Map.java revision 8b056f0b15bc1e45da8d4c504353b05e681ac013
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 29import java.io.Serializable; 30import java.util.function.BiConsumer; 31import java.util.function.BiFunction; 32import java.util.function.Function; 33 34 35/** 36 * An object that maps keys to values. A map cannot contain duplicate keys; 37 * each key can map to at most one value. 38 * 39 * <p>This interface takes the place of the <tt>Dictionary</tt> class, which 40 * was a totally abstract class rather than an interface. 41 * 42 * <p>The <tt>Map</tt> interface provides three <i>collection views</i>, which 43 * allow a map's contents to be viewed as a set of keys, collection of values, 44 * or set of key-value mappings. The <i>order</i> of a map is defined as 45 * the order in which the iterators on the map's collection views return their 46 * elements. Some map implementations, like the <tt>TreeMap</tt> class, make 47 * specific guarantees as to their order; others, like the <tt>HashMap</tt> 48 * class, do not. 49 * 50 * <p>Note: great care must be exercised if mutable objects are used as map 51 * keys. The behavior of a map is not specified if the value of an object is 52 * changed in a manner that affects <tt>equals</tt> comparisons while the 53 * object is a key in the map. A special case of this prohibition is that it 54 * is not permissible for a map to contain itself as a key. While it is 55 * permissible for a map to contain itself as a value, extreme caution is 56 * advised: the <tt>equals</tt> and <tt>hashCode</tt> methods are no longer 57 * well defined on such a map. 58 * 59 * <p>All general-purpose map implementation classes should provide two 60 * "standard" constructors: a void (no arguments) constructor which creates an 61 * empty map, and a constructor with a single argument of type <tt>Map</tt>, 62 * which creates a new map with the same key-value mappings as its argument. 63 * In effect, the latter constructor allows the user to copy any map, 64 * producing an equivalent map of the desired class. There is no way to 65 * enforce this recommendation (as interfaces cannot contain constructors) but 66 * all of the general-purpose map implementations in the JDK comply. 67 * 68 * <p>The "destructive" methods contained in this interface, that is, the 69 * methods that modify the map on which they operate, are specified to throw 70 * <tt>UnsupportedOperationException</tt> if this map does not support the 71 * operation. If this is the case, these methods may, but are not required 72 * to, throw an <tt>UnsupportedOperationException</tt> if the invocation would 73 * have no effect on the map. For example, invoking the {@link #putAll(Map)} 74 * method on an unmodifiable map may, but is not required to, throw the 75 * exception if the map whose mappings are to be "superimposed" is empty. 76 * 77 * <p>Some map implementations have restrictions on the keys and values they 78 * may contain. For example, some implementations prohibit null keys and 79 * values, and some have restrictions on the types of their keys. Attempting 80 * to insert an ineligible key or value throws an unchecked exception, 81 * typically <tt>NullPointerException</tt> or <tt>ClassCastException</tt>. 82 * Attempting to query the presence of an ineligible key or value may throw an 83 * exception, or it may simply return false; some implementations will exhibit 84 * the former behavior and some will exhibit the latter. More generally, 85 * attempting an operation on an ineligible key or value whose completion 86 * would not result in the insertion of an ineligible element into the map may 87 * throw an exception or it may succeed, at the option of the implementation. 88 * Such exceptions are marked as "optional" in the specification for this 89 * interface. 90 * 91 * <p>This interface is a member of the 92 * <a href="{@docRoot}/../technotes/guides/collections/index.html"> 93 * Java Collections Framework</a>. 94 * 95 * <p>Many methods in Collections Framework interfaces are defined 96 * in terms of the {@link Object#equals(Object) equals} method. For 97 * example, the specification for the {@link #containsKey(Object) 98 * containsKey(Object key)} method says: "returns <tt>true</tt> if and 99 * only if this map contains a mapping for a key <tt>k</tt> such that 100 * <tt>(key==null ? k==null : key.equals(k))</tt>." This specification should 101 * <i>not</i> be construed to imply that invoking <tt>Map.containsKey</tt> 102 * with a non-null argument <tt>key</tt> will cause <tt>key.equals(k)</tt> to 103 * be invoked for any key <tt>k</tt>. Implementations are free to 104 * implement optimizations whereby the <tt>equals</tt> invocation is avoided, 105 * for example, by first comparing the hash codes of the two keys. (The 106 * {@link Object#hashCode()} specification guarantees that two objects with 107 * unequal hash codes cannot be equal.) More generally, implementations of 108 * the various Collections Framework interfaces are free to take advantage of 109 * the specified behavior of underlying {@link Object} methods wherever the 110 * implementor deems it appropriate. 