1 /*
2 * Copyright (c) 1997, 2011, 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.util.function.BiConsumer;
29 import java.util.function.BiFunction;
30 import java.util.function.Function;
31
32 /**
33 * An object that maps keys to values. A map cannot contain duplicate keys;
34 * each key can map to at most one value.
35 *
36 * <p>This interface takes the place of the <tt>Dictionary</tt> class, which
37 * was a totally abstract class rather than an interface.
38 *
39 * <p>The <tt>Map</tt> interface provides three <i>collection views</i>, which
40 * allow a map's contents to be viewed as a set of keys, collection of values,
41 * or set of key-value mappings. The <i>order</i> of a map is defined as
42 * the order in which the iterators on the map's collection views return their
43 * elements. Some map implementations, like the <tt>TreeMap</tt> class, make
44 * specific guarantees as to their order; others, like the <tt>HashMap</tt>
45 * class, do not.
46 *
47 * <p>Note: great care must be exercised if mutable objects are used as map
48 * keys. The behavior of a map is not specified if the value of an object is
49 * changed in a manner that affects <tt>equals</tt> comparisons while the
50 * object is a key in the map. A special case of this prohibition is that it
51 * is not permissible for a map to contain itself as a key. While it is
52 * permissible for a map to contain itself as a value, extreme caution is
53 * advised: the <tt>equals</tt> and <tt>hashCode</tt> methods are no longer
54 * well defined on such a map.
55 *
56 * <p>All general-purpose map implementation classes should provide two
57 * "standard" constructors: a void (no arguments) constructor which creates an
58 * empty map, and a constructor with a single argument of type <tt>Map</tt>,
59 * which creates a new map with the same key-value mappings as its argument.
60 * In effect, the latter constructor allows the user to copy any map,
61 * producing an equivalent map of the desired class. There is no way to
62 * enforce this recommendation (as interfaces cannot contain constructors) but
63 * all of the general-purpose map implementations in the JDK comply.
64 *
65 * <p>The "destructive" methods contained in this interface, that is, the
66 * methods that modify the map on which they operate, are specified to throw
67 * <tt>UnsupportedOperationException</tt> if this map does not support the
68 * operation. If this is the case, these methods may, but are not required
69 * to, throw an <tt>UnsupportedOperationException</tt> if the invocation would
70 * have no effect on the map. For example, invoking the {@link #putAll(Map)}
71 * method on an unmodifiable map may, but is not required to, throw the
72 * exception if the map whose mappings are to be "superimposed" is empty.
73 *
74 * <p>Some map implementations have restrictions on the keys and values they
75 * may contain. For example, some implementations prohibit null keys and
76 * values, and some have restrictions on the types of their keys. Attempting
77 * to insert an ineligible key or value throws an unchecked exception,
78 * typically <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.
79 * Attempting to query the presence of an ineligible key or value may throw an
80 * exception, or it may simply return false; some implementations will exhibit
81 * the former behavior and some will exhibit the latter. More generally,
82 * attempting an operation on an ineligible key or value whose completion
83 * would not result in the insertion of an ineligible element into the map may
84 * throw an exception or it may succeed, at the option of the implementation.
85 * Such exceptions are marked as "optional" in the specification for this
86 * interface.
87 *
88 * <p>This interface is a member of the
89 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
90 * Java Collections Framework</a>.
91 *
92 * <p>Many methods in Collections Framework interfaces are defined
93 * in terms of the {@link Object#equals(Object) equals} method. For
94 * example, the specification for the {@link #containsKey(Object)
95 * containsKey(Object key)} method says: "returns <tt>true</tt> if and
96 * only if this map contains a mapping for a key <tt>k</tt> such that
97 * <tt>(key==null ? k==null : key.equals(k))</tt>." This specification should
98 * <i>not</i> be construed to imply that invoking <tt>Map.containsKey</tt>
99 * with a non-null argument <tt>key</tt> will cause <tt>key.equals(k)</tt> to
100 * be invoked for any key <tt>k</tt>. Implementations are free to
101 * implement optimizations whereby the <tt>equals</tt> invocation is avoided,
102 * for example, by first comparing the hash codes of the two keys. (The
103 * {@link Object#hashCode()} specification guarantees that two objects with
104 * unequal hash codes cannot be equal.) More generally, implementations of
105 * the various Collections Framework interfaces are free to take advantage of
106 * the specified behavior of underlying {@link Object} methods wherever the
107 * implementor deems it appropriate.
