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 import java.io.Serializable;
32
33 /**
34 * An object that maps keys to values. A map cannot contain duplicate keys;
35 * each key can map to at most one value.
36 *
37 * <p>This interface takes the place of the <tt>Dictionary</tt> class, which
38 * was a totally abstract class rather than an interface.
39 *
40 * <p>The <tt>Map</tt> interface provides three <i>collection views</i>, which
41 * allow a map's contents to be viewed as a set of keys, collection of values,
42 * or set of key-value mappings. The <i>order</i> of a map is defined as
43 * the order in which the iterators on the map's collection views return their
44 * elements. Some map implementations, like the <tt>TreeMap</tt> class, make
45 * specific guarantees as to their order; others, like the <tt>HashMap</tt>
46 * class, do not.
47 *
48 * <p>Note: great care must be exercised if mutable objects are used as map
49 * keys. The behavior of a map is not specified if the value of an object is
50 * changed in a manner that affects <tt>equals</tt> comparisons while the
51 * object is a key in the map. A special case of this prohibition is that it
52 * is not permissible for a map to contain itself as a key. While it is
53 * permissible for a map to contain itself as a value, extreme caution is
54 * advised: the <tt>equals</tt> and <tt>hashCode</tt> methods are no longer
55 * well defined on such a map.
56 *
57 * <p>All general-purpose map implementation classes should provide two
58 * "standard" constructors: a void (no arguments) constructor which creates an
59 * empty map, and a constructor with a single argument of type <tt>Map</tt>,
60 * which creates a new map with the same key-value mappings as its argument.
61 * In effect, the latter constructor allows the user to copy any map,
62 * producing an equivalent map of the desired class. There is no way to
63 * enforce this recommendation (as interfaces cannot contain constructors) but
64 * all of the general-purpose map implementations in the JDK comply.
65 *
66 * <p>The "destructive" methods contained in this interface, that is, the
67 * methods that modify the map on which they operate, are specified to throw
68 * <tt>UnsupportedOperationException</tt> if this map does not support the
69 * operation. If this is the case, these methods may, but are not required
70 * to, throw an <tt>UnsupportedOperationException</tt> if the invocation would
71 * have no effect on the map. For example, invoking the {@link #putAll(Map)}
72 * method on an unmodifiable map may, but is not required to, throw the
73 * exception if the map whose mappings are to be "superimposed" is empty.
74 *
75 * <p>Some map implementations have restrictions on the keys and values they
76 * may contain. For example, some implementations prohibit null keys and
77 * values, and some have restrictions on the types of their keys. Attempting
78 * to insert an ineligible key or value throws an unchecked exception,
79 * typically <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.
80 * Attempting to query the presence of an ineligible key or value may throw an
81 * exception, or it may simply return false; some implementations will exhibit
82 * the former behavior and some will exhibit the latter. More generally,
83 * attempting an operation on an ineligible key or value whose completion
84 * would not result in the insertion of an ineligible element into the map may
85 * throw an exception or it may succeed, at the option of the implementation.
86 * Such exceptions are marked as "optional" in the specification for this
87 * interface.
88 *
89 * <p>Many methods in Collections Framework interfaces are defined
90 * in terms of the {@link Object#equals(Object) equals} method. For
91 * example, the specification for the {@link #containsKey(Object)
92 * containsKey(Object key)} method says: "returns <tt>true</tt> if and
93 * only if this map contains a mapping for a key <tt>k</tt> such that
94 * <tt>(key==null ? k==null : key.equals(k))</tt>." This specification should
95 * <i>not</i> be construed to imply that invoking <tt>Map.containsKey</tt>
96 * with a non-null argument <tt>key</tt> will cause <tt>key.equals(k)</tt> to
97 * be invoked for any key <tt>k</tt>. Implementations are free to
98 * implement optimizations whereby the <tt>equals</tt> invocation is avoided,
99 * for example, by first comparing the hash codes of the two keys. (The
100 * {@link Object#hashCode()} specification guarantees that two objects with
101 * unequal hash codes cannot be equal.) More generally, implementations of
102 * the various Collections Framework interfaces are free to take advantage of
103 * the specified behavior of underlying {@link Object} methods wherever the
104 * implementor deems it appropriate.
105 *
106 * <p>Some map operations which perform recursive traversal of the map may fail
107 * with an exception for self-referential instances where the map directly or
108 * indirectly contains itself. This includes the {@code clone()},
109 * {@code equals()}, {@code hashCode()} and {@code toString()} methods.
110 * Implementations may optionally handle the self-referential scenario, however
111 * most current implementations do not do so.
112 *
113 * <p>This interface is a member of the
114 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
115 * Java Collections Framework</a>.
116 *
117 * @param <K> the type of keys maintained by this map
118 * @param <V> the type of mapped values
119 *
120 * @author Josh Bloch
121 * @see HashMap
122 * @see TreeMap
123 * @see Hashtable
124 * @see SortedMap
125 * @see Collection
126 * @see Set
127 * @since 1.2
128 */
129 public interface Map<K,V> {
130 // Query Operations
131
132 /**
133 * Returns the number of key-value mappings in this map. If the
134 * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
135 * <tt>Integer.MAX_VALUE</tt>.
