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src/java.base/share/classes/java/util/Comparator.java

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   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  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).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.util;
  27 
  28 import java.io.Serializable;
  29 import java.util.function.Function;
  30 import java.util.function.ToIntFunction;
  31 import java.util.function.ToLongFunction;
  32 import java.util.function.ToDoubleFunction;
  33 import java.util.Comparators;
  34 
  35 /**
  36  * A comparison function, which imposes a <i>total ordering</i> on some
  37  * collection of objects.  Comparators can be passed to a sort method (such
  38  * as {@link Collections#sort(List,Comparator) Collections.sort} or {@link
  39  * Arrays#sort(Object[],Comparator) Arrays.sort}) to allow precise control
  40  * over the sort order.  Comparators can also be used to control the order of
  41  * certain data structures (such as {@link SortedSet sorted sets} or {@link
  42  * SortedMap sorted maps}), or to provide an ordering for collections of
  43  * objects that don't have a {@link Comparable natural ordering}.<p>
  44  *
  45  * The ordering imposed by a comparator <tt>c</tt> on a set of elements
  46  * <tt>S</tt> is said to be <i>consistent with equals</i> if and only if
  47  * <tt>c.compare(e1, e2)==0</tt> has the same boolean value as
  48  * <tt>e1.equals(e2)</tt> for every <tt>e1</tt> and <tt>e2</tt> in
  49  * <tt>S</tt>.<p>
  50  *
  51  * Caution should be exercised when using a comparator capable of imposing an
  52  * ordering inconsistent with equals to order a sorted set (or sorted map).
  53  * Suppose a sorted set (or sorted map) with an explicit comparator <tt>c</tt>


  89  * equals(Object)} method(s):<pre>
  90  *     {(x, y) such that x.equals(y)}. </pre>
  91  *
  92  * <p>Unlike {@code Comparable}, a comparator may optionally permit
  93  * comparison of null arguments, while maintaining the requirements for
  94  * an equivalence relation.
  95  *
  96  * <p>This interface is a member of the
  97  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
  98  * Java Collections Framework</a>.
  99  *
 100  * @param <T> the type of objects that may be compared by this comparator
 101  *
 102  * @author  Josh Bloch
 103  * @author  Neal Gafter
 104  * @see Comparable
 105  * @see java.io.Serializable
 106  * @since 1.2
 107  */
 108 @FunctionalInterface
 109 public interface Comparator<T> {
 110     /**
 111      * Compares its two arguments for order.  Returns a negative integer,
 112      * zero, or a positive integer as the first argument is less than, equal
 113      * to, or greater than the second.<p>
 114      *
 115      * In the foregoing description, the notation
 116      * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical
 117      * <i>signum</i> function, which is defined to return one of <tt>-1</tt>,
 118      * <tt>0</tt>, or <tt>1</tt> according to whether the value of
 119      * <i>expression</i> is negative, zero or positive.<p>
 120      *
 121      * The implementor must ensure that <tt>sgn(compare(x, y)) ==
 122      * -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>.  (This
 123      * implies that <tt>compare(x, y)</tt> must throw an exception if and only
 124      * if <tt>compare(y, x)</tt> throws an exception.)<p>
 125      *
 126      * The implementor must also ensure that the relation is transitive:
 127      * <tt>((compare(x, y)&gt;0) &amp;&amp; (compare(y, z)&gt;0))</tt> implies
 128      * <tt>compare(x, z)&gt;0</tt>.<p>
 129      *


 165      * that two distinct comparators impose the same order.
 166      *
 167      * @param   obj   the reference object with which to compare.
 168      * @return  <code>true</code> only if the specified object is also
 169      *          a comparator and it imposes the same ordering as this
 170      *          comparator.
 171      * @see Object#equals(Object)
 172      * @see Object#hashCode()
 173      */
 174     boolean equals(Object obj);
 175 
 176     /**
 177      * Returns a comparator that imposes the reverse ordering of this
 178      * comparator.
 179      *
 180      * @return a comparator that imposes the reverse ordering of this
 181      *         comparator.
 182      * @since 1.8
 183      */
 184     default Comparator<T> reversed() {
 185         return Collections.reverseOrder(this);
 186     }
 187 
 188     /**
 189      * Returns a lexicographic-order comparator with another comparator.
