Map.java revision 51b1b6997fd3f980076b8081f7f1165ccc2a4008
1/* 2 * Copyright (c) 1997, 2006, 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 26package 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 */ 117public 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 148 * (<a href="Collection.html#optional-restrictions">optional</a>) 149 * @throws NullPointerException if the specified key is null and this map 150 * does not permit null keys 151 * (<a href="Collection.html#optional-restrictions">optional</a>) 152 */ 153 boolean containsKey(Object key); 154 155 /** 156 * Returns <tt>true</tt> if this map maps one or more keys to the 157 * specified value. More formally, returns <tt>true</tt> if and only if 158 * this map contains at least one mapping to a value <tt>v</tt> such that 159 * <tt>(value==null ? v==null : value.equals(v))</tt>. This operation 160 * will probably require time linear in the map size for most 161 * implementations of the <tt>Map</tt> interface. 162 * 163 * @param value value whose presence in this map is to be tested 164 * @return <tt>true</tt> if this map maps one or more keys to the 165 * specified value 166 * @throws ClassCastException if the value is of an inappropriate type for 167 * this map 168 * (<a href="Collection.html#optional-restrictions">optional</a>) 169 * @throws NullPointerException if the specified value is null and this 170 * map does not permit null values 171 * (<a href="Collection.html#optional-restrictions">optional</a>) 172 */ 173 boolean containsValue(Object value); 174 175 /** 176 * Returns the value to which the specified key is mapped, 177 * or {@code null} if this map contains no mapping for the key. 178 * 179 * <p>More formally, if this map contains a mapping from a key 180 * {@code k} to a value {@code v} such that {@code (key==null ? k==null : 181 * key.equals(k))}, then this method returns {@code v}; otherwise 182 * it returns {@code null}. (There can be at most one such mapping.) 183 * 184 * <p>If this map permits null values, then a return value of 185 * {@code null} does not <i>necessarily</i> indicate that the map 186 * contains no mapping for the key; it's also possible that the map 187 * explicitly maps the key to {@code null}. The {@link #containsKey 188 * containsKey} operation may be used to distinguish these two cases. 189 * 190 * @param key the key whose associated value is to be returned 191 * @return the value to which the specified key is mapped, or 192 * {@code null} if this map contains no mapping for the key 193 * @throws ClassCastException if the key is of an inappropriate type for 194 * this map 195 * (<a href="Collection.html#optional-restrictions">optional</a>) 196 * @throws NullPointerException if the specified key is null and this map 197 * does not permit null keys 198 * (<a href="Collection.html#optional-restrictions">optional</a>) 199 */ 200 V get(Object key); 201 202 // Modification Operations 203 204 /** 205 * Associates the specified value with the specified key in this map 206 * (optional operation). If the map previously contained a mapping for 207 * the key, the old value is replaced by the specified value. (A map 208 * <tt>m</tt> is said to contain a mapping for a key <tt>k</tt> if and only 209 * if {@link #containsKey(Object) m.containsKey(k)} would return 210 * <tt>true</tt>.) 211 * 212 * @param key key with which the specified value is to be associated 213 * @param value value to be associated with the specified key 214 * @return the previous value associated with <tt>key</tt>, or 215 * <tt>null</tt> if there was no mapping for <tt>key</tt>. 216 * (A <tt>null</tt> return can also indicate that the map 217 * previously associated <tt>null</tt> with <tt>key</tt>, 218 * if the implementation supports <tt>null</tt> values.) 219 * @throws UnsupportedOperationException if the <tt>put</tt> operation 220 * is not supported by this map 221 * @throws ClassCastException if the class of the specified key or value 222 * prevents it from being stored in this map 223 * @throws NullPointerException if the specified key or value is null 224 * and this map does not permit null keys or values 225 * @throws IllegalArgumentException if some property of the specified key 226 * or value prevents it from being stored in this map 227 */ 228 V put(K key, V value); 229 230 /** 231 * Removes the mapping for a key from this map if it is present 232 * (optional operation). More formally, if this map contains a mapping 233 * from key <tt>k</tt> to value <tt>v</tt> such that 234 * <code>(key==null ? k==null : key.equals(k))</code>, that mapping 235 * is removed. (The map can contain at most one such mapping.) 236 * 237 * <p>Returns the value to which this map previously associated the key, 238 * or <tt>null</tt> if the map contained no mapping for the key. 239 * 240 * <p>If this map permits null values, then a return value of 241 * <tt>null</tt> does not <i>necessarily</i> indicate that the map 242 * contained no mapping for the key; it's also possible that the map 243 * explicitly mapped the key to <tt>null</tt>. 