136 *
137 * @return the number of key-value mappings in this map
138 */
139 int size();
140
141 /**
142 * Returns <tt>true</tt> if this map contains no key-value mappings.
143 *
144 * @return <tt>true</tt> if this map contains no key-value mappings
145 */
146 boolean isEmpty();
147
148 /**
149 * Returns <tt>true</tt> if this map contains a mapping for the specified
150 * key. More formally, returns <tt>true</tt> if and only if
151 * this map contains a mapping for a key <tt>k</tt> such that
152 * <tt>(key==null ? k==null : key.equals(k))</tt>. (There can be
153 * at most one such mapping.)
154 *
155 * @param key key whose presence in this map is to be tested
156 * @return <tt>true</tt> if this map contains a mapping for the specified
157 * key
158 * @throws ClassCastException if the key is of an inappropriate type for
159 * this map
160 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
161 * @throws NullPointerException if the specified key is null and this map
162 * does not permit null keys
163 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
164 */
165 boolean containsKey(Object key);
166
167 /**
168 * Returns <tt>true</tt> if this map maps one or more keys to the
169 * specified value. More formally, returns <tt>true</tt> if and only if
170 * this map contains at least one mapping to a value <tt>v</tt> such that
171 * <tt>(value==null ? v==null : value.equals(v))</tt>. This operation
172 * will probably require time linear in the map size for most
173 * implementations of the <tt>Map</tt> interface.
174 *
175 * @param value value whose presence in this map is to be tested
176 * @return <tt>true</tt> if this map maps one or more keys to the
177 * specified value
178 * @throws ClassCastException if the value is of an inappropriate type for
179 * this map
180 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
181 * @throws NullPointerException if the specified value is null and this
182 * map does not permit null values
183 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
184 */
185 boolean containsValue(Object value);
186
187 /**
188 * Returns the value to which the specified key is mapped,
189 * or {@code null} if this map contains no mapping for the key.
190 *
191 * <p>More formally, if this map contains a mapping from a key
192 * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
193 * key.equals(k))}, then this method returns {@code v}; otherwise
194 * it returns {@code null}. (There can be at most one such mapping.)
195 *
196 * <p>If this map permits null values, then a return value of
200 * containsKey} operation may be used to distinguish these two cases.
201 *
202 * @param key the key whose associated value is to be returned
203 * @return the value to which the specified key is mapped, or
204 * {@code null} if this map contains no mapping for the key
205 * @throws ClassCastException if the key is of an inappropriate type for
206 * this map
207 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
208 * @throws NullPointerException if the specified key is null and this map
209 * does not permit null keys
210 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
211 */
212 V get(Object key);
213
214 // Modification Operations
215
216 /**
217 * Associates the specified value with the specified key in this map
218 * (optional operation). If the map previously contained a mapping for
219 * the key, the old value is replaced by the specified value. (A map
220 * <tt>m</tt> is said to contain a mapping for a key <tt>k</tt> if and only
221 * if {@link #containsKey(Object) m.containsKey(k)} would return
222 * <tt>true</tt>.)
223 *
224 * @param key key with which the specified value is to be associated
225 * @param value value to be associated with the specified key
226 * @return the previous value associated with <tt>key</tt>, or
227 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
228 * (A <tt>null</tt> return can also indicate that the map
229 * previously associated <tt>null</tt> with <tt>key</tt>,
230 * if the implementation supports <tt>null</tt> values.)
231 * @throws UnsupportedOperationException if the <tt>put</tt> operation
232 * is not supported by this map
233 * @throws ClassCastException if the class of the specified key or value
234 * prevents it from being stored in this map
235 * @throws NullPointerException if the specified key or value is null
236 * and this map does not permit null keys or values
237 * @throws IllegalArgumentException if some property of the specified key
238 * or value prevents it from being stored in this map
239 */
240 V put(K key, V value);
241
242 /**
243 * Removes the mapping for a key from this map if it is present
244 * (optional operation). More formally, if this map contains a mapping
245 * from key <tt>k</tt> to value <tt>v</tt> such that
246 * <code>(key==null ? k==null : key.equals(k))</code>, that mapping
247 * is removed. (The map can contain at most one such mapping.)
248 *
249 * <p>Returns the value to which this map previously associated the key,
250 * or <tt>null</tt> if the map contained no mapping for the key.
251 *
252 * <p>If this map permits null values, then a return value of
253 * <tt>null</tt> does not <i>necessarily</i> indicate that the map
254 * contained no mapping for the key; it's also possible that the map
255 * explicitly mapped the key to <tt>null</tt>.
256 *
257 * <p>The map will not contain a mapping for the specified key once the
258 * call returns.