 190      * If this {@code Comparator} considers two elements equal, i.e.
 191      * {@code compare(a, b) == 0}, {@code other} is used to determine the order.
 192      *
 193      * <p>The returned comparator is serializable if the specified comparator
 194      * is also serializable.
 195      *
 196      * @apiNote
 197      * For example, to sort a collection of {@code String} based on the length
 198      * and then case-insensitive natural ordering, the comparator can be
 199      * composed using following code,
 200      *
 201      * <pre>{@code
 202      *     Comparator<String> cmp = Comparator.comparingInt(String::length)
 203      *             .thenComparing(String.CASE_INSENSITIVE_ORDER);
 204      * }</pre>
 205      *


 218         };
 219     }
 220 
 221     /**
 222      * Returns a lexicographic-order comparator with a function that
 223      * extracts a key to be compared with the given {@code Comparator}.
 224      *
 225      * @implSpec This default implementation behaves as if {@code
 226      *           thenComparing(comparing(keyExtractor, cmp))}.
 227      *
 228      * @param  <U>  the type of the sort key
 229      * @param  keyExtractor the function used to extract the sort key
 230      * @param  keyComparator the {@code Comparator} used to compare the sort key
 231      * @return a lexicographic-order comparator composed of this comparator
 232      *         and then comparing on the key extracted by the keyExtractor function
 233      * @throws NullPointerException if either argument is null.
 234      * @see #comparing(Function, Comparator)
 235      * @see #thenComparing(Comparator)
 236      * @since 1.8
 237      */
 238     default <U> Comparator<T> thenComparing(
 239             Function<? super T, ? extends U> keyExtractor,
 240             Comparator<? super U> keyComparator)
 241     {
 242         return thenComparing(comparing(keyExtractor, keyComparator));
 243     }
 244 
 245     /**
 246      * Returns a lexicographic-order comparator with a function that
 247      * extracts a {@code Comparable} sort key.
 248      *
 249      * @implSpec This default implementation behaves as if {@code
 250      *           thenComparing(comparing(keyExtractor))}.
 251      *
 252      * @param  <U>  the type of the {@link Comparable} sort key
 253      * @param  keyExtractor the function used to extract the {@link
 254      *         Comparable} sort key
 255      * @return a lexicographic-order comparator composed of this and then the
 256      *         {@link Comparable} sort key.
 257      * @throws NullPointerException if the argument is null.
 258      * @see #comparing(Function)
 259      * @see #thenComparing(Comparator)
 260      * @since 1.8
 261      */
 262     default <U extends Comparable<? super U>> Comparator<T> thenComparing(
 263             Function<? super T, ? extends U> keyExtractor)
 264     {
 265         return thenComparing(comparing(keyExtractor));
 266     }
 267 
 268     /**
 269      * Returns a lexicographic-order comparator with a function that
 270      * extracts a {@code int} sort key.
 271      *
 272      * @implSpec This default implementation behaves as if {@code
 273      *           thenComparing(comparingInt(keyExtractor))}.
 274      *
 275      * @param  keyExtractor the function used to extract the integer sort key
 276      * @return a lexicographic-order comparator composed of this and then the
 277      *         {@code int} sort key
 278      * @throws NullPointerException if the argument is null.
 279      * @see #comparingInt(ToIntFunction)
 280      * @see #thenComparing(Comparator)
 281      * @since 1.8
 282      */
 283     default Comparator<T> thenComparingInt(ToIntFunction<? super T> keyExtractor) {
 284         return thenComparing(comparingInt(keyExtractor));
 285     }
 286 
 287     /**
 288      * Returns a lexicographic-order comparator with a function that
 289      * extracts a {@code long} sort key.
 290      *
 291      * @implSpec This default implementation behaves as if {@code
 292      *           thenComparing(comparingLong(keyExtractor))}.
 293      *
 294      * @param  keyExtractor the function used to extract the long sort key
 295      * @return a lexicographic-order comparator composed of this and then the
 296      *         {@code long} sort key
 297      * @throws NullPointerException if the argument is null.
