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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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25
26 package javany.util;
27
28 import java.io.Serializable;
29 import java.util.Objects;
30
31 import javany.util.function.Function;
32
33 /**
34 * A comparison function, which imposes a <i>total ordering</i> on some
35 * collection of objects. Comparators can be passed to a sort method (such
36 * as {@link Collections#sort(List,Comparator) Collections.sort} or {@link
37 * Arrays#sort(Object[],Comparator) Arrays.sort}) to allow precise control
38 * over the sort order. Comparators can also be used to control the order of
39 * certain data structures (such as {@link SortedSet sorted sets} or {@link
40 * SortedMap sorted maps}), or to provide an ordering for collections of
41 * objects that don't have a {@link Comparable natural ordering}.<p>
42 *
43 * The ordering imposed by a comparator <tt>c</tt> on a set of elements
44 * <tt>S</tt> is said to be <i>consistent with equals</i> if and only if
45 * <tt>c.compare(e1, e2)==0</tt> has the same boolean value as
46 * <tt>e1.equals(e2)</tt> for every <tt>e1</tt> and <tt>e2</tt> in
47 * <tt>S</tt>.<p>
48 *
49 * Caution should be exercised when using a comparator capable of imposing an
50 * ordering inconsistent with equals to order a sorted set (or sorted map).
51 * Suppose a sorted set (or sorted map) with an explicit comparator <tt>c</tt>
52 * is used with elements (or keys) drawn from a set <tt>S</tt>. If the
53 * ordering imposed by <tt>c</tt> on <tt>S</tt> is inconsistent with equals,
54 * the sorted set (or sorted map) will behave "strangely." In particular the
55 * sorted set (or sorted map) will violate the general contract for set (or
56 * map), which is defined in terms of <tt>equals</tt>.<p>
57 *
58 * For example, suppose one adds two elements {@code a} and {@code b} such that
59 * {@code (a.equals(b) && c.compare(a, b) != 0)}
60 * to an empty {@code TreeSet} with comparator {@code c}.
61 * The second {@code add} operation will return
62 * true (and the size of the tree set will increase) because {@code a} and
63 * {@code b} are not equivalent from the tree set's perspective, even though
64 * this is contrary to the specification of the
65 * {@link Set#add Set.add} method.<p>
66 *
67 * Note: It is generally a good idea for comparators to also implement
68 * <tt>java.io.Serializable</tt>, as they may be used as ordering methods in
69 * serializable data structures (like {@link TreeSet}, {@link TreeMap}). In
70 * order for the data structure to serialize successfully, the comparator (if
71 * provided) must implement <tt>Serializable</tt>.<p>
72 *
73 * For the mathematically inclined, the <i>relation</i> that defines the
74 * <i>imposed ordering</i> that a given comparator <tt>c</tt> imposes on a
75 * given set of objects <tt>S</tt> is:<pre>
76 * {(x, y) such that c.compare(x, y) <= 0}.
77 * </pre> The <i>quotient</i> for this total order is:<pre>
78 * {(x, y) such that c.compare(x, y) == 0}.
79 * </pre>
80 *
81 * It follows immediately from the contract for <tt>compare</tt> that the
82 * quotient is an <i>equivalence relation</i> on <tt>S</tt>, and that the
83 * imposed ordering is a <i>total order</i> on <tt>S</tt>. When we say that
84 * the ordering imposed by <tt>c</tt> on <tt>S</tt> is <i>consistent with
85 * equals</i>, we mean that the quotient for the ordering is the equivalence
86 * relation defined by the objects' {@link Object#equals(Object)
87 * equals(Object)} method(s):<pre>
88 * {(x, y) such that x.equals(y)}. </pre>
89 *
90 * <p>Unlike {@code Comparable}, a comparator may optionally permit
91 * comparison of null arguments, while maintaining the requirements for
92 * an equivalence relation.
93 *
94 * <p>This interface is a member of the
95 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
96 * Java Collections Framework</a>.