259 *
260 * @param key key whose mapping is to be removed from the map
261 * @return the previous value associated with <tt>key</tt>, or
262 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
263 * @throws UnsupportedOperationException if the <tt>remove</tt> operation
264 * is not supported by this map
265 * @throws ClassCastException if the key is of an inappropriate type for
266 * this map
267 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
268 * @throws NullPointerException if the specified key is null and this
269 * map does not permit null keys
270 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
271 */
272 V remove(Object key);
273
274
275 // Bulk Operations
276
277 /**
278 * Copies all of the mappings from the specified map to this map
279 * (optional operation). The effect of this call is equivalent to that
280 * of calling {@link #put(Object,Object) put(k, v)} on this map once
281 * for each mapping from key <tt>k</tt> to value <tt>v</tt> in the
282 * specified map. The behavior of this operation is undefined if the
283 * specified map is modified while the operation is in progress.
284 *
285 * @param m mappings to be stored in this map
286 * @throws UnsupportedOperationException if the <tt>putAll</tt> operation
287 * is not supported by this map
288 * @throws ClassCastException if the class of a key or value in the
289 * specified map prevents it from being stored in this map
290 * @throws NullPointerException if the specified map is null, or if
291 * this map does not permit null keys or values, and the
292 * specified map contains null keys or values
293 * @throws IllegalArgumentException if some property of a key or value in
294 * the specified map prevents it from being stored in this map
295 */
296 void putAll(Map<? extends K, ? extends V> m);
297
298 /**
299 * Removes all of the mappings from this map (optional operation).
300 * The map will be empty after this call returns.
301 *
302 * @throws UnsupportedOperationException if the <tt>clear</tt> operation
303 * is not supported by this map
304 */
305 void clear();
306
307
308 // Views
309
310 /**
311 * Returns a {@link Set} view of the keys contained in this map.
312 * The set is backed by the map, so changes to the map are
313 * reflected in the set, and vice-versa. If the map is modified
314 * while an iteration over the set is in progress (except through
315 * the iterator's own <tt>remove</tt> operation), the results of
316 * the iteration are undefined. The set supports element removal,
317 * which removes the corresponding mapping from the map, via the
318 * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
319 * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
320 * operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
321 * operations.
322 *
323 * @return a set view of the keys contained in this map
324 */
325 Set<K> keySet();
326
327 /**
328 * Returns a {@link Collection} view of the values contained in this map.
329 * The collection is backed by the map, so changes to the map are
330 * reflected in the collection, and vice-versa. If the map is
331 * modified while an iteration over the collection is in progress
332 * (except through the iterator's own <tt>remove</tt> operation),
333 * the results of the iteration are undefined. The collection
334 * supports element removal, which removes the corresponding
335 * mapping from the map, via the <tt>Iterator.remove</tt>,
336 * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
337 * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
338 * support the <tt>add</tt> or <tt>addAll</tt> operations.
339 *
340 * @return a collection view of the values contained in this map
341 */
342 Collection<V> values();
343
344 /**
345 * Returns a {@link Set} view of the mappings contained in this map.
346 * The set is backed by the map, so changes to the map are
347 * reflected in the set, and vice-versa. If the map is modified
348 * while an iteration over the set is in progress (except through
349 * the iterator's own <tt>remove</tt> operation, or through the
350 * <tt>setValue</tt> operation on a map entry returned by the
351 * iterator) the results of the iteration are undefined. The set
352 * supports element removal, which removes the corresponding
353 * mapping from the map, via the <tt>Iterator.remove</tt>,
354 * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
355 * <tt>clear</tt> operations. It does not support the
356 * <tt>add</tt> or <tt>addAll</tt> operations.
357 *
358 * @return a set view of the mappings contained in this map
359 */
360 Set<Map.Entry<K, V>> entrySet();
361
362 /**
363 * A map entry (key-value pair). The <tt>Map.entrySet</tt> method returns
364 * a collection-view of the map, whose elements are of this class. The
365 * <i>only</i> way to obtain a reference to a map entry is from the
366 * iterator of this collection-view. These <tt>Map.Entry</tt> objects are
367 * valid <i>only</i> for the duration of the iteration; more formally,
368 * the behavior of a map entry is undefined if the backing map has been
369 * modified after the entry was returned by the iterator, except through
370 * the <tt>setValue</tt> operation on the map entry.
371 *
372 * @see Map#entrySet()
373 * @since 1.2
374 */
375 interface Entry<K,V> {
376 /**
377 * Returns the key corresponding to this entry.
378 *
379 * @return the key corresponding to this entry
380 * @throws IllegalStateException implementations may, but are not
381 * required to, throw this exception if the entry has been
382 * removed from the backing map.
383 */
384 K getKey();
385
386 /**
387 * Returns the value corresponding to this entry. If the mapping
388 * has been removed from the backing map (by the iterator's
389 * <tt>remove</tt> operation), the results of this call are undefined.
390 *
391 * @return the value corresponding to this entry
392 * @throws IllegalStateException implementations may, but are not
393 * required to, throw this exception if the entry has been
394 * removed from the backing map.