111 * 112 * @param <K> the type of keys maintained by this map 113 * @param <V> the type of mapped values 114 * 115 * @author Josh Bloch 116 * @see HashMap 117 * @see TreeMap 118 * @see Hashtable 119 * @see SortedMap 120 * @see Collection 121 * @see Set 122 * @since 1.2 123 */ 124public interface Map<K,V> { 125 // Query Operations 126 127 /** 128 * Returns the number of key-value mappings in this map. If the 129 * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns 130 * <tt>Integer.MAX_VALUE</tt>. 131 * 132 * @return the number of key-value mappings in this map 133 */ 134 int size(); 135 136 /** 137 * Returns <tt>true</tt> if this map contains no key-value mappings. 138 * 139 * @return <tt>true</tt> if this map contains no key-value mappings 140 */ 141 boolean isEmpty(); 142 143 /** 144 * Returns <tt>true</tt> if this map contains a mapping for the specified 145 * key. More formally, returns <tt>true</tt> if and only if 146 * this map contains a mapping for a key <tt>k</tt> such that 147 * <tt>(key==null ? k==null : key.equals(k))</tt>. (There can be 148 * at most one such mapping.) 149 * 150 * @param key key whose presence in this map is to be tested 151 * @return <tt>true</tt> if this map contains a mapping for the specified 152 * key 153 * @throws ClassCastException if the key is of an inappropriate type for 154 * this map 155 * (<a href="Collection.html#optional-restrictions">optional</a>) 156 * @throws NullPointerException if the specified key is null and this map 157 * does not permit null keys 158 * (<a href="Collection.html#optional-restrictions">optional</a>) 159 */ 160 boolean containsKey(Object key); 161 162 /** 163 * Returns <tt>true</tt> if this map maps one or more keys to the 164 * specified value. More formally, returns <tt>true</tt> if and only if 165 * this map contains at least one mapping to a value <tt>v</tt> such that 166 * <tt>(value==null ? v==null : value.equals(v))</tt>. This operation 167 * will probably require time linear in the map size for most 168 * implementations of the <tt>Map</tt> interface. 169 * 170 * @param value value whose presence in this map is to be tested 171 * @return <tt>true</tt> if this map maps one or more keys to the 172 * specified value 173 * @throws ClassCastException if the value is of an inappropriate type for 174 * this map 175 * (<a href="Collection.html#optional-restrictions">optional</a>) 176 * @throws NullPointerException if the specified value is null and this 177 * map does not permit null values 178 * (<a href="Collection.html#optional-restrictions">optional</a>) 179 */ 180 boolean containsValue(Object value); 181 182 /** 183 * Returns the value to which the specified key is mapped, 184 * or {@code null} if this map contains no mapping for the key. 185 * 186 * <p>More formally, if this map contains a mapping from a key 187 * {@code k} to a value {@code v} such that {@code (key==null ? k==null : 188 * key.equals(k))}, then this method returns {@code v}; otherwise 189 * it returns {@code null}. (There can be at most one such mapping.) 190 * 191 * <p>If this map permits null values, then a return value of 192 * {@code null} does not <i>necessarily</i> indicate that the map 193 * contains no mapping for the key; it's also possible that the map 194 * explicitly maps the key to {@code null}. The {@link #containsKey 195 * containsKey} operation may be used to distinguish these two cases. 196 * 197 * @param key the key whose associated value is to be returned 198 * @return the value to which the specified key is mapped, or 199 * {@code null} if this map contains no mapping for the key 200 * @throws ClassCastException if the key is of an inappropriate type for 201 * this map 202 * (<a href="Collection.html#optional-restrictions">optional</a>) 203 * @throws NullPointerException if the specified key is null and this map 204 * does not permit null keys 205 * (<a href="Collection.html#optional-restrictions">optional</a>) 206 */ 207 V get(Object key); 208 209 // Modification Operations 210 211 /** 212 * Associates the specified value with the specified key in this map 213 * (optional operation). If the map previously contained a mapping for 214 * the key, the old value is replaced by the specified value. (A map 215 * <tt>m</tt> is said to contain a mapping for a key <tt>k</tt> if and only 216 * if {@link #containsKey(Object) m.containsKey(k)} would return 217 * <tt>true</tt>.) 218 * 219 * @param key key with which the specified value is to be associated 220 * @param value value to be associated with the specified key 221 * @return the previous value associated with <tt>key</tt>, or 222 * <tt>null</tt> if there was no mapping for <tt>key</tt>. 