244 * 245 * <p>The map will not contain a mapping for the specified key once the 246 * call returns. 247 * 248 * @param key key whose mapping is to be removed from the map 249 * @return the previous value associated with <tt>key</tt>, or 250 * <tt>null</tt> if there was no mapping for <tt>key</tt>. 251 * @throws UnsupportedOperationException if the <tt>remove</tt> operation 252 * is not supported by this map 253 * @throws ClassCastException if the key is of an inappropriate type for 254 * this map 255 * (<a href="Collection.html#optional-restrictions">optional</a>) 256 * @throws NullPointerException if the specified key is null and this 257 * map does not permit null keys 258 * (<a href="Collection.html#optional-restrictions">optional</a>) 259 */ 260 V remove(Object key); 261 262 263 // Bulk Operations 264 265 /** 266 * Copies all of the mappings from the specified map to this map 267 * (optional operation). The effect of this call is equivalent to that 268 * of calling {@link #put(Object,Object) put(k, v)} on this map once 269 * for each mapping from key <tt>k</tt> to value <tt>v</tt> in the 270 * specified map. The behavior of this operation is undefined if the 271 * specified map is modified while the operation is in progress. 272 * 273 * @param m mappings to be stored in this map 274 * @throws UnsupportedOperationException if the <tt>putAll</tt> operation 275 * is not supported by this map 276 * @throws ClassCastException if the class of a key or value in the 277 * specified map prevents it from being stored in this map 278 * @throws NullPointerException if the specified map is null, or if 279 * this map does not permit null keys or values, and the 280 * specified map contains null keys or values 281 * @throws IllegalArgumentException if some property of a key or value in 282 * the specified map prevents it from being stored in this map 283 */ 284 void putAll(Map<? extends K, ? extends V> m); 285 286 /** 287 * Removes all of the mappings from this map (optional operation). 288 * The map will be empty after this call returns. 289 * 290 * @throws UnsupportedOperationException if the <tt>clear</tt> operation 291 * is not supported by this map 292 */ 293 void clear(); 294 295 296 // Views 297 298 /** 299 * Returns a {@link Set} view of the keys contained in this map. 300 * The set is backed by the map, so changes to the map are 301 * reflected in the set, and vice-versa. If the map is modified 302 * while an iteration over the set is in progress (except through 303 * the iterator's own <tt>remove</tt> operation), the results of 304 * the iteration are undefined. The set supports element removal, 305 * which removes the corresponding mapping from the map, via the 306 * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, 307 * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> 308 * operations. It does not support the <tt>add</tt> or <tt>addAll</tt> 309 * operations. 310 * 311 * @return a set view of the keys contained in this map 312 */ 313 Set<K> keySet(); 314 315 /** 316 * Returns a {@link Collection} view of the values contained in this map. 317 * The collection is backed by the map, so changes to the map are 318 * reflected in the collection, and vice-versa. If the map is 319 * modified while an iteration over the collection is in progress 320 * (except through the iterator's own <tt>remove</tt> operation), 321 * the results of the iteration are undefined. The collection 322 * supports element removal, which removes the corresponding 323 * mapping from the map, via the <tt>Iterator.remove</tt>, 324 * <tt>Collection.remove</tt>, <tt>removeAll</tt>, 325 * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not 326 * support the <tt>add</tt> or <tt>addAll</tt> operations. 327 * 328 * @return a collection view of the values contained in this map 329 */ 330 Collection<V> values(); 331 332 /** 333 * Returns a {@link Set} view of the mappings contained in this map. 334 * The set is backed by the map, so changes to the map are 335 * reflected in the set, and vice-versa. If the map is modified 336 * while an iteration over the set is in progress (except through 337 * the iterator's own <tt>remove</tt> operation, or through the 338 * <tt>setValue</tt> operation on a map entry returned by the 339 * iterator) the results of the iteration are undefined. The set 340 * supports element removal, which removes the corresponding 341 * mapping from the map, via the <tt>Iterator.remove</tt>, 342 * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and 343 * <tt>clear</tt> operations. It does not support the 344 * <tt>add</tt> or <tt>addAll</tt> operations. 