395 */
396 V getValue();
397
398 /**
399 * Replaces the value corresponding to this entry with the specified
400 * value (optional operation). (Writes through to the map.) The
401 * behavior of this call is undefined if the mapping has already been
402 * removed from the map (by the iterator's <tt>remove</tt> operation).
403 *
404 * @param value new value to be stored in this entry
405 * @return old value corresponding to the entry
406 * @throws UnsupportedOperationException if the <tt>put</tt> operation
407 * is not supported by the backing map
408 * @throws ClassCastException if the class of the specified value
409 * prevents it from being stored in the backing map
410 * @throws NullPointerException if the backing map does not permit
411 * null values, and the specified value is null
412 * @throws IllegalArgumentException if some property of this value
413 * prevents it from being stored in the backing map
414 * @throws IllegalStateException implementations may, but are not
415 * required to, throw this exception if the entry has been
416 * removed from the backing map.
417 */
418 V setValue(V value);
419
420 /**
421 * Compares the specified object with this entry for equality.
422 * Returns <tt>true</tt> if the given object is also a map entry and
423 * the two entries represent the same mapping. More formally, two
424 * entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping
425 * if<pre>
426 * (e1.getKey()==null ?
427 * e2.getKey()==null : e1.getKey().equals(e2.getKey())) &&
428 * (e1.getValue()==null ?
429 * e2.getValue()==null : e1.getValue().equals(e2.getValue()))
430 * </pre>
431 * This ensures that the <tt>equals</tt> method works properly across
432 * different implementations of the <tt>Map.Entry</tt> interface.
433 *
434 * @param o object to be compared for equality with this map entry
435 * @return <tt>true</tt> if the specified object is equal to this map
436 * entry
437 */
438 boolean equals(Object o);
439
440 /**
441 * Returns the hash code value for this map entry. The hash code
442 * of a map entry <tt>e</tt> is defined to be: <pre>
443 * (e.getKey()==null ? 0 : e.getKey().hashCode()) ^
444 * (e.getValue()==null ? 0 : e.getValue().hashCode())
445 * </pre>
446 * This ensures that <tt>e1.equals(e2)</tt> implies that
447 * <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries
448 * <tt>e1</tt> and <tt>e2</tt>, as required by the general
449 * contract of <tt>Object.hashCode</tt>.
450 *
451 * @return the hash code value for this map entry
452 * @see Object#hashCode()
453 * @see Object#equals(Object)
454 * @see #equals(Object)
455 */
456 int hashCode();
457
458 /**
459 * Returns a comparator that compares {@link Map.Entry} in natural order on key.
460 *
461 * <p>The returned comparator is serializable and throws {@link
462 * NullPointerException} when comparing an entry with a null key.
463 *
464 * @param <K> the {@link Comparable} type of then map keys
465 * @param <V> the type of the map values
466 * @return a comparator that compares {@link Map.Entry} in natural order on key.
467 * @see Comparable
468 * @since 1.8
469 */
515 * <p>The returned comparator is serializable if the specified comparator
516 * is also serializable.
517 *
518 * @param <K> the type of the map keys
519 * @param <V> the type of the map values
520 * @param cmp the value {@link Comparator}
521 * @return a comparator that compares {@link Map.Entry} by the value.
522 * @since 1.8
523 */
524 public static <K, V> Comparator<Map.Entry<K, V>> comparingByValue(Comparator<? super V> cmp) {
525 Objects.requireNonNull(cmp);
526 return (Comparator<Map.Entry<K, V>> & Serializable)
527 (c1, c2) -> cmp.compare(c1.getValue(), c2.getValue());
528 }
529 }
530
531 // Comparison and hashing
532
533 /**
534 * Compares the specified object with this map for equality. Returns
535 * <tt>true</tt> if the given object is also a map and the two maps
536 * represent the same mappings. More formally, two maps <tt>m1</tt> and
537 * <tt>m2</tt> represent the same mappings if
538 * <tt>m1.entrySet().equals(m2.entrySet())</tt>. This ensures that the
539 * <tt>equals</tt> method works properly across different implementations
540 * of the <tt>Map</tt> interface.
541 *
542 * @param o object to be compared for equality with this map
543 * @return <tt>true</tt> if the specified object is equal to this map
544 */
545 boolean equals(Object o);
546
547 /**
548 * Returns the hash code value for this map. The hash code of a map is
549 * defined to be the sum of the hash codes of each entry in the map's
550 * <tt>entrySet()</tt> view. This ensures that <tt>m1.equals(m2)</tt>
551 * implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two maps
552 * <tt>m1</tt> and <tt>m2</tt>, as required by the general contract of
553 * {@link Object#hashCode}.
554 *
555 * @return the hash code value for this map
556 * @see Map.Entry#hashCode()
557 * @see Object#equals(Object)
558 * @see #equals(Object)
559 */
560 int hashCode();
561
562 // Defaultable methods
563
564 /**
565 * Returns the value to which the specified key is mapped, or
566 * {@code defaultValue} if this map contains no mapping for the key.
