1 /*
   2  * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   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>
  54  * is used with elements (or keys) drawn from a set <tt>S</tt>.  If the
  55  * ordering imposed by <tt>c</tt> on <tt>S</tt> 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 <tt>equals</tt>.<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  * <tt>java.io.Serializable</tt>, 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 <tt>Serializable</tt>.<p>
  74  *
  75  * For the mathematically inclined, the <i>relation</i> that defines the
  76  * <i>imposed ordering</i> that a given comparator <tt>c</tt> imposes on a
  77  * given set of objects <tt>S</tt> 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 <tt>compare</tt> that the
  84  * quotient is an <i>equivalence relation</i> on <tt>S</tt>, and that the
  85  * imposed ordering is a <i>total order</i> on <tt>S</tt>.  When we say that
  86  * the ordering imposed by <tt>c</tt> on <tt>S</tt> 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}/../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      *
 130      * Finally, the implementor must ensure that <tt>compare(x, y)==0</tt>
 131      * implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all
 132      * <tt>z</tt>.<p>
 133      *
 134      * It is generally the case, but <i>not</i> strictly required that
 135      * <tt>(compare(x, y)==0) == (x.equals(y))</tt>.  Generally speaking,
 136      * any comparator that violates this condition should clearly indicate
 137      * this fact.  The recommended language is "Note: this comparator
 138      * imposes orderings that are inconsistent with equals."
 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      * <tt>true</tt> <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)</code> implies that <tt>sgn(comp1.compare(o1,
 159      * o2))==sgn(comp2.compare(o1, o2))</tt> for every object reference
 160      * <tt>o1</tt> and <tt>o2</tt>.<p>
 161      *
 162      * Note that it is <i>always</i> safe <i>not</i> to override
 163      * <tt>Object.equals(Object)</tt>.  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</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      * Constructs 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      * To sort a collection of {@code String} based on the length and then
 198      * case-insensitive natural ordering, the comparator can be construcred
 199      * using following code,
 200      *
 201      * <pre>{@code
 202      *     Comparator<String> cmp = Comparator.comparing(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      * Constructs 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 calls {@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 and then the
 232      *         key comparator.
 233      * @throws NullPointerException if the argument is null.
 234      * @see #comparing(Function, Comparator)
 235      * @see #thenComparing(Comparator)
 236      * @since 1.8
 237      */
 238     default <U extends Comparable<? super 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      * Constructs a lexicographic order comparator with a function that
 247      * extracts a {@code Comparable} sort key.
 248      *
 249      * @implSpec This default implementation calls {@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      * Constructs a lexicographic order comparator with a function that
 270      * extracts a {@code int} sort key.
 271      *
 272      * @implSpec This default implementation calls {@code
 273      *           thenComparing(comparing(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 #comparing(ToIntFunction)
 280      * @see #thenComparing(Comparator)
 281      * @since 1.8
 282      */
 283     default Comparator<T> thenComparing(ToIntFunction<? super T> keyExtractor) {
 284         return thenComparing(comparing(keyExtractor));
 285     }
 286 
 287     /**
 288      * Constructs a lexicographic order comparator with a function that
 289      * extracts a {@code long} sort key.
 290      *
 291      * @implSpec This default implementation calls {@code
 292      *           thenComparing(comparing(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 #comparing(ToLongFunction)
 299      * @see #thenComparing(Comparator)
 300      * @since 1.8
 301      */
 302     default Comparator<T> thenComparing(ToLongFunction<? super T> keyExtractor) {
 303         return thenComparing(comparing(keyExtractor));
 304     }
 305 
 306     /**
 307      * Constructs a lexicographic order comparator with a function that
 308      * extracts a {@code double} sort key.
 309      *
 310      * @implSpec This default implementation calls {@code
 311      *           thenComparing(comparing(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 #comparing(ToDoubleFunction)
 318      * @see #thenComparing(Comparator)
 319      * @since 1.8
 320      */
 321     default Comparator<T> thenComparing(ToDoubleFunction<? super T> keyExtractor) {
 322         return thenComparing(comparing(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 a collection of objects that implement
 335      *         the {@link Comparable} interface.
