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.
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5 * This code is free software; you can redistribute it and/or modify it
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10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
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14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
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25
26 package java.util;
27
28 import java.io.Serializable;
29 import java.util.function.Function;
30 import java.util.function.ToIntFunction;
31 import java.util.function.ToLongFunction;
32 import java.util.function.ToDoubleFunction;
33 import java.util.Comparators;
34
35 /**
36 * A comparison function, which imposes a <i>total ordering</i> on some
37 * collection of objects. Comparators can be passed to a sort method (such
38 * as {@link Collections#sort(List,Comparator) Collections.sort} or {@link
39 * Arrays#sort(Object[],Comparator) Arrays.sort}) to allow precise control
40 * over the sort order. Comparators can also be used to control the order of
41 * certain data structures (such as {@link SortedSet sorted sets} or {@link
42 * SortedMap sorted maps}), or to provide an ordering for collections of
43 * objects that don't have a {@link Comparable natural ordering}.<p>
44 *
45 * The ordering imposed by a comparator <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 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 }