 298      * @see #comparingLong(ToLongFunction)
 299      * @see #thenComparing(Comparator)
 300      * @since 1.8
 301      */
 302     default Comparator<T> thenComparingLong(ToLongFunction<? super T> keyExtractor) {
 303         return thenComparing(comparingLong(keyExtractor));
 304     }
 305 
 306     /**
 307      * Returns a lexicographic-order comparator with a function that
 308      * extracts a {@code double} sort key.
 309      *
 310      * @implSpec This default implementation behaves as if {@code
 311      *           thenComparing(comparingDouble(keyExtractor))}.
 312      *
 313      * @param  keyExtractor the function used to extract the double sort key
 314      * @return a lexicographic-order comparator composed of this and then the
 315      *         {@code double} sort key
 316      * @throws NullPointerException if the argument is null.
 317      * @see #comparingDouble(ToDoubleFunction)
 318      * @see #thenComparing(Comparator)
 319      * @since 1.8
 320      */
 321     default Comparator<T> thenComparingDouble(ToDoubleFunction<? super T> keyExtractor) {
 322         return thenComparing(comparingDouble(keyExtractor));
 323     }
 324 
 325     /**
 326      * Returns a comparator that imposes the reverse of the <em>natural
 327      * ordering</em>.
 328      *
 329      * <p>The returned comparator is serializable and throws {@link
 330      * NullPointerException} when comparing {@code null}.
 331      *
 332      * @param  <T> the {@link Comparable} type of element to be compared
 333      * @return a comparator that imposes the reverse of the <i>natural
 334      *         ordering</i> on {@code Comparable} objects.
 335      * @see Comparable
 336      * @since 1.8
 337      */
 338     public static <T extends Comparable<? super T>> Comparator<T> reverseOrder() {
 339         return Collections.reverseOrder();
 340     }
 341 
 342     /**
 343      * Returns a comparator that compares {@link Comparable} objects in natural
 344      * order.
 345      *
 346      * <p>The returned comparator is serializable and throws {@link
 347      * NullPointerException} when comparing {@code null}.
 348      *
 349      * @param  <T> the {@link Comparable} type of element to be compared
 350      * @return a comparator that imposes the <i>natural ordering</i> on {@code
 351      *         Comparable} objects.
 352      * @see Comparable
 353      * @since 1.8
 354      */
 355     @SuppressWarnings("unchecked")
 356     public static <T extends Comparable<? super T>> Comparator<T> naturalOrder() {
 357         return (Comparator<T>) Comparators.NaturalOrderComparator.INSTANCE;
 358     }
 359 
 360     /**
 361      * Returns a null-friendly comparator that considers {@code null} to be
 362      * less than non-null. When both are {@code null}, they are considered
 363      * equal. If both are non-null, the specified {@code Comparator} is used
 364      * to determine the order. If the specified comparator is {@code null},
 365      * then the returned comparator considers all non-null values to be equal.
 366      *
 367      * <p>The returned comparator is serializable if the specified comparator
 368      * is serializable.
 369      *
 370      * @param  <T> the type of the elements to be compared
 371      * @param  comparator a {@code Comparator} for comparing non-null values
 372      * @return a comparator that considers {@code null} to be less than
 373      *         non-null, and compares non-null objects with the supplied
 374      *         {@code Comparator}.
 375      * @since 1.8
 376      */
 377     public static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator) {


 409      *
 410      * @apiNote
 411      * For example, to obtain a {@code Comparator} that compares {@code
 412      * Person} objects by their last name ignoring case differences,
 413      *
 414      * <pre>{@code
 415      *     Comparator<Person> cmp = Comparator.comparing(
 416      *             Person::getLastName,
 417      *             String.CASE_INSENSITIVE_ORDER);
 418      * }</pre>
 419      *
 420      * @param  <T> the type of element to be compared
 421      * @param  <U> the type of the sort key
 422      * @param  keyExtractor the function used to extract the sort key
 423      * @param  keyComparator the {@code Comparator} used to compare the sort key
 424      * @return a comparator that compares by an extracted key using the
 425      *         specified {@code Comparator}
 426      * @throws NullPointerException if either argument is null
 427      * @since 1.8
 428      */
 429     public static <T, U> Comparator<T> comparing(
 430             Function<? super T, ? extends U> keyExtractor,
 431             Comparator<? super U> keyComparator)
 432     {
 433         Objects.requireNonNull(keyExtractor);
 434         Objects.requireNonNull(keyComparator);
 435         return (Comparator<T> & Serializable)
 436             (c1, c2) -> keyComparator.compare(keyExtractor.apply(c1),
 437                                               keyExtractor.apply(c2));
 438     }
 439 
 440     /**
 441      * Accepts a function that extracts a {@link java.lang.Comparable
 442      * Comparable} sort key from a type {@code T}, and returns a {@code
 443      * Comparator<T>} that compares by that sort key.