567 *
568 * @implSpec
569 * The default implementation makes no guarantees about synchronization
570 * or atomicity properties of this method. Any implementation providing
571 * atomicity guarantees must override this method and document its
572 * concurrency properties.
|
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 import java.io.Serializable;
32
33 /**
34 * An object that maps keys to values. A map cannot contain duplicate keys;
35 * each key can map to at most one value.
36 *
37 * <p>This interface takes the place of the {@code Dictionary} class, which
38 * was a totally abstract class rather than an interface.
39 *
40 * <p>The {@code Map} interface provides three <i>collection views</i>, which
41 * allow a map's contents to be viewed as a set of keys, collection of values,
42 * or set of key-value mappings. The <i>order</i> of a map is defined as
43 * the order in which the iterators on the map's collection views return their
44 * elements. Some map implementations, like the {@code TreeMap} class, make
45 * specific guarantees as to their order; others, like the {@code HashMap}
46 * class, do not.
47 *
48 * <p>Note: great care must be exercised if mutable objects are used as map
49 * keys. The behavior of a map is not specified if the value of an object is
50 * changed in a manner that affects {@code equals} comparisons while the
51 * object is a key in the map. A special case of this prohibition is that it
52 * is not permissible for a map to contain itself as a key. While it is
53 * permissible for a map to contain itself as a value, extreme caution is
54 * advised: the {@code equals} and {@code hashCode} methods are no longer
55 * well defined on such a map.
56 *
57 * <p>All general-purpose map implementation classes should provide two
58 * "standard" constructors: a void (no arguments) constructor which creates an
59 * empty map, and a constructor with a single argument of type {@code Map},
60 * which creates a new map with the same key-value mappings as its argument.
61 * In effect, the latter constructor allows the user to copy any map,
62 * producing an equivalent map of the desired class. There is no way to
63 * enforce this recommendation (as interfaces cannot contain constructors) but
64 * all of the general-purpose map implementations in the JDK comply.
65 *
66 * <p>The "destructive" methods contained in this interface, that is, the
67 * methods that modify the map on which they operate, are specified to throw
68 * {@code UnsupportedOperationException} if this map does not support the
69 * operation. If this is the case, these methods may, but are not required
70 * to, throw an {@code UnsupportedOperationException} if the invocation would
71 * have no effect on the map. For example, invoking the {@link #putAll(Map)}
72 * method on an unmodifiable map may, but is not required to, throw the
73 * exception if the map whose mappings are to be "superimposed" is empty.
74 *
75 * <p>Some map implementations have restrictions on the keys and values they
76 * may contain. For example, some implementations prohibit null keys and
77 * values, and some have restrictions on the types of their keys. Attempting
78 * to insert an ineligible key or value throws an unchecked exception,
79 * typically {@code NullPointerException} or {@code ClassCastException}.
80 * Attempting to query the presence of an ineligible key or value may throw an
81 * exception, or it may simply return false; some implementations will exhibit
82 * the former behavior and some will exhibit the latter. More generally,
83 * attempting an operation on an ineligible key or value whose completion
84 * would not result in the insertion of an ineligible element into the map may
85 * throw an exception or it may succeed, at the option of the implementation.
86 * Such exceptions are marked as "optional" in the specification for this
87 * interface.
88 *
89 * <p>Many methods in Collections Framework interfaces are defined
90 * in terms of the {@link Object#equals(Object) equals} method. For
91 * example, the specification for the {@link #containsKey(Object)
92 * containsKey(Object key)} method says: "returns {@code true} if and
93 * only if this map contains a mapping for a key {@code k} such that
94 * {@code (key==null ? k==null : key.equals(k))}." This specification should
95 * <i>not</i> be construed to imply that invoking {@code Map.containsKey}
96 * with a non-null argument {@code key} will cause {@code key.equals(k)} to
97 * be invoked for any key {@code k}. Implementations are free to
98 * implement optimizations whereby the {@code equals} invocation is avoided,
99 * for example, by first comparing the hash codes of the two keys. (The
100 * {@link Object#hashCode()} specification guarantees that two objects with
101 * unequal hash codes cannot be equal.) More generally, implementations of
102 * the various Collections Framework interfaces are free to take advantage of
103 * the specified behavior of underlying {@link Object} methods wherever the
104 * implementor deems it appropriate.
105 *
106 * <p>Some map operations which perform recursive traversal of the map may fail
107 * with an exception for self-referential instances where the map directly or
108 * indirectly contains itself. This includes the {@code clone()},
109 * {@code equals()}, {@code hashCode()} and {@code toString()} methods.
110 * Implementations may optionally handle the self-referential scenario, however
111 * most current implementations do not do so.
112 *
113 * <p>This interface is a member of the
114 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
115 * Java Collections Framework</a>.
116 *
117 * @param <K> the type of keys maintained by this map
118 * @param <V> the type of mapped values
119 *
120 * @author Josh Bloch
121 * @see HashMap
122 * @see TreeMap
123 * @see Hashtable
124 * @see SortedMap
125 * @see Collection
126 * @see Set
127 * @since 1.2
128 */
129 public interface Map<K,V> {
130 // Query Operations
131
132 /**
133 * Returns the number of key-value mappings in this map. If the
134 * map contains more than {@code Integer.MAX_VALUE} elements, returns
135 * {@code Integer.MAX_VALUE}.
