001 /*
002 * Copyright 1997-2006 Sun Microsystems, Inc. All Rights Reserved.
003 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
004 *
005 * This code is free software; you can redistribute it and/or modify it
006 * under the terms of the GNU General Public License version 2 only, as
007 * published by the Free Software Foundation. Sun designates this
008 * particular file as subject to the "Classpath" exception as provided
009 * by Sun in the LICENSE file that accompanied this code.
010 *
011 * This code is distributed in the hope that it will be useful, but WITHOUT
012 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
013 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
014 * version 2 for more details (a copy is included in the LICENSE file that
015 * accompanied this code).
016 *
017 * You should have received a copy of the GNU General Public License version
018 * 2 along with this work; if not, write to the Free Software Foundation,
019 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
020 *
021 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
022 * CA 95054 USA or visit www.sun.com if you need additional information or
023 * have any questions.
024 */
025
026 package java.util;
027
028 /**
029 * An object that maps keys to values. A map cannot contain duplicate keys;
030 * each key can map to at most one value.
031 *
032 * <p>This interface takes the place of the <tt>Dictionary</tt> class, which
033 * was a totally abstract class rather than an interface.
034 *
035 * <p>The <tt>Map</tt> interface provides three <i>collection views</i>, which
036 * allow a map's contents to be viewed as a set of keys, collection of values,
037 * or set of key-value mappings. The <i>order</i> of a map is defined as
038 * the order in which the iterators on the map's collection views return their
039 * elements. Some map implementations, like the <tt>TreeMap</tt> class, make
040 * specific guarantees as to their order; others, like the <tt>HashMap</tt>
041 * class, do not.
042 *
043 * <p>Note: great care must be exercised if mutable objects are used as map
044 * keys. The behavior of a map is not specified if the value of an object is
045 * changed in a manner that affects <tt>equals</tt> comparisons while the
046 * object is a key in the map. A special case of this prohibition is that it
047 * is not permissible for a map to contain itself as a key. While it is
048 * permissible for a map to contain itself as a value, extreme caution is
049 * advised: the <tt>equals</tt> and <tt>hashCode</tt> methods are no longer
050 * well defined on such a map.
051 *
052 * <p>All general-purpose map implementation classes should provide two
053 * "standard" constructors: a void (no arguments) constructor which creates an
054 * empty map, and a constructor with a single argument of type <tt>Map</tt>,
055 * which creates a new map with the same key-value mappings as its argument.
056 * In effect, the latter constructor allows the user to copy any map,
057 * producing an equivalent map of the desired class. There is no way to
058 * enforce this recommendation (as interfaces cannot contain constructors) but
059 * all of the general-purpose map implementations in the JDK comply.
060 *
061 * <p>The "destructive" methods contained in this interface, that is, the
062 * methods that modify the map on which they operate, are specified to throw
063 * <tt>UnsupportedOperationException</tt> if this map does not support the
064 * operation. If this is the case, these methods may, but are not required
065 * to, throw an <tt>UnsupportedOperationException</tt> if the invocation would
066 * have no effect on the map. For example, invoking the {@link #putAll(Map)}
067 * method on an unmodifiable map may, but is not required to, throw the
068 * exception if the map whose mappings are to be "superimposed" is empty.
069 *
070 * <p>Some map implementations have restrictions on the keys and values they
071 * may contain. For example, some implementations prohibit null keys and
072 * values, and some have restrictions on the types of their keys. Attempting
073 * to insert an ineligible key or value throws an unchecked exception,
074 * typically <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.
075 * Attempting to query the presence of an ineligible key or value may throw an
076 * exception, or it may simply return false; some implementations will exhibit
077 * the former behavior and some will exhibit the latter. More generally,
078 * attempting an operation on an ineligible key or value whose completion
079 * would not result in the insertion of an ineligible element into the map may
080 * throw an exception or it may succeed, at the option of the implementation.
081 * Such exceptions are marked as "optional" in the specification for this
082 * interface.
083 *
084 * <p>This interface is a member of the
085 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
086 * Java Collections Framework</a>.