223 * (A <tt>null</tt> return can also indicate that the map 224 * previously associated <tt>null</tt> with <tt>key</tt>, 225 * if the implementation supports <tt>null</tt> values.) 226 * @throws UnsupportedOperationException if the <tt>put</tt> operation 227 * is not supported by this map 228 * @throws ClassCastException if the class of the specified key or value 229 * prevents it from being stored in this map 230 * @throws NullPointerException if the specified key or value is null 231 * and this map does not permit null keys or values 232 * @throws IllegalArgumentException if some property of the specified key 233 * or value prevents it from being stored in this map 234 */ 235 V put(K key, V value); 236 237 /** 238 * Removes the mapping for a key from this map if it is present 239 * (optional operation). More formally, if this map contains a mapping 240 * from key <tt>k</tt> to value <tt>v</tt> such that 241 * <code>(key==null ? k==null : key.equals(k))</code>, that mapping 242 * is removed. (The map can contain at most one such mapping.) 243 * 244 * <p>Returns the value to which this map previously associated the key, 245 * or <tt>null</tt> if the map contained no mapping for the key. 246 * 247 * <p>If this map permits null values, then a return value of 248 * <tt>null</tt> does not <i>necessarily</i> indicate that the map 249 * contained no mapping for the key; it's also possible that the map 250 * explicitly mapped the key to <tt>null</tt>. 251 * 252 * <p>The map will not contain a mapping for the specified key once the 253 * call returns. 254 * 255 * @param key key whose mapping is to be removed from the map 256 * @return the previous value associated with <tt>key</tt>, or 257 * <tt>null</tt> if there was no mapping for <tt>key</tt>. 258 * @throws UnsupportedOperationException if the <tt>remove</tt> operation 259 * is not supported by this map 260 * @throws ClassCastException if the key is of an inappropriate type for 261 * this map 262 * (<a href="Collection.html#optional-restrictions">optional</a>) 263 * @throws NullPointerException if the specified key is null and this 264 * map does not permit null keys 265 * (<a href="Collection.html#optional-restrictions">optional</a>) 266 */ 267 V remove(Object key); 268 269 270 // Bulk Operations 271 272 /** 273 * Copies all of the mappings from the specified map to this map 274 * (optional operation). The effect of this call is equivalent to that 275 * of calling {@link #put(Object,Object) put(k, v)} on this map once 276 * for each mapping from key <tt>k</tt> to value <tt>v</tt> in the 277 * specified map. The behavior of this operation is undefined if the 278 * specified map is modified while the operation is in progress. 279 * 280 * @param m mappings to be stored in this map 281 * @throws UnsupportedOperationException if the <tt>putAll</tt> operation 282 * is not supported by this map 283 * @throws ClassCastException if the class of a key or value in the 284 * specified map prevents it from being stored in this map 285 * @throws NullPointerException if the specified map is null, or if 286 * this map does not permit null keys or values, and the 287 * specified map contains null keys or values 288 * @throws IllegalArgumentException if some property of a key or value in 289 * the specified map prevents it from being stored in this map 290 */ 291 void putAll(Map<? extends K, ? extends V> m); 292 293 /** 294 * Removes all of the mappings from this map (optional operation). 295 * The map will be empty after this call returns. 296 * 297 * @throws UnsupportedOperationException if the <tt>clear</tt> operation 298 * is not supported by this map 299 */ 300 void clear(); 301 302 303 // Views 304 305 /** 306 * Returns a {@link Set} view of the keys contained in this map. 307 * The set is backed by the map, so changes to the map are 308 * reflected in the set, and vice-versa. If the map is modified 309 * while an iteration over the set is in progress (except through 310 * the iterator's own <tt>remove</tt> operation), the results of 311 * the iteration are undefined. The set supports element removal, 312 * which removes the corresponding mapping from the map, via the 313 * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, 314 * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> 315 * operations. It does not support the <tt>add</tt> or <tt>addAll</tt> 316 * operations. 