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    1   /*
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    9    * by Oracle in the LICENSE file that accompanied this code.
   10    *
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   13    * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   14    * version 2 for more details (a copy is included in the LICENSE file that
   15    * accompanied this code).
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   25   
   26   package java.util;
   27   
   28   /**
   29    * A comparison function, which imposes a <i>total ordering</i> on some
   30    * collection of objects.  Comparators can be passed to a sort method (such
   31    * as {@link Collections#sort(List,Comparator) Collections.sort} or {@link
   32    * Arrays#sort(Object[],Comparator) Arrays.sort}) to allow precise control
   33    * over the sort order.  Comparators can also be used to control the order of
   34    * certain data structures (such as {@link SortedSet sorted sets} or {@link
   35    * SortedMap sorted maps}), or to provide an ordering for collections of
   36    * objects that don't have a {@link Comparable natural ordering}.<p>
   37    *
   38    * The ordering imposed by a comparator <tt>c</tt> on a set of elements
   39    * <tt>S</tt> is said to be <i>consistent with equals</i> if and only if
   40    * <tt>c.compare(e1, e2)==0</tt> has the same boolean value as
   41    * <tt>e1.equals(e2)</tt> for every <tt>e1</tt> and <tt>e2</tt> in
   42    * <tt>S</tt>.<p>
   43    *
   44    * Caution should be exercised when using a comparator capable of imposing an
   45    * ordering inconsistent with equals to order a sorted set (or sorted map).
   46    * Suppose a sorted set (or sorted map) with an explicit comparator <tt>c</tt>
   47    * is used with elements (or keys) drawn from a set <tt>S</tt>.  If the
   48    * ordering imposed by <tt>c</tt> on <tt>S</tt> is inconsistent with equals,
   49    * the sorted set (or sorted map) will behave "strangely."  In particular the
   50    * sorted set (or sorted map) will violate the general contract for set (or
   51    * map), which is defined in terms of <tt>equals</tt>.<p>
   52    *
   53    * For example, suppose one adds two elements {@code a} and {@code b} such that
   54    * {@code (a.equals(b) && c.compare(a, b) != 0)}
   55    * to an empty {@code TreeSet} with comparator {@code c}.
   56    * The second {@code add} operation will return
   57    * true (and the size of the tree set will increase) because {@code a} and
   58    * {@code b} are not equivalent from the tree set's perspective, even though
   59    * this is contrary to the specification of the
   60    * {@link Set#add Set.add} method.<p>
   61    *
   62    * Note: It is generally a good idea for comparators to also implement
   63    * <tt>java.io.Serializable</tt>, as they may be used as ordering methods in
   64    * serializable data structures (like {@link TreeSet}, {@link TreeMap}).  In
   65    * order for the data structure to serialize successfully, the comparator (if
   66    * provided) must implement <tt>Serializable</tt>.<p>
   67    *
   68    * For the mathematically inclined, the <i>relation</i> that defines the
   69    * <i>imposed ordering</i> that a given comparator <tt>c</tt> imposes on a
   70    * given set of objects <tt>S</tt> is:<pre>
   71    *       {(x, y) such that c.compare(x, y) &lt;= 0}.
   72    * </pre> The <i>quotient</i> for this total order is:<pre>
   73    *       {(x, y) such that c.compare(x, y) == 0}.
   74    * </pre>
   75    *
   76    * It follows immediately from the contract for <tt>compare</tt> that the
   77    * quotient is an <i>equivalence relation</i> on <tt>S</tt>, and that the
   78    * imposed ordering is a <i>total order</i> on <tt>S</tt>.  When we say that
   79    * the ordering imposed by <tt>c</tt> on <tt>S</tt> is <i>consistent with
   80    * equals</i>, we mean that the quotient for the ordering is the equivalence
   81    * relation defined by the objects' {@link Object#equals(Object)
   82    * equals(Object)} method(s):<pre>
   83    *     {(x, y) such that x.equals(y)}. </pre>
   84    *
   85    * <p>Unlike {@code Comparable}, a comparator may optionally permit
   86    * comparison of null arguments, while maintaining the requirements for
   87    * an equivalence relation.
