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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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10 *
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15 * accompanied this code).
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24 */
25
26 package java.util;
27
28 /**
29 * An object that maps keys to values. A map cannot contain duplicate keys;
30 * each key can map to at most one value.
31 *
32 * <p>This interface takes the place of the <tt>Dictionary</tt> class, which
33 * was a totally abstract class rather than an interface.
34 *
35 * <p>The <tt>Map</tt> interface provides three <i>collection views</i>, which
36 * allow a map's contents to be viewed as a set of keys, collection of values,
37 * or set of key-value mappings. The <i>order</i> of a map is defined as
38 * the order in which the iterators on the map's collection views return their
39 * elements. Some map implementations, like the <tt>TreeMap</tt> class, make
40 * specific guarantees as to their order; others, like the <tt>HashMap</tt>
41 * class, do not.
42 *
43 * <p>Note: great care must be exercised if mutable objects are used as map
44 * keys. The behavior of a map is not specified if the value of an object is
45 * changed in a manner that affects <tt>equals</tt> comparisons while the
46 * object is a key in the map. A special case of this prohibition is that it
47 * is not permissible for a map to contain itself as a key. While it is
48 * permissible for a map to contain itself as a value, extreme caution is
49 * advised: the <tt>equals</tt> and <tt>hashCode</tt> methods are no longer
50 * well defined on such a map.
51 *
52 * <p>All general-purpose map implementation classes should provide two
53 * "standard" constructors: a void (no arguments) constructor which creates an
54 * empty map, and a constructor with a single argument of type <tt>Map</tt>,
55 * which creates a new map with the same key-value mappings as its argument.
56 * In effect, the latter constructor allows the user to copy any map,
57 * producing an equivalent map of the desired class. There is no way to
58 * enforce this recommendation (as interfaces cannot contain constructors) but
59 * all of the general-purpose map implementations in the JDK comply.
60 *
61 * <p>The "destructive" methods contained in this interface, that is, the
62 * methods that modify the map on which they operate, are specified to throw
63 * <tt>UnsupportedOperationException</tt> if this map does not support the
64 * operation. If this is the case, these methods may, but are not required
65 * to, throw an <tt>UnsupportedOperationException</tt> if the invocation would
66 * have no effect on the map. For example, invoking the {@link #putAll(Map)}
67 * method on an unmodifiable map may, but is not required to, throw the
68 * exception if the map whose mappings are to be "superimposed" is empty.
69 *
70 * <p>Some map implementations have restrictions on the keys and values they
71 * may contain. For example, some implementations prohibit null keys and
72 * values, and some have restrictions on the types of their keys. Attempting
73 * to insert an ineligible key or value throws an unchecked exception,
74 * typically <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.
75 * Attempting to query the presence of an ineligible key or value may throw an
76 * exception, or it may simply return false; some implementations will exhibit
77 * the former behavior and some will exhibit the latter. More generally,
78 * attempting an operation on an ineligible key or value whose completion
79 * would not result in the insertion of an ineligible element into the map may
80 * throw an exception or it may succeed, at the option of the implementation.
81 * Such exceptions are marked as "optional" in the specification for this
82 * interface.
83 *
84 * <p>This interface is a member of the
85 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
86 * Java Collections Framework</a>.
87 *
88 * <p>Many methods in Collections Framework interfaces are defined
89 * in terms of the {@link Object#equals(Object) equals} method. For
90 * example, the specification for the {@link #containsKey(Object)
91 * containsKey(Object key)} method says: "returns <tt>true</tt> if and
92 * only if this map contains a mapping for a key <tt>k</tt> such that
93 * <tt>(key==null ? k==null : key.equals(k))</tt>." This specification should
94 * <i>not</i> be construed to imply that invoking <tt>Map.containsKey</tt>
95 * with a non-null argument <tt>key</tt> will cause <tt>key.equals(k)</tt> to
96 * be invoked for any key <tt>k</tt>. Implementations are free to
97 * implement optimizations whereby the <tt>equals</tt> invocation is avoided,
98 * for example, by first comparing the hash codes of the two keys. (The
99 * {@link Object#hashCode()} specification guarantees that two objects with
100 * unequal hash codes cannot be equal.) More generally, implementations of
101 * the various Collections Framework interfaces are free to take advantage of
102 * the specified behavior of underlying {@link Object} methods wherever the
103 * implementor deems it appropriate.
104 *
105 * @param <K> the type of keys maintained by this map
106 * @param <V> the type of mapped values
107 *
108 * @author Josh Bloch
109 * @see HashMap
110 * @see TreeMap
111 * @see Hashtable
112 * @see SortedMap
113 * @see Collection
114 * @see Set
115 * @since 1.2
116 */
117 public interface Map<K,V> {
118 // Query Operations
119
120 /**
121 * Returns the number of key-value mappings in this map. If the
122 * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
123 * <tt>Integer.MAX_VALUE</tt>.