 444      *
 445      * <p>The returned comparator is serializable if the specified function
 446      * is also serializable.
 447      *
 448      * @apiNote
 449      * For example, to obtain a {@code Comparator} that compares {@code
 450      * Person} objects by their last name,
 451      *
 452      * <pre>{@code
 453      *     Comparator<Person> byLastName = Comparator.comparing(Person::getLastName);
 454      * }</pre>
 455      *
 456      * @param  <T> the type of element to be compared
 457      * @param  <U> the type of the {@code Comparable} sort key
 458      * @param  keyExtractor the function used to extract the {@link
 459      *         Comparable} sort key
 460      * @return a comparator that compares by an extracted key
 461      * @throws NullPointerException if the argument is null
 462      * @since 1.8
 463      */
 464     public static <T, U extends Comparable<? super U>> Comparator<T> comparing(
 465             Function<? super T, ? extends U> keyExtractor)
 466     {
 467         Objects.requireNonNull(keyExtractor);
 468         return (Comparator<T> & Serializable)
 469             (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
 470     }
 471 
 472     /**
 473      * Accepts a function that extracts an {@code int} sort key from a type
 474      * {@code T}, and returns a {@code Comparator<T>} that compares by that
 475      * sort key.
 476      *
 477      * <p>The returned comparator is serializable if the specified function
 478      * is also serializable.
 479      *
 480      * @param  <T> the type of element to be compared
 481      * @param  keyExtractor the function used to extract the integer sort key
 482      * @return a comparator that compares by an extracted key
 483      * @see #comparing(Function)
 484      * @throws NullPointerException if the argument is null
 485      * @since 1.8
 486      */
 487     public static <T> Comparator<T> comparingInt(ToIntFunction<? super T> keyExtractor) {
 488         Objects.requireNonNull(keyExtractor);
 489         return (Comparator<T> & Serializable)
 490             (c1, c2) -> Integer.compare(keyExtractor.applyAsInt(c1), keyExtractor.applyAsInt(c2));
 491     }
 492 
 493     /**
 494      * Accepts a function that extracts a {@code long} sort key from a type
 495      * {@code T}, and returns a {@code Comparator<T>} that compares by that
 496      * sort key.
 497      *
 498      * <p>The returned comparator is serializable if the specified function is
 499      * also serializable.
 500      *
 501      * @param  <T> the type of element to be compared
 502      * @param  keyExtractor the function used to extract the long sort key
 503      * @return a comparator that compares by an extracted key
 504      * @see #comparing(Function)
 505      * @throws NullPointerException if the argument is null
 506      * @since 1.8
 507      */
 508     public static <T> Comparator<T> comparingLong(ToLongFunction<? super T> keyExtractor) {
 509         Objects.requireNonNull(keyExtractor);
 510         return (Comparator<T> & Serializable)
 511             (c1, c2) -> Long.compare(keyExtractor.applyAsLong(c1), keyExtractor.applyAsLong(c2));
 512     }
 513 
 514     /**
 515      * Accepts a function that extracts a {@code double} sort key from a type
 516      * {@code T}, and returns a {@code Comparator<T>} that compares by that
 517      * sort key.
 518      *
 519      * <p>The returned comparator is serializable if the specified function
 520      * is also serializable.
