1 /*
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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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 <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) <= 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)>0) && (compare(y, z)>0))</tt> implies
128 * <tt>compare(x, z)>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 "equal to" 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 (Comparator<T>) Collections.reverseOrder(this);
186 }
187
188 /**
189 * Returns a comparator that imposes the reverse of the <em>natural
190 * ordering</em>.
191 *
192 * <p>The returned comparator is serializable. Try to compare null with
193 * returned comparator will throw {@link NullPointerException}.
194 *
195 * @param <T> The {@link Comparable} type of element to be compared
196 * @return A comparator that imposes the reverse of the <i>natural
197 * ordering</i> on a collection of objects that implement
198 * the {@link Comparable} interface.
199 * @see Comparable
200 * @since 1.8
201 */
202 public static <T extends Comparable<? super T>> Comparator<T> reverseOrder() {
203 return Collections.reverseOrder();
204 }
205
206 /**
207 * Returns a comparator compares {@link Comparable} type in natural order.
208 *
209 * <p>The returned comparator is serializable. Try to compare null with
210 * returned comparator will throw {@link NullPointerException}.
211 *
212 * @param <T> The {@link Comparable} type of element to be compared
213 * @return A comparator that imposes the <i>natural ordering</i> on a
214 * collection of objects that implement the {@link Comparable}
215 * interface.
216 * @see Comparable
217 * @since 1.8
218 */
219 public static <T extends Comparable<? super T>> Comparator<T> naturalOrder() {
220 return (Comparator<T>) Comparators.NaturalOrderComparator.INSTANCE;
221 }
222
223 /**
224 * Returns a null-friendly comparator that considers {@code null} to be
225 * less than non-null. When both are {@code null}, they are considered
226 * equal. If both are non-null, the specified {@code Comparator} is used
227 * to determine the order.
228 *
229 * <p>The returned comparator is serializable if the specified comparator
230 * is serializable.
231 *
232 * @param <T> the type of the elements to be compared
233 * @param comparator A {@code Comparator} for comparing non-null values
234 * @return A comparator that considers {@code null} to be less than non-null.
235 * @since 1.8
236 */
237 public static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator) {
238 return new Comparators.NullComparator(-1, comparator);
239 }
240
241 /**
242 * Returns a null-friendly comparator that considers {@code null} to be
243 * greater than non-null. When both are {@code null}, they are considered
244 * equal. If both are non-null, the specified {@code Comparator} is used
245 * to determine the order.
246 *
247 * <p>The returned comparator is serializable if the specified comparator
248 * is serializable.
249 *
250 * @param <T> the type of the elements to be compared
251 * @param comparator A {@code Comparator} for comparing non-null values
252 * @return A comparator that considers {@code null} to be greater than non-null.
253 * @since 1.8
254 */
255 public static <T> Comparator<T> nullsLast(Comparator<? super T> comparator) {
256 return new Comparators.NullComparator(1, comparator);
257 }
258
259 /**
260 * Accepts a function that extracts a sort key from a type {@code T}, and
261 * returns a {@code Comparator<T>} that compares by that sort key using
262 * the specified {@link Comparator}. For example, to obtain a {@code
263 * Comparator} that compares {@code Person} objects by their last name
264 * ignoring case differences,
265 *
266 * <pre>{@code
267 * Comparator<People> cmp = Comparator.comparing(
268 * Person::getLastName,
269 * String.CASE_INSENSITIVE_ORDER);
270 * }</pre>
271 *
272 * <p> The returned comparator is serializable if the specified function
273 * and comparator are both serializable.
274 *
275 * @param <T> The type of element to be compared
276 * @param <U> The type of the sort key
277 * @param keyExtractor the function used to extract the sort key
278 * @param keyComparator the {@code Comparator} used to compare the sort key
279 * @return A comparator that compares by an extracted key using the
280 * specified {@code Comparator}
281 * @throws NullPointerException if the argument is null
282 * @since 1.8
283 */
284 public static <T, U> Comparator<T> comparing(
285 Function<? super T, ? extends U> keyExtractor,
286 Comparator<? super U> keyComparator)
287 {
288 Objects.requireNonNull(keyExtractor);
289 Objects.requireNonNull(keyComparator);
290 return (Comparator<T> & Serializable)
291 (c1, c2) -> keyComparator.compare(keyExtractor.apply(c1),
292 keyExtractor.apply(c2));
293 }
294
295 /**
296 * Accepts a function that extracts a {@link java.lang.Comparable
297 * Comparable} sort key from a type {@code T}, and returns a {@code
298 * Comparator<T>} that compares by that sort key. For example, to obtain
299 * a {@code Comparator} that compares {@code Person} objects by their last
300 * name,
301 *
302 * <pre>{@code
303 * Comparator<People> byLastName = Comparator.comparing(Person::getLastName);
304 * }</pre>
305 *
306 * <p> The returned comparator is serializable if the specified function
307 * is also serializable.