124 *
125 * @return the number of key-value mappings in this map
126 */
127 int size();
128
129 /**
130 * Returns <tt>true</tt> if this map contains no key-value mappings.
131 *
132 * @return <tt>true</tt> if this map contains no key-value mappings
133 */
134 boolean isEmpty();
135
136 /**
137 * Returns <tt>true</tt> if this map contains a mapping for the specified
138 * key. More formally, returns <tt>true</tt> if and only if
139 * this map contains a mapping for a key <tt>k</tt> such that
140 * <tt>(key==null ? k==null : key.equals(k))</tt>. (There can be
141 * at most one such mapping.)
142 *
143 * @param key key whose presence in this map is to be tested
144 * @return <tt>true</tt> if this map contains a mapping for the specified
145 * key
146 * @throws ClassCastException if the key is of an inappropriate type for
147 * this map (optional)
148 * @throws NullPointerException if the specified key is null and this map
149 * does not permit null keys (optional)
150 */
151 boolean containsKey(Object key);
152
153 /**
154 * Returns <tt>true</tt> if this map maps one or more keys to the
155 * specified value. More formally, returns <tt>true</tt> if and only if
156 * this map contains at least one mapping to a value <tt>v</tt> such that
157 * <tt>(value==null ? v==null : value.equals(v))</tt>. This operation
158 * will probably require time linear in the map size for most
159 * implementations of the <tt>Map</tt> interface.
160 *
161 * @param value value whose presence in this map is to be tested
162 * @return <tt>true</tt> if this map maps one or more keys to the
163 * specified value
164 * @throws ClassCastException if the value is of an inappropriate type for
165 * this map (optional)
166 * @throws NullPointerException if the specified value is null and this
167 * map does not permit null values (optional)
168 */
169 boolean containsValue(Object value);
170
171 /**
172 * Returns the value to which the specified key is mapped,
173 * or {@code null} if this map contains no mapping for the key.
174 *
175 * <p>More formally, if this map contains a mapping from a key
176 * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
177 * key.equals(k))}, then this method returns {@code v}; otherwise
178 * it returns {@code null}. (There can be at most one such mapping.)
179 *
180 * <p>If this map permits null values, then a return value of
181 * {@code null} does not <i>necessarily</i> indicate that the map
182 * contains no mapping for the key; it's also possible that the map
183 * explicitly maps the key to {@code null}. The {@link #containsKey
184 * containsKey} operation may be used to distinguish these two cases.
185 *
186 * @param key the key whose associated value is to be returned
187 * @return the value to which the specified key is mapped, or
188 * {@code null} if this map contains no mapping for the key
189 * @throws ClassCastException if the key is of an inappropriate type for
190 * this map (optional)
191 * @throws NullPointerException if the specified key is null and this map
192 * does not permit null keys (optional)
193 */
194 V get(Object key);
195
196 // Modification Operations
197
198 /**
199 * Associates the specified value with the specified key in this map
200 * (optional operation). If the map previously contained a mapping for
201 * the key, the old value is replaced by the specified value. (A map
202 * <tt>m</tt> is said to contain a mapping for a key <tt>k</tt> if and only
203 * if {@link #containsKey(Object) m.containsKey(k)} would return
204 * <tt>true</tt>.)
205 *
206 * @param key key with which the specified value is to be associated
207 * @param value value to be associated with the specified key
208 * @return the previous value associated with <tt>key</tt>, or
209 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
210 * (A <tt>null</tt> return can also indicate that the map
211 * previously associated <tt>null</tt> with <tt>key</tt>,
212 * if the implementation supports <tt>null</tt> values.)
213 * @throws UnsupportedOperationException if the <tt>put</tt> operation
214 * is not supported by this map
215 * @throws ClassCastException if the class of the specified key or value
216 * prevents it from being stored in this map
217 * @throws NullPointerException if the specified key or value is null
218 * and this map does not permit null keys or values
219 * @throws IllegalArgumentException if some property of the specified key
220 * or value prevents it from being stored in this map
221 */
222 V put(K key, V value);
223
224 /**
225 * Removes the mapping for a key from this map if it is present
226 * (optional operation). More formally, if this map contains a mapping
227 * from key <tt>k</tt> to value <tt>v</tt> such that
228 * <code>(key==null ? k==null : key.equals(k))</code>, that mapping
229 * is removed. (The map can contain at most one such mapping.)