97 *
98 * @param <T> the type of objects that may be compared by this comparator
99 *
100 * @author Josh Bloch
101 * @author Neal Gafter
102 * @see Comparable
103 * @see java.io.Serializable
104 * @since 1.2
105 */
106 @FunctionalInterface
107 public interface Comparator<any T> {
108 /**
109 * Compares its two arguments for order. Returns a negative integer,
110 * zero, or a positive integer as the first argument is less than, equal
111 * to, or greater than the second.<p>
112 *
113 * In the foregoing description, the notation
114 * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical
115 * <i>signum</i> function, which is defined to return one of <tt>-1</tt>,
116 * <tt>0</tt>, or <tt>1</tt> according to whether the value of
117 * <i>expression</i> is negative, zero or positive.<p>
118 *
119 * The implementor must ensure that <tt>sgn(compare(x, y)) ==
120 * -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>. (This
121 * implies that <tt>compare(x, y)</tt> must throw an exception if and only
122 * if <tt>compare(y, x)</tt> throws an exception.)<p>
123 *
124 * The implementor must also ensure that the relation is transitive:
125 * <tt>((compare(x, y)>0) && (compare(y, z)>0))</tt> implies
126 * <tt>compare(x, z)>0</tt>.<p>
127 *
128 * Finally, the implementor must ensure that <tt>compare(x, y)==0</tt>
129 * implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all
130 * <tt>z</tt>.<p>
131 *
132 * It is generally the case, but <i>not</i> strictly required that
133 * <tt>(compare(x, y)==0) == (x.equals(y))</tt>. Generally speaking,
134 * any comparator that violates this condition should clearly indicate
135 * this fact. The recommended language is "Note: this comparator
136 * imposes orderings that are inconsistent with equals."
137 *
138 * @param o1 the first object to be compared.
139 * @param o2 the second object to be compared.
140 * @return a negative integer, zero, or a positive integer as the
141 * first argument is less than, equal to, or greater than the
142 * second.
143 * @throws NullPointerException if an argument is null and this
144 * comparator does not permit null arguments
145 * @throws ClassCastException if the arguments' types prevent them from
146 * being compared by this comparator.
147 */
148 int compare(T o1, T o2);
149
150 /**
151 * Indicates whether some other object is "equal to" this
152 * comparator. This method must obey the general contract of
153 * {@link Object#equals(Object)}. Additionally, this method can return
154 * <tt>true</tt> <i>only</i> if the specified object is also a comparator
155 * and it imposes the same ordering as this comparator. Thus,
156 * <code>comp1.equals(comp2)</code> implies that <tt>sgn(comp1.compare(o1,
157 * o2))==sgn(comp2.compare(o1, o2))</tt> for every object reference
158 * <tt>o1</tt> and <tt>o2</tt>.<p>
159 *
160 * Note that it is <i>always</i> safe <i>not</i> to override
161 * <tt>Object.equals(Object)</tt>. However, overriding this method may,
162 * in some cases, improve performance by allowing programs to determine
163 * that two distinct comparators impose the same order.
164 *
165 * @param obj the reference object with which to compare.
166 * @return <code>true</code> only if the specified object is also
167 * a comparator and it imposes the same ordering as this
168 * comparator.
169 * @see Object#equals(Object)
170 * @see Object#hashCode()
171 */
172 boolean equals(Object obj);
173
174 /**
175 * Returns a comparator that imposes the reverse ordering of this
176 * comparator.
177 *
178 * @return a comparator that imposes the reverse ordering of this
179 * comparator.
180 * @since 1.8
181 */
182 default Comparator<T> reversed() {
183 return new Comparators.ReverseComparator<>(this);
184 }
185
186 /**
187 * Returns a lexicographic-order comparator with another comparator.
188 * If this {@code Comparator} considers two elements equal, i.e.
189 * {@code compare(a, b) == 0}, {@code other} is used to determine the order.
190 *
191 * <p>The returned comparator is serializable if the specified comparator
192 * is also serializable.
193 *
194 * @apiNote
195 * For example, to sort a collection of {@code String} based on the length
196 * and then case-insensitive natural ordering, the comparator can be
197 * composed using following code,
198 *
199 * <pre>{@code
200 * Comparator<String> cmp = Comparator.comparingInt(String::length)
201 * .thenComparing(String.CASE_INSENSITIVE_ORDER);
202 * }</pre>
203 *
204 * @param other the other comparator to be used when this comparator
205 * compares two objects that are equal.
