/* Comparator.java -- Interface for objects that specify an ordering
   Copyright (C) 1998, 2001 Free Software Foundation, Inc.

This file is part of GNU Classpath.

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package java.util;

/**
 * Interface for objects that specify an ordering between objects. The ordering
 * should be <em>total</em>, such that any two objects of the correct type
 * can be compared, and the comparison is reflexive, anti-symmetric, and
 * transitive.  It is also recommended that the comparator be <em>consistent
 * with equals</em>, although this is not a strict requirement. A relation
 * is consistent with equals if these two statements always have the same
 * results (if no exceptions occur):<br>
 * <code>compare((Object) e1, (Object) e2) == 0</code> and
 * <code>e1.equals((Object) e2)</code><br>
 * Comparators that violate consistency with equals may cause strange behavior
 * in sorted lists and sets.  For example, a case-sensitive dictionary order
 * comparison of Strings is consistent with equals, but if it is
 * case-insensitive it is not, because "abc" and "ABC" compare as equal even
 * though "abc".equals("ABC") returns false.
 * <P>
 * In general, Comparators should be Serializable, because when they are passed
 * to Serializable data structures such as SortedMap or SortedSet, the entire
 * data structure will only serialize correctly if the comparator is
 * Serializable.
 *
 * @author Original author unknown
 * @author Eric Blake <ebb9@email.byu.edu>
 * @see Comparable
 * @see TreeMap
 * @see TreeSet
 * @see SortedMap
 * @see SortedSet
 * @see Arrays#sort(Object[], Comparator)
 * @see java.io.Serializable
 * @since 1.2
 * @status updated to 1.4
 */
public interface Comparator
{
  /**
   * Return an integer that is negative, zero or positive depending on whether
   * the first argument is less than, equal to or greater than the second
   * according to this ordering. This method should obey the following
   * contract:
   * <ul>
   *   <li>if compare(a, b) &lt; 0 then compare(b, a) &gt; 0</li>
   *   <li>if compare(a, b) throws an exception, so does compare(b, a)</li>
   *   <li>if compare(a, b) &lt; 0 and compare(b, c) &lt; 0 then compare(a, c)
   *       &lt; 0</li>
   *   <li>if compare(a, b) == 0 then compare(a, c) and compare(b, c) must
   *       have the same sign</li
   * </ul>
   * To be consistent with equals, the following additional constraint is
   * in place:
   * <ul>
   *   <li>if a.equals(b) or both a and b are null, then
   *       compare(a, b) == 0.</li>
   * </ul><p>
   *
   * Although it is permissible for a comparator to provide an order
   * inconsistent with equals, that should be documented.
   *
   * @param o1 the first object
   * @param o2 the second object
   * @return the comparison
   * @throws ClassCastException if the elements are not of types that can be
   *         compared by this ordering.
   */
  int compare(Object o1, Object o2);

  /**
   * Return true if the object is equal to this object.  To be
   * considered equal, the argument object must satisfy the constraints
   * of <code>Object.equals()</code>, be a Comparator, and impose the
   * same ordering as this Comparator. The default implementation
   * inherited from Object is usually adequate.
   *
   * @param obj The object
   * @return true if it is a Comparator that imposes the same order
   * @see Object#equals(Object)
   */
  boolean equals(Object obj);
}