 336      * @see Comparable
 337      * @since 1.8
 338      */
 339     public static <T extends Comparable<? super T>> Comparator<T> reverseOrder() {
 340         return Collections.reverseOrder();
 341     }
 342 
 343     /**
 344      * Returns a comparator that compares {@link Comparable} type in natural
 345      * order.
 346      *
 347      * <p>The returned comparator is serializable and throws {@link
 348      * NullPointerException} when comparing {@code null}.
 349      *
 350      * @param  <T> The {@link Comparable} type of element to be compared
 351      * @return A comparator that imposes the <i>natural ordering</i> on a
 352      *         collection of objects that implement the {@link Comparable}
 353      *         interface.
 354      * @see Comparable
 355      * @since 1.8
 356      */
 357     public static <T extends Comparable<? super T>> Comparator<T> naturalOrder() {
 358         return (Comparator<T>) Comparators.NaturalOrderComparator.INSTANCE;
 359     }
 360 
 361     /**
 362      * Returns a null-friendly comparator that considers {@code null} to be
 363      * less than non-null. When both are {@code null}, they are considered
 364      * equal. If both are non-null, the specified {@code Comparator} is used
 365      * to determine the order. If the specified comparator is {@code null},
 366      * then the returned comparator considers all non-null values to be equal.
 367      *
 368      * <p>The returned comparator is serializable if the specified comparator
 369      * is serializable.
 370      *
 371      * @param  <T> the type of the elements to be compared
 372      * @param  comparator A {@code Comparator} for comparing non-null values
 373      * @return A comparator that considers {@code null} to be less than non-null.
 374      * @since 1.8
 375      */
 376     public static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator) {
 377         return new Comparators.NullComparator(true, comparator);
 378     }
 379 
 380     /**
 381      * Returns a null-friendly comparator that considers {@code null} to be
 382      * greater than non-null. When both are {@code null}, they are considered
 383      * equal. If both are non-null, the specified {@code Comparator} is used
 384      * to determine the order. If the specified comparator is {@code null},
 385      * then the returned comparator considers all non-null values to be equal.
 386      *
 387      * <p>The returned comparator is serializable if the specified comparator
 388      * is serializable.
 389      *
 390      * @param  <T> the type of the elements to be compared
 391      * @param  comparator A {@code Comparator} for comparing non-null values
 392      * @return A comparator that considers {@code null} to be greater than non-null.
 393      * @since 1.8
 394      */
 395     public static <T> Comparator<T> nullsLast(Comparator<? super T> comparator) {
 396         return new Comparators.NullComparator(false, comparator);
 397     }
 398 
 399     /**
 400      * Accepts a function that extracts a sort key from a type {@code T}, and
 401      * returns a {@code Comparator<T>} that compares by that sort key using
 402      * the specified {@link Comparator}.
 403       *
 404      * <p>The returned comparator is serializable if the specified function
 405      * and comparator are both serializable.
 406      *
 407      * @apiNote
 408      * To obtain a {@code Comparator} that compares {@code Person} objects by their last name
 409      * ignoring case differences,
 410      *
 411      * <pre>{@code
 412      *     Comparator<People> cmp = Comparator.comparing(
 413      *             Person::getLastName,
 414      *             String.CASE_INSENSITIVE_ORDER);
 415      * }</pre>
 416      *
 417      * @param  <T> The type of element to be compared
 418      * @param  <U> The type of the sort key
 419      * @param  keyExtractor the function used to extract the sort key
 420      * @param  keyComparator the {@code Comparator} used to compare the sort key
 421      * @return A comparator that compares by an extracted key using the
 422      *         specified {@code Comparator}
 423      * @throws NullPointerException if the argument is null
 424      * @since 1.8
 425      */
 426     public static <T, U> Comparator<T> comparing(
 427             Function<? super T, ? extends U> keyExtractor,
 428             Comparator<? super U> keyComparator)
 429     {
 430         Objects.requireNonNull(keyExtractor);
 431         Objects.requireNonNull(keyComparator);
 432         return (Comparator<T> & Serializable)
 433             (c1, c2) -> keyComparator.compare(keyExtractor.apply(c1),
 434                                               keyExtractor.apply(c2));
 435     }
 436 
 437     /**
 438      * Accepts a function that extracts a {@link java.lang.Comparable
 439      * Comparable} sort key from a type {@code T}, and returns a {@code
 440      * Comparator<T>} that compares by that sort key.