136 *
137 * @return the number of key-value mappings in this map
138 */
139 int size();
140
141 /**
142 * Returns {@code true} if this map contains no key-value mappings.
143 *
144 * @return {@code true} if this map contains no key-value mappings
145 */
146 boolean isEmpty();
147
148 /**
149 * Returns {@code true} if this map contains a mapping for the specified
150 * key. More formally, returns {@code true} if and only if
151 * this map contains a mapping for a key {@code k} such that
152 * {@code (key==null ? k==null : key.equals(k))}. (There can be
153 * at most one such mapping.)
154 *
155 * @param key key whose presence in this map is to be tested
156 * @return {@code true} if this map contains a mapping for the specified
157 * key
158 * @throws ClassCastException if the key is of an inappropriate type for
159 * this map
160 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
161 * @throws NullPointerException if the specified key is null and this map
162 * does not permit null keys
163 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
164 */
165 boolean containsKey(Object key);
166
167 /**
168 * Returns {@code true} if this map maps one or more keys to the
169 * specified value. More formally, returns {@code true} if and only if
170 * this map contains at least one mapping to a value {@code v} such that
171 * {@code (value==null ? v==null : value.equals(v))}. This operation
172 * will probably require time linear in the map size for most
173 * implementations of the {@code Map} interface.
174 *
175 * @param value value whose presence in this map is to be tested
176 * @return {@code true} if this map maps one or more keys to the
177 * specified value
178 * @throws ClassCastException if the value is of an inappropriate type for
179 * this map
180 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
181 * @throws NullPointerException if the specified value is null and this
182 * map does not permit null values
183 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
184 */
185 boolean containsValue(Object value);
186
187 /**
188 * Returns the value to which the specified key is mapped,
189 * or {@code null} if this map contains no mapping for the key.
190 *
191 * <p>More formally, if this map contains a mapping from a key
192 * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
193 * key.equals(k))}, then this method returns {@code v}; otherwise
194 * it returns {@code null}. (There can be at most one such mapping.)
195 *
196 * <p>If this map permits null values, then a return value of
200 * containsKey} operation may be used to distinguish these two cases.
201 *
202 * @param key the key whose associated value is to be returned
203 * @return the value to which the specified key is mapped, or
204 * {@code null} if this map contains no mapping for the key
205 * @throws ClassCastException if the key is of an inappropriate type for
206 * this map
207 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
208 * @throws NullPointerException if the specified key is null and this map
209 * does not permit null keys
210 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
211 */
212 V get(Object key);
213
214 // Modification Operations
215
216 /**
217 * Associates the specified value with the specified key in this map
218 * (optional operation). If the map previously contained a mapping for
219 * the key, the old value is replaced by the specified value. (A map
220 * {@code m} is said to contain a mapping for a key {@code k} if and only
221 * if {@link #containsKey(Object) m.containsKey(k)} would return
222 * {@code true}.)
223 *
224 * @param key key with which the specified value is to be associated
225 * @param value value to be associated with the specified key
226 * @return the previous value associated with {@code key}, or
227 * {@code null} if there was no mapping for {@code key}.
228 * (A {@code null} return can also indicate that the map
229 * previously associated {@code null} with {@code key},
230 * if the implementation supports {@code null} values.)
231 * @throws UnsupportedOperationException if the {@code put} operation
232 * is not supported by this map
233 * @throws ClassCastException if the class of the specified key or value
234 * prevents it from being stored in this map
235 * @throws NullPointerException if the specified key or value is null
236 * and this map does not permit null keys or values
237 * @throws IllegalArgumentException if some property of the specified key
238 * or value prevents it from being stored in this map
239 */
240 V put(K key, V value);
241
242 /**
243 * Removes the mapping for a key from this map if it is present
244 * (optional operation). More formally, if this map contains a mapping
245 * from key {@code k} to value {@code v} such that
246 * {@code (key==null ? k==null : key.equals(k))}, that mapping
247 * is removed. (The map can contain at most one such mapping.)
248 *
249 * <p>Returns the value to which this map previously associated the key,
250 * or {@code null} if the map contained no mapping for the key.
251 *
252 * <p>If this map permits null values, then a return value of
253 * {@code null} does not <i>necessarily</i> indicate that the map
254 * contained no mapping for the key; it's also possible that the map
255 * explicitly mapped the key to {@code null}.
256 *
257 * <p>The map will not contain a mapping for the specified key once the
258 * call returns.
259 *
260 * @param key key whose mapping is to be removed from the map
261 * @return the previous value associated with {@code key}, or
262 * {@code null} if there was no mapping for {@code key}.