317 * 318 * @return a set view of the keys contained in this map 319 */ 320 Set<K> keySet(); 321 322 /** 323 * Returns a {@link Collection} view of the values contained in this map. 324 * The collection is backed by the map, so changes to the map are 325 * reflected in the collection, and vice-versa. If the map is 326 * modified while an iteration over the collection is in progress 327 * (except through the iterator's own <tt>remove</tt> operation), 328 * the results of the iteration are undefined. The collection 329 * supports element removal, which removes the corresponding 330 * mapping from the map, via the <tt>Iterator.remove</tt>, 331 * <tt>Collection.remove</tt>, <tt>removeAll</tt>, 332 * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not 333 * support the <tt>add</tt> or <tt>addAll</tt> operations. 334 * 335 * @return a collection view of the values contained in this map 336 */ 337 Collection<V> values(); 338 339 /** 340 * Returns a {@link Set} view of the mappings contained in this map. 341 * The set is backed by the map, so changes to the map are 342 * reflected in the set, and vice-versa. If the map is modified 343 * while an iteration over the set is in progress (except through 344 * the iterator's own <tt>remove</tt> operation, or through the 345 * <tt>setValue</tt> operation on a map entry returned by the 346 * iterator) the results of the iteration are undefined. The set 347 * supports element removal, which removes the corresponding 348 * mapping from the map, via the <tt>Iterator.remove</tt>, 349 * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and 350 * <tt>clear</tt> operations. It does not support the 351 * <tt>add</tt> or <tt>addAll</tt> operations. 352 * 353 * @return a set view of the mappings contained in this map 354 */ 355 Set<Map.Entry<K, V>> entrySet(); 356 357 /** 358 * A map entry (key-value pair). The <tt>Map.entrySet</tt> method returns 359 * a collection-view of the map, whose elements are of this class. The 360 * <i>only</i> way to obtain a reference to a map entry is from the 361 * iterator of this collection-view. These <tt>Map.Entry</tt> objects are 362 * valid <i>only</i> for the duration of the iteration; more formally, 363 * the behavior of a map entry is undefined if the backing map has been 364 * modified after the entry was returned by the iterator, except through 365 * the <tt>setValue</tt> operation on the map entry. 366 * 367 * @see Map#entrySet() 368 * @since 1.2 369 */ 370 interface Entry<K,V> { 371 /** 372 * Returns the key corresponding to this entry. 373 * 374 * @return the key corresponding to this entry 375 * @throws IllegalStateException implementations may, but are not 376 * required to, throw this exception if the entry has been 377 * removed from the backing map. 378 */ 379 K getKey(); 380 381 /** 382 * Returns the value corresponding to this entry. If the mapping 383 * has been removed from the backing map (by the iterator's 384 * <tt>remove</tt> operation), the results of this call are undefined. 385 * 386 * @return the value corresponding to this entry 387 * @throws IllegalStateException implementations may, but are not 388 * required to, throw this exception if the entry has been 389 * removed from the backing map. 390 */ 391 V getValue(); 392 393 /** 394 * Replaces the value corresponding to this entry with the specified 395 * value (optional operation). (Writes through to the map.) The 396 * behavior of this call is undefined if the mapping has already been 397 * removed from the map (by the iterator's <tt>remove</tt> operation). 398 * 399 * @param value new value to be stored in this entry 400 * @return old value corresponding to the entry 401 * @throws UnsupportedOperationException if the <tt>put</tt> operation 402 * is not supported by the backing map 403 * @throws ClassCastException if the class of the specified value 404 * prevents it from being stored in the backing map 405 * @throws NullPointerException if the backing map does not permit 406 * null values, and the specified value is null 407 * @throws IllegalArgumentException if some property of this value 408 * prevents it from being stored in the backing map 409 * @throws IllegalStateException implementations may, but are not 410 * required to, throw this exception if the entry has been 411 * removed from the backing map. 412 */ 413 V setValue(V value); 414 415 /** 416 * Compares the specified object with this entry for equality. 417 * Returns <tt>true</tt> if the given object is also a map entry and 418 * the two entries represent the same mapping. More formally, two 419 * entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping 420 * if<pre> 421 * (e1.getKey()==null ? 422 * e2.getKey()==null : e1.getKey().equals(e2.getKey())) && 423 * (e1.getValue()==null ? 424 * e2.getValue()==null : e1.getValue().equals(e2.getValue())) 425 * </pre> 426 * This ensures that the <tt>equals</tt> method works properly across 427 * different implementations of the <tt>Map.Entry</tt> interface. 428 * 429 * @param o object to be compared for equality with this map entry 430 * @return <tt>true</tt> if the specified object is equal to this map 431 * entry 432 */ 433 boolean equals(Object o); 434 435 /** 436 * Returns the hash code value for this map entry. The hash code 437 * of a map entry <tt>e</tt> is defined to be: <pre> 438 * (e.getKey()==null ? 