087 *
088 * <p>Many methods in Collections Framework interfaces are defined
089 * in terms of the {@link Object#equals(Object) equals} method. For
090 * example, the specification for the {@link #containsKey(Object)
091 * containsKey(Object key)} method says: "returns <tt>true</tt> if and
092 * only if this map contains a mapping for a key <tt>k</tt> such that
093 * <tt>(key==null ? k==null : key.equals(k))</tt>." This specification should
094 * <i>not</i> be construed to imply that invoking <tt>Map.containsKey</tt>
095 * with a non-null argument <tt>key</tt> will cause <tt>key.equals(k)</tt> to
096 * be invoked for any key <tt>k</tt>. Implementations are free to
097 * implement optimizations whereby the <tt>equals</tt> invocation is avoided,
098 * for example, by first comparing the hash codes of the two keys. (The
099 * {@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 * @version 1.62, 05/05/07
110 * @see HashMap
111 * @see TreeMap
112 * @see Hashtable
113 * @see SortedMap
114 * @see Collection
115 * @see Set
116 * @since 1.2
117 */
118 public interface Map<K, V> {
119 // Query Operations
120
121 /**
122 * Returns the number of key-value mappings in this map. If the
123 * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
124 * <tt>Integer.MAX_VALUE</tt>.
125 *
126 * @return the number of key-value mappings in this map
127 */
128 int size();
129
130 /**
131 * Returns <tt>true</tt> if this map contains no key-value mappings.
132 *
133 * @return <tt>true</tt> if this map contains no key-value mappings
134 */
135 boolean isEmpty();
136
137 /**
138 * Returns <tt>true</tt> if this map contains a mapping for the specified
139 * key. More formally, returns <tt>true</tt> if and only if
140 * this map contains a mapping for a key <tt>k</tt> such that
141 * <tt>(key==null ? k==null : key.equals(k))</tt>. (There can be
142 * at most one such mapping.)
143 *
144 * @param key key whose presence in this map is to be tested
145 * @return <tt>true</tt> if this map contains a mapping for the specified
146 * key
147 * @throws ClassCastException if the key is of an inappropriate type for
148 * this map (optional)
149 * @throws NullPointerException if the specified key is null and this map
150 * does not permit null keys (optional)
151 */
152 boolean containsKey(Object key);
153
154 /**
155 * Returns <tt>true</tt> if this map maps one or more keys to the
156 * specified value. More formally, returns <tt>true</tt> if and only if
157 * this map contains at least one mapping to a value <tt>v</tt> such that
158 * <tt>(value==null ? v==null : value.equals(v))</tt>. This operation
159 * will probably require time linear in the map size for most
160 * implementations of the <tt>Map</tt> interface.
161 *
162 * @param value value whose presence in this map is to be tested
163 * @return <tt>true</tt> if this map maps one or more keys to the
164 * specified value
165 * @throws ClassCastException if the value is of an inappropriate type for
166 * this map (optional)
167 * @throws NullPointerException if the specified value is null and this
168 * map does not permit null values (optional)
169 */
170 boolean containsValue(Object value);
171
172 /**
173 * Returns the value to which the specified key is mapped,
174 * or {@code null} if this map contains no mapping for the key.
175 *
176 * <p>More formally, if this map contains a mapping from a key
177 * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
178 * key.equals(k))}, then this method returns {@code v}; otherwise
179 * it returns {@code null}. (There can be at most one such mapping.)
180 *
181 * <p>If this map permits null values, then a return value of
182 * {@code null} does not <i>necessarily</i> indicate that the map
183 * contains no mapping for the key; it's also possible that the map
184 * explicitly maps the key to {@code null}. The {@link #containsKey
185 * containsKey} operation may be used to distinguish these two cases.
186 *
187 * @param key the key whose associated value is to be returned
188 * @return the value to which the specified key is mapped, or
189 * {@code null} if this map contains no mapping for the key
190 * @throws ClassCastException if the key is of an inappropriate type for
191 * this map (optional)
192 * @throws NullPointerException if the specified key is null and this map
193 * does not permit null keys (optional)
194 */
195 V get(Object key);
196
197 // Modification Operations
198
199 /**
200 * Associates the specified value with the specified key in this map
201 * (optional operation). If the map previously contained a mapping for
202 * the key, the old value is replaced by the specified value. (A map
203 * <tt>m</tt> is said to contain a mapping for a key <tt>k</tt> if and only
204 * if {@link #containsKey(Object) m.containsKey(k)} would return
205 * <tt>true</tt>.)
206 *
207 * @param key key with which the specified value is to be associated
208 * @param value value to be associated with the specified key
209 * @return the previous value associated with <tt>key</tt>, or
210 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
211 * (A <tt>null</tt> return can also indicate that the map
212 * previously associated <tt>null</tt> with <tt>key</tt>,
213 * if the implementation supports <tt>null</tt> values.)
