Source for java.util.Comparator

   1: /* Comparator.java -- Interface for objects that specify an ordering
   2:    Copyright (C) 1998, 2001, 2004, 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., 51 Franklin Street, Fifth Floor, Boston, MA
  19: 02110-1301 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<T>
  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(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: }