108 *
109 * @param <K> the type of keys maintained by this map
110 * @param <V> the type of mapped values
111 *
112 * @author Josh Bloch
113 * @see HashMap
114 * @see TreeMap
115 * @see Hashtable
116 * @see SortedMap
117 * @see Collection
118 * @see Set
119 * @since 1.2
120 */
121 public interface Map<K,V> {
122
123 // Query Operations
124
125 /**
126 * Returns the number of key-value mappings in this map. If the
127 * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
128 * <tt>Integer.MAX_VALUE</tt>.
129 *
130 * @return the number of key-value mappings in this map
131 */
132 int size();
133
134 /**
135 * Returns <tt>true</tt> if this map contains no key-value mappings.
136 *
137 * @return <tt>true</tt> if this map contains no key-value mappings
138 */
139 boolean isEmpty();
140
141 /**
142 * Returns <tt>true</tt> if this map contains a mapping for the specified
143 * key. More formally, returns <tt>true</tt> if and only if
144 * this map contains a mapping for a key <tt>k</tt> such that
145 * <tt>(key==null ? k==null : key.equals(k))</tt>. (There can be
146 * at most one such mapping.)
147 *
148 * @param key key whose presence in this map is to be tested
149 * @return <tt>true</tt> if this map contains a mapping for the specified
150 * key
151 * @throws ClassCastException if the key is of an inappropriate type for
152 * this map
153 * (<a href="Collection.html#optional-restrictions">optional</a>)
154 * @throws NullPointerException if the specified key is null and this map
155 * does not permit null keys
156 * (<a href="Collection.html#optional-restrictions">optional</a>)
157 */
158 boolean containsKey(Object key);
159
160 /**
161 * Returns <tt>true</tt> if this map maps one or more keys to the
162 * specified value. More formally, returns <tt>true</tt> if and only if
163 * this map contains at least one mapping to a value <tt>v</tt> such that
164 * <tt>(value==null ? v==null : value.equals(v))</tt>. This operation
165 * will probably require time linear in the map size for most
166 * implementations of the <tt>Map</tt> interface.
167 *
168 * @param value value whose presence in this map is to be tested
169 * @return <tt>true</tt> if this map maps one or more keys to the
170 * specified value
171 * @throws ClassCastException if the value is of an inappropriate type for
172 * this map
173 * (<a href="Collection.html#optional-restrictions">optional</a>)
174 * @throws NullPointerException if the specified value is null and this
175 * map does not permit null values
176 * (<a href="Collection.html#optional-restrictions">optional</a>)
177 */
178 boolean containsValue(Object value);
179
180 /**
181 * Returns the value to which the specified key is mapped,
182 * or {@code null} if this map contains no mapping for the key.
183 *
184 * <p>More formally, if this map contains a mapping from a key
185 * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
186 * key.equals(k))}, then this method returns {@code v}; otherwise
187 * it returns {@code null}. (There can be at most one such mapping.)
188 *
189 * <p>If this map permits null values, then a return value of
190 * {@code null} does not <i>necessarily</i> indicate that the map
191 * contains no mapping for the key; it's also possible that the map
192 * explicitly maps the key to {@code null}. The {@link #containsKey
193 * containsKey} operation may be used to distinguish these two cases.
194 *
195 * @param key the key whose associated value is to be returned
196 * @return the value to which the specified key is mapped, or
197 * {@code null} if this map contains no mapping for the key
198 * @throws ClassCastException if the key is of an inappropriate type for
199 * this map
200 * (<a href="Collection.html#optional-restrictions">optional</a>)
201 * @throws NullPointerException if the specified key is null and this map
202 * does not permit null keys
203 * (<a href="Collection.html#optional-restrictions">optional</a>)
204 */
205 V get(Object key);
206
207 // Modification Operations
208
209 /**
210 * Associates the specified value with the specified key in this map
211 * (optional operation). If the map previously contained a mapping for
212 * the key, the old value is replaced by the specified value. (A map
213 * <tt>m</tt> is said to contain a mapping for a key <tt>k</tt> if and only
214 * if {@link #containsKey(Object) m.containsKey(k)} would return
215 * <tt>true</tt>.)
216 *
217 * @param key key with which the specified value is to be associated
218 * @param value value to be associated with the specified key
219 * @return the previous value associated with <tt>key</tt>, or
220 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
221 * (A <tt>null</tt> return can also indicate that the map
222 * previously associated <tt>null</tt> with <tt>key</tt>,
223 * if the implementation supports <tt>null</tt> values.)
