < 0</li> 086 * <li>if compare(a, b) == 0 then compare(a, c) and compare(b, c) must 087 * have the same sign</li> 088 * </ul> 089 * To be consistent with equals, the following additional constraint is 090 * in place: 091 * <ul> 092 * <li>if a.equals(b) or both a and b are null, then 093 * compare(a, b) == 0.</li> 094 * </ul><p> 095 * 096 * Although it is permissible for a comparator to provide an order 097 * inconsistent with equals, that should be documented. 098 * 099 * @param o1 the first object 100 * @param o2 the second object 101 * @return the comparison 102 * @throws ClassCastException if the elements are not of types that can be 103 * compared by this ordering. 104 */ 105 int compare(T o1, T o2); 106 107 /** 108 * Return true if the object is equal to this object. To be 109 * considered equal, the argument object must satisfy the constraints 110 * of <code>Object.equals()</code>, be a Comparator, and impose the 111 * same ordering as this Comparator. The default implementation 112 * inherited from Object is usually adequate. 113 * 114 * @param obj The object 115 * @return true if it is a Comparator that imposes the same order 116 * @see Object#equals(Object) 117 */ 118 boolean equals(Object obj); 119}