263 * @throws UnsupportedOperationException if the {@code remove} operation
264 * is not supported by this map
265 * @throws ClassCastException if the key is of an inappropriate type for
266 * this map
267 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
268 * @throws NullPointerException if the specified key is null and this
269 * map does not permit null keys
270 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
271 */
272 V remove(Object key);
273
274
275 // Bulk Operations
276
277 /**
278 * Copies all of the mappings from the specified map to this map
279 * (optional operation). The effect of this call is equivalent to that
280 * of calling {@link #put(Object,Object) put(k, v)} on this map once
281 * for each mapping from key {@code k} to value {@code v} in the
282 * specified map. The behavior of this operation is undefined if the
283 * specified map is modified while the operation is in progress.
284 *
285 * @param m mappings to be stored in this map
286 * @throws UnsupportedOperationException if the {@code putAll} operation
287 * is not supported by this map
288 * @throws ClassCastException if the class of a key or value in the
289 * specified map prevents it from being stored in this map
290 * @throws NullPointerException if the specified map is null, or if
291 * this map does not permit null keys or values, and the
292 * specified map contains null keys or values
293 * @throws IllegalArgumentException if some property of a key or value in
294 * the specified map prevents it from being stored in this map
295 */
296 void putAll(Map<? extends K, ? extends V> m);
297
298 /**
299 * Removes all of the mappings from this map (optional operation).
300 * The map will be empty after this call returns.
301 *
302 * @throws UnsupportedOperationException if the {@code clear} operation
303 * is not supported by this map
304 */
305 void clear();
306
307
308 // Views
309
310 /**
311 * Returns a {@link Set} view of the keys contained in this map.
312 * The set is backed by the map, so changes to the map are
313 * reflected in the set, and vice-versa. If the map is modified
314 * while an iteration over the set is in progress (except through
315 * the iterator's own {@code remove} operation), the results of
316 * the iteration are undefined. The set supports element removal,
317 * which removes the corresponding mapping from the map, via the
318 * {@code Iterator.remove}, {@code Set.remove},
319 * {@code removeAll}, {@code retainAll}, and {@code clear}
320 * operations. It does not support the {@code add} or {@code addAll}
321 * operations.
322 *
323 * @return a set view of the keys contained in this map
324 */
325 Set<K> keySet();
326
327 /**
328 * Returns a {@link Collection} view of the values contained in this map.
329 * The collection is backed by the map, so changes to the map are
330 * reflected in the collection, and vice-versa. If the map is
331 * modified while an iteration over the collection is in progress
332 * (except through the iterator's own {@code remove} operation),
333 * the results of the iteration are undefined. The collection
334 * supports element removal, which removes the corresponding
335 * mapping from the map, via the {@code Iterator.remove},
336 * {@code Collection.remove}, {@code removeAll},
337 * {@code retainAll} and {@code clear} operations. It does not
338 * support the {@code add} or {@code addAll} operations.
339 *
340 * @return a collection view of the values contained in this map
341 */
342 Collection<V> values();
343
344 /**
345 * Returns a {@link Set} view of the mappings contained in this map.
346 * The set is backed by the map, so changes to the map are
347 * reflected in the set, and vice-versa. If the map is modified
348 * while an iteration over the set is in progress (except through
349 * the iterator's own {@code remove} operation, or through the
350 * {@code setValue} operation on a map entry returned by the
351 * iterator) the results of the iteration are undefined. The set
352 * supports element removal, which removes the corresponding
353 * mapping from the map, via the {@code Iterator.remove},
354 * {@code Set.remove}, {@code removeAll}, {@code retainAll} and
355 * {@code clear} operations. It does not support the
356 * {@code add} or {@code addAll} operations.
357 *
358 * @return a set view of the mappings contained in this map
359 */
360 Set<Map.Entry<K, V>> entrySet();
361
362 /**
363 * A map entry (key-value pair). The {@code Map.entrySet} method returns
364 * a collection-view of the map, whose elements are of this class. The
365 * <i>only</i> way to obtain a reference to a map entry is from the
366 * iterator of this collection-view. These {@code Map.Entry} objects are
367 * valid <i>only</i> for the duration of the iteration; more formally,
368 * the behavior of a map entry is undefined if the backing map has been
369 * modified after the entry was returned by the iterator, except through
370 * the {@code setValue} operation on the map entry.
371 *
372 * @see Map#entrySet()
373 * @since 1.2
374 */
375 interface Entry<K,V> {
376 /**
377 * Returns the key corresponding to this entry.
378 *
379 * @return the key corresponding to this entry
380 * @throws IllegalStateException implementations may, but are not
381 * required to, throw this exception if the entry has been
382 * removed from the backing map.
383 */
384 K getKey();
385
386 /**
387 * Returns the value corresponding to this entry. If the mapping
388 * has been removed from the backing map (by the iterator's
389 * {@code remove} operation), the results of this call are undefined.
390 *
391 * @return the value corresponding to this entry
392 * @throws IllegalStateException implementations may, but are not
393 * required to, throw this exception if the entry has been
394 * removed from the backing map.
395 */
396 V getValue();
397
398 /**
399 * Replaces the value corresponding to this entry with the specified
400 * value (optional operation). (Writes through to the map.) The
401 * behavior of this call is undefined if the mapping has already been
402 * removed from the map (by the iterator's {@code remove} operation).