0 : e.getKey().hashCode()) ^ 439 * (e.getValue()==null ? 0 : e.getValue().hashCode()) 440 * </pre> 441 * This ensures that <tt>e1.equals(e2)</tt> implies that 442 * <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries 443 * <tt>e1</tt> and <tt>e2</tt>, as required by the general 444 * contract of <tt>Object.hashCode</tt>. 445 * 446 * @return the hash code value for this map entry 447 * @see Object#hashCode() 448 * @see Object#equals(Object) 449 * @see #equals(Object) 450 */ 451 int hashCode(); 452 453 /** 454 * Returns a comparator that compares {@link Map.Entry} in natural order on key. 455 * 456 * <p>The returned comparator is serializable and throws {@link 457 * NullPointerException} when comparing an entry with a null key. 458 * 459 * @param <K> the {@link Comparable} type of then map keys 460 * @param <V> the type of the map values 461 * @return a comparator that compares {@link Map.Entry} in natural order on key. 462 * @see Comparable 463 * @since 1.8 464 */ 465 public static <K extends Comparable<? super K>, V> Comparator<Map.Entry<K,V>> comparingByKey() { 466 return (Comparator<Map.Entry<K, V>> & Serializable) 467 (c1, c2) -> c1.getKey().compareTo(c2.getKey()); 468 } 469 470 /** 471 * Returns a comparator that compares {@link Map.Entry} by key using the given 472 * {@link Comparator}. 473 * 474 * <p>The returned comparator is serializable if the specified comparator 475 * is also serializable. 476 * 477 * @param <K> the type of the map keys 478 * @param <V> the type of the map values 479 * @param cmp the key {@link Comparator} 480 * @return a comparator that compares {@link Map.Entry} by the key. 481 * @since 1.8 482 */ 483 public static <K, V> Comparator<Map.Entry<K, V>> comparingByKey(Comparator<? super K> cmp) { 484 Objects.requireNonNull(cmp); 485 return (Comparator<Map.Entry<K, V>> & Serializable) 486 (c1, c2) -> cmp.compare(c1.getKey(), c2.getKey()); 487 } 488 } 489 490 // Comparison and hashing 491 492 /** 493 * Compares the specified object with this map for equality. Returns 494 * <tt>true</tt> if the given object is also a map and the two maps 495 * represent the same mappings. More formally, two maps <tt>m1</tt> and 496 * <tt>m2</tt> represent the same mappings if 497 * <tt>m1.entrySet().equals(m2.entrySet())</tt>. This ensures that the 498 * <tt>equals</tt> method works properly across different implementations 499 * of the <tt>Map</tt> interface. 500 * 501 * @param o object to be compared for equality with this map 502 * @return <tt>true</tt> if the specified object is equal to this map 503 */ 504 boolean equals(Object o); 505 506 /** 507 * Returns the hash code value for this map. The hash code of a map is 508 * defined to be the sum of the hash codes of each entry in the map's 509 * <tt>entrySet()</tt> view. This ensures that <tt>m1.equals(m2)</tt> 510 * implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two maps 511 * <tt>m1</tt> and <tt>m2</tt>, as required by the general contract of 512 * {@link Object#hashCode}. 513 * 514 * @return the hash code value for this map 515 * @see Map.Entry#hashCode() 516 * @see Object#equals(Object) 517 * @see #equals(Object) 518 */ 519 int hashCode(); 520 521 /** 522 * Performs the given action for each entry in this map until all entries 523 * have been processed or the action throws an exception. Unless 524 * otherwise specified by the implementing class, actions are performed in 525 * the order of entry set iteration (if an iteration order is specified.) 526 * Exceptions thrown by the action are relayed to the caller. 527 * 528 * @implSpec 529 * The default implementation is equivalent to, for this {@code map}: 530 * <pre> {@code 531 * for (Map.Entry<K, V> entry : map.entrySet()) 532 * action.accept(entry.getKey(), entry.getValue()); 533 * }</pre> 534 * 535 * The default implementation makes no guarantees about synchronization 536 * or atomicity properties of this method. Any implementation providing 537 * atomicity guarantees must override this method and document its 538 * concurrency properties. 539 * 540 * @param action The action to be performed for each entry 541 * @throws NullPointerException if the specified action is null 542 * @throws ConcurrentModificationException if an entry is found to be 543 * removed during iteration 544 * @since 1.8 545 */ 546 default void forEach(BiConsumer<? super K, ? super V> action) { 547 Objects.requireNonNull(action); 548 for (Map.Entry<K, V> entry : entrySet()) { 549 K k; 550 V v; 551 try { 552 k = entry.getKey(); 553 v = entry.getValue(); 554 } catch(IllegalStateException ise) { 555 // this usually means the entry is no longer in the map. 556 throw new ConcurrentModificationException(ise); 557 } 558 action.accept(k, v); 559 } 560 } 561} 562