   88    *
   89    * <p>This interface is a member of the
   90    * <a href="{@docRoot}/../technotes/guides/collections/index.html">
   91    * Java Collections Framework</a>.
   92    *
   93    * @param <T> the type of objects that may be compared by this comparator
   94    *
   95    * @author  Josh Bloch
   96    * @author  Neal Gafter
   97    * @see Comparable
   98    * @see java.io.Serializable
   99    * @since 1.2
  100    */
  101   
  102   public interface Comparator<T> {
  103       /**
  104        * Compares its two arguments for order.  Returns a negative integer,
  105        * zero, or a positive integer as the first argument is less than, equal
  106        * to, or greater than the second.<p>
  107        *
  108        * In the foregoing description, the notation
  109        * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical
  110        * <i>signum</i> function, which is defined to return one of <tt>-1</tt>,
  111        * <tt>0</tt>, or <tt>1</tt> according to whether the value of
  112        * <i>expression</i> is negative, zero or positive.<p>
  113        *
  114        * The implementor must ensure that <tt>sgn(compare(x, y)) ==
  115        * -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>.  (This
  116        * implies that <tt>compare(x, y)</tt> must throw an exception if and only
  117        * if <tt>compare(y, x)</tt> throws an exception.)<p>
  118        *
  119        * The implementor must also ensure that the relation is transitive:
  120        * <tt>((compare(x, y)&gt;0) &amp;&amp; (compare(y, z)&gt;0))</tt> implies
  121        * <tt>compare(x, z)&gt;0</tt>.<p>
  122        *
  123        * Finally, the implementor must ensure that <tt>compare(x, y)==0</tt>
  124        * implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all
  125        * <tt>z</tt>.<p>
  126        *
  127        * It is generally the case, but <i>not</i> strictly required that
  128        * <tt>(compare(x, y)==0) == (x.equals(y))</tt>.  Generally speaking,
  129        * any comparator that violates this condition should clearly indicate
  130        * this fact.  The recommended language is "Note: this comparator
  131        * imposes orderings that are inconsistent with equals."
  132        *
  133        * @param o1 the first object to be compared.
  134        * @param o2 the second object to be compared.
  135        * @return a negative integer, zero, or a positive integer as the
  136        *         first argument is less than, equal to, or greater than the
  137        *         second.
  138        * @throws NullPointerException if an argument is null and this
  139        *         comparator does not permit null arguments
  140        * @throws ClassCastException if the arguments' types prevent them from
  141        *         being compared by this comparator.
  142        */
  143       int compare(T o1, T o2);
  144   
  145       /**
  146        * Indicates whether some other object is &quot;equal to&quot; this
  147        * comparator.  This method must obey the general contract of
  148        * {@link Object#equals(Object)}.  Additionally, this method can return
  149        * <tt>true</tt> <i>only</i> if the specified object is also a comparator
  150        * and it imposes the same ordering as this comparator.  Thus,
  151        * <code>comp1.equals(comp2)</code> implies that <tt>sgn(comp1.compare(o1,
  152        * o2))==sgn(comp2.compare(o1, o2))</tt> for every object reference
  153        * <tt>o1</tt> and <tt>o2</tt>.<p>
  154        *
  155        * Note that it is <i>always</i> safe <i>not</i> to override
  156        * <tt>Object.equals(Object)</tt>.  However, overriding this method may,
  157        * in some cases, improve performance by allowing programs to determine
  158        * that two distinct comparators impose the same order.
  159        *
  160        * @param   obj   the reference object with which to compare.
  161        * @return  <code>true</code> only if the specified object is also
  162        *          a comparator and it imposes the same ordering as this
  163        *          comparator.
  164        * @see Object#equals(Object)
  165        * @see Object#hashCode()
  166        */
  167       boolean equals(Object obj);
  168   }

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