230 *
231 * <p>Returns the value to which this map previously associated the key,
232 * or <tt>null</tt> if the map contained no mapping for the key.
233 *
234 * <p>If this map permits null values, then a return value of
235 * <tt>null</tt> does not <i>necessarily</i> indicate that the map
236 * contained no mapping for the key; it's also possible that the map
237 * explicitly mapped the key to <tt>null</tt>.
238 *
239 * <p>The map will not contain a mapping for the specified key once the
240 * call returns.
241 *
242 * @param key key whose mapping is to be removed from the map
243 * @return the previous value associated with <tt>key</tt>, or
244 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
245 * @throws UnsupportedOperationException if the <tt>remove</tt> operation
246 * is not supported by this map
247 * @throws ClassCastException if the key is of an inappropriate type for
248 * this map (optional)
249 * @throws NullPointerException if the specified key is null and this
250 * map does not permit null keys (optional)
251 */
252 V remove(Object key);
253
254
255 // Bulk Operations
256
257 /**
258 * Copies all of the mappings from the specified map to this map
259 * (optional operation). The effect of this call is equivalent to that
260 * of calling {@link #put(Object,Object) put(k, v)} on this map once
261 * for each mapping from key <tt>k</tt> to value <tt>v</tt> in the
262 * specified map. The behavior of this operation is undefined if the
263 * specified map is modified while the operation is in progress.
264 *
265 * @param m mappings to be stored in this map
266 * @throws UnsupportedOperationException if the <tt>putAll</tt> operation
267 * is not supported by this map
268 * @throws ClassCastException if the class of a key or value in the
269 * specified map prevents it from being stored in this map
270 * @throws NullPointerException if the specified map is null, or if
271 * this map does not permit null keys or values, and the
272 * specified map contains null keys or values
273 * @throws IllegalArgumentException if some property of a key or value in
274 * the specified map prevents it from being stored in this map
275 */
276 void putAll(Map<? extends K, ? extends V> m);
277
278 /**
279 * Removes all of the mappings from this map (optional operation).
280 * The map will be empty after this call returns.
281 *
282 * @throws UnsupportedOperationException if the <tt>clear</tt> operation
283 * is not supported by this map
284 */
285 void clear();
286
287
288 // Views
289
290 /**
291 * Returns a {@link Set} view of the keys contained in this map.
292 * The set is backed by the map, so changes to the map are
293 * reflected in the set, and vice-versa. If the map is modified
294 * while an iteration over the set is in progress (except through
295 * the iterator's own <tt>remove</tt> operation), the results of
296 * the iteration are undefined. The set supports element removal,
297 * which removes the corresponding mapping from the map, via the
298 * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
299 * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
300 * operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
301 * operations.
302 *
303 * @return a set view of the keys contained in this map
304 */
305 Set<K> keySet();
306
307 /**
308 * Returns a {@link Collection} view of the values contained in this map.
309 * The collection is backed by the map, so changes to the map are
310 * reflected in the collection, and vice-versa. If the map is
311 * modified while an iteration over the collection is in progress
312 * (except through the iterator's own <tt>remove</tt> operation),
313 * the results of the iteration are undefined. The collection
314 * supports element removal, which removes the corresponding
315 * mapping from the map, via the <tt>Iterator.remove</tt>,
316 * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
317 * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
318 * support the <tt>add</tt> or <tt>addAll</tt> operations.
319 *
320 * @return a collection view of the values contained in this map
321 */
322 Collection<V> values();
323
324 /**
325 * Returns a {@link Set} view of the mappings contained in this map.
326 * The set is backed by the map, so changes to the map are
327 * reflected in the set, and vice-versa. If the map is modified
328 * while an iteration over the set is in progress (except through
329 * the iterator's own <tt>remove</tt> operation, or through the
330 * <tt>setValue</tt> operation on a map entry returned by the
331 * iterator) the results of the iteration are undefined. The set
332 * supports element removal, which removes the corresponding
333 * mapping from the map, via the <tt>Iterator.remove</tt>,
334 * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
335 * <tt>clear</tt> operations. It does not support the
336 * <tt>add</tt> or <tt>addAll</tt> operations.
337 *
338 * @return a set view of the mappings contained in this map
339 */
340 Set<Map.Entry<K, V>> entrySet();
341
342 /**
343 * A map entry (key-value pair). The <tt>Map.entrySet</tt> method returns
344 * a collection-view of the map, whose elements are of this class. The
345 * <i>only</i> way to obtain a reference to a map entry is from the
346 * iterator of this collection-view. These <tt>Map.Entry</tt> objects are
347 * valid <i>only</i> for the duration of the iteration; more formally,
348 * the behavior of a map entry is undefined if the backing map has been
349 * modified after the entry was returned by the iterator, except through
350 * the <tt>setValue</tt> operation on the map entry.
