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   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
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  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).
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  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 {@code c} on a set of elements
  46  * {@code S} is said to be <i>consistent with equals</i> if and only if
  47  * {@code c.compare(e1, e2)==0} has the same boolean value as
  48  * {@code e1.equals(e2)} for every {@code e1} and {@code e2} in
  49  * {@code S}.<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 {@code c}
  54  * is used with elements (or keys) drawn from a set {@code S}.  If the
  55  * ordering imposed by {@code c} on {@code S} is inconsistent with equals,
  56  * the sorted set (or sorted map) will behave "strangely."  In particular the
  57  * sorted set (or sorted map) will violate the general contract for set (or
  58  * map), which is defined in terms of {@code equals}.<p>
  59  *
  60  * For example, suppose one adds two elements {@code a} and {@code b} such that
  61  * {@code (a.equals(b) && c.compare(a, b) != 0)}
  62  * to an empty {@code TreeSet} with comparator {@code c}.
  63  * The second {@code add} operation will return
  64  * true (and the size of the tree set will increase) because {@code a} and
  65  * {@code b} are not equivalent from the tree set's perspective, even though
  66  * this is contrary to the specification of the
  67  * {@link Set#add Set.add} method.<p>
  68  *
  69  * Note: It is generally a good idea for comparators to also implement
  70  * {@code java.io.Serializable}, as they may be used as ordering methods in
  71  * serializable data structures (like {@link TreeSet}, {@link TreeMap}).  In
  72  * order for the data structure to serialize successfully, the comparator (if
  73  * provided) must implement {@code Serializable}.<p>
  74  *
  75  * For the mathematically inclined, the <i>relation</i> that defines the
  76  * <i>imposed ordering</i> that a given comparator {@code c} imposes on a
  77  * given set of objects {@code S} is:<pre>
  78  *       {(x, y) such that c.compare(x, y) &lt;= 0}.
  79  * </pre> The <i>quotient</i> for this total order is:<pre>
  80  *       {(x, y) such that c.compare(x, y) == 0}.
  81  * </pre>
  82  *
  83  * It follows immediately from the contract for {@code compare} that the
  84  * quotient is an <i>equivalence relation</i> on {@code S}, and that the
  85  * imposed ordering is a <i>total order</i> on {@code S}.  When we say that
  86  * the ordering imposed by {@code c} on {@code S} is <i>consistent with
  87  * equals</i>, we mean that the quotient for the ordering is the equivalence
  88  * relation defined by the objects' {@link Object#equals(Object)
  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}/java.base/java/util/package-summary.html#CollectionsFramework">
  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      * The implementor must ensure that {@code sgn(compare(x, y)) ==
 116      * -sgn(compare(y, x))} for all {@code x} and {@code y}.  (This
 117      * implies that {@code compare(x, y)} must throw an exception if and only
 118      * if {@code compare(y, x)} throws an exception.)<p>
 119      *
 120      * The implementor must also ensure that the relation is transitive:
 121      * {@code ((compare(x, y)>0) && (compare(y, z)>0))} implies
 122      * {@code compare(x, z)>0}.<p>
 123      *
 124      * Finally, the implementor must ensure that {@code compare(x, y)==0}
 125      * implies that {@code sgn(compare(x, z))==sgn(compare(y, z))} for all
 126      * {@code z}.<p>
 127      *
 128      * It is generally the case, but <i>not</i> strictly required that
 129      * {@code (compare(x, y)==0) == (x.equals(y))}.  Generally speaking,
 130      * any comparator that violates this condition should clearly indicate
 131      * this fact.  The recommended language is "Note: this comparator
 132      * imposes orderings that are inconsistent with equals."<p>
 133      *
 134      * In the foregoing description, the notation
 135      * {@code sgn(}<i>expression</i>{@code )} designates the mathematical
 136      * <i>signum</i> function, which is defined to return one of {@code -1},
 137      * {@code 0}, or {@code 1} according to whether the value of
 138      * <i>expression</i> is negative, zero, or positive, respectively.
 139      *
 140      * @param o1 the first object to be compared.
 141      * @param o2 the second object to be compared.
 142      * @return a negative integer, zero, or a positive integer as the
 143      *         first argument is less than, equal to, or greater than the
 144      *         second.
 145      * @throws NullPointerException if an argument is null and this
 146      *         comparator does not permit null arguments
 147      * @throws ClassCastException if the arguments' types prevent them from
 148      *         being compared by this comparator.
 149      */
 150     int compare(T o1, T o2);
 151 
 152     /**
 153      * Indicates whether some other object is &quot;equal to&quot; this
 154      * comparator.  This method must obey the general contract of
 155      * {@link Object#equals(Object)}.  Additionally, this method can return
 156      * {@code true} <i>only</i> if the specified object is also a comparator
 157      * and it imposes the same ordering as this comparator.  Thus,
 158      * {@code comp1.equals(comp2)} implies that {@code sgn(comp1.compare(o1,
 159      * o2))==sgn(comp2.compare(o1, o2))} for every object reference
 160      * {@code o1} and {@code o2}.<p>
 161      *
 162      * Note that it is <i>always</i> safe <i>not</i> to override
 163      * {@code Object.equals(Object)}.  However, overriding this method may,
 164      * in some cases, improve performance by allowing programs to determine
 165      * that two distinct comparators impose the same order.
 166      *
 167      * @param   obj   the reference object with which to compare.
 168      * @return  {@code true} 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      *
 206      * @param  other the other comparator to be used when this comparator
 207      *         compares two objects that are equal.
 208      * @return a lexicographic-order comparator composed of this and then the
 209      *         other comparator
 210      * @throws NullPointerException if the argument is null.
 211      * @since 1.8
 212      */
 213     default Comparator<T> thenComparing(Comparator<? super T> other) {
 214         Objects.requireNonNull(other);
 215         return (Comparator<T> & Serializable) (c1, c2) -> {
 216             int res = compare(c1, c2);
 217             return (res != 0) ? res : other.compare(c1, c2);
 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 an {@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) {
 378         return new Comparators.NullComparator<>(true, comparator);
 379     }
 380 
 381     /**
 382      * Returns a null-friendly comparator that considers {@code null} to be
 383      * greater than non-null. When both are {@code null}, they are considered
 384      * equal. If both are non-null, the specified {@code Comparator} is used
 385      * to determine the order. If the specified comparator is {@code null},
 386      * then the returned comparator considers all non-null values to be equal.
 387      *
 388      * <p>The returned comparator is serializable if the specified comparator
 389      * is serializable.
 390      *
 391      * @param  <T> the type of the elements to be compared
 392      * @param  comparator a {@code Comparator} for comparing non-null values
 393      * @return a comparator that considers {@code null} to be greater than
 394      *         non-null, and compares non-null objects with the supplied
 395      *         {@code Comparator}.
 396      * @since 1.8
 397      */
 398     public static <T> Comparator<T> nullsLast(Comparator<? super T> comparator) {
 399         return new Comparators.NullComparator<>(false, comparator);
 400     }
 401 
 402     /**
 403      * Accepts a function that extracts a sort key from a type {@code T}, and
 404      * returns a {@code Comparator<T>} that compares by that sort key using
 405      * the specified {@link Comparator}.
 406       *
 407      * <p>The returned comparator is serializable if the specified function
 408      * and comparator are both serializable.
 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 }