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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 <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      * 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 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) {
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 }