224 * @throws UnsupportedOperationException if the <tt>put</tt> operation
225 * is not supported by this map
226 * @throws ClassCastException if the class of the specified key or value
227 * prevents it from being stored in this map
228 * @throws NullPointerException if the specified key or value is null
229 * and this map does not permit null keys or values
230 * @throws IllegalArgumentException if some property of the specified key
231 * or value prevents it from being stored in this map
232 */
233 V put(K key, V value);
234
235 /**
236 * Removes the mapping for a key from this map if it is present
237 * (optional operation). More formally, if this map contains a mapping
238 * from key <tt>k</tt> to value <tt>v</tt> such that
239 * <code>(key==null ? k==null : key.equals(k))</code>, that mapping
240 * is removed. (The map can contain at most one such mapping.)
241 *
242 * <p>Returns the value to which this map previously associated the key,
243 * or <tt>null</tt> if the map contained no mapping for the key.
244 *
245 * <p>If this map permits null values, then a return value of
246 * <tt>null</tt> does not <i>necessarily</i> indicate that the map
247 * contained no mapping for the key; it's also possible that the map
248 * explicitly mapped the key to <tt>null</tt>.
249 *
250 * <p>The map will not contain a mapping for the specified key once the
251 * call returns.
252 *
253 * @param key key whose mapping is to be removed from the map
254 * @return the previous value associated with <tt>key</tt>, or
255 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
256 * @throws UnsupportedOperationException if the <tt>remove</tt> operation
257 * is not supported by this map
258 * @throws ClassCastException if the key is of an inappropriate type for
259 * this map
260 * (<a href="Collection.html#optional-restrictions">optional</a>)
261 * @throws NullPointerException if the specified key is null and this
262 * map does not permit null keys
263 * (<a href="Collection.html#optional-restrictions">optional</a>)
264 */
265 V remove(Object key);
266
267
268 // Bulk Operations
269
270 /**
271 * Copies all of the mappings from the specified map to this map
272 * (optional operation). The effect of this call is equivalent to that
273 * of calling {@link #put(Object,Object) put(k, v)} on this map once
274 * for each mapping from key <tt>k</tt> to value <tt>v</tt> in the
275 * specified map. The behavior of this operation is undefined if the
276 * specified map is modified while the operation is in progress.
277 *
278 * @param m mappings to be stored in this map
279 * @throws UnsupportedOperationException if the <tt>putAll</tt> operation
280 * is not supported by this map
281 * @throws ClassCastException if the class of a key or value in the
282 * specified map prevents it from being stored in this map
283 * @throws NullPointerException if the specified map is null, or if
284 * this map does not permit null keys or values, and the
285 * specified map contains null keys or values
286 * @throws IllegalArgumentException if some property of a key or value in
287 * the specified map prevents it from being stored in this map
288 */
289 void putAll(Map<? extends K, ? extends V> m);
290
291 /**
292 * Removes all of the mappings from this map (optional operation).
293 * The map will be empty after this call returns.
294 *
295 * @throws UnsupportedOperationException if the <tt>clear</tt> operation
296 * is not supported by this map
297 */
298 void clear();
299
300
301 // Views
302
303 /**
304 * Returns a {@link Set} view of the keys contained in this map.
305 * The set is backed by the map, so changes to the map are
306 * reflected in the set, and vice-versa. If the map is modified
307 * while an iteration over the set is in progress (except through
308 * the iterator's own <tt>remove</tt> operation), the results of
309 * the iteration are undefined. The set supports element removal,
310 * which removes the corresponding mapping from the map, via the
311 * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
312 * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
313 * operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
314 * operations.
315 *
316 * @return a set view of the keys contained in this map
317 */
318 Set<K> keySet();
319
320 /**
321 * Returns a {@link Collection} view of the values contained in this map.
322 * The collection is backed by the map, so changes to the map are
323 * reflected in the collection, and vice-versa. If the map is
324 * modified while an iteration over the collection is in progress
325 * (except through the iterator's own <tt>remove</tt> operation),
326 * the results of the iteration are undefined. The collection
327 * supports element removal, which removes the corresponding
328 * mapping from the map, via the <tt>Iterator.remove</tt>,
329 * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
330 * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
331 * support the <tt>add</tt> or <tt>addAll</tt> operations.
332 *
333 * @return a collection view of the values contained in this map
334 */
335 Collection<V> values();
336
337 /**
338 * Returns a {@link Set} view of the mappings contained in this map.
