libjava/java/util/Comparator.java

   1 /* Comparator.java -- Interface for objects that specify an ordering
   2    Copyright (C) 1998, 2001, 2005  Free Software Foundation, Inc.
   3
   4 This file is part of GNU Classpath.
   5
   6 GNU Classpath is free software; you can redistribute it and/or modify
   7 it under the terms of the GNU General Public License as published by
   8 the Free Software Foundation; either version 2, or (at your option)
   9 any later version.
  10
  11 GNU Classpath is distributed in the hope that it will be useful, but
  12 WITHOUT ANY WARRANTY; without even the implied warranty of
  13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14 General Public License for more details.
  15
  16 You should have received a copy of the GNU General Public License
  17 along with GNU Classpath; see the file COPYING.  If not, write to the
  18 Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  19 02111-1307 USA.
  20
  21 Linking this library statically or dynamically with other modules is
  22 making a combined work based on this library.  Thus, the terms and
  23 conditions of the GNU General Public License cover the whole
  24 combination.
  25
  26 As a special exception, the copyright holders of this library give you
  27 permission to link this library with independent modules to produce an
  28 executable, regardless of the license terms of these independent
  29 modules, and to copy and distribute the resulting executable under
  30 terms of your choice, provided that you also meet, for each linked
  31 independent module, the terms and conditions of the license of that
  32 module.  An independent module is a module which is not derived from
  33 or based on this library.  If you modify this library, you may extend
  34 this exception to your version of the library, but you are not
  35 obligated to do so.  If you do not wish to do so, delete this
  36 exception statement from your version. */
  37
  38
  39 package java.util;
  40
  41 /**
  42  * Interface for objects that specify an ordering between objects. The ordering
  43  * should be <em>total</em>, such that any two objects of the correct type
  44  * can be compared, and the comparison is reflexive, anti-symmetric, and
  45  * transitive.  It is also recommended that the comparator be <em>consistent
  46  * with equals</em>, although this is not a strict requirement. A relation
  47  * is consistent with equals if these two statements always have the same
  48  * results (if no exceptions occur):<br>
  49  * <code>compare((Object) e1, (Object) e2) == 0</code> and
  50  * <code>e1.equals((Object) e2)</code><br>
  51  * Comparators that violate consistency with equals may cause strange behavior
  52  * in sorted lists and sets.  For example, a case-sensitive dictionary order
  53  * comparison of Strings is consistent with equals, but if it is
  54  * case-insensitive it is not, because "abc" and "ABC" compare as equal even
  55  * though "abc".equals("ABC") returns false.
  56  * <P>
  57  * In general, Comparators should be Serializable, because when they are passed
  58  * to Serializable data structures such as SortedMap or SortedSet, the entire
  59  * data structure will only serialize correctly if the comparator is
  60  * Serializable.
  61  *
  62  * @author Original author unknown
  63  * @author Eric Blake (ebb9@email.byu.edu)
  64  * @see Comparable
  65  * @see TreeMap
  66  * @see TreeSet
  67  * @see SortedMap
  68  * @see SortedSet
  69  * @see Arrays#sort(Object[], Comparator)
  70  * @see java.io.Serializable
  71  * @since 1.2
  72  * @status updated to 1.4
  73  */
  74 public interface Comparator
  75 {
  76   /**
  77    * Return an integer that is negative, zero or positive depending on whether
  78    * the first argument is less than, equal to or greater than the second
  79    * according to this ordering. This method should obey the following
  80    * contract:
  81    * <ul>
  82    *   <li>if compare(a, b) &lt; 0 then compare(b, a) &gt; 0</li>
  83    *   <li>if compare(a, b) throws an exception, so does compare(b, a)</li>
  84    *   <li>if compare(a, b) &lt; 0 and compare(b, c) &lt; 0 then compare(a, c)
  85    *       &lt; 0</li>
  86    *   <li>if compare(a, b) == 0 then compare(a, c) and compare(b, c) must
  87    *       have the same sign</li>
  88    * </ul>
  89    * To be consistent with equals, the following additional constraint is
  90    * in place:
  91    * <ul>
  92    *   <li>if a.equals(b) or both a and b are null, then
  93    *       compare(a, b) == 0.</li>
  94    * </ul><p>
  95    *
  96    * Although it is permissible for a comparator to provide an order
  97    * inconsistent with equals, that should be documented.
  98    *
  99    * @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(Object o1, Object 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 }