 521      *
 522      * @param  <T> the type of element to be compared
 523      * @param  keyExtractor the function used to extract the double sort key
 524      * @return a comparator that compares by an extracted key
 525      * @see #comparing(Function)
 526      * @throws NullPointerException if the argument is null
 527      * @since 1.8
 528      */
 529     public static<T> Comparator<T> comparingDouble(ToDoubleFunction<? super T> keyExtractor) {
 530         Objects.requireNonNull(keyExtractor);
 531         return (Comparator<T> & Serializable)
 532             (c1, c2) -> Double.compare(keyExtractor.applyAsDouble(c1), keyExtractor.applyAsDouble(c2));
 533     }
 534 }


   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  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).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package javany.util;
  27 
  28 import java.io.Serializable;
  29 import java.util.Objects;
  30 
  31 import javany.util.function.Function;


  32 
  33 /**
  34  * A comparison function, which imposes a <i>total ordering</i> on some
  35  * collection of objects.  Comparators can be passed to a sort method (such
  36  * as {@link Collections#sort(List,Comparator) Collections.sort} or {@link
  37  * Arrays#sort(Object[],Comparator) Arrays.sort}) to allow precise control
  38  * over the sort order.  Comparators can also be used to control the order of
  39  * certain data structures (such as {@link SortedSet sorted sets} or {@link
  40  * SortedMap sorted maps}), or to provide an ordering for collections of
  41  * objects that don't have a {@link Comparable natural ordering}.<p>
  42  *
  43  * The ordering imposed by a comparator <tt>c</tt> on a set of elements
  44  * <tt>S</tt> is said to be <i>consistent with equals</i> if and only if
  45  * <tt>c.compare(e1, e2)==0</tt> has the same boolean value as
  46  * <tt>e1.equals(e2)</tt> for every <tt>e1</tt> and <tt>e2</tt> in
  47  * <tt>S</tt>.<p>
  48  *
  49  * Caution should be exercised when using a comparator capable of imposing an
  50  * ordering inconsistent with equals to order a sorted set (or sorted map).
  51  * Suppose a sorted set (or sorted map) with an explicit comparator <tt>c</tt>


  87  * equals(Object)} method(s):<pre>
  88  *     {(x, y) such that x.equals(y)}. </pre>
  89  *
  90  * <p>Unlike {@code Comparable}, a comparator may optionally permit
  91  * comparison of null arguments, while maintaining the requirements for
  92  * an equivalence relation.
  93  *
  94  * <p>This interface is a member of the
  95  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
  96  * Java Collections Framework</a>.
  97  *
  98  * @param <T> the type of objects that may be compared by this comparator
  99  *
 100  * @author  Josh Bloch
 101  * @author  Neal Gafter
 102  * @see Comparable
 103  * @see java.io.Serializable
 104  * @since 1.2
 105  */
 106 @FunctionalInterface
 107 public interface Comparator<any T> {
 108     /**
 109      * Compares its two arguments for order.  Returns a negative integer,
 110      * zero, or a positive integer as the first argument is less than, equal
 111      * to, or greater than the second.<p>
 112      *
 113      * In the foregoing description, the notation
 114      * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical
 115      * <i>signum</i> function, which is defined to return one of <tt>-1</tt>,
 116      * <tt>0</tt>, or <tt>1</tt> according to whether the value of
 117      * <i>expression</i> is negative, zero or positive.<p>
 118      *
 119      * The implementor must ensure that <tt>sgn(compare(x, y)) ==
 120      * -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>.  (This
 121      * implies that <tt>compare(x, y)</tt> must throw an exception if and only
 122      * if <tt>compare(y, x)</tt> throws an exception.)<p>
 123      *
 124      * The implementor must also ensure that the relation is transitive:
 125      * <tt>((compare(x, y)&gt;0) &amp;&amp; (compare(y, z)&gt;0))</tt> implies
 126      * <tt>compare(x, z)&gt;0</tt>.<p>
 127      *


 163      * that two distinct comparators impose the same order.
 164      *
 165      * @param   obj   the reference object with which to compare.
 166      * @return  <code>true</code> only if the specified object is also
 167      *          a comparator and it imposes the same ordering as this
 168      *          comparator.
 169      * @see Object#equals(Object)
 170      * @see Object#hashCode()
 171      */
 172     boolean equals(Object obj);
 173 
 174     /**
 175      * Returns a comparator that imposes the reverse ordering of this
 176      * comparator.
 177      *
 178      * @return a comparator that imposes the reverse ordering of this
 179      *         comparator.
 180      * @since 1.8
 181      */
 182     default Comparator<T> reversed() {
 183         return new Comparators.ReverseComparator<>(this);
 184     }
 185 
 186     /**
 187      * Returns a lexicographic-order comparator with another comparator.