214 * @throws UnsupportedOperationException if the <tt>put</tt> operation
215 * is not supported by this map
216 * @throws ClassCastException if the class of the specified key or value
217 * prevents it from being stored in this map
218 * @throws NullPointerException if the specified key or value is null
219 * and this map does not permit null keys or values
220 * @throws IllegalArgumentException if some property of the specified key
221 * or value prevents it from being stored in this map
222 */
223 V put(K key, V value);
224
225 /**
226 * Removes the mapping for a key from this map if it is present
227 * (optional operation). More formally, if this map contains a mapping
228 * from key <tt>k</tt> to value <tt>v</tt> such that
229 * <code>(key==null ? k==null : key.equals(k))</code>, that mapping
230 * is removed. (The map can contain at most one such mapping.)
231 *
232 * <p>Returns the value to which this map previously associated the key,
233 * or <tt>null</tt> if the map contained no mapping for the key.
234 *
235 * <p>If this map permits null values, then a return value of
236 * <tt>null</tt> does not <i>necessarily</i> indicate that the map
237 * contained no mapping for the key; it's also possible that the map
238 * explicitly mapped the key to <tt>null</tt>.
239 *
240 * <p>The map will not contain a mapping for the specified key once the
241 * call returns.
242 *
243 * @param key key whose mapping is to be removed from the map
244 * @return the previous value associated with <tt>key</tt>, or
245 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
246 * @throws UnsupportedOperationException if the <tt>remove</tt> operation
247 * is not supported by this map
248 * @throws ClassCastException if the key is of an inappropriate type for
249 * this map (optional)
250 * @throws NullPointerException if the specified key is null and this
251 * map does not permit null keys (optional)
252 */
253 V remove(Object key);
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 // Views
288
289 /**
290 * Returns a {@link Set} view of the keys contained in this map.
291 * The set is backed by the map, so changes to the map are
292 * reflected in the set, and vice-versa. If the map is modified
293 * while an iteration over the set is in progress (except through
294 * the iterator's own <tt>remove</tt> operation), the results of
295 * the iteration are undefined. The set supports element removal,
296 * which removes the corresponding mapping from the map, via the
297 * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
298 * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
299 * operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
300 * operations.
301 *
302 * @return a set view of the keys contained in this map
303 */
304 Set<K> keySet();
305
306 /**
307 * Returns a {@link Collection} view of the values contained in this map.
308 * The collection is backed by the map, so changes to the map are
309 * reflected in the collection, and vice-versa. If the map is
310 * modified while an iteration over the collection is in progress
311 * (except through the iterator's own <tt>remove</tt> operation),
312 * the results of the iteration are undefined. The collection
313 * supports element removal, which removes the corresponding
314 * mapping from the map, via the <tt>Iterator.remove</tt>,
315 * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
316 * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
317 * support the <tt>add</tt> or <tt>addAll</tt> operations.
318 *
319 * @return a collection view of the values contained in this map
320 */
321 Collection<V> values();
322
323 /**
324 * Returns a {@link Set} view of the mappings contained in this map.
325 * The set is backed by the map, so changes to the map are
326 * reflected in the set, and vice-versa. If the map is modified
327 * while an iteration over the set is in progress (except through
328 * the iterator's own <tt>remove</tt> operation, or through the
329 * <tt>setValue</tt> operation on a map entry returned by the
330 * iterator) the results of the iteration are undefined. The set
331 * supports element removal, which removes the corresponding
332 * mapping from the map, via the <tt>Iterator.remove</tt>,
333 * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
334 * <tt>clear</tt> operations. It does not support the
335 * <tt>add</tt> or <tt>addAll</tt> operations.
336 *
337 * @return a set view of the mappings contained in this map
338 */
339 Set<Map.Entry<K, V>> entrySet();
340
341 /**
342 * A map entry (key-value pair). The <tt>Map.entrySet</tt> method returns
343 * a collection-view of the map, whose elements are of this class. The
344 * <i>only</i> way to obtain a reference to a map entry is from the
345 * iterator of this collection-view. These <tt>Map.Entry</tt> objects are
346 * valid <i>only</i> for the duration of the iteration; more formally,
347 * the behavior of a map entry is undefined if the backing map has been
348 * modified after the entry was returned by the iterator, except through
349 * the <tt>setValue</tt> operation on the map entry.