 441      *
 442      * <p>The returned comparator is serializable if the specified function
 443      * is also serializable.
 444      *
 445      * @apiNote
 446      * To obtain a {@code Comparator} that compares {@code Person} objects by their last
 447      * name,
 448      *
 449      * <pre>{@code
 450      *     Comparator<People> byLastName = Comparator.comparing(Person::getLastName);
 451      * }</pre>
 452      *
 453      * @param  <T> The type of element to be compared
 454      * @param  <U> The type of the {@code Comparable} sort key
 455      * @param  keyExtractor the function used to extract the {@link
 456      *         Comparable} sort key
 457      * @return A comparator that compares by an extracted key
 458      * @throws NullPointerException if the argument is null
 459      * @since 1.8
 460      */
 461     public static <T, U extends Comparable<? super U>> Comparator<T> comparing(
 462             Function<? super T, ? extends U> keyExtractor)
 463     {
 464         Objects.requireNonNull(keyExtractor);
 465         return (Comparator<T> & Serializable)
 466             (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
 467     }
 468 
 469     /**
 470      * Accepts a function that extracts an {@code int} sort key from a type
 471      * {@code T}, and returns a {@code Comparator<T>} that compares by that
 472      * sort key.
 473      *
 474      * <p>The returned comparator is serializable if the specified function
 475      * is also serializable.
 476      *
 477      * @param  <T> The type of element to be compared
 478      * @param  keyExtractor the function used to extract the integer sort key
 479      * @return A comparator that compares by an extracted key
 480      * @see #comparing(Function)
 481      * @throws NullPointerException if the argument is null
 482      * @since 1.8
 483      */
 484     public static <T> Comparator<T> comparing(ToIntFunction<? super T> keyExtractor) {
 485         Objects.requireNonNull(keyExtractor);
 486         return (Comparator<T> & Serializable)
 487             (c1, c2) -> Integer.compare(keyExtractor.applyAsInt(c1), keyExtractor.applyAsInt(c2));
 488     }
 489 
 490     /**
 491      * Accepts a function that extracts a {@code long} sort key from a type
 492      * {@code T}, and returns a {@code Comparator<T>} that compares by that
 493      * sort key.
 494      *
 495      * <p>The returned comparator is serializable if the specified function is
 496      * also serializable.
 497      *
 498      * @param  <T> The type of element to be compared
 499      * @param  keyExtractor the function used to extract the long sort key
 500      * @return A comparator that compares by an extracted key
 501      * @see #comparing(Function)
 502      * @throws NullPointerException if the argument is null
 503      * @since 1.8
 504      */
 505     public static <T> Comparator<T> comparing(ToLongFunction<? super T> keyExtractor) {
 506         Objects.requireNonNull(keyExtractor);
 507         return (Comparator<T> & Serializable)
 508             (c1, c2) -> Long.compare(keyExtractor.applyAsLong(c1), keyExtractor.applyAsLong(c2));
 509     }
 510 
 511     /**
 512      * Accepts a function that extracts a {@code double} sort key from a type
 513      * {@code T}, and returns a {@code Comparator<T>} that compares by that
 514      * sort key.
 515      *
 516      * <p>The returned comparator is serializable if the specified function
 517      * is also serializable.
 518      *
 519      * @param  <T> The type of element to be compared
 520      * @param  keyExtractor the function used to extract the double sort key
 521      * @return A comparator that compares by an extracted key
 522      * @see #comparing(Function)
 523      * @throws NullPointerException if the argument is null
 524      * @since 1.8
 525      */
 526     public static<T> Comparator<T> comparing(ToDoubleFunction<? super T> keyExtractor) {
 527         Objects.requireNonNull(keyExtractor);
 528         return (Comparator<T> & Serializable)
 529             (c1, c2) -> Double.compare(keyExtractor.applyAsDouble(c1), keyExtractor.applyAsDouble(c2));
 530     }
 531 }