351 *
352 * @see Map#entrySet()
353 * @since 1.2
354 */
355 interface Entry<K,V> {
356 /**
357 * Returns the key corresponding to this entry.
358 *
359 * @return the key corresponding to this entry
360 * @throws IllegalStateException implementations may, but are not
361 * required to, throw this exception if the entry has been
362 * removed from the backing map.
363 */
364 K getKey();
365
366 /**
367 * Returns the value corresponding to this entry. If the mapping
368 * has been removed from the backing map (by the iterator's
369 * <tt>remove</tt> operation), the results of this call are undefined.
370 *
371 * @return the value corresponding to this entry
372 * @throws IllegalStateException implementations may, but are not
373 * required to, throw this exception if the entry has been
374 * removed from the backing map.
375 */
376 V getValue();
377
378 /**
379 * Replaces the value corresponding to this entry with the specified
380 * value (optional operation). (Writes through to the map.) The
381 * behavior of this call is undefined if the mapping has already been
382 * removed from the map (by the iterator's <tt>remove</tt> operation).
383 *
384 * @param value new value to be stored in this entry
385 * @return old value corresponding to the entry
386 * @throws UnsupportedOperationException if the <tt>put</tt> operation
387 * is not supported by the backing map
388 * @throws ClassCastException if the class of the specified value
389 * prevents it from being stored in the backing map
390 * @throws NullPointerException if the backing map does not permit
391 * null values, and the specified value is null
392 * @throws IllegalArgumentException if some property of this value
393 * prevents it from being stored in the backing map
394 * @throws IllegalStateException implementations may, but are not
395 * required to, throw this exception if the entry has been
396 * removed from the backing map.
397 */
398 V setValue(V value);
399
400 /**
401 * Compares the specified object with this entry for equality.
402 * Returns <tt>true</tt> if the given object is also a map entry and
403 * the two entries represent the same mapping. More formally, two
404 * entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping
405 * if<pre>
406 * (e1.getKey()==null ?
407 * e2.getKey()==null : e1.getKey().equals(e2.getKey())) &&
408 * (e1.getValue()==null ?
409 * e2.getValue()==null : e1.getValue().equals(e2.getValue()))
410 * </pre>
411 * This ensures that the <tt>equals</tt> method works properly across
412 * different implementations of the <tt>Map.Entry</tt> interface.
413 *
414 * @param o object to be compared for equality with this map entry
415 * @return <tt>true</tt> if the specified object is equal to this map
416 * entry
417 */
418 boolean equals(Object o);
419
420 /**
421 * Returns the hash code value for this map entry. The hash code
422 * of a map entry <tt>e</tt> is defined to be: <pre>
423 * (e.getKey()==null ? 0 : e.getKey().hashCode()) ^
424 * (e.getValue()==null ? 0 : e.getValue().hashCode())
425 * </pre>
426 * This ensures that <tt>e1.equals(e2)</tt> implies that
427 * <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries
428 * <tt>e1</tt> and <tt>e2</tt>, as required by the general
429 * contract of <tt>Object.hashCode</tt>.
430 *
431 * @return the hash code value for this map entry
432 * @see Object#hashCode()
433 * @see Object#equals(Object)
434 * @see #equals(Object)
435 */
436 int hashCode();
437 }
438
439 // Comparison and hashing
440
441 /**
442 * Compares the specified object with this map for equality. Returns
443 * <tt>true</tt> if the given object is also a map and the two maps
444 * represent the same mappings. More formally, two maps <tt>m1</tt> and
445 * <tt>m2</tt> represent the same mappings if
446 * <tt>m1.entrySet().equals(m2.entrySet())</tt>. This ensures that the
447 * <tt>equals</tt> method works properly across different implementations
448 * of the <tt>Map</tt> interface.
449 *
450 * @param o object to be compared for equality with this map
451 * @return <tt>true</tt> if the specified object is equal to this map
452 */
453 boolean equals(Object o);
454
455 /**
456 * Returns the hash code value for this map. The hash code of a map is
457 * defined to be the sum of the hash codes of each entry in the map's
458 * <tt>entrySet()</tt> view. This ensures that <tt>m1.equals(m2)</tt>
459 * implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two maps
460 * <tt>m1</tt> and <tt>m2</tt>, as required by the general contract of
461 * {@link Object#hashCode}.
462 *
463 * @return the hash code value for this map
464 * @see Map.Entry#hashCode()
465 * @see Object#equals(Object)
466 * @see #equals(Object)
467 */
468 int hashCode();
469 }