350 *
351 * @see Map#entrySet()
352 * @since 1.2
353 */
354 interface Entry<K, V> {
355 /**
356 * Returns the key corresponding to this entry.
357 *
358 * @return the key corresponding to this entry
359 * @throws IllegalStateException implementations may, but are not
360 * required to, throw this exception if the entry has been
361 * removed from the backing map.
362 */
363 K getKey();
364
365 /**
366 * Returns the value corresponding to this entry. If the mapping
367 * has been removed from the backing map (by the iterator's
368 * <tt>remove</tt> operation), the results of this call are undefined.
369 *
370 * @return the value corresponding to this entry
371 * @throws IllegalStateException implementations may, but are not
372 * required to, throw this exception if the entry has been
373 * removed from the backing map.
374 */
375 V getValue();
376
377 /**
378 * Replaces the value corresponding to this entry with the specified
379 * value (optional operation). (Writes through to the map.) The
380 * behavior of this call is undefined if the mapping has already been
381 * removed from the map (by the iterator's <tt>remove</tt> operation).
382 *
383 * @param value new value to be stored in this entry
384 * @return old value corresponding to the entry
385 * @throws UnsupportedOperationException if the <tt>put</tt> operation
386 * is not supported by the backing map
387 * @throws ClassCastException if the class of the specified value
388 * prevents it from being stored in the backing map
389 * @throws NullPointerException if the backing map does not permit
390 * null values, and the specified value is null
391 * @throws IllegalArgumentException if some property of this value
392 * prevents it from being stored in the backing map
393 * @throws IllegalStateException implementations may, but are not
394 * required to, throw this exception if the entry has been
395 * removed from the backing map.
396 */
397 V setValue(V value);
398
399 /**
400 * Compares the specified object with this entry for equality.
401 * Returns <tt>true</tt> if the given object is also a map entry and
402 * the two entries represent the same mapping. More formally, two
403 * entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping
404 * if<pre>
405 * (e1.getKey()==null ?
406 * e2.getKey()==null : e1.getKey().equals(e2.getKey())) &&
407 * (e1.getValue()==null ?
408 * e2.getValue()==null : e1.getValue().equals(e2.getValue()))
409 * </pre>
410 * This ensures that the <tt>equals</tt> method works properly across
411 * different implementations of the <tt>Map.Entry</tt> interface.
412 *
413 * @param o object to be compared for equality with this map entry
414 * @return <tt>true</tt> if the specified object is equal to this map
415 * entry
416 */
417 boolean equals(Object o);
418
419 /**
420 * Returns the hash code value for this map entry. The hash code
421 * of a map entry <tt>e</tt> is defined to be: <pre>
422 * (e.getKey()==null ? 0 : e.getKey().hashCode()) ^
423 * (e.getValue()==null ? 0 : e.getValue().hashCode())
424 * </pre>
425 * This ensures that <tt>e1.equals(e2)</tt> implies that
426 * <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries
427 * <tt>e1</tt> and <tt>e2</tt>, as required by the general
428 * contract of <tt>Object.hashCode</tt>.
429 *
430 * @return the hash code value for this map entry
431 * @see Object#hashCode()
432 * @see Object#equals(Object)
433 * @see #equals(Object)
434 */
435 int hashCode();
436 }
437
438 // Comparison and hashing
439
440 /**
441 * Compares the specified object with this map for equality. Returns
442 * <tt>true</tt> if the given object is also a map and the two maps
443 * represent the same mappings. More formally, two maps <tt>m1</tt> and
444 * <tt>m2</tt> represent the same mappings if
445 * <tt>m1.entrySet().equals(m2.entrySet())</tt>. This ensures that the
446 * <tt>equals</tt> method works properly across different implementations
447 * of the <tt>Map</tt> interface.
448 *
449 * @param o object to be compared for equality with this map
450 * @return <tt>true</tt> if the specified object is equal to this map
451 */
452 boolean equals(Object o);
453
454 /**
455 * Returns the hash code value for this map. The hash code of a map is
456 * defined to be the sum of the hash codes of each entry in the map's
457 * <tt>entrySet()</tt> view. This ensures that <tt>m1.equals(m2)</tt>
458 * implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two maps
459 * <tt>m1</tt> and <tt>m2</tt>, as required by the general contract of
460 * {@link Object#hashCode}.
461 *
462 * @return the hash code value for this map
463 * @see Map.Entry#hashCode()
464 * @see Object#equals(Object)
465 * @see #equals(Object)
466 */
467 int hashCode();
468 }
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