345 * 346 * @return a set view of the mappings contained in this map 347 */ 348 Set<Map.Entry<K, V>> entrySet(); 349 350 /** 351 * A map entry (key-value pair). The <tt>Map.entrySet</tt> method returns 352 * a collection-view of the map, whose elements are of this class. The 353 * <i>only</i> way to obtain a reference to a map entry is from the 354 * iterator of this collection-view. These <tt>Map.Entry</tt> objects are 355 * valid <i>only</i> for the duration of the iteration; more formally, 356 * the behavior of a map entry is undefined if the backing map has been 357 * modified after the entry was returned by the iterator, except through 358 * the <tt>setValue</tt> operation on the map entry. 359 * 360 * @see Map#entrySet() 361 * @since 1.2 362 */ 363 interface Entry<K,V> { 364 /** 365 * Returns the key corresponding to this entry. 366 * 367 * @return the key corresponding to this entry 368 * @throws IllegalStateException implementations may, but are not 369 * required to, throw this exception if the entry has been 370 * removed from the backing map. 371 */ 372 K getKey(); 373 374 /** 375 * Returns the value corresponding to this entry. If the mapping 376 * has been removed from the backing map (by the iterator's 377 * <tt>remove</tt> operation), the results of this call are undefined. 378 * 379 * @return the value 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 V getValue(); 385 386 /** 387 * Replaces the value corresponding to this entry with the specified 388 * value (optional operation). (Writes through to the map.) The 389 * behavior of this call is undefined if the mapping has already been 390 * removed from the map (by the iterator's <tt>remove</tt> operation). 391 * 392 * @param value new value to be stored in this entry 393 * @return old value corresponding to the entry 394 * @throws UnsupportedOperationException if the <tt>put</tt> operation 395 * is not supported by the backing map 396 * @throws ClassCastException if the class of the specified value 397 * prevents it from being stored in the backing map 398 * @throws NullPointerException if the backing map does not permit 399 * null values, and the specified value is null 400 * @throws IllegalArgumentException if some property of this value 401 * prevents it from being stored in the backing map 402 * @throws IllegalStateException implementations may, but are not 403 * required to, throw this exception if the entry has been 404 * removed from the backing map. 405 */ 406 V setValue(V value); 407 408 /** 409 * Compares the specified object with this entry for equality. 410 * Returns <tt>true</tt> if the given object is also a map entry and 411 * the two entries represent the same mapping. More formally, two 412 * entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping 413 * if<pre> 414 * (e1.getKey()==null ? 415 * e2.getKey()==null : e1.getKey().equals(e2.getKey())) && 416 * (e1.getValue()==null ? 417 * e2.getValue()==null : e1.getValue().equals(e2.getValue())) 418 * </pre> 419 * This ensures that the <tt>equals</tt> method works properly across 420 * different implementations of the <tt>Map.Entry</tt> interface. 421 * 422 * @param o object to be compared for equality with this map entry 423 * @return <tt>true</tt> if the specified object is equal to this map 424 * entry 425 */ 426 boolean equals(Object o); 427 428 /** 429 * Returns the hash code value for this map entry. The hash code 430 * of a map entry <tt>e</tt> is defined to be: <pre> 431 * (e.getKey()==null ? 0 : e.getKey().hashCode()) ^ 432 * (e.getValue()==null ? 0 : e.getValue().hashCode()) 433 * </pre> 434 * This ensures that <tt>e1.equals(e2)</tt> implies that 435 * <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries 436 * <tt>e1</tt> and <tt>e2</tt>, as required by the general 437 * contract of <tt>Object.hashCode</tt>. 438 * 439 * @return the hash code value for this map entry 440 * @see Object#hashCode() 441 * @see Object#equals(Object) 442 * @see #equals(Object) 443 */ 444 int hashCode(); 445 } 446 447 // Comparison and hashing 448 449 /** 450 * Compares the specified object with this map for equality. Returns 451 * <tt>true</tt> if the given object is also a map and the two maps 452 * represent the same mappings. More formally, two maps <tt>m1</tt> and 453 * <tt>m2</tt> represent the same mappings if 454 * <tt>m1.entrySet().equals(m2.entrySet())</tt>. This ensures that the 455 * <tt>equals</tt> method works properly across different implementations 456 * of the <tt>Map</tt> interface. 457 * 458 * @param o object to be compared for equality with this map 459 * @return <tt>true</tt> if the specified object is equal to this map 460 */ 461 boolean equals(Object o); 462 463 /** 464 * Returns the hash code value for this map. The hash code of a map is 465 * defined to be the sum of the hash codes of each entry in the map's 466 * <tt>entrySet()</tt> view. This ensures that <tt>m1.equals(m2)</tt> 467 * implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two maps 468 * <tt>m1</tt> and <tt>m2</tt>, as required by the general contract of 469 * {@link Object#hashCode}. 470 * 471 * @return the hash code value for this map 472 * @see Map.Entry#hashCode() 473 * @see Object#equals(Object) 474 * @see #equals(Object) 475 */ 476 int hashCode(); 477 478} 479