 188      * If this {@code Comparator} considers two elements equal, i.e.
 189      * {@code compare(a, b) == 0}, {@code other} is used to determine the order.
 190      *
 191      * <p>The returned comparator is serializable if the specified comparator
 192      * is also serializable.
 193      *
 194      * @apiNote
 195      * For example, to sort a collection of {@code String} based on the length
 196      * and then case-insensitive natural ordering, the comparator can be
 197      * composed using following code,
 198      *
 199      * <pre>{@code
 200      *     Comparator<String> cmp = Comparator.comparingInt(String::length)
 201      *             .thenComparing(String.CASE_INSENSITIVE_ORDER);
 202      * }</pre>
 203      *


 216         };
 217     }
 218 
 219     /**
 220      * Returns a lexicographic-order comparator with a function that
 221      * extracts a key to be compared with the given {@code Comparator}.
 222      *
 223      * @implSpec This default implementation behaves as if {@code
 224      *           thenComparing(comparing(keyExtractor, cmp))}.
 225      *
 226      * @param  <U>  the type of the sort key
 227      * @param  keyExtractor the function used to extract the sort key
 228      * @param  keyComparator the {@code Comparator} used to compare the sort key
 229      * @return a lexicographic-order comparator composed of this comparator
 230      *         and then comparing on the key extracted by the keyExtractor function
 231      * @throws NullPointerException if either argument is null.
 232      * @see #comparing(Function, Comparator)
 233      * @see #thenComparing(Comparator)
 234      * @since 1.8
 235      */
 236     default <any U> Comparator<T> thenComparing(
 237             Function<? super T, ? extends U> keyExtractor,
 238             Comparator<? super U> keyComparator)
 239     {
 240         return thenComparing(comparing(keyExtractor, keyComparator));
 241     }
 242 
 243     /**
 244      * Returns a lexicographic-order comparator with a function that
 245      * extracts a {@code Comparable} sort key.
 246      *
 247      * @implSpec This default implementation behaves as if {@code
 248      *           thenComparing(comparing(keyExtractor))}.
 249      *
 250      * @param  <U>  the type of the {@link Comparable} sort key
 251      * @param  keyExtractor the function used to extract the {@link
 252      *         Comparable} sort key
 253      * @return a lexicographic-order comparator composed of this and then the
 254      *         {@link Comparable} sort key.
 255      * @throws NullPointerException if the argument is null.
 256      * @see #comparing(Function)
 257      * @see #thenComparing(Comparator)
 258      * @since 1.8
 259      */
 260     default <U extends Comparable<? super U>> Comparator<T> thenComparing(
 261             Function<? super T, ? extends U> keyExtractor)
 262     {
 263         return thenComparing(comparing(keyExtractor));
 264     }
 265 
 266     /**
 267      * Returns a lexicographic-order comparator with a function that
 268      * extracts a {@code int} sort key.
 269      *
 270      * @implSpec This default implementation behaves as if {@code
 271      *           thenComparing(comparingInt(keyExtractor))}.
 272      *
 273      * @param  keyExtractor the function used to extract the integer sort key
 274      * @return a lexicographic-order comparator composed of this and then the
 275      *         {@code int} sort key
 276      * @throws NullPointerException if the argument is null.
 277      * @see #comparingInt(Function)
 278      * @see #thenComparing(Comparator)
 279      * @since 1.8
 280      */
 281     default Comparator<T> thenComparingInt(Function<? super T, int> keyExtractor) {
 282         return thenComparing(comparingInt(keyExtractor));
 283     }
 284 
 285     /**
 286      * Returns a lexicographic-order comparator with a function that
 287      * extracts a {@code long} sort key.
 288      *
 289      * @implSpec This default implementation behaves as if {@code
 290      *           thenComparing(comparingLong(keyExtractor))}.
 291      *
 292      * @param  keyExtractor the function used to extract the long sort key
 293      * @return a lexicographic-order comparator composed of this and then the
 294      *         {@code long} sort key
 295      * @throws NullPointerException if the argument is null.