206 * @return a lexicographic-order comparator composed of this and then the
207 * other comparator
208 * @throws NullPointerException if the argument is null.
209 * @since 1.8
210 */
211 default Comparator<T> thenComparing(Comparator<? super T> other) {
212 Objects.requireNonNull(other);
213 return (Comparator<T> & Serializable) (c1, c2) -> {
214 int res = compare(c1, c2);
215 return (res != 0) ? res : other.compare(c1, c2);
216 };
217 }
218
219 /**
220 * Returns a lexicographic-order comparator with a function that
221 * extracts a key to be compared with the given {@code Comparator}.
222 *
223 * @implSpec This default implementation behaves as if {@code
224 * thenComparing(comparing(keyExtractor, cmp))}.
225 *
226 * @param <U> the type of the sort key
227 * @param keyExtractor the function used to extract the sort key
228 * @param keyComparator the {@code Comparator} used to compare the sort key
229 * @return a lexicographic-order comparator composed of this comparator
230 * and then comparing on the key extracted by the keyExtractor function
231 * @throws NullPointerException if either argument is null.
232 * @see #comparing(Function, Comparator)
233 * @see #thenComparing(Comparator)
234 * @since 1.8
235 */
236 default <any U> Comparator<T> thenComparing(
237 Function<? super T, ? extends U> keyExtractor,
238 Comparator<? super U> keyComparator)
239 {
240 return thenComparing(comparing(keyExtractor, keyComparator));
241 }
242
243 /**
244 * Returns a lexicographic-order comparator with a function that
245 * extracts a {@code Comparable} sort key.
246 *
247 * @implSpec This default implementation behaves as if {@code
248 * thenComparing(comparing(keyExtractor))}.
249 *
250 * @param <U> the type of the {@link Comparable} sort key
251 * @param keyExtractor the function used to extract the {@link
252 * Comparable} sort key
253 * @return a lexicographic-order comparator composed of this and then the
254 * {@link Comparable} sort key.
255 * @throws NullPointerException if the argument is null.
256 * @see #comparing(Function)
257 * @see #thenComparing(Comparator)
258 * @since 1.8
259 */
260 default <U extends Comparable<? super U>> Comparator<T> thenComparing(
261 Function<? super T, ? extends U> keyExtractor)
262 {
263 return thenComparing(comparing(keyExtractor));
264 }
265
266 /**
267 * Returns a lexicographic-order comparator with a function that
268 * extracts a {@code int} sort key.
269 *
270 * @implSpec This default implementation behaves as if {@code
271 * thenComparing(comparingInt(keyExtractor))}.
272 *
273 * @param keyExtractor the function used to extract the integer sort key
274 * @return a lexicographic-order comparator composed of this and then the
275 * {@code int} sort key
276 * @throws NullPointerException if the argument is null.
277 * @see #comparingInt(Function)
278 * @see #thenComparing(Comparator)
279 * @since 1.8
280 */
281 default Comparator<T> thenComparingInt(Function<? super T, int> keyExtractor) {
282 return thenComparing(comparingInt(keyExtractor));
283 }
284
285 /**
286 * Returns a lexicographic-order comparator with a function that
287 * extracts a {@code long} sort key.
288 *
289 * @implSpec This default implementation behaves as if {@code
290 * thenComparing(comparingLong(keyExtractor))}.
291 *
292 * @param keyExtractor the function used to extract the long sort key
293 * @return a lexicographic-order comparator composed of this and then the
294 * {@code long} sort key
295 * @throws NullPointerException if the argument is null.
296 * @see #comparingLong(Function)
297 * @see #thenComparing(Comparator)
298 * @since 1.8
299 */
300 default Comparator<T> thenComparingLong(Function<? super T, long> keyExtractor) {
301 return thenComparing(comparingLong(keyExtractor));
302 }
303
304 /**
305 * Returns a lexicographic-order comparator with a function that
306 * extracts a {@code double} sort key.
307 *
308 * @implSpec This default implementation behaves as if {@code
309 * thenComparing(comparingDouble(keyExtractor))}.
310 *
311 * @param keyExtractor the function used to extract the double sort key
312 * @return a lexicographic-order comparator composed of this and then the
313 * {@code double} sort key
314 * @throws NullPointerException if the argument is null.