339 * The set is backed by the map, so changes to the map are
340 * reflected in the set, and vice-versa. If the map is modified
341 * while an iteration over the set is in progress (except through
342 * the iterator's own <tt>remove</tt> operation, or through the
343 * <tt>setValue</tt> operation on a map entry returned by the
344 * iterator) the results of the iteration are undefined. The set
345 * supports element removal, which removes the corresponding
346 * mapping from the map, via the <tt>Iterator.remove</tt>,
347 * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
348 * <tt>clear</tt> operations. It does not support the
349 * <tt>add</tt> or <tt>addAll</tt> operations.
350 *
351 * @return a set view of the mappings contained in this map
352 */
353 Set<Map.Entry<K, V>> entrySet();
354
355 /**
356 * A map entry (key-value pair). The <tt>Map.entrySet</tt> method returns
357 * a collection-view of the map, whose elements are of this class. The
358 * <i>only</i> way to obtain a reference to a map entry is from the
359 * iterator of this collection-view. These <tt>Map.Entry</tt> objects are
360 * valid <i>only</i> for the duration of the iteration; more formally,
361 * the behavior of a map entry is undefined if the backing map has been
362 * modified after the entry was returned by the iterator, except through
363 * the <tt>setValue</tt> operation on the map entry.
364 *
365 * @see Map#entrySet()
366 * @since 1.2
367 */
368 interface Entry<K,V> {
369 /**
370 * Returns the key corresponding to this entry.
371 *
372 * @return the key corresponding to this entry
373 * @throws IllegalStateException implementations may, but are not
374 * required to, throw this exception if the entry has been
375 * removed from the backing map.
376 */
377 K getKey();
378
379 /**
380 * Returns the value corresponding to this entry. If the mapping
381 * has been removed from the backing map (by the iterator's
382 * <tt>remove</tt> operation), the results of this call are undefined.
383 *
384 * @return the value corresponding to this entry
385 * @throws IllegalStateException implementations may, but are not
386 * required to, throw this exception if the entry has been
387 * removed from the backing map.
388 */
389 V getValue();
390
391 /**
392 * Replaces the value corresponding to this entry with the specified
393 * value (optional operation). (Writes through to the map.) The
394 * behavior of this call is undefined if the mapping has already been
395 * removed from the map (by the iterator's <tt>remove</tt> operation).
396 *
397 * @param value new value to be stored in this entry
398 * @return old value corresponding to the entry
399 * @throws UnsupportedOperationException if the <tt>put</tt> operation
400 * is not supported by the backing map
401 * @throws ClassCastException if the class of the specified value
402 * prevents it from being stored in the backing map
403 * @throws NullPointerException if the backing map does not permit
404 * null values, and the specified value is null
405 * @throws IllegalArgumentException if some property of this value
406 * prevents it from being stored in the backing map
407 * @throws IllegalStateException implementations may, but are not
408 * required to, throw this exception if the entry has been
409 * removed from the backing map.
410 */
411 V setValue(V value);
412
413 /**
414 * Compares the specified object with this entry for equality.
415 * Returns <tt>true</tt> if the given object is also a map entry and
416 * the two entries represent the same mapping. More formally, two
417 * entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping
418 * if<pre>
419 * (e1.getKey()==null ?
420 * e2.getKey()==null : e1.getKey().equals(e2.getKey())) &&
421 * (e1.getValue()==null ?
422 * e2.getValue()==null : e1.getValue().equals(e2.getValue()))
423 * </pre>
424 * This ensures that the <tt>equals</tt> method works properly across
425 * different implementations of the <tt>Map.Entry</tt> interface.
426 *
427 * @param o object to be compared for equality with this map entry
428 * @return <tt>true</tt> if the specified object is equal to this map
429 * entry
430 */
431 boolean equals(Object o);
432
433 /**
434 * Returns the hash code value for this map entry. The hash code
435 * of a map entry <tt>e</tt> is defined to be: <pre>
436 * (e.getKey()==null ? 0 : e.getKey().hashCode()) ^
437 * (e.getValue()==null ? 0 : e.getValue().hashCode())
438 * </pre>
439 * This ensures that <tt>e1.equals(e2)</tt> implies that
440 * <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries
441 * <tt>e1</tt> and <tt>e2</tt>, as required by the general
442 * contract of <tt>Object.hashCode</tt>.
443 *
444 * @return the hash code value for this map entry
445 * @see Object#hashCode()
446 * @see Object#equals(Object)
447 * @see #equals(Object)
448 */
449 int hashCode();
450 }
451
452 // Comparison and hashing
453
454 /**
455 * Compares the specified object with this map for equality. Returns
456 * <tt>true</tt> if the given object is also a map and the two maps
457 * represent the same mappings. More formally, two maps <tt>m1</tt> and
458 * <tt>m2</tt> represent the same mappings if
459 * <tt>m1.entrySet().equals(m2.entrySet())</tt>. This ensures that the
460 * <tt>equals</tt> method works properly across different implementations
461 * of the <tt>Map</tt> interface.