 296      * @see #comparingLong(Function)
 297      * @see #thenComparing(Comparator)
 298      * @since 1.8
 299      */
 300     default Comparator<T> thenComparingLong(Function<? super T, long> keyExtractor) {
 301         return thenComparing(comparingLong(keyExtractor));
 302     }
 303 
 304     /**
 305      * Returns a lexicographic-order comparator with a function that
 306      * extracts a {@code double} sort key.
 307      *
 308      * @implSpec This default implementation behaves as if {@code
 309      *           thenComparing(comparingDouble(keyExtractor))}.
 310      *
 311      * @param  keyExtractor the function used to extract the double sort key
 312      * @return a lexicographic-order comparator composed of this and then the
 313      *         {@code double} sort key
 314      * @throws NullPointerException if the argument is null.
 315      * @see #comparingDouble(Function)
 316      * @see #thenComparing(Comparator)
 317      * @since 1.8
 318      */
 319     default Comparator<T> thenComparingDouble(Function<? super T, double> keyExtractor) {
 320         return thenComparing(comparingDouble(keyExtractor));
 321     }
 322 
 323     /**
 324      * Returns a comparator that imposes the reverse of the <em>natural
 325      * ordering</em>.
 326      *
 327      * <p>The returned comparator is serializable and throws {@link
 328      * NullPointerException} when comparing {@code null}.
 329      *
 330      * @param  <T> the {@link Comparable} type of element to be compared
 331      * @return a comparator that imposes the reverse of the <i>natural
 332      *         ordering</i> on {@code Comparable} objects.
 333      * @see Comparable
 334      * @since 1.8
 335      */
 336     public static <any T> Comparator<T> reverseOrder() {
 337         return Comparators.<T>naturalOrder().reversed();
 338     }
 339 
 340     /**
 341      * Returns a comparator that compares {@link Comparable} objects in natural
 342      * order.
 343      *
 344      * <p>The returned comparator is serializable and throws {@link
 345      * NullPointerException} when comparing {@code null}.
 346      *
 347      * @param  <T> the {@link Comparable} type of element to be compared
 348      * @return a comparator that imposes the <i>natural ordering</i> on {@code
 349      *         Comparable} objects.
 350      * @see Comparable
 351      * @since 1.8
 352      */
 353     @SuppressWarnings("unchecked")
 354     public static <any T> Comparator<T> naturalOrder() {
 355         return Comparators.naturalOrder();
 356     }
 357 
 358     /**
 359      * Returns a null-friendly comparator that considers {@code null} to be
 360      * less than non-null. When both are {@code null}, they are considered
 361      * equal. If both are non-null, the specified {@code Comparator} is used
 362      * to determine the order. If the specified comparator is {@code null},
 363      * then the returned comparator considers all non-null values to be equal.
 364      *
 365      * <p>The returned comparator is serializable if the specified comparator
 366      * is serializable.
 367      *
 368      * @param  <T> the type of the elements to be compared
 369      * @param  comparator a {@code Comparator} for comparing non-null values
 370      * @return a comparator that considers {@code null} to be less than
 371      *         non-null, and compares non-null objects with the supplied
 372      *         {@code Comparator}.
 373      * @since 1.8
 374      */
 375     public static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator) {


 407      *
 408      * @apiNote
 409      * For example, to obtain a {@code Comparator} that compares {@code
 410      * Person} objects by their last name ignoring case differences,
 411      *
 412      * <pre>{@code
 413      *     Comparator<Person> cmp = Comparator.comparing(
 414      *             Person::getLastName,
 415      *             String.CASE_INSENSITIVE_ORDER);
 416      * }</pre>
 417      *
 418      * @param  <T> the type of element to be compared
 419      * @param  <U> the type of the sort key
 420      * @param  keyExtractor the function used to extract the sort key
 421      * @param  keyComparator the {@code Comparator} used to compare the sort key
 422      * @return a comparator that compares by an extracted key using the
 423      *         specified {@code Comparator}
 424      * @throws NullPointerException if either argument is null
 425      * @since 1.8
 426      */
 427     public static <any T, any U> Comparator<T> comparing(
 428             Function<? super T, ? extends U> keyExtractor,
 429             Comparator<? super U> keyComparator)
 430     {
 431         Objects.requireNonNull(keyExtractor);
 432         Objects.requireNonNull(keyComparator);
 433         return (Comparator<T> & Serializable)
 434             (c1, c2) -> keyComparator.compare(keyExtractor.apply(c1),
 435                                               keyExtractor.apply(c2));
 436     }
 437 
 438     /**
 439      * Accepts a function that extracts a {@link java.lang.Comparable
 440      * Comparable} sort key from a type {@code T}, and returns a {@code
 441      * Comparator<T>} that compares by that sort key.