308 *
309 * @param <T> The type of element to be compared
310 * @param <U> The type of the {@code Comparable} sort key
311 * @param keyExtractor the function used to extract the {@link
312 * Comparable} sort key
313 * @return A comparator that compares by an extracted key
314 * @throws NullPointerException if the argument is null
315 * @since 1.8
316 */
317 public static <T, U extends Comparable<? super U>> Comparator<T> comparing(
318 Function<? super T, ? extends U> keyExtractor)
319 {
320 Objects.requireNonNull(keyExtractor);
321 return (Comparator<T> & Serializable)
322 (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
323 }
324
325 /**
326 * Accepts a function that extracts an {@code int} sort key from a type
327 * {@code T}, and returns a {@code Comparator<T>} that compares by that
328 * sort key. For example, to obtain a {@code Comparator} that compares
329 * {@code Person} objects by their age,
330 *
331 * <pre>{@code
332 * Comparator<People> byAge = Comparator.comparing(Person::getAge);
333 * }</pre>
334 *
335 * <p> The returned comparator is serializable if the specified function
336 * is also serializable.
337 *
338 * @param <T> The type of element to be compared
339 * @param keyExtractor the function used to extract the integer sort key
340 * @return A comparator that compares by an extracted key
341 * @see #comparing(Function)
342 * @throws NullPointerException if the argument is null
343 * @since 1.8
344 */
345 public static <T> Comparator<T> comparing(ToIntFunction<? super T> keyExtractor) {
346 Objects.requireNonNull(keyExtractor);
347 return (Comparator<T> & Serializable)
348 (c1, c2) -> Integer.compare(keyExtractor.applyAsInt(c1), keyExtractor.applyAsInt(c2));
349 }
350
351 /**
352 * Accepts a function that extracts a {@code long} sort key from a type
353 * {@code T}, and returns a {@code Comparator<T>} that compares by that
354 * sort key.
355 *
356 * <p> The returned comparator is serializable if the specified function
357 * is also serializable.
358 *
359 * @param <T> The type of element to be compared
360 * @param keyExtractor the function used to extract the long sort key
361 * @return A comparator that compares by an extracted key
362 * @see #comparing(Function)
363 * @throws NullPointerException if the argument is null
364 * @since 1.8
365 */
366 public static <T> Comparator<T> comparing(ToLongFunction<? super T> keyExtractor) {
367 Objects.requireNonNull(keyExtractor);
368 return (Comparator<T> & Serializable)
369 (c1, c2) -> Long.compare(keyExtractor.applyAsLong(c1), keyExtractor.applyAsLong(c2));
370 }
371
372 /**
373 * Accepts a function that extracts a {@code double} sort key from a type
374 * {@code T}, and returns a {@code Comparator<T>} that compares by that
375 * sort key.
376 *
377 * <p> The returned comparator is serializable if the specified function
378 * is also serializable.
379 *
380 * @param <T> The type of element to be compared
381 * @param keyExtractor the function used to extract the double sort key
382 * @return A comparator that compares by an extracted key
383 * @see #comparing(Function)
384 * @throws NullPointerException if the argument is null
385 * @since 1.8
386 */
387 public static<T> Comparator<T> comparing(ToDoubleFunction<? super T> keyExtractor) {
388 Objects.requireNonNull(keyExtractor);
389 return (Comparator<T> & Serializable)
390 (c1, c2) -> Double.compare(keyExtractor.applyAsDouble(c1), keyExtractor.applyAsDouble(c2));
391 }
392
393 /**
394 * Constructs a lexicographic order comparator with another comparator.
395 * If this {@code Comparator} considers two elements equal, i.e.
396 * {@code compare(a, b) == 0}, {@code other} is used to determine the order.
397 *
398 * <p> For example, to sort a collection of {@code String} based on the
399 * length and then case-insensitive natural ordering, the comparator can
400 * be construcred using following code,
401 *
402 * <pre>{@code
403 * Comparator<String> cmp = Comparator.comparing(String::length)
404 * .thenComparing(String.CASE_INSENSITIVE_ORDER);
405 * }</pre>
406 *
407 * <p> The returned comparator is serializable if the specified comparator
408 * is also serializable.
409 *
410 * @param <T> The type of elements compared by the returned comparator.
411 * @param other the other comparator to be used when this comparator
412 * compares two objects that are equal.
413 * @return A lexicographic order comparator composed of this and then the
414 * other comparator
415 * @throws NullPointerException if the argument is null.