462 *
463 * @param o object to be compared for equality with this map
464 * @return <tt>true</tt> if the specified object is equal to this map
465 */
466 boolean equals(Object o);
467
468 /**
469 * Returns the hash code value for this map. The hash code of a map is
470 * defined to be the sum of the hash codes of each entry in the map's
471 * <tt>entrySet()</tt> view. This ensures that <tt>m1.equals(m2)</tt>
472 * implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two maps
473 * <tt>m1</tt> and <tt>m2</tt>, as required by the general contract of
474 * {@link Object#hashCode}.
475 *
476 * @return the hash code value for this map
477 * @see Map.Entry#hashCode()
478 * @see Object#equals(Object)
479 * @see #equals(Object)
480 */
481 int hashCode();
482
483 // Defaultable methods
484
485 /**
486 * Execute the specified {@code BiConsumer} with the key and value of
487 * each entry in this map.
488 *
489 * @param block the {@code BiConsumer} to which entries will be applied
490 */
491 default void forEach(BiConsumer<? super K, ? super V> block) {
492 Objects.requireNonNull(block);
493 for (Map.Entry<K, V> entry : entrySet()) {
494 block.accept(entry.getKey(), entry.getValue());
495 }
496 }
497
498 /**
499 * Apply the specified function to each entry in this map, replacing
500 * each entry's value with the result of calling the function's
501 * {@link BiFunction#apply(Object, Object) BiFunction.apply(K key, V, value)}
502 * method with the current entry's key and value.
503 *
504 * @param function the function to apply to each entry
505 */
506 default void replaceAll(BiFunction<K, V, V> function) {
507 Objects.requireNonNull(function);
508 for (Map.Entry<K, V> entry : entrySet()) {
509 entry.setValue(function.apply(entry.getKey(), entry.getValue()));
510 }
511 }
512
513 /**
514 * If the specified key is not already associated with a value,
515 * associates it with the given value and returns {@code null},
516 * else returns the current value.
517 *
518 * <p>The default implementation is equivalent to, for this {@code
519 * map}:
520 *
521 * <pre> {@code
522 * if (!map.containsKey(key))
523 * return map.put(key, value);
524 * else
525 * return map.get(key);}</pre>
526 *
527 * <p>The default implementation makes no guarantees about
528 * synchronization or atomicity properties of this method. Any
529 * class overriding this method must specify its concurrency
530 * properties. In particular, all implementations of
531 * subinterface {@link java.util.concurrent.ConcurrentMap}
532 * must ensure that this operation is performed atomically.
533 *
534 * @param key key with which the specified value is to be associated
535 * @param value value to be associated with the specified key
536 * @return the previous value associated with the specified key, or
537 * <tt>null</tt> if there was no mapping for the key.
538 * (A <tt>null</tt> return can also indicate that the map
539 * previously associated <tt>null</tt> with the key,
540 * if the implementation supports null values.)
541 * @throws UnsupportedOperationException if the <tt>put</tt> operation
542 * is not supported by this map
543 * @throws ClassCastException if the class of the specified key or value
544 * prevents it from being stored in this map
545 * @throws NullPointerException if the specified key or value is null,
546 * and this map does not permit null keys or values
547 * @throws IllegalArgumentException if some property of the specified key
548 * or value prevents it from being stored in this map
549 */
550 default V putIfAbsent(K key, V value) {
551 return containsKey(key) ? get(key) : put(key, value);
552 }
553
554 /**
555 * Removes the entry for the specified key only if it is currently
556 * mapped to the specified value.
557 *
558 * <p>The default implementation is equivalent to, for this {@code map}:
559 *
560 * <pre> {@code
561 * if (map.containsKey(key) && map.get(key).equals(value)) {
562 * map.remove(key);
563 * return true;
564 * } else
565 * return false;}</pre>
566 *
567 * <p>The default implementation makes no guarantees about
568 * synchronization or atomicity properties of this method. Any
569 * class overriding this method must specify its concurrency
570 * properties. In particular, all implementations of
571 * subinterface {@link java.util.concurrent.ConcurrentMap}
572 * must ensure that this operation is performed atomically.