315 * @see #comparingDouble(Function)
316 * @see #thenComparing(Comparator)
317 * @since 1.8
318 */
319 default Comparator<T> thenComparingDouble(Function<? super T, double> keyExtractor) {
320 return thenComparing(comparingDouble(keyExtractor));
321 }
322
323 /**
324 * Returns a comparator that imposes the reverse of the <em>natural
325 * ordering</em>.
326 *
327 * <p>The returned comparator is serializable and throws {@link
328 * NullPointerException} when comparing {@code null}.
329 *
330 * @param <T> the {@link Comparable} type of element to be compared
331 * @return a comparator that imposes the reverse of the <i>natural
332 * ordering</i> on {@code Comparable} objects.
333 * @see Comparable
334 * @since 1.8
335 */
336 public static <any T> Comparator<T> reverseOrder() {
337 return Comparators.<T>naturalOrder().reversed();
338 }
339
340 /**
341 * Returns a comparator that compares {@link Comparable} objects in natural
342 * order.
343 *
344 * <p>The returned comparator is serializable and throws {@link
345 * NullPointerException} when comparing {@code null}.
346 *
347 * @param <T> the {@link Comparable} type of element to be compared
348 * @return a comparator that imposes the <i>natural ordering</i> on {@code
349 * Comparable} objects.
350 * @see Comparable
351 * @since 1.8
352 */
353 @SuppressWarnings("unchecked")
354 public static <any T> Comparator<T> naturalOrder() {
355 return Comparators.naturalOrder();
356 }
357
358 /**
359 * Returns a null-friendly comparator that considers {@code null} to be
360 * less than non-null. When both are {@code null}, they are considered
361 * equal. If both are non-null, the specified {@code Comparator} is used
362 * to determine the order. If the specified comparator is {@code null},
363 * then the returned comparator considers all non-null values to be equal.
364 *
365 * <p>The returned comparator is serializable if the specified comparator
366 * is serializable.
367 *
368 * @param <T> the type of the elements to be compared
369 * @param comparator a {@code Comparator} for comparing non-null values
370 * @return a comparator that considers {@code null} to be less than
371 * non-null, and compares non-null objects with the supplied
372 * {@code Comparator}.
373 * @since 1.8
374 */
375 public static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator) {
376 return new Comparators.NullComparator<>(true, comparator);
377 }
378
379 /**
380 * Returns a null-friendly comparator that considers {@code null} to be
381 * greater than non-null. When both are {@code null}, they are considered
382 * equal. If both are non-null, the specified {@code Comparator} is used
383 * to determine the order. If the specified comparator is {@code null},
384 * then the returned comparator considers all non-null values to be equal.
385 *
386 * <p>The returned comparator is serializable if the specified comparator
387 * is serializable.
388 *
389 * @param <T> the type of the elements to be compared
390 * @param comparator a {@code Comparator} for comparing non-null values
391 * @return a comparator that considers {@code null} to be greater than
392 * non-null, and compares non-null objects with the supplied
393 * {@code Comparator}.
394 * @since 1.8
395 */
396 public static <T> Comparator<T> nullsLast(Comparator<? super T> comparator) {
397 return new Comparators.NullComparator<>(false, comparator);
398 }
399
400 /**
401 * Accepts a function that extracts a sort key from a type {@code T}, and
402 * returns a {@code Comparator<T>} that compares by that sort key using
403 * the specified {@link Comparator}.
404 *
405 * <p>The returned comparator is serializable if the specified function
406 * and comparator are both serializable.
407 *
408 * @apiNote
409 * For example, to obtain a {@code Comparator} that compares {@code
410 * Person} objects by their last name ignoring case differences,
411 *
412 * <pre>{@code
413 * Comparator<Person> cmp = Comparator.comparing(
414 * Person::getLastName,
415 * String.CASE_INSENSITIVE_ORDER);
416 * }</pre>
417 *
418 * @param <T> the type of element to be compared
419 * @param <U> the type of the sort key
420 * @param keyExtractor the function used to extract the sort key
421 * @param keyComparator the {@code Comparator} used to compare the sort key
422 * @return a comparator that compares by an extracted key using the
423 * specified {@code Comparator}
424 * @throws NullPointerException if either argument is null
425 * @since 1.8
426 */
427 public static <any T, any U> Comparator<T> comparing(
428 Function<? super T, ? extends U> keyExtractor,
429 Comparator<? super U> keyComparator)
430 {
431 Objects.requireNonNull(keyExtractor);
432 Objects.requireNonNull(keyComparator);
433 return (Comparator<T> & Serializable)
434 (c1, c2) -> keyComparator.compare(keyExtractor.apply(c1),
435 keyExtractor.apply(c2));
436 }
437
438 /**
439 * Accepts a function that extracts a {@link java.lang.Comparable
440 * Comparable} sort key from a type {@code T}, and returns a {@code
441 * Comparator<T>} that compares by that sort key.