 442      *
 443      * <p>The returned comparator is serializable if the specified function
 444      * is also serializable.
 445      *
 446      * @apiNote
 447      * For example, to obtain a {@code Comparator} that compares {@code
 448      * Person} objects by their last name,
 449      *
 450      * <pre>{@code
 451      *     Comparator<Person> byLastName = Comparator.comparing(Person::getLastName);
 452      * }</pre>
 453      *
 454      * @param  <T> the type of element to be compared
 455      * @param  <U> the type of the {@code Comparable} sort key
 456      * @param  keyExtractor the function used to extract the {@link
 457      *         Comparable} sort key
 458      * @return a comparator that compares by an extracted key
 459      * @throws NullPointerException if the argument is null
 460      * @since 1.8
 461      */
 462     public static <any T, U extends Comparable<? super U>> Comparator<T> comparing(
 463             Function<? super T, ? extends U> keyExtractor)
 464     {
 465         Objects.requireNonNull(keyExtractor);
 466         return (Comparator<T> & Serializable)
 467             (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
 468     }
 469 
 470     /**
 471      * Accepts a function that extracts an {@code int} sort key from a type
 472      * {@code T}, and returns a {@code Comparator<T>} that compares by that
 473      * sort key.
 474      *
 475      * <p>The returned comparator is serializable if the specified function
 476      * is also serializable.
 477      *
 478      * @param  <T> the type of element to be compared
 479      * @param  keyExtractor the function used to extract the integer sort key
 480      * @return a comparator that compares by an extracted key
 481      * @see #comparing(Function)
 482      * @throws NullPointerException if the argument is null
 483      * @since 1.8
 484      */
 485     public static <any T> Comparator<T> comparingInt(Function<? super T, int> keyExtractor) {
 486         Objects.requireNonNull(keyExtractor);
 487         return (Comparator<T> & Serializable)
 488             (c1, c2) -> Integer.compare(keyExtractor.apply(c1), keyExtractor.apply(c2));
 489     }
 490 
 491     /**
 492      * Accepts a function that extracts a {@code long} sort key from a type
 493      * {@code T}, and returns a {@code Comparator<T>} that compares by that
 494      * sort key.
 495      *
 496      * <p>The returned comparator is serializable if the specified function is
 497      * also serializable.
 498      *
 499      * @param  <T> the type of element to be compared
 500      * @param  keyExtractor the function used to extract the long sort key
 501      * @return a comparator that compares by an extracted key
 502      * @see #comparing(Function)
 503      * @throws NullPointerException if the argument is null
 504      * @since 1.8
 505      */
 506     public static <any T> Comparator<T> comparingLong(Function<? super T, long> keyExtractor) {
 507         Objects.requireNonNull(keyExtractor);
 508         return (Comparator<T> & Serializable)
 509             (c1, c2) -> Long.compare(keyExtractor.apply(c1), keyExtractor.apply(c2));
 510     }
 511 
 512     /**
 513      * Accepts a function that extracts a {@code double} sort key from a type
 514      * {@code T}, and returns a {@code Comparator<T>} that compares by that
 515      * sort key.
 516      *
 517      * <p>The returned comparator is serializable if the specified function
 518      * is also serializable.
 519      *
 520      * @param  <T> the type of element to be compared
 521      * @param  keyExtractor the function used to extract the double sort key
 522      * @return a comparator that compares by an extracted key
 523      * @see #comparing(Function)
 524      * @throws NullPointerException if the argument is null
 525      * @since 1.8
 526      */
 527     public static <any T> Comparator<T> comparingDouble(Function<? super T, double> keyExtractor) {
 528         Objects.requireNonNull(keyExtractor);
 529         return (Comparator<T> & Serializable)
 530             (c1, c2) -> Double.compare(keyExtractor.apply(c1), keyExtractor.apply(c2));
 531     }
 532 }
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