416 * @since 1.8
417 */
418 default Comparator<T> thenComparing(Comparator<? super T> other) {
419 Objects.requireNonNull(other);
420 return (Comparator<T> & Serializable) (c1, c2) -> {
421 int res = compare(c1, c2);
422 return (res != 0) ? res : other.compare(c1, c2);
423 };
424 }
425
426 /**
427 * Constructs a lexicographic order comparator with a function that
428 * extracts a key to be compared with the given {@code Comparator}.
429 *
430 * @implSpec This default implementation calls {@code
431 * thenComparing(comparing(keyExtractor, cmp))}.
432 *
433 * @param <T> The type of elements compared by the returned comparator.
434 * @param <U> The type of the sort key
435 * @param keyExtractor the function used to extract the sort key
436 * @param keyComparator the {@code Comparator} used to compare the sort key
437 * @return A lexicographic order comparator composed of this and then the
438 * key comparator.
439 * @throws NullPointerException if the argument is null.
440 * @see #comparing(Function, Comparator)
441 * @see #thenComparing(Comparator)
442 * @since 1.8
443 */
444 default <U extends Comparable<? super U>> Comparator<T> thenComparing(
445 Function<? super T, ? extends U> keyExtractor,
446 Comparator<? super U> keyComparator)
447 {
448 return thenComparing(comparing(keyExtractor, keyComparator));
449 }
450
451 /**
452 * Constructs a lexicographic order comparator with a function that
453 * extracts a {@code Comparable} sort key.
454 *
455 * @implSpec This default implementation calls {@code
456 * thenComparing(comparing(keyExtractor))}.
457 *
458 * @param <T> The type of elements compared by the returned comparator.
459 * @param <U> The type of the {@link Comparable} sort key
460 * @param keyExtractor the function used to extract the {@link
461 * Comparable} sort key
462 * @return A lexicographic order comparator composed of this and then the
463 * {@link Comparable} sort key.
464 * @throws NullPointerException if the argument is null.
465 * @see #comparing(Function)
466 * @see #thenComparing(Comparator)
467 * @since 1.8
468 */
469 default <U extends Comparable<? super U>> Comparator<T> thenComparing(
470 Function<? super T, ? extends U> keyExtractor)
471 {
472 return thenComparing(comparing(keyExtractor));
473 }
474
475 /**
476 * Constructs a lexicographic order comparator with a function that
477 * extracts a {@code int} sort key.
478 *
479 * @implSpec This default implementation calls {@code
480 * thenComparing(comparing(keyExtractor))}.
481 *
482 * @param <T> The type of elements compared by the returned comparator.
483 * @param keyExtractor the function used to extract the integer sort key
484 * @return A lexicographic order comparator composed of this and then the
485 * {@code int} sort key
486 * @throws NullPointerException if the argument is null.
487 * @see #comparing(ToIntFunction)
488 * @see #thenComparing(Comparator)
489 * @since 1.8
490 */
491 default Comparator<T> thenComparing(ToIntFunction<? super T> keyExtractor) {
492 return thenComparing(comparing(keyExtractor));
493 }
494
495 /**
496 * Constructs a lexicographic order comparator with a function that
497 * extracts a {@code long} sort key.
498 *
499 * @implSpec This default implementation calls {@code
500 * thenComparing(comparing(keyExtractor))}.
501 *
502 * @param <T> The type of elements compared by the returned comparator.
503 * @param keyExtractor the function used to extract the long sort key
504 * @return A lexicographic order comparator composed of this and then the
505 * {@code long} sort key
506 * @throws NullPointerException if the argument is null.
507 * @see #comparing(ToLongFunction)
508 * @see #thenComparing(Comparator)
509 * @since 1.8
510 */
511 default Comparator<T> thenComparing(ToLongFunction<? super T> keyExtractor) {
512 return thenComparing(comparing(keyExtractor));
513 }
514
515 /**
516 * Constructs a lexicographic order comparator with a function that
517 * extracts a {@code double} sort key.
518 *
519 * @implSpec This default implementation calls {@code
520 * thenComparing(comparing(keyExtractor))}.
521 *
522 * @param <T> The type of elements compared by the returned comparator.
523 * @param keyExtractor the function used to extract the double sort key
524 * @return A lexicographic order comparator composed of this and then the
525 * {@code double} sort key
526 * @throws NullPointerException if the argument is null.
527 * @see #comparing(ToDoubleFunction)
528 * @see #thenComparing(Comparator)
529 * @since 1.8
530 */
531 default Comparator<T> thenComparing(ToDoubleFunction<? super T> keyExtractor) {
532 return thenComparing(comparing(keyExtractor));
533 }
534 }