573 *
574 * @param key key with which the specified value is associated
575 * @param value value expected to be associated with the specified key
576 * @return <tt>true</tt> if the value was removed
577 * @throws UnsupportedOperationException if the <tt>remove</tt> operation
578 * is not supported by this map
579 * @throws ClassCastException if the key or value is of an inappropriate
580 * type for this map
581 * (<a href="Collection.html#optional-restrictions">optional</a>)
582 * @throws NullPointerException if the specified key or value is null,
583 * and this map does not permit null keys or values
584 * (<a href="Collection.html#optional-restrictions">optional</a>)
585 */
586 default boolean remove(Object key, Object value) {
587 if (!containsKey(key) || !get(key).equals(value))
588 return false;
589 remove(key);
590 return true;
591 }
592
593 /**
594 * Replaces the entry for the specified key only if currently
595 * mapped to the specified value.
596 *
597 * <p>The default implementation is equivalent to, for this {@code map}:
598 *
599 * <pre> {@code
600 * if (map.containsKey(key) && map.get(key).equals(oldValue)) {
601 * map.put(key, newValue);
602 * return true;
603 * } else
604 * return false;}</pre>
605 *
606 * <p>The default implementation makes no guarantees about
607 * synchronization or atomicity properties of this method. Any
608 * class overriding this method must specify its concurrency
609 * properties. In particular, all implementations of
610 * subinterface {@link java.util.concurrent.ConcurrentMap}
611 * must ensure that this operation is performed atomically.
612 *
613 * @param key key with which the specified value is associated
614 * @param oldValue value expected to be associated with the specified key
615 * @param newValue value to be associated with the specified key
616 * @return <tt>true</tt> if the value was replaced
617 * @throws UnsupportedOperationException if the <tt>put</tt> operation
618 * is not supported by this map
619 * @throws ClassCastException if the class of a specified key or value
620 * prevents it from being stored in this map
621 * @throws NullPointerException if a specified key or value is null,
622 * and this map does not permit null keys or values
623 * @throws IllegalArgumentException if some property of a specified key
624 * or value prevents it from being stored in this map
625 */
626 default boolean replace(K key, V oldValue, V newValue) {
627 if (!containsKey(key) || !get(key).equals(oldValue))
628 return false;
629 put(key, newValue);
630 return true;
631 }
632
633 /**
634 * Replaces the entry for the specified key only if it is
635 * currently mapped to some value.
636 *
637 * <p>The default implementation is equivalent to, for this {@code map}:
638 *
639 * <pre> {@code
640 * if (map.containsKey(key)) {
641 * return map.put(key, value);
642 * } else
643 * return null;}</pre>
644 *
645 * <p>The default implementation makes no guarantees about
646 * synchronization or atomicity properties of this method. Any
647 * class overriding this method must specify its concurrency
648 * properties. In particular, all implementations of
649 * subinterface {@link java.util.concurrent.ConcurrentMap}
650 * must ensure that this operation is performed atomically.
651 *
652 * @param key key with which the specified value is associated
653 * @param value value to be associated with the specified key
654 * @return the previous value associated with the specified key, or
655 * <tt>null</tt> if there was no mapping for the key.
656 * (A <tt>null</tt> return can also indicate that the map
657 * previously associated <tt>null</tt> with the key,
658 * if the implementation supports null values.)
659 * @throws UnsupportedOperationException if the <tt>put</tt> operation
660 * is not supported by this map
661 * @throws ClassCastException if the class of the specified key or value
662 * prevents it from being stored in this map
663 * @throws NullPointerException if the specified key or value is null,
664 * and this map does not permit null keys or values
665 * @throws IllegalArgumentException if some property of the specified key
666 * or value prevents it from being stored in this map
667 */
668 default V replace(K key, V value) {
669 return containsKey(key) ? put(key, value) : null;
670 }
671
672 /**
673 * If the specified key is not already associated with a value (or
674 * is mapped to {@code null}), attempts to compute its value using
675 * the given mapping function and enters it into this map unless
676 * {@code null}.
677 *
678 * <p>The default implementation is equivalent to the following
679 * steps for this {@code map}, then returning the current value or
680 * {@code null} if now absent:
681 *
682 * <pre> {@code
683 * if (map.get(key) == null) {
684 * V newValue = mappingFunction.apply(key);
685 * if (newValue != null)
686 * map.putIfAbsent(key, newValue);
687 * }}</pre>
688 *
689 * If the function returns {@code null} no mapping is recorded. If
690 * the function itself throws an (unchecked) exception, the
691 * exception is rethrown, and no mapping is recorded. The most
692 * common usage is to construct a new object serving as an initial
693 * mapped value or memoized result, as in:
694 *
695 * <pre> {@code
696 * map.computeIfAbsent(key, k -> new Value(f(k)));}</pre>
697 *
698 * <p>The default implementation makes no guarantees about
699 * synchronization or atomicity properties of this method or the
700 * application of the mapping function. Any class overriding this
701 * method must specify its concurrency properties. In particular,
702 * all implementations of subinterface {@link
703 * java.util.concurrent.ConcurrentMap} must document whether the
704 * function is applied once atomically only if the value is not
705 * present. Any class that permits null values must document
706 * whether and how this method distinguishes absence from null
707 * mappings.
