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.
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021Linking this library statically or dynamically with other modules is
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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) &lt; 0 then compare(b, a) &gt; 0</li>
083   *   <li>if compare(a, b) throws an exception, so does compare(b, a)</li>
084   *   <li>if compare(a, b) &lt; 0 and compare(b, c) &lt; 0 then compare(a, c)
085   *       &lt; 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}