442 *
443 * <p>The returned comparator is serializable if the specified function
444 * is also serializable.
445 *
446 * @apiNote
447 * For example, to obtain a {@code Comparator} that compares {@code
448 * Person} objects by their last name,
449 *
450 * <pre>{@code
451 * Comparator<Person> byLastName = Comparator.comparing(Person::getLastName);
452 * }</pre>
453 *
454 * @param <T> the type of element to be compared
455 * @param <U> the type of the {@code Comparable} sort key
456 * @param keyExtractor the function used to extract the {@link
457 * Comparable} sort key
458 * @return a comparator that compares by an extracted key
459 * @throws NullPointerException if the argument is null
460 * @since 1.8
461 */
462 public static <any T, U extends Comparable<? super U>> Comparator<T> comparing(
463 Function<? super T, ? extends U> keyExtractor)
464 {
465 Objects.requireNonNull(keyExtractor);
466 return (Comparator<T> & Serializable)
467 (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
468 }
469
470 /**
471 * Accepts a function that extracts an {@code int} sort key from a type
472 * {@code T}, and returns a {@code Comparator<T>} that compares by that
473 * sort key.
474 *
475 * <p>The returned comparator is serializable if the specified function
476 * is also serializable.
477 *
478 * @param <T> the type of element to be compared
479 * @param keyExtractor the function used to extract the integer sort key
480 * @return a comparator that compares by an extracted key
481 * @see #comparing(Function)
482 * @throws NullPointerException if the argument is null
483 * @since 1.8
484 */
485 public static <any T> Comparator<T> comparingInt(Function<? super T, int> keyExtractor) {
486 Objects.requireNonNull(keyExtractor);
487 return (Comparator<T> & Serializable)
488 (c1, c2) -> Integer.compare(keyExtractor.apply(c1), keyExtractor.apply(c2));
489 }
490
491 /**
492 * Accepts a function that extracts a {@code long} sort key from a type
493 * {@code T}, and returns a {@code Comparator<T>} that compares by that
494 * sort key.
495 *
496 * <p>The returned comparator is serializable if the specified function is
497 * also serializable.
498 *
499 * @param <T> the type of element to be compared
500 * @param keyExtractor the function used to extract the long sort key
501 * @return a comparator that compares by an extracted key
502 * @see #comparing(Function)
503 * @throws NullPointerException if the argument is null
504 * @since 1.8
505 */
506 public static <any T> Comparator<T> comparingLong(Function<? super T, long> keyExtractor) {
507 Objects.requireNonNull(keyExtractor);
508 return (Comparator<T> & Serializable)
509 (c1, c2) -> Long.compare(keyExtractor.apply(c1), keyExtractor.apply(c2));
510 }
511
512 /**
513 * Accepts a function that extracts a {@code double} sort key from a type
514 * {@code T}, and returns a {@code Comparator<T>} that compares by that
515 * sort key.
516 *
517 * <p>The returned comparator is serializable if the specified function
518 * is also serializable.
519 *
520 * @param <T> the type of element to be compared
521 * @param keyExtractor the function used to extract the double sort key
522 * @return a comparator that compares by an extracted key
523 * @see #comparing(Function)
524 * @throws NullPointerException if the argument is null
525 * @since 1.8
526 */
527 public static <any T> Comparator<T> comparingDouble(Function<? super T, double> keyExtractor) {
528 Objects.requireNonNull(keyExtractor);
529 return (Comparator<T> & Serializable)
530 (c1, c2) -> Double.compare(keyExtractor.apply(c1), keyExtractor.apply(c2));
531 }
532 }