403 *
404 * @param value new value to be stored in this entry
405 * @return old value corresponding to the entry
406 * @throws UnsupportedOperationException if the {@code put} operation
407 * is not supported by the backing map
408 * @throws ClassCastException if the class of the specified value
409 * prevents it from being stored in the backing map
410 * @throws NullPointerException if the backing map does not permit
411 * null values, and the specified value is null
412 * @throws IllegalArgumentException if some property of this value
413 * prevents it from being stored in the backing map
414 * @throws IllegalStateException implementations may, but are not
415 * required to, throw this exception if the entry has been
416 * removed from the backing map.
417 */
418 V setValue(V value);
419
420 /**
421 * Compares the specified object with this entry for equality.
422 * Returns {@code true} if the given object is also a map entry and
423 * the two entries represent the same mapping. More formally, two
424 * entries {@code e1} and {@code e2} represent the same mapping
425 * if<pre>
426 * (e1.getKey()==null ?
427 * e2.getKey()==null : e1.getKey().equals(e2.getKey())) &&
428 * (e1.getValue()==null ?
429 * e2.getValue()==null : e1.getValue().equals(e2.getValue()))
430 * </pre>
431 * This ensures that the {@code equals} method works properly across
432 * different implementations of the {@code Map.Entry} interface.
433 *
434 * @param o object to be compared for equality with this map entry
435 * @return {@code true} if the specified object is equal to this map
436 * entry
437 */
438 boolean equals(Object o);
439
440 /**
441 * Returns the hash code value for this map entry. The hash code
442 * of a map entry {@code e} is defined to be: <pre>
443 * (e.getKey()==null ? 0 : e.getKey().hashCode()) ^
444 * (e.getValue()==null ? 0 : e.getValue().hashCode())
445 * </pre>
446 * This ensures that {@code e1.equals(e2)} implies that
447 * {@code e1.hashCode()==e2.hashCode()} for any two Entries
448 * {@code e1} and {@code e2}, as required by the general
449 * contract of {@code Object.hashCode}.
450 *
451 * @return the hash code value for this map entry
452 * @see Object#hashCode()
453 * @see Object#equals(Object)
454 * @see #equals(Object)
455 */
456 int hashCode();
457
458 /**
459 * Returns a comparator that compares {@link Map.Entry} in natural order on key.
460 *
461 * <p>The returned comparator is serializable and throws {@link
462 * NullPointerException} when comparing an entry with a null key.
463 *
464 * @param <K> the {@link Comparable} type of then map keys
465 * @param <V> the type of the map values
466 * @return a comparator that compares {@link Map.Entry} in natural order on key.
467 * @see Comparable
468 * @since 1.8
469 */
515 * <p>The returned comparator is serializable if the specified comparator
516 * is also serializable.
517 *
518 * @param <K> the type of the map keys
519 * @param <V> the type of the map values
520 * @param cmp the value {@link Comparator}
521 * @return a comparator that compares {@link Map.Entry} by the value.
522 * @since 1.8
523 */
524 public static <K, V> Comparator<Map.Entry<K, V>> comparingByValue(Comparator<? super V> cmp) {
525 Objects.requireNonNull(cmp);
526 return (Comparator<Map.Entry<K, V>> & Serializable)
527 (c1, c2) -> cmp.compare(c1.getValue(), c2.getValue());
528 }
529 }
530
531 // Comparison and hashing
532
533 /**
534 * Compares the specified object with this map for equality. Returns
535 * {@code true} if the given object is also a map and the two maps
536 * represent the same mappings. More formally, two maps {@code m1} and
537 * {@code m2} represent the same mappings if
538 * {@code m1.entrySet().equals(m2.entrySet())}. This ensures that the
539 * {@code equals} method works properly across different implementations
540 * of the {@code Map} interface.
541 *
542 * @param o object to be compared for equality with this map
543 * @return {@code true} if the specified object is equal to this map
544 */
545 boolean equals(Object o);
546
547 /**
548 * Returns the hash code value for this map. The hash code of a map is
549 * defined to be the sum of the hash codes of each entry in the map's
550 * {@code entrySet()} view. This ensures that {@code m1.equals(m2)}
551 * implies that {@code m1.hashCode()==m2.hashCode()} for any two maps
552 * {@code m1} and {@code m2}, as required by the general contract of
553 * {@link Object#hashCode}.
554 *
555 * @return the hash code value for this map
556 * @see Map.Entry#hashCode()
557 * @see Object#equals(Object)
558 * @see #equals(Object)
559 */
560 int hashCode();
561
562 // Defaultable methods
563
564 /**
565 * Returns the value to which the specified key is mapped, or
566 * {@code defaultValue} if this map contains no mapping for the key.
567 *
568 * @implSpec
569 * The default implementation makes no guarantees about synchronization
570 * or atomicity properties of this method. Any implementation providing
571 * atomicity guarantees must override this method and document its
572 * concurrency properties.
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