708 *
709 * @param key key with which the specified value is to be associated
710 * @param mappingFunction the function to compute a value
711 * @return the current (existing or computed) value associated with
712 * the specified key, or null if the computed value is null
713 * @throws NullPointerException if the specified key is null and
714 * this map does not support null keys, or the
715 * mappingFunction is null
716 * @throws UnsupportedOperationException if the <tt>put</tt> operation
717 * is not supported by this map
718 * @throws ClassCastException if the class of the specified key or value
719 * prevents it from being stored in this map
720 * @throws RuntimeException or Error if the mappingFunction does so,
721 * in which case the mapping is left unestablished
722 */
723 default V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
724 V v, newValue;
725 return ((v = get(key)) == null &&
726 (newValue = mappingFunction.apply(key)) != null &&
727 (v = putIfAbsent(key, newValue)) == null) ? newValue : v;
728 }
729
730 /**
731 * If the value for the specified key is present and non-null,
732 * attempts to compute a new mapping given the key and its current
733 * mapped value.
734 *
735 * <p>The default implementation is equivalent to performing the
736 * following steps for this {@code map}, then returning the
737 * current value or {@code null} if now absent:
738 *
739 * <pre> {@code
740 * if (map.get(key) != null) {
741 * V oldValue = map.get(key);
742 * V newValue = remappingFunction.apply(key, oldValue);
743 * if (newValue != null)
744 * map.replace(key, oldValue, newValue);
745 * else
746 * map.remove(key, oldValue);
747 * }}</pre>
748 *
749 * In concurrent contexts, the default implementation may retry
750 * these steps when multiple threads attempt updates. If the
751 * function returns {@code null}, the mapping is removed (or
752 * remains absent if initially absent). If the function itself
753 * throws an (unchecked) exception, the exception is rethrown, and
754 * the current mapping is left unchanged.
755 *
756 * <p>The default implementation makes no guarantees about
757 * synchronization or atomicity properties of this method or the
758 * application of the remapping function. Any class overriding
759 * this method must specify its concurrency properties. In
760 * particular, all implementations of subinterface {@link
761 * java.util.concurrent.ConcurrentMap} must document whether the
762 * function is applied once atomically only if the value is
763 * present. Any class that permits null values must document
764 * whether and how this method distinguishes absence from null
765 * mappings.
766 *
767 * @param key key with which the specified value is to be associated
768 * @param remappingFunction the function to compute a value
769 * @return the new value associated with the specified key, or null if none
770 * @throws NullPointerException if the specified key is null and
771 * this map does not support null keys, or the
772 * remappingFunction is null
773 * @throws UnsupportedOperationException if the <tt>put</tt> operation
774 * is not supported by this map
775 * @throws ClassCastException if the class of the specified key or value
776 * prevents it from being stored in this map
777 * @throws RuntimeException or Error if the remappingFunction does so,
778 * in which case the mapping is unchanged
779 */
780 default V computeIfPresent(K key,
781 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
782 V v;
783 while ((v = get(key)) != null) {
784 V newValue = remappingFunction.apply(key, v);
785 if (newValue != null) {
786 if (replace(key, v, newValue))
787 return newValue;
788 }
789 else if (remove(key, v))
790 return null;
791 }
792 return v;
793 }
794
795 /**
796 * Attempts to compute a mapping for the specified key and its
797 * current mapped value (or {@code null} if there is no current
798 * mapping). For example, to either create or append a {@code
799 * String msg} to a value mapping:
800 *
801 * <pre> {@code
802 * map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))}</pre>
803 * (Method {@link #merge} is often simpler to use for such purposes.)
