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/* Comparator.java -- Interface for objects that specify an ordering

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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<T>

{

  /**

   * 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(T o1, T 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);

}