804 *
805 * <p>The default implementation is equivalent to
806 * performing the following steps for this {@code map}, then
807 * returning the current value or {@code null} if absent:
808 *
809 * <pre> {@code
810 * V oldValue = map.get(key);
811 * V newValue = remappingFunction.apply(key, oldValue);
812 * if (newValue != null)
813 * map.replace(key, oldValue, newValue);
814 * else
815 * map.remove(key, oldValue);
816 * }</pre>
817 *
818 * In concurrent contexts, the default implementation may retry
819 * these steps when multiple threads attempt updates. If the
820 * function returns {@code null}, the mapping is removed (or
821 * remains absent if initially absent). If the function itself
822 * throws an (unchecked) exception, the exception is rethrown, and
823 * the current mapping is left unchanged.
824 *
825 * <p>The default implementation makes no guarantees about
826 * synchronization or atomicity properties of this method or the
827 * application of the remapping function. Any class overriding
828 * this method must specify its concurrency properties. In
829 * particular, all implementations of subinterface {@link
830 * java.util.concurrent.ConcurrentMap} must document whether the
831 * function is applied exactly once atomically. Any class that
832 * permits null values must document whether and how this method
833 * distinguishes absence from null mappings.
834 *
835 * @param key key with which the specified value is to be associated
836 * @param remappingFunction the function to compute a value
837 * @return the new value associated with the specified key, or null if none
838 * @throws NullPointerException if the specified key is null and
839 * this map does not support null keys, or the
840 * remappingFunction is null
841 * @throws UnsupportedOperationException if the <tt>put</tt> operation
842 * is not supported by this map
843 * @throws ClassCastException if the class of the specified key or value
844 * prevents it from being stored in this map
845 * @throws RuntimeException or Error if the remappingFunction does so,
846 * in which case the mapping is unchanged
847 */
848 default V compute(K key,
849 BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
850 for (;;) {
851 V oldValue = get(key);
852 V newValue = remappingFunction.apply(key, oldValue);
853 if (newValue != null) {
854 if (replace(key, oldValue, newValue))
855 return newValue;
856 }
857 else if (remove(key, oldValue))
858 return null;
859 }
860 }
861
862 /**
863 * If the specified key is not already associated with a
864 * (non-null) value, associates it with the given value.
865 * Otherwise, replaces the value with the results of the given
866 * remapping function, or removes if {@code null}. This method may
867 * be of use when combining multiple mapped values for a key. For
868 * example. to either create or append a {@code String msg} to a
869 * value mapping:
870 *
871 * <pre> {@code
872 * map.merge(key, msg, String::concat)}</pre>
873 *
874 * <p>The default implementation is equivalent to performing the
875 * following steps for this {@code map}, then returning the
876 * current value or {@code null} if absent:
877 *
878 * <pre> {@code
879 * V oldValue = map.get(key);
880 * V newValue = (oldValue == null) ? value :
881 * remappingFunction.apply(oldValue, value);
882 * if (newValue == null)
883 * map.remove(key, oldValue);
884 * else if (oldValue == null)
885 * map.putIfAbsent(key, newValue);
886 * else
887 * map.replace(key, oldValue, newValue);
888 * }</pre>
889 *
890 * In concurrent contexts, the default implementation may retry
891 * these steps when multiple threads attempt updates. If the
892 * function returns {@code null}, the mapping is removed (or
893 * remains absent if initially absent). If the function itself
894 * throws an (unchecked) exception, the exception is rethrown, and
895 * the current mapping is left unchanged.
896 *
897 * <p>The default implementation makes no guarantees about
898 * synchronization or atomicity properties of this method or the
899 * application of the remapping function. Any class overriding
900 * this method must specify its concurrency properties. In
901 * particular, all implementations of subinterface {@link
902 * java.util.concurrent.ConcurrentMap} must document whether the
903 * function is applied exactly once atomically. Any class that
904 * permits null values must document whether and how this method
905 * distinguishes absence from null mappings.
906 *
907 * @param key key with which the specified value is to be associated
908 * @param value the value to use if absent
909 * @param remappingFunction the function to recompute a value if present
910 * @return the new value associated with the specified key, or null if none
911 * @throws NullPointerException if the specified key is null and
912 * this map does not support null keys, or the
913 * remappingFunction is null
914 * @throws RuntimeException or Error if the remappingFunction does so,
915 * in which case the mapping is unchanged
916 */
917 default V merge(K key, V value,
918 BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
919 for (;;) {
920 V oldValue, newValue;
921 if ((oldValue = get(key)) == null) {
922 if (value == null || putIfAbsent(key, value) == null)
923 return value;
924 }
925 else if ((newValue = remappingFunction.apply(oldValue, value)) != null) {
926 if (replace(key, oldValue, newValue))
927 return newValue;
928 }
929 else if (remove(key, oldValue))
930 return null;
931 }
932 }
933 }